Ca2+-Activated K+ Channels and the Regulation of the Uteroplacental Circulation

Adequate uteroplacental blood supply is essential for the development and growth of the placenta and fetus during pregnancy. Aberrant uteroplacental perfusion is associated with pregnancy complications such as preeclampsia, fetal growth restriction (FGR), and gestational diabetes. The regulation of uteroplacental blood flow is thus vital to the well-being of the mother and fetus. Ca2+-activated K+ (KCa) channels of small, intermediate, and large conductance participate in setting and regulating the resting membrane potential of vascular smooth muscle cells (VSMCs) and endothelial cells (ECs) and play a critical role in controlling vascular tone and blood pressure. KCa channels are important mediators of estrogen/pregnancy-induced adaptive changes in the uteroplacental circulation. Activation of the channels hyperpolarizes uteroplacental VSMCs/ECs, leading to attenuated vascular tone, blunted vasopressor responses, and increased uteroplacental blood flow. However, the regulation of uteroplacental vascular function by KCa channels is compromised in pregnancy complications. This review intends to provide a comprehensive overview of roles of KCa channels in the regulation of the uteroplacental circulation under physiological and pathophysiological conditions.


Introduction
Vascular tone in small arteries/arterioles governs vascular resistance and hence blood perfusion of a given tissue/organ. It is determined by the contractile state of vascular smooth muscle cells (VSMCs), which is regulated by dynamic changes in intracellular Ca 2+ concentrations ([Ca 2+ ] i ) [1]. Vasoconstriction is initiated by an increase in [Ca 2+ ] i primarily due to Ca 2+ influx mediated by the L-type voltage-dependent Ca 2+ (Ca V 1.2) channel in the plasma membrane and/or Ca 2+ release mediated by ryanodine (RyR)/inositol trisphosphate (IP 3 R) receptors in the sarcoplasmic reticulum (SR) membrane. The activity of the Ca V 1.2 channel in VSMCs is regulated by the membrane potential. Potassium (K + ) channels are dominant ion conductive pathways in the vasculature to set/regulate the membrane potential. Their activities in endothelial cells (ECs) and VSMCs participate in regulating Ca 2+ homeostasis and vascular tone [2,3]. Membrane hyperpolarization induced by the opening of K + channels closes the Ca V 1.2 channel in VSMCs, leading to a fall in [Ca 2+ ] i and subsequent vasodilatation. In contrast, membrane depolarization caused by the closing of K + channels opens the Ca V 1.2 channel, resulting in an increase in [Ca 2+ ] i and vasoconstriction. Therefore, the dynamic interplay between Ca V 1.2 and K + channels in the vasculature plays a pivotal role in regulating vascular tone. Among various types of K + channels in ECs and VSMCs, Ca 2+ -activated K + (K Ca ) channels are instrumental in the regulation of vascular tone [2][3][4][5][6][7].
Uteroplacental blood flow increases dramatically during pregnancy. Adequate uteroplacental blood perfusion is essential for the growth/development of the placenta and fetus, as well as the well-being of the mother. Uteroplacental blood flow is inversely proportional to uteroplacental vascular resistance. Increased uteroplacental blood flow in pregnancy is primarily achieved by lowering uteroplacental vascular resistance owing to the structural remodeling of spiral arteries, establishment of the placenta, and vasodilation [8,9]. Adaptive changes in the uteroplacental circulation are impaired in pregnancy complications such as preeclampsia, fetal growth restriction (FGR, also known as intrauterine growth restriction), and gestational diabetes, leading to insufficient perfusion of the placenta [9][10][11]. These disorders are interrelated. For example, early-onset preeclampsia is often associated with FGR, whereas gestational diabetes is a risk factor for preeclampsia [12,13]. Pregnancy complications are associated with maternal and perinatal morbidity and mortality [14][15][16] and predispose the mother and offspring to metabolic and cardiovascular diseases in later life [17][18][19][20]. The first experimental evidence that K Ca channels participate in regulating uteroplacental blood flow was presented by Rosenfeld's group in 2000 [21]. Since then, molecular and functional expression of K Ca channels in uteroplacental vessels has received considerable attention. In this review, we highlight current knowledge on the roles of K Ca channels in the regulation of uteroplacental circulation in physiological and pathophysiological conditions.

Overview of K Ca Channels
K Ca channels are a large family of K + channels, which are activated by intracellular Ca 2+ and selectively transport K + ions. K Ca channels contain six/seven-transmembrane domains, and are classified into two groups based on their biophysical properties [22]. One group includes the BK Ca channel that has large single-channel conductance ranging from 100 to 300 pS [23,24] and is activated by micromolar [Ca 2+ ] i and membrane depolarization [23,25]. The other group comprises small-conductance (SK Ca ) (K Ca 2.1-2.3) and intermediate-conductance (IK Ca , K Ca 3.1) K Ca channels that are voltage-insensitive and are activated by sub-micromolar [Ca 2+ ] i . The SK Ca channel has single-channel conductance of 5-20 pS [26,27], whereas the IK Ca channel has unitary conductance of 20-40 pS [28,29].
A functional BK Ca channel is composed of a tetramer of α-subunit that is encoded by the KCNMA1 gene. The BK Ca channel achieves its functional diversity primarily through the association of α subunits with accessory subunits and other proteins, alternative splicing, and post-translational modifications such as phosphorylation, oxidation, and palmitoylation [30][31][32][33][34][35][36][37]. Each BK Ca channel α subunit (125-140 kDa) contains seven transmembrane spanning segments (S0-S6) and a large cytoplasmic COOH-terminus ( Figure 1). They form three main structural domains that serve distinct functions [38]. S1-S4 segments constitute the voltage-sensing domain that detects changes in the membrane potential. S5-S6 segments line the pore to control K + permeation [39,40]. Two tandem RCK (regulator of conductance for K + ) domains (RCK1 and RCK2) in the cytoplasmic COOH-terminus from each subunit form a Ca 2+ gating ring and function as a Ca 2+ sensor [41].
The BK Ca channel is ubiquitously distributed among mammalian tissues [39] and usually associates with auxiliary β-subunits (~20 kDa). These accessory proteins are expressed in a cell-specific manner and display unique regulatory effects on the channel. Four distinct β-subunits, β1-4, are encoded by KCNMB1-4 [22]. The β1 subunit is primarily expressed in smooth muscle [42], whereas β2, β3, and β4 subunits are mostly expressed in neurons, chromaffin cells, kidney, heart, liver, and lung, among others [43][44][45]. The β-subunit consists of two transmembrane domains with intracellular N-and C-termini and a long extracellular loop ( Figure 1). Up to four β-subunits could co-assemble with pore-forming α subunits [46,47]. Co-assembling with these auxiliary subunits alters the channel's apparent sensitivity to Ca 2+ and voltage as well as kinetic properties [35]. A group of leucine-rich repeat-containing (LRRC) proteins (~35 kDa) are identified as auxiliary γ subunits of the BKCa channel [48]. The expression of LRRC proteins is also tissue-dependent [49]. These LRRC proteins are structurally distinct from the β-subunit. They consist of a large, extracellular domain with six leucine-rich repeat units (LRR1-6), and a single transmembrane segment (Figure 1). In a manner similar to the β subunit, the association of γ subunits to α subunits also alters channel gating properties by increasing voltage sensitivity even in the absence of Ca 2+ [35]. SKCa channels are encoded by KCNN1-3, whereas the IKCa channel is encoded by KCNN4. SKCa and IKCa channels share a similar topology to members of the KV channel superfamily and consist of six transmembrane segments (S1-S6) [50] ( Figure 1). They are also tetrameric structures. The channel pore is formed by S5 and S6. However, the S4 segment of SKCa and IKCa channels contains fewer charged residues than its counterparts in the KV and BKCa channels, resulting in a lack of voltage dependence. These channels are expressed primarily in neurons and ECs. Although the activities of SKCa and IKCa channels are also controlled by intracellular Ca 2+ levels, Ca 2+ does not directly bind to channels. Instead, the Ca 2+ sensitivity of these channels is achieved through the binding of Ca 2+ to calmodulin (CaM) constitutively bound to the C-terminus of the channel [51,52].

KCa Channels in VSMCs
The BKCa channel α subunit is abundantly expressed in VSMCs of virtually all vascular beds. BKCa channel accessory β and γ subunits are also found in VSMCs [46,53-55]. The predominant β isoform in VSMCs is the β1 subunit [42]. Although β2 and β4 subunits are also present in VSMCs of some vessels, their expression is extremely low [56][57][58]. The association of accessory subunits with α subunits alters channel biophysical properties. Both β1 and γ subunits increase BKCa channel sensitivity to both Ca 2+ and voltage in VSMCs [42,53]. The α subunit contains the S0 segment, voltage sensing domain (S1-S4 segments), pore gate domain (S5 and S6 segments), and cytosolic domain containing RCK1 and RCK2. Ca 2+ sensitivity is conferred by binding of Ca 2+ to RCK1 and RCK2. (B) Membrane topology of BK Ca channel β and γ subunits. The β subunit is comprised of two transmembrane domains, whereas the γ subunit possesses only one transmembrane domain with six leucine-rich repeat segments in its extracellular domain. (C) Membrane topology of SK Ca and IK Ca channels. Both SK Ca and IK Ca channels contain six transmembrane domains. Ca 2+ sensitivity is conferred by constitutively bound calmodulin (CaM) to the intracellular C-terminus. (D) K Ca channels are either heterotetrameric (BK Ca ) or homotetrameric (SK Ca and IK Ca ) assemblies of subunits.
A group of leucine-rich repeat-containing (LRRC) proteins (~35 kDa) are identified as auxiliary γ subunits of the BK Ca channel [48]. The expression of LRRC proteins is also tissue-dependent [49]. These LRRC proteins are structurally distinct from the β-subunit. They consist of a large, extracellular domain with six leucine-rich repeat units (LRR1-6), and a single transmembrane segment (Figure 1). In a manner similar to the β subunit, the association of γ subunits to α subunits also alters channel gating properties by increasing voltage sensitivity even in the absence of Ca 2+ [35]. SK Ca channels are encoded by KCNN1-3, whereas the IK Ca channel is encoded by KCNN4. SK Ca and IK Ca channels share a similar topology to members of the K V channel superfamily and consist of six transmembrane segments (S1-S6) [50] ( Figure 1). They are also tetrameric structures. The channel pore is formed by S5 and S6. However, the S4 segment of SK Ca and IK Ca channels contains fewer charged residues than its counterparts in the K V and BK Ca channels, resulting in a lack of voltage dependence. These channels are expressed primarily in neurons and ECs. Although the activities of SK Ca and IK Ca channels are also controlled by intracellular Ca 2+ levels, Ca 2+ does not directly bind to channels. Instead, the Ca 2+ sensitivity of these channels is achieved through the binding of Ca 2+ to calmodulin (CaM) constitutively bound to the C-terminus of the channel [51,52].

K Ca Channels in VSMCs
The BK Ca channel α subunit is abundantly expressed in VSMCs of virtually all vascular beds. BK Ca channel accessory β and γ subunits are also found in VSMCs [46,53-55]. The predominant β isoform in VSMCs is the β1 subunit [42]. Although β2 and β4 subunits are also present in VSMCs of some vessels, their expression is extremely low [56][57][58]. The association of accessory subunits with α subunits alters channel biophysical properties. Both β1 and γ subunits increase BK Ca channel sensitivity to both Ca 2+

Activation of BK Ca Channels in VSMCs
Given the large conductance and copious expression of the BK Ca channel in VSMCs, small changes in the open probability of the channel have a significant impact on the membrane potential of VSMCs and vascular tone. BK Ca channel activation in VSMCs is primarily linked to Ca 2+ release events from the SR through RYRs and/or Ca 2+ influx through Ca V 1.2 channels or nonselective cation ion channels ( Figure 2) [106]. Thus, the formation of Ca 2+ microdomains/macromolecular complexes provides a rapid feedback and elicits an efficient regulation of Ca 2+ signaling in VSMCs.

Figure 2.
Regulation of vascular function by cross-talks among ion channels. The BKCa channel is preferentially expressed in vascular smooth muscle cells (VSMCs), whereas SKCa and IKCa channels are primarily expressed in endothelial cells (ECs). Vasoconstriction is triggered by an increase in intracellular Ca 2+ ([Ca 2+ ]i) in VSMCs due to Ca 2+ release from the sarcoplasmic reticulum (SR) and/or Ca 2+ influx through the CaV 1.2 channel in the plasma membrane. Vasoconstriction is counteracted by activities of SKCa/IKCa channels in ECs and BKCa channels in VSMCs. The activation of SKCa and IKCa channels in ECs triggered by inositol triphosphate receptor (IP3R)-mediated Ca 2+ release and/or transient receptor potential (TRP) channel-mediated Ca 2+ influx promotes hyperpolarization and release of nitric oxide (NO)/endothelium-derived hyperpolarizing factor (EDRF). In addition, hyperpolarization in ECs could be transmitted to VSMCs via myoendothelial gag junctions (MEGJs). The BKCa channel is activated by RyR-mediated Ca 2+ sparks in VSMCs. BKCa channel activity is also subject to regulation by EC-derived NO and EDHF. Moreover, the accumulation of K + ions in the intercellular space hyperpolarizes VSMCs by activating the inwardly rectifying K + (Kir) channel. Overall, these events promote hyperpolarization of VSMCs, which in turn leads to the closure of the CaV 1.2 channel and subsequent vasodilation. ACh, acetylcholine; BK, bradykinin; SP, substance P; GPCR, G protein-coupled receptor; PLC, phospholipase C; NOS, nitric oxide synthases; GC, guanylate cyclase; cGMP, cyclic guanosine monophosphate; PKG, protein kinase G.

BKCa Channels and Vascular Tone
VSMCs of small arteries/arterioles possess intrinsic properties to constrict in response to an increase in intralumenal pressure and to dilate following a decrease in intralumenal pressure [107]. An increase in intralumenal pressure depolarizes the plasma membrane leading to the opening of the CaV1.2 channel and vasoconstriction/myogenic tone. However, myogenic vasoconstriction is regulated by a negative feedback mechanism conferred by the BKCa channel [5]. Membrane depolarization promotes Ca 2+ sparks in VSMCs. In addition, Ca 2+ entry through CaV1.2 and TRPV4 channels also enhances Ca 2+ sparks that in turn activate the BKCa channel [96]. Activation of the BKCa channel in VSMCs triggers STOCs and subsequent membrane hyperpolarization, leading to CaV1.2 channel closure and vasodilation [94]. Therefore, the BKCa channel functions as a 'brake' to prevent excessive vasoconstriction. The importance of the BKCa channel in the regulation of vascular function has been well demonstrated by pharmacological and genetic manipulations. The blockade of the BKCa channel with iberiotoxin or tetraethylammonium (TEA) induces membrane depolarization, followed by an elevation of [Ca 2+ ]i, vasoconstriction, and elevated blood pressure [42, [108][109][110]. Genetic ablation of the BKCa channel α subunit leads to hypertension [111], suggesting an essential role of this channel in regulating blood pressure and controlling blood perfusion to organs. The BKCa channel β1 subunit is also vital in regulating vascular tone. The BKCa channel in VSMCs from β1 null mice has decreased Ca 2+ sensitivity and reduced channel activity due to uncoupling the channel from Ca 2+ sparks. These changes result in VSMC membrane depolarization and enhancement of vasoconstriction, which ultimately lead to the development of hypertension [42,100,112,113]. ] i ) in VSMCs due to Ca 2+ release from the sarcoplasmic reticulum (SR) and/or Ca 2+ influx through the Ca V 1.2 channel in the plasma membrane. Vasoconstriction is counteracted by activities of SK Ca /IK Ca channels in ECs and BK Ca channels in VSMCs. The activation of SK Ca and IK Ca channels in ECs triggered by inositol triphosphate receptor (IP 3 R)-mediated Ca 2+ release and/or transient receptor potential (TRP) channel-mediated Ca 2+ influx promotes hyperpolarization and release of nitric oxide (NO)/endothelium-derived hyperpolarizing factor (EDRF). In addition, hyperpolarization in ECs could be transmitted to VSMCs via myoendothelial gag junctions (MEGJs). The BK Ca channel is activated by RyR-mediated Ca 2+ sparks in VSMCs. BK Ca channel activity is also subject to regulation by EC-derived NO and EDHF. Moreover, the accumulation of K + ions in the intercellular space hyperpolarizes VSMCs by activating the inwardly rectifying K + (K ir ) channel. Overall, these events promote hyperpolarization of VSMCs, which in turn leads to the closure of the Ca V 1.2 channel and subsequent vasodilation. ACh, acetylcholine; BK, bradykinin; SP, substance P; GPCR, G protein-coupled receptor; PLC, phospholipase C; NOS, nitric oxide synthases; GC, guanylate cyclase; cGMP, cyclic guanosine monophosphate; PKG, protein kinase G.

BK Ca Channels and Vascular Tone
VSMCs of small arteries/arterioles possess intrinsic properties to constrict in response to an increase in intralumenal pressure and to dilate following a decrease in intralumenal pressure [107]. An increase in intralumenal pressure depolarizes the plasma membrane leading to the opening of the Ca V 1.2 channel and vasoconstriction/myogenic tone. However, myogenic vasoconstriction is regulated by a negative feedback mechanism conferred by the BK Ca channel [5]. Membrane depolarization promotes Ca 2+ sparks in VSMCs. In addition, Ca 2+ entry through Ca V 1.2 and TRPV4 channels also enhances Ca 2+ sparks that in turn activate the BK Ca channel [96]. Activation of the BK Ca channel in VSMCs triggers STOCs and subsequent membrane hyperpolarization, leading to Ca V 1.2 channel closure and vasodilation [94]. Therefore, the BK Ca channel functions as a 'brake' to prevent excessive vasoconstriction. The importance of the BK Ca channel in the regulation of vascular function has been well demonstrated by pharmacological and genetic manipulations. The blockade of the BK Ca channel with iberiotoxin or tetraethylammonium (TEA) induces membrane depolarization, followed by an elevation of [Ca 2+ ] i , vasoconstriction, and elevated blood pressure [42, [108][109][110]. Genetic ablation of the BK Ca channel α subunit leads to hypertension [111], suggesting an essential role of this channel in regulating blood pressure and controlling blood perfusion to organs. The BK Ca channel β1 subunit is also vital in regulating vascular tone. The BK Ca channel in VSMCs from β1 null mice has decreased Ca 2+ sensitivity and reduced channel activity due to uncoupling the channel from Ca 2+ sparks. These changes result in VSMC membrane depolarization and enhancement of vaso-constriction, which ultimately lead to the development of hypertension [42,100,112,113]. Not surprisingly, the expression of the BK Ca channel β1 subunit in VSMCs is reduced in hypertension in patients [114] and in animal models [115][116][117]. In contrast, a gain-of-function mutation of the BK Ca channel β1 subunit is associated with a low prevalence of hypertension in human studies [118][119][120]. In addition, the expression of the BK Ca channel β1 subunit in VSMCs of rat mesenteric arteries is upregulated after hemorrhagic shock [121]. This upregulation enhances Ca 2+ sensitivity of the BK Ca channel, promotes VSMC membrane hyperpolarization, and reduces vasoconstriction to norepinephrine. Diabetes is also associated with suppressed expression of the BK Ca channel β1 subunit in VSMCs [122,123].
NO can also regulate BK Ca channel activity in VSMCs by altering the trafficking of the BK Ca channel β1 subunit. NO is found to stimulate rapid surface trafficking of the BK Ca channel β1 subunit via cGMP-PKG-and cAMP-PKA-dependent pathways, resulting in increased channel Ca 2+ sensitivity/channel activity, and vasodilation [137]. Moreover, NO is able to directly activate the BK Ca channel in VSMCs [138,139].

Activation of SK Ca and IK Ca Channels in ECs
The vascular endothelium plays a key role in regulating vascular tone. Activation of SK Ca and IK Ca channels is an essential process for endothelium-dependent vasorelaxation conferred by various vasoactive agents [60,81, [140][141][142][143]. Endothelium-dependent vasodilators and physical stimuli such as fluid shear stress increase [Ca 2+ ] i in ECs by triggering IP 3 R-mediated Ca 2+ release from SR, store-operated Ca 2+ entry, and TRPV4-mediated Ca 2+ influx [144]. Ca 2+ subsequently binds to calmodulin constitutively bound to SK Ca and IK Ca channels, resulting in channel conformational changes and channel activation [145].

SK Ca and IK Ca Channels and Vascular Tone
Opening endothelial SK Ca and IK Ca channels induces hyperpolarization, which could be transmitted to adjacent VSMCs via MEGJ, leading to hyperpolarization of VSMCs, closure of the Ca V 1.2 channel, and subsequent vasodilation ( Figure 2) [2,[146][147][148]. In addition, K + ion accumulated in the extracellular space between ECs and VSMCs due to activation of endothelial SK Ca and IK Ca channels is proposed to cause hyperpolarization and relaxation of the VSMCs through activating the inwardly-rectifying K + (K ir ) channel and/or the Na + -K + -ATPase [149,150]. Furthermore, both SK Ca and IK Ca channels also participate in regulating NO synthesis and release from ECs [151][152][153]. The blockade of the SK Ca channel with apamin and of the IK Ca channel with charybdotoxin or triarylmethane-34 (TRAM-34) attenuates NO production in ECs [151,152]. Activation of endothelial SK Ca and IK Ca channels also promotes the release of endothelium-derived hyperpolarizing factor (EDHF) [154]. Depending on the size of the vessels, different mechanisms may be involved in the actions of SK Ca and IK Ca channels. Activating endothelial SK Ca and IK Ca channels causes vasorelaxation mainly via the release of NO in large arteries and EDHFs in small arteries, respectively [155,156]. NO and EDHFs released from ECs subsequently trigger BK Ca channel activation in VSMCs, leading to vasorelaxation [139,[157][158][159]. Pharmacologic blockade or genetic ablation of SK Ca and/or IK Ca channels depolarizes ECs and decreases vasoactive agent-evoked hyperpolarization of ECs and VSMCs, resulting in impaired vasorelaxation and reduced blood flow [59, 151,152,[160][161][162][163][164]. Conversely, SK Ca and IK Ca channel activation decreases vascular tone/blood pressure and increases blood flow [153,163,[165][166][167]. The functional importance of SK Ca and IK Ca channels is further-more supported by observations that deletion of either or both SK Ca and IK Ca genes is associated with the development of hypertension [59, 164,168]. Consistent with these findings, the expression of SK Ca 2.3 and/or IK Ca channels was reduced in mesenteric arteries from spontaneously or ANG II-induced hypertensive rats [169,170]. However, the IK Ca channel is upregulated under certain pathophysiological conditions such as myocardial infarction, and atherosclerosis [64, [171][172][173]. In addition, the expression of SK Ca 2.3 and IK Ca channels is differently altered by chronic hypoxia in pulmonary arteries. Exposure to chronic hypoxia causes upregulation of the SK Ca 2.3 channel, but downregulation of the IK Ca channel [174].

K Ca Channels in Uteroplacental Vasculature
Both real-time polymerase chain reaction (RT-PCR) and Western blot reveal the expression of BK Ca channel α, β1, and β2 subunits in the uterine arteries of humans and sheep [21,57,[230][231][232][233][234][235]. The β1 subunit is the predominant β isoform in uterine arteries, and the expression level of the β2 subunit is low. Immunohistochemistry further reveals that these BK Ca channel subunits are located in VSMCs, but not in the endothelium, of uterine arteries [57, [230][231][232]. The BK Ca channel in VSMCs of uterine arteries is activated by an increase in [Ca 2+ ] i , and has unitary conductance of 100-200 pS [21,236]. The BK Ca channel γ subunit is also detected in both human and mouse uterine arteries [55,236]. SK Ca and IK Ca channels are also expressed in uterine arteries [68,237]. IK Ca channel mRNA is detected in cultured human uterine microvascular ECs [238]. Both SK Ca and IK Ca channels have been visualized in the endothelium of human and sheep uterine arteries with immunohistochemistry [68,239]. Of interest, K Ca 2.2 and K Ca 2.3 channels are present in VSMCs of sheep uterine arteries [68]. BK Ca , IK Ca , and K Ca 2.3 channels are also detected in VSMCs and/or ECs of placental chorionic plate arteries of pregnant women [240,241].

Estrogen as a Key Determinant of K Ca Channel Upregulation
The expression of K Ca channels in uteroplacental vessels is under the influence of estrogen during the ovarian cycle and pregnancy. Khan et al. demonstrate that the BK Ca channel α subunit protein in ovine uterine arteries remains constant during both follicular and luteal phases of the ovarian cycle [232]. The protein level of the BK Ca channel β1 subunit is higher in uterine arteries from follicular phase ewes than in vessels from luteal phase animals. Similarly, protein abundance of the BK Ca channel α subunit in uterine arteries is negligibly affected by gestation, whereas the expression of the BK Ca channel β1 subunit is upregulated in uterine arteries from pregnant sheep [57,233]. The upregulation of the BK Ca channel β1 subunit expression in uterine arteries during the follicular phase of the ovarian cycle and during pregnancy is paralleling with elevated plasma estrogen levels [57,232]. Remarkably, prolonged treatment of nonpregnant sheep or isolated uterine arteries from nonpregnant animals with 17β-estradiol increases the BK Ca channel β1 subunit expression in the uterine vasculature, resembling those changes that occurred during the ovarian cycle and gestation [230,233,235]. Similarly, estrogen treatment and pregnancy also increase BK Ca channel β1 subunit expression in rat uterus [242]. These observations implicate estrogen as an initiator for the upregulation of BK Ca channel expression in the uterus and its vascular beds in pregnancy. The expression of the BK Ca channel β2 subunit in uterine arteries remains low and unchanged during pregnancy [57]. The increased expression of the BK Ca channel β1 subunit alters channel stoichiometry and increases Ca 2+ sensitivity. In addition, pregnancy and prolonged treatment of nonpregnant sheep with 17β-estradiol also upregulate the expression of NOS, PKG-1α, and cGMP in uterine arteries [57,230,232,243,244]. The upregulation of the NO-cGMP-cPKG pathway could stimulate the BK Ca channel through phosphorylation [245]. The enhanced BK Ca channel activity subsequently contributes to reduced uterine vascular resistance [233].
Pregnancy also upregulates SK Ca channel expression in uterine arteries [68]. This upregulation is also simulated by ex vivo estrogen treatment of isolated uterine arteries from nonpregnant sheep. The expression of K Ca 2.3 and IK Ca channels in the aorta is increased in pregnant mice [246]. Similarly, estrogen replacement in ovariectomized rats increases the K Ca 2.3 channel expression in the uterus and nonvascular smooth muscle [247,248]. In contrast, ovariectomy reduces K Ca 2.3 channel activity and endothelium-dependent vasorelaxation in mouse mesenteric arteries [249]. Likewise, incubating human uterine microvascular ECs with high concentrations of estrogen or serum from normal pregnant women promotes SK Ca 2.3 and IK Ca channel expression [246]. Moreover, the treatment with serum from normal pregnant women increases plasma membrane abundance of SK Ca 2.3 and IK Ca channels in human uterine microvascular ECs [250]. As expected, estrogen replacement in ovariectomized rats enhances EDHF-mediated vasodilation of uterine arteries [251]. However, estrogen replacement in ovariectomized mice reduces K Ca 2.3 channel expression in the uterus [252].

Mechanisms Underlying Estrogen-Mediated K Ca Channel Upregulation
Estrogen usually regulates gene expression via interacting with its classical receptors, ERα and ERβ. The binding of estrogen results in conformational changes of estrogen receptors, allowing these receptors to interact with estrogen response elements (EREs) in the promoter region of target genes to regulate transcription [253]. However, examination of the cloned ovine KCNMB1 promoter sequences reveals that this promoter contains no EREs [235]. Instead, ERα interacts with Sp1 and binds to Sp1 binding sites to regulate KCNMB1 expression in ovine uterine arteries. Several putative transcription factor binding sites, containing CpG dinucleotides in or near their core binding sequences, have been identified in ovine KCNMB1 promoter, including Sp1 at −380 and AP1 at −652, −879, and −1202. Among these sites, the Sp1 -380 binding element is essential for ovine KCNMB1 gene expression as deletion of this site significantly decreases the KCNMB1 promoter activity [235]. The importance of Sp1 in the regulation of expression of KCNMB1 is also demonstrated in nonvascular smooth muscle. Overexpression of Sp1 in smooth muscle cells of rabbit sphincter of Oddi enhances KCNMB1 promoter activity [254].
DNA methylation, the covalent addition of a methyl group (-CH3) to the base cytosine in the dinucleotide 5 -CpG-3 catalyzed by DNA methyltransferases (DNMTs), is an important epigenetic mechanism controlling gene expression [255]. DNA methylation is usually associated with gene repression. CpG dinucleotides of the Sp1 binding site at the KCNMB1 gene promoter are highly methylated in the uterine arteries of nonpregnant sheep, resulting in low transcription factor binding and KCNMB1 promoter activity. Ten-eleven translocation methylcytosine dioxygenases (TETs) catalyze the conversion of 5-methylcytosine (5mC) to 5-hydroxymethylcytosine (5hmC) in active DNA demethylation. Pregnancy via estrogen upregulates TET1 which in turn decreases CpG methylation at the Sp1 binding site and facilitates Sp1/ERα binding to the Sp1 binding site of KCNMB1, leading to the upregulation of the BK Ca channel β1 subunit in uterine arteries [235,256] (Figure 3).   259]. In addition, the blockade of the BKCa channel with TEA also does not alter the myogenic tone of uterine arteries [233]. Moreover, basal uterine blood flow in nonpregnant sheep is negligibly altered by local infusion of TEA [21]. These findings suggest that the BKCa channel in the uterine arteries of nonpregnant sheep is quiescent and contributes minimally to the The increased SK Ca channel expression in uterine arteries during pregnancy is also mediated by estrogen [68]. Estrogen regulates SK Ca 2.3 gene (KCNN3) expression through interactions between ERα and Sp1 in Cos7 and L6 cells [257]. Moreover, estrogen treatment stimulates the expression of the SK Ca 2.3 transcript in human myometrial cells overexpress-ing Sp1 [252]. These observations suggest an important role of Sp1 in the expression of the KCNN3 gene.
Vascular endothelial growth factor (VEGF) appears to play role in the pregnancyinduced upregulation of SK Ca 2.3 and IK Ca channels. The upregulation of SK Ca 2.3 and IK Ca channels induced by exposure to serum from normal pregnant women in cultured human uterine microvascular ECs is diminished by blocking VEGF receptors [246]. Serum from normal pregnant women and VEGF increases H 2 O 2 generation and promote SK Ca 2.3 and IK Ca channel expression via the H 2 O 2 /FYN/ERK pathway [246]. VEGF receptor activation also causes the downregulation of caveolin-1 and subsequently inhibits the internalization of SK Ca 2.3 and IK Ca channels, leading to their high abundance in the plasma membrane in uterine vascular ECs in pregnancy [250]. It should be noted that placental VEGF expression is also subject to regulation by estrogen in pregnancy [258].

K Ca Channels and the Adaptation of the Uteroplacental Circulation
Findings from in vivo and in vitro studies exploring the functional roles of K Ca channels in the uterine circulation of nonpregnant sheep are quite intriguing. Despite the expression of the BK Ca channel in uterine arteries of nonpregnant animals, stimulation of the BK Ca channel with NS 1619 fails to promote vasorelaxation of these vessels [68,259]. In addition, the blockade of the BK Ca channel with TEA also does not alter the myogenic tone of uterine arteries [233]. Moreover, basal uterine blood flow in nonpregnant sheep is negligibly altered by local infusion of TEA [21]. These findings suggest that the BK Ca channel in the uterine arteries of nonpregnant sheep is quiescent and contributes minimally to the regulation of uterine vascular tone, vascular reactivity, and basal uterine blood flow. Interestingly, pregnancy 'awakes' the BK Ca channel and the channel becomes active in ovine uterine arteries. Activation of the BK Ca channel promotes vasorelaxation of uterine arteries from pregnant sheep [68,259], whereas inhibition of the BK Ca channel increases the myogenic tone of uterine arteries [233]. Moreover, local infusion of TEA into uterine arteries decreases basal uterine blood flow by~50% in pregnant sheep [182,260].
It is currently unknown why the BK Ca channel is dormant in the uterine arteries of nonpregnant sheep. One possible explanation is the low abundance of the channel in uterine arteries. The other scenario is that the majority of the BK Ca channel β1 subunit in uterine arteries of nonpregnant sheep are in the cytoplasm and do not form complexes with the α subunit at the surface membrane as observed in rat mesenteric and human cerebral arteries [137]. Leo et al. [137] demonstrate that NO stimulates rapid trafficking of the BK Ca channel β1 subunit to the plasma membrane via a PKG-dependent pathway. Pregnancy is accompanied by parallel increases in NO, cGMP, protein kinase G-1α and the BK Ca channel β1 subunit in uterine arteries [57, 203,233]. We recently demonstrated that pregnancy increases the association of α and β1 subunits in uterine arteries [261]. The association of BK Ca channel β1 and α subunits has been shown to increase channel activity by enhancing the channel's Ca 2+ sensitivity [42]. It is reasonable to speculate that the enhanced NO-PKG pathway in uterine arteries could stimulate the trafficking of the BK Ca channel β1 subunit to the plasma membrane of VSMCs in addition to increased BK Ca channel β1 subunit expression, thus facilitating the transition from the dormant BK Ca channel in the nonpregnant state to the active channel in pregnancy.
BK Ca channel activity is subject to modulation by protein kinases [37,124]. Activation of protein kinase C inhibits the BK Ca channel in uterine arteries [233,262]. Thus, vasoconstriction induced by α-adrenergic ligands and thromboxane may involve PKC-mediated inhibition of the BK Ca channel in this vessel [263,264]. Notably, PKC activity in uterine arteries is suppressed in pregnancy [193,264,265]. On the other hand, the production of vasodilators such as NO, calcitonin gene-related peptide, and adrenomedullin is increased in pregnancy and they produce vasorelaxation of uterine arteries apparently via cGMPmediated activation of the BK Ca channel [231,266,267]. Inhibition of the BK Ca channel enhances uterine vasoconstriction induced by α-adrenergic ligands, thromboxane, and PKC activator in intact sheep and in isolated vessels [231,262,268,269]. Therefore, activation of the BK Ca channel could offset vasoconstriction and prevents vasospasm of uterine arteries, which probably contributes to the refractoriness of uterine arteries to vasoconstrictors during normal pregnancy.
In VSMCs, the BK Ca channel is primarily activated by Ca 2+ sparks mediated by RyRs [270]. Activated BK Ca channels then mediate K + efflux in the form of STOCs, leading to membrane hyperpolarization, Ca V 1.2 channel closure, and vasorelaxation. We recently demonstrated that pregnancy-induced decreases in the myogenic tone of uterine arteries also involve the upregulation of RyR expression/function and enhanced Ca 2+ sparks [271]. Moreover, pregnancy promotes the colocalization of RyR1/2 and the BK Ca channel β1 subunit, leading to enhanced Ca 2+ spark-STOC coupling [261]. The increased Ca 2+ spark-STOC coupling then boosts STOCs, resulting in reduced uterine arterial myogenic tone in pregnancy [261,271].
NO and hydrogen sulfide (H 2 S) are recognized as important regulators of vascular function. Pregnancy increases NO and H 2 S production in both human and sheep uterine arteries, which contributes to estrogen-induced uterine vasodilation in pregnancy [244,[272][273][274]. NO is a potent stimulator of the BK Ca channel in VSMCs [139]. It is expected that NO also triggers BK Ca activation in uterine arteries to promote vasodilation in pregnancy as there is a parallel increase in both the production of NO and cGMP and expression of the BK Ca channel in uterine arteries during pregnancy [57, 203,233]. A recent study reveals that H 2 S elicits vasodilation of uterine arteries via activating the BK Ca channel [236].
EDHF plays an important role in regulating uterine vascular contractility during pregnancy [220,275]. Endothelial SK Ca 2.3 and IK Ca channels mediate endothelial membrane hyperpolarization and participate in EDHF-mediated vasodilator response [148,276]. Pregnancy significantly potentiates EDHF-mediated vasodilation of uterine arteries [204,277]. For example, EDHF contributes to~30% of endothelium-dependent vasorelaxation of uterine arteries in nonpregnant rats and this fraction increases to~70% in pregnant animals [277]. A combination of apamin plus charybdotoxin or TRAM 34, but not of apamin plus the BK Ca channel blocker iberiotoxin, abolished the EDHF-mediated dilation of human and rat uterine arteries, suggesting that SK Ca and IK Ca channels are major mediators of EDHF responses in uterine arteries [204,275,278]. MEGJs provide direct contact between the ECs and VSMCs. MEGJs are the primary pathway of EDHF-mediated relaxation of myometrial arteries in pregnancy [148]. The SK Ca channel may also mediate NO-induced relaxation of uterine arteries [279]. In addition, the SK Ca channel in uterine VSMCs participates in regulating the myogenic tone of uterine arteries [68].
The SK Ca 2.3 and IK Ca channels also participate in uteroplacental angiogenesis and vascular remodeling during pregnancy. Inhibiting SK Ca 2.3 and IK Ca channels in HUVECs with apamin and TRAM 34, respectively, inhibits the secretion of angiogenic factors, proliferation/migration, and tube formation [280]. On the other hand, overexpression of the SK Ca 2.3 channel increases the diameter of uterine arteries [281]. Similarly, SK Ca 2.3 channel overexpression also increases the ratio of VEGF to sFlt-1 and vessel size/numbers in the placenta [282].

Aberrant Expression/Function of Uteroplacental Vascular K Ca in Pregnancy Complications
The expression of the BK Ca channel β1 subunit is repressed in human placental chorionic plate arteries in preeclampsia, which is associated with impaired NO-induced vasodilation [69]. In addition, preeclampsia also reduces the expression of the BK Ca channel β1 subunit in umbilical vein ECs [283]. In a sheep model of preeclampsia, it is found that high-altitude acclimatization downregulates the BK Ca channel β1 subunit in uterine arteries leading to increased uterine vascular tone [234,284]. The expression of the BK Ca channel β1 subunit is also downregulated in the uterine arteries of a mouse model of preeclampsia induced by electrical stimulation, leading to increased uteroplacental vascular resistance [285].
Both SK Ca and IK Ca channels are downregulated in human placental chorionic plate arteries in preeclampsia [241]. The IK Ca channel is also downregulated in ECs of the umbilical artery and vein from preeclamptic pregnancy [238,283]. The contribution of MEGJs to EDHF-induced relaxation of myometrial arteries is diminished in preeclampsia [214]. Treating cultured HUVECs with plasma from preeclamptic women mimics the impacts of preeclampsia on IK Ca channel expression [238]. An increase in circulating testosterone level is an important risk factor for preeclampsia [286][287][288]. In a rat model of preeclampsia/FGR, elevated levels of plasma testosterone result in FGR [237]. Uterine arteries from pregnant rats chronically treated with testosterone display augmented vasoconstriction to thromboxane, phenylephrine, and angiotensin II. In addition, the prolonged testosterone treatment also downregulates the SK Ca 2.3 channel in uterine arteries, leading to diminished EDHF-mediated relaxation [237]. In pregnant guinea pigs, chronic hypoxia attenuates EDHF-mediated relaxation of uterine arteries [289], possibly due to impaired SK Ca /IK Ca channel expression/function.
Gestational diabetes is associated with the downregulation of both BK Ca channel α and β1 subunits in human umbilical arterial smooth muscle cells [290]. Using a rat model in which gestational diabetes is induced by the injection of streptozotocin during pregnancy, Gokina's group demonstrates that EDHF-induced uteroplacental vasodilation is impaired owing to reduced basal and agonist-stimulated [Ca 2+ ] i in ECs [291]. Moreover, they also provide evidence that diabetes selectively causes dysfunction of the IK Ca channel in uteroplacental arteries, which attributes to the impaired EDHF response [292,293]. Likewise, EDHF-induced vasorelaxation is reduced in uterine arteries of streptozotocin-treated pregnant mice [294].

Hypoxia and HIFs
Hypoxia during gestation is a major insult to maternal cardiovascular homeostasis and complicates adaptive changes in the uteroplacental circulation [295,296]. HIFs play a crucial role in cellular (mal)adaptation in response to hypoxia. Levels of HIF-1α increase in preeclamptic placentas, in placentas from human high-altitude pregnancy, in uterine arteries of high-altitude acclimatized pregnant sheep, and in placentas of a hypoxic rodent model of preeclampsia [297][298][299][300]. There are complex interplays among HIFs, ROS/endoplasmic reticulum (ER) stress, and epigenetic regulation [296]. For example, HIF-1α is stabilized by mitochondrial ROS [301], whereas HIF-1α through miR-210-induced downregulation of ISCU promotes mitochondrial ROS production [302]. Moreover, DNMT expression is upregulated by HIF-1α [303]. These factors can act alone and in concert to contribute to the pathogenesis of preeclampsia.
Gestational hypoxia attenuates the pregnancy-induced rise in uteroplacental blood flow, leading to increased incidence of preeclampsia and IUGR [299,[304][305][306][307]. K Ca channels in vascular beds are major targets of hypoxia [37,308]. Gestational hypoxia directly downregulates the BK Ca channel β1 subunit and suppresses the upregulation of the BK Ca channel β1 subunit and SK Ca channels in ovine uterine arteries during pregnancy [68,234]. The attenuated expression of K Ca channels culminates in decreased channel activities, leading to increased myogenic tone and diminished K Ca channel-mediated vasorelaxation.

Epigenetic Regulation
MicroRNAs (miRs) are non-coding RNAs and play important roles in regulating gene expression. miRs regulate gene expression by interacting with the 3 -untranslated region (3 -UTR) of target mRNAs to induce mRNA degradation and translational repression [309]. Circulating and uteroplacental levels of miR-210, a target of HIF-1α, are increased in preeclampsia, in high-altitude pregnancy, and in a high-altitude hypoxic sheep model of preeclampsia [284,[310][311][312][313]. KCNMB1 and RYR2 each contain a miR-210 complementary binding site in their 3 -UTRs and both of them are targets of miR-210 [313]. Indeed, gestational hypoxia via miR-210-mediated downregulation of RyR2 and BK Ca channel β1 subunit disrupts the Ca 2+ spark-STOC coupling in uterine arteries and hence increases uterine arterial myogenic tone [313].
The dynamic of DNA methylation and demethylation is also an important epigenetic mechanism to fine-tune gene expression. DNA methylation catalyzed by a family of DNMTs transfers a methyl group from S-adenyl methionine to the cytosine residue in a CpG dinucleotide(s) to form 5-methylcytosine (5mC). In general, methylation in the promoter regions of genes is associated with the repression of transcription [314]. On the other hand, active DNA demethylation is initiated by TETs which mediate the oxidation of 5mC to 5-hydroxymethylcytosine (5hmC), thus reviving gene transcription [315]. Gestational hypoxia is found to upregulate DNMT3b in uterine arteries, hence enhancing DNA methylation [316] (Figure 3). TET1 is also a target of miR-210 and gestational hypoxia via miR-210 triggers the downregulation of TET1 in uterine arteries [284,317]. TET1 deficiency nullifies pregnancy-induced DNA demethylation [235,284,317]. Overall, these changes lead to hypermethylation of KCNMB1, downregulation of the BK Ca channel β1 subunit in uterine arteries, and increased myogenic tone [284,316,317]. Gestational hypoxia also suppresses the expression of ERα in uterine arteries through hypermethylating the Erαencoding gene ESR1, which could in turn impairs pregnancy-and estrogen-induced BK Ca channel β1 subunit upregulation [318][319][320].

Oxidative/ER Stress
Pregnancy complications are in a state of exaggerated oxidative stress [321]. Reactive oxygen species (ROS) have been implicated in the pathogenesis of various cardiovascular disorders. Mitochondria and NADPH oxidases (NOX) are major sources of ROS in the vasculature [322]. Preeclampsia and gestational hypoxia are found to increase the expression/activity of NOX2 and ROS in the uterine arteries of pregnant sheep and HUVECs [283,300]. Mitochondrial ROS are increased in the placenta of a rat model of preeclampsia produced by reduced uterine perfusion pressure [323]. Likewise, gestational hypoxia also increases mitochondrial ROS via miR-210-mediated downregulation of ISCU and subsequent perturbation of mitochondrial respiration in uterine arteries [324]. ROS could exert its impacts on K Ca channels directly or indirectly. Cys911 oxidation in the BK Ca channel α subunit decreases Ca 2+ sensitivity and impairs channel function [325]. Acute inhibition of ROS with apocynin (a NOX inhibitor) or N-acetylcysteine/EUK-134 (antioxidants) increases BK Ca channel activity in uterine arterial VSMCs of pregnant sheep experiencing gestational hypoxia [259,300,326], suggesting that the BK Ca channel in uterine arteries is tonically inhibited by ROS under hypoxia. Moreover, antioxidant treatment with N-acetylcysteine in ex vivo studies restores the capacity of estrogen to stimulate molecular and functional expression of the BK Ca channel β1 subunit [259,326]. These findings suggest that gestational hypoxia-induced oxidative stress also impairs BK Ca channel function by suppressing estrogen-induced KCNMB1 expression in uterine arteries. The Ca 2+ spark-STOC coupling is disrupted by mitochondrial ROS, leading to increased myogenic tone. ROS derived from NOX2 also repress the expression of the BK Ca channel β1 subunit in HU-VECs from preeclamptic pregnancy [283]. Impaired uteroplacental perfusion in mice with gestational diabetes is associated with elevated oxidative stress in uterine arteries [224]. Although the impact of ROS on BK Ca channel expression/function is not examined in uteroplacental VSMCs of gestational diabetes, NOX-derived ROS have been shown to mediate the downregulation of the BK Ca channel β1 subunit in VSMCs of other vascular beds in diabetic mice [327].
The expression of the SK Ca channel is downregulated by NOX2-derived ROS in umbilical vessels and HUVECs from preeclamptic pregnancy [238,283]. This downregulation is imitated by treating HUVECs with serum from women with preeclampsia, oxidized low-density lipoprotein, palmitic acid, and the superoxide donor xanthine/xanthine oxidase mixture [238,328]. Similarly, exogenous H 2 O 2 suppresses the expression of IK Ca and/or SK Ca channels in cultured HUVECs [329]. In human uterine microvascular ECs, NOX4-derived superoxide mediates the downregulation of K Ca 2.3 and K Ca 3.1 channels induced by serum from preeclamptic women [246]. In addition, NOX4-derived ROS also promote the internalization of K Ca 2.3 and K Ca 3.1 channels by increasing the association of these channels with caveolin-1, clathrin, and Rab5c in human uterine microvascular ECs [250]. Testosterone suppresses mitochondrial respiration in uteroplacental and vascular cells [330,331]. Thus, the downregulation of the SK Ca channel in uterine arteries of pregnant rats chronically treated with testosterone is probably mediated by mitochondrial ROS [237]. Chronic administration of Mito-Tempo in diabetic mice also normalizes the impaired SK Ca activity in heart ECs [332].
Endoplasmic reticulum (ER) stress occurs when ER homeostasis is perturbed. Placentas from preeclamptic pregnancy, FGR, and diabetic pregnancy undergo ER stress [333][334][335][336]. Gestational hypoxia also triggers ER stress and activates unfolded protein response (UPR) in the human placenta and in ovine uterine arteries [337,338]. The ER stress inhibitor tauroursodeoxycholic acid and PERK inhibitor GSK2606414 relieve hypoxia-mediated suppression of Ca 2+ sparks/STOCs and decrease myogenic tone in uterine arteries [337]. ER stress is found to cause downregulation of the BK Ca channel β1 subunit and suppression of BK Ca channel activity in VSMCs [339]. Similarly, SK Ca 2.3 and IK Ca channel activities are also suppressed by ER stress in ECs [340]. Thus, ER stress also contributes to the maladaptation of the uteroplacental circulation by impairing K Ca expression/function in pregnancy complications.

PKC
Preeclamptic serum increases PKC signaling in cultured HUVECs [341,342]. Gestational hypoxia upregulates PKC in the uterine arteries of pregnant sheep [343]. Activation of PKC inhibits BK Ca channel activity and increases myogenic tone in the uterine arteries of pregnant sheep [233]. This mechanism also contributes to gestational hypoxia-induced suppression of SK Ca channel activity [262]. Peroxisome proliferator-activated receptor-γ (PPARγ), a ligand-activated transcription factor, has been implicated in the pathogenesis of preeclampsia [344]. Mesenteric arteries from transgenic mice expressing dominant-negative mutant PPARγ displays increased myogenic tone, due to PKC-mediated inhibition of the BK Ca channel in VSMCs [345]. Similarly, chronic inhibition of PPARγ during rat pregnancy attenuates uterine vasodilation and causes FGR [346]. Moreover, elevated expression of PKCβ in diabetic mouse aortas promotes the BK Ca channel β1 subunit downregulation by impairing AKT signaling [327].

Conclusions
Uteroplacental blood flow increases markedly in pregnancy to meet the demand for placental and fetal growth. Uteroplacental vessels undergo extensive structural and functional changes to accommodate increased uteroplacental perfusion. However, these adaptative changes are impaired in pregnancy complications. Precise mechanisms underlying the adaptation/maladaptation of the uteroplacental circulation are not completely understood. Findings over the past twenty years have suggested important roles of K Ca channels in the regulation of the uteroplacental circulation under physiological and pathophysiological conditions. Notably, estrogen plays a central role in upregulating K Ca channel expression/function leading to reduced uterine vascular tone in normal pregnancy. Lines of evidence suggest that multiple mechanisms including HIFs/miR-210, oxidative stress/ER stress, and PKC contribute to K Ca channel dysfunction in uteroplacental vessels, resulting in the maladaptation of the uteroplacental circulation in pregnancy complications. Thus, restoring K Ca channel expression/function by targeting HIFs/miR-210, oxidative stress/ER stress, and PKC may offer avenues for the development of therapeutics for pregnancy complications.