4.1. Dietary EAE Improved Hypoxia Tolerance and Suppressed Hypoxia–Reoxygenation-Induecd Oxidative Lesions in Branchiae and Red Blood Corpuscles of Fish
Our earlier work revealed that supplementing carp with dietary EAE for 30 days notably enhanced their growth [
29]. In this study, we found that dietary EAE supplementation improved the DT while reducing the OCR in carps exposed to hypoxia. Such outcomes suggest a heightened hypoxia tolerance in fish due to EAE. Consistently, it aligns with the prior literature suggesting a link between fish growth and their ability to withstand hypoxic conditions [
40]. The primary roles of branchiae involve nitrogenous waste elimination and facilitating gas exchange essential for fish respiration [
3]. Red blood corpuscles, on the other hand, primarily facilitate O
2 transport and orchestrate CO
2 production during respiration [
41]. However, Hb can autonomously undergo oxidation, producing ROS and Met-Hb that cannot bind O
2 [
42]. Reactive oxygen species such as O
2·−, H
2O
2 and ·OH can potentially oxidize cellular components, leading to the formation of MDA and PC, which can subsequently inactivate essential cellular enzymes [
17]. Notably, fish branchiae possess abundant red blood corpuscles [
3], suggesting that branchial functions might be influenced by their ROS levels. In this context, we observed that EAE supplementation mitigated the rise in oxidative markers in both branchiae and red blood corpuscles of carps subjected to hypoxia–reoxygenation. Such findings emphasize that hypoxia–reoxygenation can induce ROS production and oxidative damage in these cells, echoing previous studies [
13]. Furthermore, our observations highlight that dietary EAE can counteract such oxidative challenges, reminiscent of findings reported in fish digestive organs [
24]. Hence, our findings provide evidence that hypoxia–reoxygenation can promote oxidative stress in fish branchiae and red blood corpuscles, but dietary EAE can offer protective effects against such oxidative damages.
Antioxidant enzymes present in fish cells play a pivotal role in neutralizing intracellular ROS and preventing oxidative damage to cellular components [
43]. Our investigations underscored that dietary EAE counteracted the diminished activity of GPx and CAT in the carp’s red blood corpuscles upon exposure to hypoxia–reoxygenation. This observation resonates with previous findings, indicating enhanced activity of antioxidant enzymes in the hepatopancreas and intestines of fish supplemented with AsE [
24]. Serving as a primary non-enzymatic antioxidant, GSH directly neutralizes intracellular ROS [
44]. Furthermore, the enzymatic conversion of oxidized GSH (GSSG) back to its reduced form, aided by GR, ensures high intracellular GSH levels [
29]. GST assisted in conjugating lipid peroxides’ cleavage products by taking GSH as the substrate [
45]. In the current research, dietary EAE assisted in enhancing the above indicator and improved the hypoxia–reoxygenation- caused decrease in GSH level and GST activity in carp branchiae, which well supported a report on the hepatopancreas and intestines of fish [
24]. As revealed, dietary EAE could quench hypoxia–reoxygenation-caused oxidative lesions by elevating fish branchiae or red blood corpuscles’ enzymatic antioxidant activity and non-enzymatic antioxidant levels.
4.2. Dietary EAE Suppressed Cu-Caused Oxidative Lesions and Apoptosis in Branchiae of Fish
Studies have highlighted the branchiae as the primary organ susceptible to Cu toxicity in fish [
1]. Na
+,K
+-ATPase plays a vital role in maintaining ion electrochemical gradients across fish branchial plasma membranes [
46]. Disruptions to the activity of this enzyme could compromise both respiratory and ionoregulatory mechanisms [
1]. This investigation revealed that dietary EAE mitigated the adverse impact of Cu on hypoxia tolerance and this particular enzyme in carp branchiae. When considering protein metabolism, GPT and GOT emerge as crucial enzymes, and their activity levels provide insights into amino acid utilization within fish organs [
25]. Our data indicate that dietary EAE countered the reduction in the GPT and GOT activities triggered by Cu in the carp’s branchiae. This observation aligns with outcomes observed in trichlorfon-exposed fish [
8]. Cu exposure seemed to compromise specific functionalities in fish branchiae, but the detrimental effects were attenuated with dietary EAE supplementation. Furthermore, the study identified a reduction in H
2O
2 and MDA levels in carp branchiae when they were provided with dietary EAE, despite Cu exposure. Drawing from the above, Cu’s presence appears to promote ROS generation, leading to the oxidation of cellular components in the branchiae, a notion reinforced by the report [
18]. The overarching conclusion from these findings suggests that Cu induces oxidative damage in fish branchiae, but dietary EAE supplementation offers protection, mirroring outcomes observed in fish digestive systems [
25].
Luzio et al., 2013 [
47] have documented that Cu triggers apoptosis in fish branchiae through both intrinsic pathways implicating caspase-9 and 3 and extrinsic pathways implicating caspase-8 and 3. In our present investigation, the administration of dietary EAE was observed to counteract the upregulation of caspases 3 and 8 induced by Cu. This suggests that EAE can act as a defensive mechanism against apoptosis in fish branchiae triggered by Cu, which aligns with previous findings in carp exposed to trichlorfon [
8]. The involvement of ROS in facilitating Cu-induced apoptosis in aquatic organisms, particularly in fish branchiae or hemocytes, is noteworthy [
11,
15]. Interestingly, our research emphasizes that EAE supplementation amplifies the activity of antioxidant enzymes like SOD, GPx, and CAT. It also offers a protective shield against Cu-induced reductions in CAT and SOD enzymatic activities in carp branchiae. This mirrors previous observations where Cu was identified to diminish the activities of enzymatic antioxidants in branchiae [
18], while dietary AsE was found to thwart this decrease in fish hepatopancreas and intestines [
25]. Collectively, our results insinuate that dietary EAE potentially combats oxidative damage and apoptosis provoked by Cu in fish branchiae, chiefly by bolstering their enzymatic antioxidant defenses. This concurs with patterns observed in trichlorfon-exposed fish [
8]. The exact mechanisms through which AsE influences these observed changes in Cu-exposed fish branchiae warrant further investigation.
4.3. Dietary EAE Suppressed Cu-Caused Oxidative Lesions as Well as Apoptosis in Fish Red Blood Corpuscles
Na
+,K
+-ATPase plays an essential role in preserving the cytoplasmic ionic environment, which is pivotal to preventing colloidal osmotic lysis in red blood corpuscles [
41]. LDH, an indispensable enzyme in anaerobic glycolysis, facilitates energy production [
48], which in turn safeguards the function of Na
+,K
+-ATPase within these cells [
41]. Our investigation reveals that carp red blood corpuscles benefit from dietary EAE supplementation, which counteracts the detrimental effects of Cu on the activities of Na
+,K
+-ATPase and LDH. These observations are consistent with previous findings in trichlorfon-exposed carp [
8]. According to Tiihonenk and Nikinmaa 1991 [
49], carp’s red blood corpuscles can harness amino acids as an energy substrate. GPT and GOT are instrumental in channeling amino acids into the tricarboxylic acid cycle through deamination, facilitating energy production [
50,
51]. Our study highlights that dietary EAE alleviates the adverse effects of Cu on GPT and GOT activities within these cells. The data align with earlier research focused on carp branchiae. Additionally, dietary EAE moderates the impact of Cu, as evident by attenuating a reduction in hematological indices like Hct, RBC, HbC and MCH. This suggests Cu’s propensity to diminish red blood corpuscle counts and hemoglobin content, whereas dietary EAE appears to bolster these hematological markers in fish blood, resonating with findings in rainbow trout [
52] and trichlorfon-exposed carp [
8]. Limited studies explore the effects of Cu on GOT and GPT in fish’s red blood corpuscles or the potential role of AsE in influencing hematological indices in Cu-exposed fish. In summary, our findings underscore Cu’s adverse impacts on fish red blood corpuscles, predominantly through metabolic disruption and compromised O
2 transportation. Conversely, dietary EAE shows potential in mitigating these Cu-induced challenges.
The interplay between Cu’s impact on the functional attributes and the ROS status of fish’s red blood corpuscles merits attention. Our study highlights that when carp are exposed to Cu, dietary EAE successfully mitigates the rise in the levels of ·OH and Met-Hb within their red blood corpuscles. This underscores Cu’s propensity to induce ROS generation and, conversely, EAE’s capability to counteract this induction. Such observations are congruent with previously documented findings that Cu exposure has been shown to amplify ROS production in white shrimp’s hemocytes [
11], while dietary AsE enhances the capacity to neutralize ROS in the digestive organs of fish exposed to Cu [
25]. An overarching insight from our research indicates that Cu does more than merely boost ROS production—it also precipitates a decline in functional efficiency, suggesting that it triggers oxidative damage in fish red blood corpuscles. However, the administration of dietary EAE appears to provide a protective buffer against such Cu-induced oxidative disruptions.
It has been shown that ROS stimulates caspases 8 and 9, respectively, through activating clustered receptors of death (extrinsic pathway) and cytochrome c (intrinsic pathway) that are released, thus activating caspase-3 and successively exposing PS to the surface of fish red blood corpuscles [
16]. Here, due to Cu, PS exposure, ROS levels and caspase-8 and 3 activities were enhanced in carp red blood corpuscles, which showed that Cu initiates apoptosis via the extrinsic pathway involved with death receptors and caspase-8 and was in accordance with an article on zebrafish branchia treated with Cu [
47]. Moreover, Cu led to an increase in cytochrome c levels and caspase-9 activity in fish red blood corpuscles, which indicated that Cu induces apoptosis through a mitochondria-mediated pathway and supported a report on chicken hepatocytes [
53]. Therefore, apoptosis seems to be initiated via two different pathways in fish red blood corpuscles treated with Cu. Not much data can be found on the way that CuSO
4 influences the apoptotic mechanism in fish red blood corpuscles. However, dietary EAE prevented the increase in PS exposure, cytochrome c levels and caspase-3, caspase-8 and caspase-9 activities in the carp red blood corpuscles caused by Cu. These results confirmed that EAE had the function of protecting red blood corpuscles against apoptosis by decreasing the cytochrome c level and the caspase activity in fish caused by Cu, which well conformed to the report that AsE remarkably decreases keratinocyte apoptosis during catagen in mice [
54].
GSH plays a pivotal role as a buffer for sulfhydryl groups (–SH), crucial in ensuring a reduced –SH state within antioxidant enzymes and Hb [
41]. In our observations, it became evident that dietary EAE effectively counteracts the decline in the GSH level induced by Cu in fish’s red blood corpuscles. Such findings align consistently with the documented impacts of Cu exposure on the liver, gills, and intestines of fish [
18,
55], as well as the effects on the hepatopancreas and intestines of carp fed with dietary AsE [
24,
25]. From a broader perspective, these insights underscore the potential of dietary AsE to alleviate oxidative damage and apoptosis triggered by Cu by bolstering a non-enzymatic antioxidant presence within fish red blood corpuscles.
The protective roles of AsE against oxidative damage and apoptosis in red blood corpuscles might stem from its primary constituents. Notably, ligustilide and ferulic acid have been identified as the principal active components of
Angelica Sinensis [
56]. Within the EAE examined in this study, ligustilide and ferulic acid were present at concentrations of 4.604% and 0.561%, respectively. Intriguingly, the ferulic acid concentration in EAE is notably elevated, approximately 13-fold, compared to its levels (0.040–0.044%) in
Angelica Sinensis sourced from Sichuan, China [
57]. Other research has shed light onto the positive impacts of ligustilide, emphasizing its potential to mitigate lipid and protein oxidation, reduce neuronal apoptosis, and bolster the activities of key enzymes like SOD and CAT within the mouse brain [
58,
59]. Ferulic acid can be absorbed effectually through the intestinal mucosa in mice [
60], easily move into the streams of blood [
61], lower the ROS level in rats’ aortas [
62] and abrogate the elevation of caspase-3 levels as well as cytochrome c release in the ischemic cortex in rats [
63]. However, information regarding the effect of ligustilide and ferulic acid on red blood corpuscles and branchiae is scarce in fish.