Jellyfish may be the most common of the medically significant venomous marine creatures, especially in the tropic and subtropic regions. Jellyfish envenomation results in both local reactions (pain and red linear or hive-like lesions) and systemic reactions (nausea, vomiting, muscle cramps, diarrhea, dizziness, diaphoresis, coma, muscular spasms or even death) [1,2]. When human or animal victims contact jellyfish tentacles, the nematocyst threads will launch into their integument and inject venoms composed of multiple proteinaceous toxins into the victim’s body, leading to a variety of clinical symptoms, as mentioned above. Jellyfish venoms have a wide spectrum of biological activities, such as hemolytic, enzymatic, dermonecrotic, myotoxic, neurotoxic and cardiovascular toxic effects [2–4], among which the hemolytic and cardiovascular toxicities are the best studied, while the other activities are seldom mentioned. However, much about jellyfish venoms is still unknown.

We have previously found that the tentacle-only extract free of nematocysts (TOE) from the jellyfish Cyanea capillata showed similar hemolytic and cardiovascular toxicities to those of nematocyst venom [5–7]. Further studies have confirmed that cardiotoxicity is the main reason for death caused by TOE in rats [8], and hemolytic toxicity may act in synergy with the cardiovascular effects of TOE by inducing the release of K⁺ and lactic acid from the lysed erythrocytes [9]. Thus, in the present study, we adopted TOE from C. capillata as an abundant venom resource to survey the hematologic changes that occur in jellyfish envenomation and investigate the activities of jellyfish venom other than hemolytic and cardiovascular toxicities.

C. capillata, a relatively mild toxic jellyfish in comparison to Chironex fleckeri or Physalia physalis, is a common species of the coast of China [10]. Therefore, the clinical manifestations of C. capillata envenomation and physiopathologic research on this venom have a particular significance in China. In the present study, we aimed to explore the activities besides hemolytic [5] and cardiovascular [6,7] toxicities of C. capillata venom by observing the toxic symptoms and hematological changes caused by TOE.

At first, the acute toxicity of TOE was determined in mice by tail vein injection, and the LD₅₀ was calculated as 4.25 mg/kg. During the LD₅₀ determination, we observed that the toxic symptoms were dose-dependent, so we divided the doses into three grades according to the toxic symptoms (Table 1). In the lower dose grade (less than 5 mg/kg), the mice mainly showed heart failure-related symptoms, including congestion and edema in lung and edema in heart. In the highest dose grade (more than 10 mg/kg), the mice mainly showed typical central nervous system-related symptoms, including body trembling, double leg twitching, convulsion and opisthotonos. In the intermediate dose grade (5–10 mg/kg), the mice showed heart failure and nervous system-related symptoms simultaneously. In light of this, we speculate that the TOE may have strong and acute neurotoxicity in addition to its cardiovascular toxicity, leading to rapid death at high doses.

Among the arterial blood gas indexes, pH, PO₂ and PCO₂ showed irreversible changes beginning quickly after TOE administration at the dose of 5 mg/kg i.v., and other indexes, including HCO₃⁻, HCO₃std, TCO₂, BEecf and BE (B), decreased significantly within 5 min, suggesting that metabolic acidosis developed quickly, which might be attributed to a direct and severe hemolytic reaction that led to the release of both lactic acid and K⁺. Eventually, PCO₂ decreased because of compensatory hyperpnea, but PO₂ and SO₂c (%) did not decrease until rat death, suggesting that the respiratory response is not the major cause of death by TOE.

After TOE administration, Na⁺ and Ca²⁺ decreased rapidly, but K⁺ increased and reached a maximum at 5 min. These responses could be easily attributed to the severe hemolysis of TOE in vivo, which induces intracellular lactic acid and K⁺ release. However, the hemolysis of TOE in vivo may not be the only mechanism underlying electrolyte changes. Nematocyst venom from P. physalis markedly increases Ca²⁺ influx, Na⁺ influx and K⁺ efflux in a dose-dependent manner in cultured embryonic chick heart cells [11]. Another study [12] found that P. physalis nematocyst venom can induce Ca²⁺ influx into various kinds of cells, which are not limited to those from the cardiovascular system. P. physalis venom produces pore-like structures in intact cultured cells [13]. All of these results indicate that TOE may decrease circulating Ca²⁺ and Na⁺ and increase K⁺ by stimulating cellular Ca²⁺ and Na⁺ influx and K⁺ efflux through pores created in the cell membrane.

In this study, the elevation of all examined heart injury-related enzymes, including LDH, CK, CK-MB and AST, further indicated that direct cardiac toxicity is the leading cause of death and that a cardiac depressive response is the basic physiopathologic change caused by TOE in anesthetized rats [8]. Moreover, AST and ALT significantly increased, indicating that liver function might be influenced by TOE in vivo. Preliminary in vitro studies have shown that jellyfish nematocyst venoms are extremely toxic to hepatocytes by inhibiting their metabolic activities [14,15], which is consistent with our in vivo results with TOE from C. capillata. However, it should be noted that acute liver congestion, which could have resulted from acute total heart failure, may also have contributed to the elevation of AST and ALT. sCr markedly increased, but BUN remained normal. In addition, we have reported the negative results of kidney pathological examinations previously [8]. Thus, these results indicate that the kidney might function normally at least 3 h after TOE administration. The sCr elevation may suggest that TOE has muscular toxicity, but our data do not allow us to distinguish whether the sCr elevation was simply due to cardiac muscle injury or both skeletal muscle and cardiac muscle injuries.

Blood glucose had an initial transient rise and then lowered to the normal level gradually within 180 min. Nemocyst venom from the “Irukandji” jellyfish Alatina nr mordens can cause marked increases of plasma epinephrine and norepinephrine in anesthetized rats [16], so we presume that the fluctuation of glucose level could be attributed to catecholamines or components contributing to release of endogenous catecholamines. Similarly, the initial pressor responses caused by C. fleckeri and C. barnesi venoms are also attributed to the over-release of endogenous catecholamines in rats [17,18]. Mature erythrocytes of mammals have no mitochondria, so glycolysis is their only glucose utilization pathway. In this study, the quickly increased lactic acid might be explained as the consequence of hemolysis induced by TOE. On the other hand, lactic acid elevation can result in a pH decrease, metabolic acidosis and K⁺ elevation. After TOE administration, hemodiastase did not significantly increase, suggesting that pancreatic function remained normal. Hct% and THbc showed an initial increase, then a quick recovery, indicating that hypovolemic shock, characterized by loss of numerous extracellular fluids, was not the reason for hypotension caused by TOE in vivo.

The present study on the toxic symptoms and hematological changes induced by TOE advances our understanding of the in vivo pathophysiological effects of jellyfish envenomation. In addition to hemolytic and cardiovascular toxicities, TOE from C. capillata may also exhibit neurotoxicity, liver toxicity and muscular toxicity, but no kidney or pancreatic toxicity.