Prothrombin activator-like toxin appears to mediate cardiovascular collapse following envenoming by Pseudonaja textilis

Histopathological Changes in the Liver, Heart and Kidneys Following Malayan Pit Viper (Calloselasma rhodostoma) Envenoming and the Neutralising Effects of Hemato Polyvalent Snake Antivenom

Calloselasma rhodostoma (Malayan pit viper) is a medically important snake species that is widely distributed across Southeast Asia. Systemic coagulopathy causing severe haemorrhage and local tissue injury is commonly observed following C. rhodostoma envenoming. However, nephrotoxicity and congestive heart failure were previously reported in a patient who had a long length of hospital stay. In this study, we determined the effect of C. rhodostoma envenoming on cardiovascular disturbances and the associated morphological changes in the liver, heart and kidneys using animal models. We also evaluated the efficacy of Hemato polyvalent antivenom (HPAV; Queen Saovabha Memorial Institute (QSMI) of the Thai Red Cross Society, Thailand) in neutralising the histopathological effects of C. rhodostoma venom. The intravenous (i.v.) administration of C. rhodostoma venom (1000 µg/kg) caused a rapid decrease in mean arterial pressure (MAP) followed by complete cardiac collapse in anaesthetized rats. Moreover, the intraperitoneal (i.p.) administration of C. rhodostoma venom (11.1 mg/kg; 3 × LD₅₀) for 24 h caused cellular lesions in the liver and heart tissues. C. rhodostoma venom also induced nephrotoxicity, as indicated by the presence of tubular injury, interstitial vascular congestion and inflammatory infiltration in the whole area of the kidney. The administration of HPAV, at manufacturer-recommended doses, 15 min prior to or after the addition of C. rhodostoma venom reduced the extent of the morphological changes in the liver, heart and kidneys. This study found that experimental C. rhodostoma envenoming induced cardiovascular disturbances, hepatotoxicity and nephrotoxicity. We also highlighted the potential broad utility of HPAV to neutralise the histopathological effects of C. rhodostoma venom. The early delivery of antivenom appears capable of preventing envenoming outcomes.

Role of Phospholipases A2 in Vascular Relaxation and Sympatholytic Effects of Five Australian Brown Snake, Pseudonaja spp., Venoms in Rat Isolated Tissues

Human envenoming by Australian brown snakes (Pseudonaja spp.) may result in potentially life-threatening hypotension and subsequent cardiovascular collapse. There have been relatively few studies of the cardiovascular and sympathetic effects of Pseudonaja spp. venoms. In this study, we have examined the effects of venom from five brown snake species-P. affinis, aspidorhyncha, inframacula, nuchalis, and textilis-on cardiac inotropic and chronotropic responses, vascular tone, and sympathetic nerve-induced vascular contractions in rat isolated tissues. The role of phospholipases A₂ (PLA₂s) in venom-induced effects was assessed with the sPLA₂ inhibitor varespladib. In rat isolated left and right atria, there were no physiologically relevant effects of Pseudonaja venoms (0.1-30 µg/ml) on left atrial force of contraction (inotropy) or right atrial rate (chronotropy). In contrast, in isolated small mesenteric arteries precontracted with a thromboxane mimetic, each of the five brown snake venoms (at 30 µg/ml) caused marked vasorelaxation (-60 to -90% of contractile tone). Pretreatment with varespladib (1 µM) significantly inhibited the vasorelaxation caused by P. aspidorhyncha, P. nuchalis, and P. textilis venoms. Electrically induced sympathetic nerve-mediated contractions of mesenteric arteries were significantly attenuated by only P. textilis, and P. affinis venoms (30 µg/ml) and these sympatholytic effects were inhibited by varespladib (1 µM). Based on their inhibition with the sPLA₂ inhibitor varespladib, we conclude that PLA₂ toxins in P. aspidorhyncha, P. nuchalis, and P. textilis venoms are involved in brown snake venom-induced vasorelaxation and the sympatholytic effects of P. affinis, and P. textilis venoms. Our study supports the promising potential role of varespladib as an initial (pre-referral) and/or adjunct (in combination with antivenom) therapeutic agent for brown snake envenoming.

An in vivo examination of the differences between rapid cardiovascular collapse and prolonged hypotension induced by snake venom

We investigated the cardiovascular effects of venoms from seven medically important species of snakes: Australian Eastern Brown snake (Pseudonaja textilis), Sri Lankan Russell’s viper (Daboia russelii), Javanese Russell’s viper (D. siamensis), Gaboon viper (Bitis gabonica), Uracoan rattlesnake (Crotalus vegrandis), Carpet viper (Echis ocellatus) and Puff adder (Bitis arietans), and identified two distinct patterns of effects: i.e. rapid cardiovascular collapse and prolonged hypotension. P. textilis (5 µg/kg, i.v.) and E. ocellatus (50 µg/kg, i.v.) venoms induced rapid (i.e. within 2 min) cardiovascular collapse in anaesthetised rats. P. textilis (20 mg/kg, i.m.) caused collapse within 10 min. D. russelii (100 µg/kg, i.v.) and D. siamensis (100 µg/kg, i.v.) venoms caused ‘prolonged hypotension’, characterised by a persistent decrease in blood pressure with recovery. D. russelii venom (50 mg/kg and 100 mg/kg, i.m.) also caused prolonged hypotension. A priming dose of P. textilis venom (2 µg/kg, i.v.) prevented collapse by E. ocellatus venom (50 µg/kg, i.v.), but had no significant effect on subsequent addition of D. russelii venom (1 mg/kg, i.v). Two priming doses (1 µg/kg, i.v.) of E. ocellatus venom prevented collapse by E. ocellatus venom (50 µg/kg, i.v.). B. gabonica, C. vegrandis and B. arietans (all at 200 µg/kg, i.v.) induced mild transient hypotension. Artificial respiration prevented D. russelii venom induced prolonged hypotension but not rapid cardiovascular collapse from E. ocellatus venom. D. russelii venom (0.001–1 μg/ml) caused concentration-dependent relaxation (EC₅₀ = 82.2 ± 15.3 ng/ml, R_max = 91 ± 1%) in pre-contracted mesenteric arteries. In contrast, E. ocellatus venom (1 µg/ml) only produced a maximum relaxant effect of 27 ± 14%, suggesting that rapid cardiovascular collapse is unlikely to be due to peripheral vasodilation. The prevention of rapid cardiovascular collapse, by ‘priming’ doses of venom, supports a role for depletable endogenous mediators in this phenomenon.

The Cardiovascular and Neurotoxic Effects of the Venoms of Six Bony and Cartilaginous Fish Species

Fish venoms are often poorly studied, in part due to the difficulty in obtaining, extracting, and storing them. In this study, we characterize the cardiovascular and neurotoxic effects of the venoms from the following six species of fish: the cartilaginous stingrays Neotrygon kuhlii and Himantura toshi, and the bony fish Platycephalus fucus, Girella tricuspidata, Mugil cephalus, and Dentex tumifrons. All venoms (10–100 µg/kg, i.v.), except G. tricuspidata and P. fuscus, induced a biphasic response on mean arterial pressure (MAP) in the anesthetised rat. P. fucus venom exhibited a hypotensive response, while venom from G. tricuspidata displayed a single depressor response. All venoms induced cardiovascular collapse at 200 µg/kg, i.v. The in vitro neurotoxic effects of venom were examined using the chick biventer cervicis nerve-muscle (CBCNM) preparation. N. kuhlii, H. toshi, and P. fucus venoms caused concentration-dependent inhibition of indirect twitches in the CBCNM preparation. These three venoms also inhibited responses to exogenous acetylcholine (ACh) and carbachol (CCh), but not potassium chloride (KCl), indicating a post-synaptic mode of action. Venom from G. tricuspidata, M. cephalus, and D. tumifrons had no significant effect on indirect twitches or agonist responses in the CBCNM. Our results demonstrate that envenoming by these species of fish may result in moderate cardiovascular and/or neurotoxic effects. Future studies aimed at identifying the molecules responsible for these effects could uncover potentially novel lead compounds for future pharmaceuticals, in addition to generating new knowledge about the evolutionary relationships between venomous animals.