Actions of cardiotoxins from the southern Chinese cobra (Naja naja atra) on rat cardiac tissue

Effect of Several Naja atra Antivenom Injection Methods on the Rabbit Model of Naja naja atra Bite Poisoning

Snakebite is a global public health concern, which often occurs in tropical and subtropical underdeveloped areas, but it is often neglected. In the southern China, Naja naja atra (Chinese cobra) is a common venomous snake that causes swelling and necrosis of local tissues, even amputation and death. Currently, the main therapy is the administration of Naja atra antivenom, which greatly reduces mortality. However, the antivenom is not particularly effective in the improvement of local tissue necrosis. Clinically, antivenom is mainly administered intravenously. We speculated that the method of injection influences the efficacy of antivenom. In this study, the rabbit model was used to explore the effects of different antivenom injection methods on systemic and local poisoning symptoms. If topical injection of antivenom contributes to ameliorate tissue necrosis, then we need to reconsider the use of Naja atra antivenom.

Effects of Cardiotoxins from Naja oxiana Cobra Venom on Rat Heart Muscle and Aorta: A Comparative Study of Toxin-Induced Contraction Mechanisms

Cardiotoxins (CaTxs) are a group of snake toxins that affect the cardiovascular system (CVS). Two types (S and P) of CaTxs are known, but the exact differences in the effects of these types on CVS have not been thoroughly studied. We investigated cellular mechanisms of action on CVS for Naja oxiana cobra CaTxs CTX-1 (S-type) and CTX-2 (P-type) focusing on the papillary muscle (PM) contractility and contraction of aortic rings (AR) supplemented by pharmacological analysis. It was found that CTX-1 and CTX-2 exerted dose-dependent effects manifested in PM contracture and AR contraction. CTX-2 impaired functions of PM and AR more strongly than CTX-1. Effects of CaTxs on PM were significantly reduced by nifedipine, an L-type Ca2+ channel blocker, and by KB-R7943, an inhibitor of reverse-mode Na+/Ca2+ exchange. Furthermore, 2-aminoethoxydiphenyl borate, an inhibitor of store-operated calcium entry, partially restored PM contractility damaged by CaTxs. The CaTx influence on AR contracture was significantly reduced by nifedipine and KB-R7943. The involvement of reverse-mode Na+/Ca2+ exchange in the effect of CaTxs on the rat aorta was shown for the first time. The results obtained indicate that CaTx effects on CVS are mainly associated with disturbance of transporting systems responsible for the Ca2+ influx.

Analysis of the Necrosis-Inducing Components of the Venom of Naja atra and Assessment of the Neutralization Ability of Freeze-Dried Antivenom

Patients bitten by Naja atra who are treated with bivalent freeze-dried neurotoxic antivenom in Taiwan have an improved survival rate but develop necrotic wound changes. The World Health Organization (WHO) has suggested using the minimum necrotizing dose (MND) of venom as a method of evaluating the neutralization effect of antivenom. The aim of this study was to evaluate the effectiveness of antivenom for the prevention of necrosis based on the MND and clarify which component of the venom of N. atra induces necrosis. The neurotoxins (NTXs) were removed from the crude venom (deNTXs), and different concentrations of deNTXs were injected intradermally into the dorsal skin of mice. After three days, the necrotic lesion diameter was found to be approximately 5 mm, and the MND was calculated. A reduction in the necrotic diameter of 50% was used to identify the MND₅₀. Furthermore, both phospholipase A₂ (PLA₂) and cytotoxins (CTXs) were separately removed from the deNTXs to identify the major necrosis-inducing factor, and the necrotic lesions were scored. All mice injected with deNTXs survived for three days and developed necrotic wounds. The MND of the deNTXs for mice was 0.494 ± 0.029 µg/g, that of the deNTXs-dePLA₂ (major component retained: CTXs) was 0.294 ± 0.05 µg/g, and that of the deNTX-deCTX (major component retained: PLA₂) venom was greater than 1.25 µg/g. These values show that CTX is the major factor inducing necrosis. These results suggest that the use of the deNTXs is necessary to enable the mice to survive long enough to develop venom-induced cytolytic effects. CTXs play a major role in N. atra-related necrosis. However, the MND₅₀ could not be identified in this study, which meant that the antivenom did not neutralize venom-induced necrosis.

Cardiovascular Effects of Snake Toxins: Cardiotoxicity and Cardioprotection

Snake venoms, as complex mixtures of peptides and proteins, affect various vital systems of the organism. One of the main targets of the toxic components from snake venoms is the cardiovascular system. Venom proteins and peptides can act in different ways, exhibiting either cardiotoxic or cardioprotective effects. The principal classes of these compounds are cobra cardiotoxins, phospholipases A2, and natriuretic, as well as bradykinin-potentiating peptides. There is another group of proteins capable of enhancing angiogenesis, which include, e.g., vascular endothelial growth factors possessing hypotensive and cardioprotective activities. Venom proteins and peptides exhibiting cardiotropic and vasoactive effects are promising candidates for the design of new drugs capable of preventing or constricting the development of pathological processes in cardiovascular diseases, which are currently the leading cause of death worldwide. For example, a bradykinin-potentiating peptide from Bothrops jararaca snake venom was the first snake venom compound used to create the widely used antihypertensive drugs captopril and enalapril. In this paper, we review the current state of research on snake venom components affecting the cardiovascular system and analyse the mechanisms of physiological action of these toxins and the prospects for their medical application.

Cardiotoxins from Cobra Naja oxiana Change the Force of Contraction and the Character of Rhythmoinotropic Phenomena in the Rat Myocardium

The study of the influence of cobra Naja oxiana cardiotoxins on the contractility of the rat papillary muscles and its rhythmoinotropic characteristics has shown that the presence of toxins induces a slight contractility decrease in the stimulation frequency range up to 0.1 Hz. In the stimulation frequency range from 0.1 to 0.5 Hz, a positive inotropic effect is found. However, the positive inotropic effect is replaced by a negative one with further increase in the frequency up to 3 Hz. In the presence of cardiotoxins, the positive force-frequency relationship in the region of 1-3 Hz, characteristic of healthy rat myocardium, disappears and the relationship becomes completely negative. L-type calcium channel blocker nifedipine does not affect the changes induced by toxins, while a high concentration (10 mM) of calcium prevents the effects of cardiotoxins on the muscle. The results obtained show that the impairment of the force-frequency relationship occurs long before the development of irreversible damage in the myocardium and may be the first sign of the pathological action of cardiotoxins.

Envenomation by Micrurus psyches in French Guiana

We report here the first known envenomation by Micrurus psyches, the so-called Carib coral snake, which occurred on April 2016 in the surroundings of Saint-Laurent-du-Maroni, Western French Guiana. Besides local neurological symptoms, it featured unexpected electrocardiogram changes, which were emergence of a first-degree atrioventricular block and biphasic T waves, both transient. NewWorld elapid venoms were not known for being cardiotoxic so far. The possible reasons of this cardiac injury are discussed.

Pharmacology of Heparin and Related Drugs

Heparin has been recognized as a valuable anticoagulant and antithrombotic for several decades and is still widely used in clinical practice for a variety of indications. The anticoagulant activity of heparin is mainly attributable to the action of a specific pentasaccharide sequence that acts in concert with antithrombin, a plasma coagulation factor inhibitor. This observation has led to the development of synthetic heparin mimetics for clinical use. However, it is increasingly recognized that heparin has many other pharmacological properties, including but not limited to antiviral, anti-inflammatory, and antimetastatic actions. Many of these activities are independent of its anticoagulant activity, although the mechanisms of these other activities are currently less well defined. Nonetheless, heparin is being exploited for clinical uses beyond anticoagulation and developed for a wide range of clinical disorders. This article provides a "state of the art" review of our current understanding of the pharmacology of heparin and related drugs and an overview of the status of development of such drugs.

Proteomic characterization and comparison of venoms from two elapid snakes (Bungarus multicinctus and Naja atra) from China

Bungarus multicinctus (many-banded krait) and Naja atra (Chinese cobra) are widely distributed and medically important venomous snakes in China; however, their venom proteomic profiles have not been fully compared. Here, we fractionated crude venoms and analyzed them using a combination of proteomic techniques. Three-finger toxins (3-FTx) and phospholipase A2 (PLA2) were most abundant in both species, respectively accounting for 32.6% and 66.4% of total B. multicinctus venom, and 84.3% and 12.2% of total N. atra venom. Venoms from these two species contained one common protein family and six less abundant species-specific protein families. The proteomic profiles of B. multicinctus and N. atra venoms and analysis of toxicological activity in mice suggested that 3-FTx and PLA2 are the major contributors to clinical symptoms caused by envenomation. The venoms differed in enzymatic activity, likely the result of inter-specific variation in the amount of related venom components. Antivenomics assessment revealed that a small number of venom components (3-FTxs and PLA2s in B. multicinctus, and 3-FTxs in N. atra) could not be immunocaptured completely, suggesting that we should pay attention to enhancing the immune response of these components in designing commercial antivenoms for B. multicinctus and N. atra.

BIOLOGICAL SIGNIFICANCE

The proteomic profiles of venoms from two medically important snake species - B. multicinctus and N. atra - have been explored. Quantitative and qualitative differences are evident in both venoms when proteomic profiles and transcriptomic results are compared; this is a reminder that combined approaches are needed to explore the precise composition of snake venom. Two protein families (3-FTx and PLA2) of high abundance in these snake venoms are major players in the biochemical and pharmacological effects of envenomation. Elucidation of the proteomic profiles of these snake venoms is helpful in understanding composition-function relationships and will facilitate the clinical application of antivenoms.

Biochemistry of envenomation

Venoms and toxins are of significant interest due to their ability to cause a wide range of pathophysiological conditions that can potentially result in death. Despite their wide distribution among plants and animals, the biochemical pathways associated with these pathogenic agents remain largely unexplored. Impoverished and underdeveloped regions appear especially susceptible to increased incidence and severity due to poor socioeconomic conditions and lack of appropriate medical treatment infrastructure. To facilitate better management and treatment of envenomation victims, it is essential that the biochemical mechanisms of their action be elucidated. This review aims to characterize downstream envenomation mechanisms by addressing the major neuro-, cardio-, and hemotoxins as well as ion-channel toxins. Because of their use in folk and traditional medicine, the biochemistry behind venom therapy and possible implications on conventional medicine will also be addressed.

Ventricular bigeminy following a cobra envenomation

CONTEXT

Envenoming by some species of cobras (Naja species) may include cardiotoxic effects including various dysrhythmias. However, dysrhythmias leading specifically to ventricular bigeminy have not been previously documented. We report a case of cardiotoxicity and the development of ventricular bigeminy following a cobra envenomation.

CASE DETAILS

The patient was a 23-year-old man who presented to an emergency department following an alleged cobra bite. There was transient episode of nausea, vomiting, hypotension and tachycardia. The ECG showed infrequent ventricular ectopics that progressed to ventricular bigeminy and persisted even after the vital signs normalized. Complete resolution and resumption of normal sinus rhythm occurred following an empirical administration of monovalent antivenom against Naja kaouthia venom. The patient was discharged after 24 hours of uneventful observation.

DISCUSSION

The patient's concomitant local effects, episodic cardiovascular instability and evolution of ventricular bigeminy support the likelihood of a venom-induced disease. Ventricular bigeminy can develop following a cobra envenomation. Thorough clinical evaluation, close serial observation of vital signs and early continuous cardiac monitoring are important in Naja spp. bites.

Role of heparan sulfates and glycosphingolipids in the pore formation of basic polypeptides of cobra cardiotoxin

Cobra venom contains cardiotoxins (CTXs) that induce tissue necrosis and systolic heart arrest in bitten victims. CTX-induced membrane pore formation is one of the major mechanisms responsible for the venom's designated cytotoxicity. This chapter examines how glycoconjugates such as heparan sulfates (HS) and glycosphingolipids, located respectively in the extracellular matrix and lipid bilayers of the cell membranes, facilitate CTX pore formation. Evidences for HS-facilitated cell surface retention and glycosphingolipid-facilitated membrane bilayer insertion of CTX are reviewed. We suggest that similar physical steps could play a role in the mediation of other pore forming toxins (PFT). The membrane pores formed by PFT are expected to have limited lifetime on biological cell surface as a result of membrane dynamics during endocytosis and/or rearrangement of lipid rafts.

Beta-cardiotoxin: a new three-finger toxin from Ophiophagus hannah (king cobra) venom with beta-blocker activity

Snake venoms have provided a number of novel ligands with therapeutic potential. We have constructed a partial cDNA library from the mRNA of Ophiophagus hannah (king cobra) venom gland tissue and identified five new genes encoding proteins belonging to the three-finger toxin family of snake venom proteins. We have isolated and characterized one of these beta-sheet containing proteins with a mass of 7012.43 +/- 0.91 Da from the venom. The protein was nonlethal up to a dose of 10 mg/kg when injected intraperitoneally into Swiss albino mice. However, it induces labored breathing and death at a dose of 100 mg/kg. It does not show any hemolytic or anticoagulant activity. It caused a dose-dependent decrease of heart rate in vivo (anesthetized Sprague-Dawley rats) and also ex vivo (Langendorff isolated rat heart). This is in contrast to classical cardiotoxins from snake venom that increase the heart rate in animals. Radioligand displacement studies showed that this protein targets beta-adrenergic receptors with a binding affinity (Ki) of 5.3 and 2.3 microM toward beta1 and beta2 subtypes, respectively, to bring about its effect, and hence, it was named as beta-cardiotoxin. This is the first report of a natural exogenous beta-blocker.

Cobra cardiotoxin and phospholipase A2 as GAG-binding toxins: on the path from structure to cardiotoxicity and inflammation

Glycosaminoglycans (GAGs) represent the sulfated carbohydrate moieties of proteoglycans which occur abundantly in tissues of the cardiovascular system. Many proteins bind specifically to GAGs and perform an important role in inflammation, cell proliferation, and blood coagulation processes. Recently, in vitro GAG-binding studies of cardiotoxins (CTXs) and basic phospholipase A(2) (PLA(2)) from cobra venom established the toxins as two new families of GAG-binding proteins. In particular, discontinuous basic residues in beta-sheet CTXs may form a cationic cradle suitable for heparin binding, as in the case of fibronectin module III-13. The binding specificity of beta-sheet proteins to different GAGs can be further enhanced by involving other cationic clusters near the flexible loop of the molecule. Since the three-dimensional structures of many CTXs and PLA(2) are available, these two toxins may serve as models for the elucidation of the molecular recognition of GAG-binding proteins and also as polypeptide templates for further improvement of the binding specificity suitable for future biomedical application. Research along the line of GAG-guided toxicity of cobra venom components may help us to understand the functional role of GAGs and the action mechanism of cobra venom components in the cardiovascular system.

Effect of calcium on the vascular contraction induced by cobra venom cardiotoxin

1. The cytotoxic effects of cardiotoxin (CTX) purified from Cobra venom were tested in endothelium-denuded rat aortic ring preparations in tissue organ baths and the effect of extracellular Ca2+ on the cytotoxic effect of CTX was investigated using a digital dynamic calcium imaging technique. 2. At 10 micromol/L, CTX induced a slowly developing and sustained contraction that amounted to approximately 50% of the maximal contraction induced by 80 mmol/L KCl. At high concentrations (> 15 micromol/L), CTX caused irreversible damage to the smooth muscle contractile function. However, washout of CTX at its peak contraction did not affect the subsequent contraction to either KCl or phenylephrine. 3. Contraction induced by CTX was dependent on the Ca2+ concentration in the external solution. A maximal contractile response to CTX was obtained in medium containing 1-2.5 mmol/L Ca2+. This contractile response induced by CTX decreased with higher Ca2+ concentrations and was completely diminished when 7 mmol/L Ca2+, 3 mmol/L Ni2+ or 30 micromol/L tetrandrine (a non-selective calcium channel blocker) was present in the external solution before addition of CTX to the bath. 4. The above observations were supported by the calcium imaging work performed with cultured aortic smooth muscle cells from Wistar-Kyoto rats, in which CTX was shown to induce the elevation of cytosolic Ca2+ in the presence, but not in the absence, of 2.5 mmol/L extracellular Ca2+. Increasing the extracellular Ca2+ concentration to 7 mmol/L, the addition of 3 mmol/L Ni2+ or inclusion of 30 micro mol/L tetrandrine inhibited the elevation of cytosolic Ca2+ induced by CTX. 5. These results suggest that: (i) a CTX-sensitive internal calcium store does not exist in rat aortic smooth muscle; (ii) the contractile effect CTX is associated with a Ca2+ influx process; and (iii) CTX interacts extracellularly with the plasma membrane at the level of the calcium channels, as well as anionic sites to which Ca2+ and other inorganic cations bind.

Ophidian envenomation strategies and the role of purines

Snake envenomation employs three well integrated strategies: prey immobilization via hypotension, prey immobilization via paralysis, and prey digestion. Purines (adenosine, guanosine and inosine) evidently play a central role in the envenomation strategies of most advanced snakes. Purines constitute the perfect multifunctional toxins, participating simultaneously in all three envenomation strategies. Because they are endogenous regulatory compounds in all vertebrates, it is impossible for any prey organism to develop resistance to them. Purine generation from endogenous precursors in the prey explains the presence of many hitherto unexplained enzyme activities in snake venoms: 5'-nucleotidase, endonucleases (including ribonuclease), phosphodiesterase, ATPase, ADPase, phosphomonoesterase, and NADase. Phospholipases A(2), cytotoxins, myotoxins, and heparinase also participate in purine liberation, in addition to their better known functions. Adenosine contributes to prey immobilization by activation of neuronal adenosine A(1) receptors, suppressing acetylcholine release from motor neurons and excitatory neurotransmitters from central sites. It also exacerbates venom-induced hypotension by activating A(2) receptors in the vasculature. Adenosine and inosine both activate mast cell A(3) receptors, liberating vasoactive substances and increasing vascular permeability. Guanosine probably contributes to hypotension, by augmenting vascular endothelial cGMP levels via an unknown mechanism. Novel functions are suggested for toxins that act upon blood coagulation factors, including nitric oxide production, using the prey's carboxypeptidases. Leucine aminopeptidase may link venom hemorrhagic metalloproteases and endogenous chymotrypsin-like proteases with venom L-amino acid oxidase (LAO), accelerating the latter. The primary function of LAO is probably to promote prey hypotension by activating soluble guanylate cyclase in the presence of superoxide dismutase. LAO's apoptotic activity, too slow to be relevant to prey capture, is undoubtedly secondary and probably serves principally a digestive function. It is concluded that the principal function of L-type Ca(2+) channel antagonists and muscarinic toxins, in Dendroaspis venoms, and acetylcholinesterase in other elapid venoms, is to promote hypotension. Venom dipeptidyl peptidase IV-like enzymes probably also contribute to hypotension by destroying vasoconstrictive peptides such as Peptide YY, neuropeptide Y and substance P. Purines apparently bind to other toxins which then serve as molecular chaperones to deposit the bound purines at specific subsets of purine receptors. The assignment of pharmacological activities such as transient neurotransmitter suppression, histamine release and antinociception, to a variety of proteinaceous toxins, is probably erroneous. Such effects are probably due instead to purines bound to these toxins, and/or to free venom purines.

Electrically induced intracellular Ca2+ transient in single ventricular myocytes: a useful parameter for the study of cardiac drugs

1. Fluorescent Ca2+ indicators, such as fura-2/AM and calcium green-1, have become one of the most popular tools for measuring intracellular calcium ([Ca2+]i). 2. Electrical stimulation triggers a cascade of events in the cardiac muscle, which results in a [Ca2+]i transient and, eventually, contraction. The events that occur in electrically induced cardiac myocytes mimic the normal physiological events in vivo. 3. The electrically induced [Ca2+]i transient represents influx of Ca2+ from outside and mobilization of Ca2+ from the intracellular store and is directly related to contraction. Thus, it is more important to determine the electrically induced [Ca2+]i transient than [Ca2+]i. The [Ca2+]i transient can be easily measured with the spectrofluorescence method using fura 2/AM as the Ca2+ indicator in a single ventricular myocyte preparation. 4. We made use of the results of studies on carbachol, tetrandrine and cardiotoxin to illustrate the usefulness of the electrically induced [Ca2+]i transient in the study of actions of cardiac drugs.

Changes in the arterial blood pressure, heart rate and normal ECG parameters of rat after envenomation with Egyptian cobra (Naja haje) venom

1. The effect of Egyptian cobra (Naja haje) venom on the normal electrical activity of the cardiac muscles (ECG) and arterial blood pressure of envenomated rats were investigated in this study. 2. Rats were divided into three groups. The first group was injected im with saline and considered as control group. Rats of the second and third groups were injected IM with 0.02 micrograms and 0.04 micrograms cobra venom/gim b.wt, respectively. 3. Mean blood pressure (MBP), heart rate (HR) and four different ECG parameters (PR and QT intervals, R and T wave amplitudes) were measured over 1 h following envenomation. 4. The low dose (0.02 micrograms/g) of N. haje venom caused hypotension accompanied by an increase in the HR, whereas hypertension and bradycardia developed after injection of the high dose (0.04 micrograms/g) of venom. 5. There was a decrease in the P-R interval after administration of the low dose and prolongation of it after the high dose. The Q-T interval and R-wave amplitude were significantly increased after injection of both doses. T-wave amplitude was significantly elevated only after injection of the high dose. 6. The present results indicate that the Egyptian cobra (N. haje) venom significantly alters the arterial blood pressure and ECG parameters of envenomated rats. The suggests that impairment of the electrical activity of cardiac muscle may be one of the reasons why victims of cobra bite die.

Respiratory toxicity of direct lytic factor in the venom of the southern Chinese cobra (N. naja atra) in dogs

The effects of direct lytic factor (DLF) on respiratory ventilation, gas exchange as well as hemodynamics were studied in anesthetized dogs. After an intravenous DLF dose of 1 mg/kg, the initial manifestation of intoxication was observed as follows: (1) Increase in airway impedance characterized by slowed air flow rate and increased negative transpulmonary pressure. (2) Decrease in dynamic compliance. (3) Progressive increase in venoarterial shunt (Qs/Qt) and decrease in PaO2, (4) Elevation of pulmonary artery blood pressure and fall of mean systemic blood pressure and maximal left ventricular pressure. Above actions reached the peak values at 15 min and thereafter all respiratory functional parameters, except Qs/Qt and hypoxemia, returned gradually to approach the normal levels at 50 min. The tidal volume, PaCO2 and LVEDP remained unchanged until another DLF dose of 1.5 mg/kg was given. After a second dose of DLF (total 2.5 mg/kg), the respiratory functions and the cardiac performance deteriorated as follows: (1) Further increase in Qs/Qt and hypoxemia. (2) Appearance of hypercapnea and acidosis. (3) Fall of dP/dtmax and elevation of LVEDP, widening of QRS complex of ECG. (4) Blood pressure run a downhill course. From above experimental evidence, we came to the conclusion that as well as the basic cardiotoxicity, respiratory toxicity of DLF must be considered as the primary, because of broad spectrum of action of DLF and early effect on respiratory function.

Inhibition by multivalent cations of contraction induced by Chinese cobra venom cardiotoxin in guinea pig papillary muscle

The effects of cardiotoxin (CTX), purified from the venom of Chinese Cobra (Naja naja atra) by a three-step chromatography, on the contractile responses of isolated guinea pig papillary muscle preparation and its antagonism by lanthanum ion (La3+) and divalent cations were examined. CTX induced tonic contraction following a transient augmentation of electrically evoked rhythmic contractions, which is similar to that seen in perfused heart preparation. Multivalent cations, La3+, Ca2+, Mn2+ and Mg2+, concentration-dependently blocked CTX-induced contraction. In Ca(2+)-free medium, CTX did not induce contraction and CTX-induced contraction was not modified in Na(+)-free medium. Nifedipine (1 mumol/L), effectively blocked KCl-induced contracture, but only partially inhibited CTX-induced contraction; thus suggesting that Ca2+ influx induced by CTX utilizes channels other than L-type Ca2+ channels. These cations may compete with CTX for the negatively charged membrane binding site which is responsible for the modulation of Ca2+ movement.

Possible mechanisms of action of cobra snake venom cardiotoxins and bee venom melittin

Cobra snake venom cardiotoxins and bee venom melittin share a number of pharmacological properties in intact tissues including hemolysis, cytolysis, contractures of muscle, membrane depolarization and activation of tissue phospholipase C and, to a far lesser extent, an arachidonic acid-associated phospholipase A2. The toxins have also been demonstrated to open the Ca2+ release channel (ryanodine receptor) and alter the activity of the Ca(2+)+Mg(2+)-ATPase in isolated sarcoplasmic reticulum preparations derived from cardiac or skeletal muscle. However, a relationship of these actions in isolated organelles to contracture induction has not yet been established. The toxins also bind to and, in some cases, alter the function of a number of other proteins in disrupted tissues. The most difficult tasks in understanding the mechanism of action of these toxins have been dissociating the primary from secondary effects and distinguishing between effects that only occur in disrupted tissues and those that occur in intact tissue. The use of cardiotoxin and melittin fractions contaminated with trace ('undetectable') amounts of venom-derived phospholipases A2 has continued to be common practice, despite the problems associated with the synergism between the toxins and enzymes and the availability of methods to overcome this problem. With adequate precautions taken with regard to methodology and interpretation of results, the cobra venom cardiotoxins and bee venom melittin may prove to be useful probes of a number of cell processes, including lipid metabolism and Ca2+ regulation in skeletal and cardiac muscle.

Effects of an acidic phospholipase A2 purified from Ophiophagus hannah (king cobra) venom on rat heart

An acidic phospholipase A2 (OHV A-PLA2) purified from Ophiophagus hannah venom had a cardiotoxic action on rat heart. In rats OHV A-PLA2 (2-4 mg/kg) caused ECG abnormalities including decreased heart rate, prolonged P-R interval, widened QRS complex and complete A-V block. When tested on isolated rat right atria, OHV A-PLA2 (10-20 micrograms/ml) produced a positive chronotropic effect. When tested on isolated rat left atria or papillary muscle preparations, OHV A-PLA2 (2.5-20 micrograms/ml) caused positive inotropic effect, followed by contracture. The positive inotropic effects could be abolished by high Ca2+ and enhanced by low Ca2+; both treatments accelerated contracture. The contracture could be inhibited in Mn2+ (5 mM)-containing medium and abolished by Ca(2+)-free bath solution containing 1 mM EDTA. The cardiotoxic action of OHV A-PLA2 was not influenced by verapamil, tetrodotoxin, propranolol, phentolamine, atropine or indomethacin. It is suggested that the cardiotoxic effects of OHV A-PLA2 may result from increasing intracellular levels of Ca2+.

The ocular effects of spitting cobras: II. Evidence that cardiotoxins are responsible for the corneal opacification syndrome

Fractionation of H. haemachatus, N. nigricollis, N. nivea and N. melanoleuca venoms using Amberlite CG-50 and (NH4)HCO3 elution gradient chromatography yielded 11-13 fractions for each venom. One fraction, F X, from H. haemachatus, two fractions, F X and F XI, from N. nigricollis and one fraction, F VIII, from N. melanoleuca venoms possessed the whole of ocular activity of the venoms. The fractions were the only venom fractions that caused cardiac depressant activity; their effect was reversed by raising Ca++ concentration in the physiological solution; they did not influence the twitches of the phrenic nerve hemidiaphragm and guinea-pig ileum preparations. Further purification of the fractions on Sephadex G-50 followed by fractionation on Amberlite CG-50 yielded fractions free from phospholipase A2 activity but possessing the same ocular effects. Similarly, the cardiotoxin from commercial N. nigricollis venom caused the same ocular effects as the crude venom and its purified cardiotoxic fractions. All cardiotoxic fractions as well as N. nigricollis cardiotoxin, caused extensive chemosis, blepharitis and corneal opacification with corneal and subconjunctival neovascularization. On a weight basis, the cardiotoxins were weaker in their oculotoxic activity than the corresponding parent crude venoms possibly because of the potentiating effect of phospholipase A2 in the crude venoms. It is postulated that in spitting cobras the cardiotoxins are responsible for the corneal opacification syndrome. In other cobra venoms the stable binding of cardiotoxins with acidic proteins limits their possible ocular effects. Only in the venoms of the spitting species are the cardiotoxins present in an appropriately free form to cause the ocular opacification syndrome.

A bioassay for cobra cardiotoxin activity using semi-isolated cockroach heart

A semi-isolated cockroach heart preparation was used to rapidly determine the activity of cobra cardiotoxin, monitored as a direct response on heart rate. This preparation produced a dose-response curve in the presence of active cardiotoxin and demonstrated that cardiotoxin retained its biological activity after boiling, although cardiotoxin activity was destroyed by heating in the presence of dithiothreitol. Experiments that cross-linked radiolabeled cardiotoxin to solubilized cockroach heart membranes suggested that cardiotoxin bound specifically to a 59,000 mol. wt membrane protein in this tissue.

Cardiotoxin from cobra venom affects the Ca-Mg-ATPase of cardiac sarcolemmal membrane vesicles

Highly purified sarcolemmal (SL) membrane vesicles were prepared from bovine cardiac tissue and used to evaluate the effects of cardiotoxin (CTX) on Ca2+ transport systems of the SL membrane. The addition of CTX, at 1.0 microM and 10 microM, stimulated the ATP-dependent transport of 45Ca2+ by the Ca2(+)-Mg2(+)-ATPase to 138% and 193% of control levels, respectively. The increase in Ca2+ accumulation by the SL vesicles in the presence of CTX was not affected by 100 microM ouabain but was reduced to 50% of control uptake levels by the addition of 0.5 mM dicyclohexylcarbodiimide. Additionally, no effect of CTX (1.0 microM to 10 microM) was seen on 45Ca2+ transport by the Na-Ca exchange system. These effects of CTX on the SL membrane do not appear to be due to non-specific membrane disruption since SL vesicles preloaded with 45Ca2+ did not release (efflux) the accumulated Ca2+ more rapidly in the presence of CTX (1.0 microM to 250 microM). The rate of hydrolysis of ATP by the SL Ca2(+)-Mg2(+)-ATPase was observed to increase (29% to 52%) as CTX concentration increased (1.0 microM to 10 microM). The same concentrations of CTX had no effect on ATP hydrolysis by Na(+)-K(+)-ATPase. We conclude that 10 microM CTX stimulates the rate of ATP hydrolysis and Ca2+ transport by the SL Ca2(+)-Mg2(+)-ATPase.

Do cardiotoxins possess a functional site? Structural and chemical modification studies reveal the functional site of the cardiotoxin from Naja nigricollis

Examination of the literature has revealed that regarding the amino acid sequences, cardiotoxins constitute a family of homogeneous compounds. In contrast, cardiotoxins appear heterogeneous as far as their biological and spectroscopic properties are concerned. As a result, comparison between these molecules with a view to establishing structure-activity correlations is complicated. We have therefore reviewed recent works aiming at identifying the functional site of a defined cardiotoxin, ie toxin gamma from the venom of the spitting cobra Naja nigricollis. The biological and structural properties of toxin gamma are first described. In particular, a model depicting the 3-dimensional structure of the toxin studied by NMR spectroscopy is proposed. The toxin polypeptide chain is folded into 3 adjacent loops rich in beta-sheet structure connected to a small globular core containing the 4 disulfide bonds. A number of derivatives chemically modified at a single aromatic or amino group have been prepared. The structure of each derivative was probed by emission fluorescence, circular dichroism and NMR spectroscopy. Also tested was the ability of the derivatives to kill mice, depolarize excitable cell membranes and lyse epithelial cells. Modification of some residues in the first loop, in particular Lys-12 and at the base of the second loop substantially affected biological properties, with no sign of concomitant structural modifications other than local changes. Modifications in other regions much less affected the biological properties of the toxin. A plausible functional site for toxin gamma involving loop I and the base of loop II is presented. It is stressed that the functional site of other cardiotoxins may be different.