Mechanism of action of cobra cardiotoxin in the skeletal muscle.

Cobra cardiotoxin (CTX) is a potent polypeptide in inducing irreversible contracture of the chick biventer cervicis muscle. this polypeptide is about 2000 times more potent than caffeine in inducing contracture of this muscle preparation but the rate of CTX contracture is slower. Studies on the interaction on CTX with divalent cations showed that low Ca0++ (10(-31-2 mM)-Krebs' enhanced markedly while ethylene glycol bis (beta-aminoethyl ether)-N,N'-tetraacetic acid-Krebs' or high Ca++ (12 mM) inhibited completely CTX contracture; 10 mM Mg++ enhanced but 20 mM Mg++ or 10 mM Zn++ inhibited CTX contracture. Neither Na+ nor tetrodotoxin affected CTX contracture. Comparison of CTX and caffeine contracture showed that ethylenediamine tetraacetic acid (1 mM) and Mn++ (5 mM) inhibited CTX contracture completely but inhibited caffeine contracture only partially; procaine did not inhibit CTX contracture but inhibited caffeine contracture competitively; by contrast, N-ethylmaleimide inhibited CTX but not caffeine contracture. Neither caffeine nor K+ contracture was inhibited by 12 mM Ca++. CTX could induce contracture in the depolarized muscle and the muscle with T-tubule destroyed or closed, whereas K+ failed to induce contracture in the latter. Caffeine contracture was inhibited in the muscle with a previous elicitation of CTX contracture. This antagonistic effect of CTX could be prevented by calcium. Moreover, CTX increased both 45Ca++ efflux and 45Ca++ uptake. Both Zn++ (0.6 mM) and Mg++ (10 mM) but not protamine and polylysine mimicked CTX to increase 45 Ca++ uptake. Zn++ (0.6mM) was also found to be effective in replacing Ca0++ to induce CTX contracture in 10(-6) M Ca0++. CTX increased Ca and Na but decreased K contents of the muscle. The binding of radioactive iodinated CTX was inhibited not only by unlabeled CTX but also by 10mM Ca++. All of these findings suggest that CTX may affect a membrane calcium binding site and may induce contracture by releasing the membrane calcium rather than by increasing Na+ permeability of the muscle membrane.