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

The cardiotoxic actions of southern Chinese cobra (Naja naja atra) venom have been investigated. Cardiotoxicity accounted for lethality of crude venom in anaesthetized rats and cardiotoxic polypeptides constituted the bulk of the venom when it was fractionated by CM-Sephadex chromatography. The cardiotoxic actions of these polypeptides were investigated in isolated cardiac tissue. Actions of cardiotoxins on rate, force and intracellular potentials occurred more readily in ventricular tissue and were reversible in all cardiac tissues. Bolus injections of cardiotoxins to perfused rat hearts produced profound changes in intracellular potentials, but such changes were reversed by 10 min after toxin wash-out. The changes included depolarization and loss of the fast phase (0) of the action potential. The loss of beating and contracture produced by cardiotoxins in isolated atria and ventricles showed such marked tachyphylaxis that toxicity was lost after 2-5 exposures. Both calcium and heparin ameliorated the cardiotoxic actions of cardiotoxins. Since the actions of cardiotoxin were partially selective for ventricular tissue, reversible, demonstrated tachyphylaxis and could be prevented, it is suggested that a specific molecular mechanism may be involved, rather than a non-specific action such as cell membrane destruction.