Stimulation of sodium and calcium uptake by scorpion toxin in chick embryo heart cells

Scorpion toxins, the basic miniproteins of scorpion venom, stimulated the passive uptake of Na+ and Ca2+ in chick embryo heart cells. Half-maximum stimulation was obtained for 20-30 nM Na+ and 40-50 nM Ca2+. Scorpion toxin-activated Na+ and Ca2+ uptakes were fully inhibited by tetrodotoxin, a specific inhibitor of the action potential Na+ ionophore in excitable membranes. Half-maximum inhibition was obtained with the same concentration of tetrodotoxin (10 nM) for both Na+ and Ca2+. Scorpion toxin-stimulated Ca2+ uptake was dependent on extracellular Na+ concentration and was not inhibited by Ca2+ channel blocking drugs which are inactive on heart cell action potential. Thus, in heart cells scorpion toxin affects the passive Ca2+ transport, which is coupled to passive Na+ ionphore. Other results suggest that (1) tetrodotoxin and scorpion toxin bind to different sites of the sarcolemma and (2) binding of scorpion toxin to its specific sites may unmask latent tetrodotoxin - sensitive fast channels.