The inhibitory effect of the toxic fraction from sea urchin (Toxopneustes pileolus) venom on Ca-45(2+) uptake in crude synaptosome fraction from chick brain

The effects of toxic peaks (P-I, P-II and P-III eluted from Sephadex G-200 column) from the sea urchin Toxopneustes pileolus on time-dependent Ca-45(2 +) uptake in chick Pt fraction (crude synaptosome fraction) were studied un der physiological ionic conditions. Time-dependent Ca-45(2+) uptake was inh ibited by P-II and P-III, but not by P-I, P-II had the greatest inhibitory effect, The inhibitory effect of P-II was not due to the inhibition of Ca-4 5(2+) binding on P-2 fraction, because P-II did not affect Ca-45(2+) bindin g in osmotic-shocked P-2 fraction. P-II did not affect KCI-stimulated Ca-45 (2+) uptake in P-2 fraction, (Ca2+-Mg2+)-ATPase activity in the synaptic pl asma membrane (SPM) fraction, or (Na+-K+)-ATPase and Mg2+-ATPase activities in osmotically-shocked P-2 fraction. In contrast, the Na+/Ca2+ exchanger b locker 2',4'-dichlorobenzamil (DCB; 100 mu M), with a poor specificity, inh ibited not only time-dependent Ca-45(2+) uptake but also KCI-stimulated Ca- 45(2+) uptake, (Ca2+-Mg2+)-ATPase, Mg2+-ATPase and (Na+-K+)-ATPase. Involve ment of Na+-Ca2+ exchanger in the time-dependent Ca-45(2+) uptake was ruled out, since it was not inhibited by replacement of Na+ with Li+ in reaction medium, These results suggested that the inhibition by P-II on time-depend ent Ca-45(2+) uptake appeared to be more specific than the commercially ava ilable Na+/Ca2+ exchanger blocker DCB, although the mechanism is not clear yet.