The inhibitory effect of the toxic fraction from sea urchin (Toxopneustes pileolus) venom on 45Ca2+ uptake in crude synaptosome fraction from chick brain

Abstract

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 45Ca2+ uptake in chick P2 fraction (crude synaptosome fraction) were studied under physiological ionic conditions. Time-dependent 45Ca2+ uptake was inhibited 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 45Ca2+ binding on P2 fraction, because P-II did not affect 45Ca2+ binding in osmotic-shocked P2 fraction. P-II did not affect KCl- stimulated 45Ca2+ uptake in P2 fraction, (Ca2+-Mg2+)-ATPase activity in the synaptic plasma membrane (SPM) fraction, or (Na+-K+)-ATPase and Mg2+-ATPase activities in osmotically-shocked P2 fraction. In contrast, the Na+/Ca2+ exchanger blocker 2',4'-dichlorobenzamil (DCB; 100 μM), with a poor specificity, inhibited not only time-dependent 45Ca2+ uptake but also KCl-stimulated Ca2+ uptake, (Ca2+-Mg2+)-ATPase, Mg2+- ATPase and (Na+-K+)-ATPase. Involvement of Na+-Ca2+ exchanger in the time-dependent 45Ca2+ 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-dependent 45Ca2+ uptake appeared to be more specific than the commercially available Na+/Ca2+ exchanger blocker DCB, although the mechanism is not clear yet.