CHARACTERIZATION OF PRESYNAPTIC CALCIUM CHANNELS WITH OMEGA-CONOTOXIN-MVIIC AND OMEGA-GRAMMOTOXIN-SIA - ROLE FOR A RESISTANT CALCIUM-CHANNEL TYPE IN NEUROSECRETION

The peptide Ca2+ channel antagonists omega-conotoxin (omega-CTX) MVIIC and omega-grammotoxin (omega-GTX) SIA were studied by measuring their effects on the release of [H-3]glutamate from rat brain synaptosomes. The pseudo-first-order association constant for omega-CTX MVIIC (1.1 x 10(4) M(-1) sec(-1)) was small, relative to that for omega-GTX SIA (3.6 x 10(5) M(-1) sec(-1)). Equilibrium experiments showed that omega-CTX MVIIC blocked similar to 70% of Ca2+-dependent glutamate release evoked by 30 mM KCI (IC50 similar to 200 nM), whereas omega-GTX SIA virtually eliminated release, with lower potency (IC50 similar to 700 nM). At stronger depolarizations (60 mM KCI), neither toxin (at 1 mu M) showed significant block of release, but when these or other Ca2+ channel antagonists (omega-CTX GVIA or omega-agatoxin IVA) were used in combination a substantial fraction of release was blocked. [H-3]Glutamate release that was resistant to omega-CTX MVIIC was characterized with respect to its sensitivity to block by omega-GTX SIA and the inorganic blocker Ni2+, Both omega-GTX SIA and Ni2+ were relatively weak blockers of the resistant release. These results suggest that a previously uncharacterized Ca2+ channel exists in nerve terminals acid can be distinguished on the basis of its resistance to omega-CTX MVIIC and its weak sensitivity to omega-GTX SIA and Ni2+. Thus, at least three channel types (P, N, and a ''resistant'' type) contribute to excitation-secretion coupling in nerve terminals.