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 [3H]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 approximately 70% of Ca(2+)-dependent glutamate release evoked by 30 mM KCl (IC50 approximately 200 nM), whereas omega-GTX SIA virtually eliminated release, with lower potency (IC50 approximately 700 nM). At stronger depolarizations (60 mM KCl), neither toxin (at 1 microM) 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. [3H]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 and 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.
Turner
T J
TJ
Department of Physiology, Tufts University School of Medicine, Boston, Massachusetts 02111.
Lampe
R A
RA
Dunlap
K
K
D000818:Animals; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D015220:Calcium Channels / Q000378:metabolism* / Q000502:physiology; D018698:Glutamic Acid / Q000378:metabolism; D008297:Male; D010455:Peptides / Q000494:pharmacology*; D010456:Peptides, Cyclic / Q000494:pharmacology*; D017729:Presynaptic Terminals / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D020960:omega-Conotoxins
D002121:Calcium Channel Blockers; D015220:Calcium Channels; D010455:Peptides; D010456:Peptides, Cyclic; D020960:omega-Conotoxins; C082807:omega-grammotoxin SIA; C075851:omega-conotoxin-MVIIC; D018698:Glutamic Acid