NOVEL PHARMACOLOGICAL SENSITIVITY OF THE PRESYNAPTIC CALCIUM CHANNELS CONTROLLING ACETYLCHOLINE-RELEASE IN SKATE ELECTRIC ORGAN

The presynaptic terminals of skate (Raja montagui) electric organ were tested for their sensitivity to calcium channel antagonists. Acetylcholine (ACh) release and the elevation of intraterminal Ca2+ concentrations triggered by K+ depolarisation were studied. ACh release was measured as H-3 efflux from slices of organ prelabelled with [H-3]choline. Depolarisation caused a marked, Ca2+-dependent increase in H-3 efflux that was completely blocked by 100 mu M Cd2+ and by 300 nM omega-conotoxin-MVIIC (MVIIC). Inhibition by MVIIC was concentration dependent (IC50 of similar to 20 nM) and reversible. No inhibition was seen with nifedipine (5 mu M) or the two other peptide antagonists studied. omega-conotoxin-GVIA (GVIA) at 5 mu M and omega-agatoxin-IVA (Aga-IVA) at 1 mu M. In a ''nerve plate'' preparation (a presynaptic plexus of nerve fibres, Schwann cells, and nerve terminals) changes in intraterminal Ca2+ concentrations were measured by microfluorimetry using fluo-3. An increase in fluorescence, indicating a rise in the free [Ca2+], rapidly followed K+ depolarisation, and this change was restricted to the nerve terminals. This response was insensitive to nifedipine (5 mu M), GVIA(5 mu M), and Aga-IVA (300 nM) but almost completely abolished by MVIIC (1 mu M) MVIIC inhibition was concentration dependent and partially reversible. These results show that the nerve terminals in skate electric organ have calcium channels with a pharmacological sensitivity that is markedly different from the established L, N, and P types in other systems but shares some, but not all, of the features of the recently described Q type.