EFFECTS OF CA2+ CHANNEL BLOCKERS ON TRANSMITTER RELEASE AND PRESYNAPTIC CURRENTS AT THE FROG NEUROMUSCULAR-JUNCTION

1. The effects of the calcium channel blockers, funnel-web spider toxin (FTX), omega-agatoxin IVA (omega-Aga IVA) and omega-conotoxin GVIA (omega-CgTX), were tested on transmitter release and presynaptic currents in frog motor nerve endings. 2. Evoked transmitter release was blocked by FTX (IC50 = 0.02 mu l ml(-1)) and omega-CgTX (1 mu M) but was not affected by omega-Aga TVA (0.5 mu M). When FTX (0.1 mu ml(-1)) vas assayed on spontaneous release either in normal Ringer solution or in low Ca2+-high Mg2+ solution, it was found not to affect miniature endplate potential (MEPP) amplitude but to increase MEPP frequency by similar to 2-fold in both conditions. 3. Presynaptic calcium currents (I-Ca), measured by the perineurial technique in the presence of 10 mM tetraethylammonium chloride (TEA) and 200 mu M BaCl2 to block K+ currents, were blocked by omega-CgTX (5 mu M), partially blocked by FTX (1 mu l ml(-1)) and not affected by omega-Aga IVA (0.5 mu M). 4. The presynaptic calcium-activated potassium current (I-K(Ca)) measured by the perineurial technique in the presence of 0.5 mu M 3,4-aminopyridine (DAP) to block voltage-dependent K+ currents, was strongly affected by charybdotoxin (ChTX) (300 nM) and completely abolished by BaCl2 (200 mu M). This current was also blocked by omega-CgTX (5 mu M) and by CdCl2 (200 mu M) but was not affected by FTX (1 mu l ml(-1)). The blockade by omega-CgTX could not be reversed by elevating [Ca](0) to 10 mM. 5. The results suggest that in frog synaptic terminals two omega-CgTX-sensitive populations might coexist. The transmitter release process seems to be mediated by calcium influx through a omega-CgTX- and FTX-sensitive population.