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 microliter ml-1) and omega-CgTX (1 microM) but was not affected by omega-Aga IVA (0.5 microM). When FTX (0.1 microliter ml-1) was assayed on spontaneous release either in normal Ringer solution or in low Ca(2+)-high Mg2+ solution, it was found not to affect miniature endplate potential (MEPP) amplitude but to increase MEPP frequency by approximately 2-fold in both conditions. 3. Presynaptic calcium currents (ICa), measured by the perineurial technique in the presence of 10 mM tetraethylammonium chloride (TEA) and 200 microM BaCl2 to block K+ currents, were blocked by omega-CgTX (5 microM), partially blocked by FTX (1 microliter ml-1) and not affected by omega-Aga IVA (0.5 microM). 4. The presynaptic calcium-activated potassium current (IK(Ca)) measured by the perineurial technique in the presence of 0.5 microM 3,4-aminopyridine (DAP) to block voltage-dependent K+ currents, was strongly affected by charybdotoxin (ChTX) (300 nM) and completely abolished by BaCl2 (200 microM). This current was also blocked by omega-CgTX (5 microM) and by CdCl2 (200 microM) but was not affected by FTX (1 microliter ml-1). The blockade by omega-CgTX could not be reversed by elevating [Ca]o 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.
D000818:Animals; D002104:Cadmium / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism / Q000502:physiology; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D005071:Evoked Potentials / Q000187:drug effects; D066298:In Vitro Techniques; D008274:Magnesium / Q000378:metabolism / Q000502:physiology; D009045:Motor Endplate / Q000187:drug effects; D009469:Neuromuscular Junction / Q000187:drug effects / Q000378:metabolism*; D018377:Neurotransmitter Agents / Q000378:metabolism*; D010455:Peptides / Q000494:pharmacology; D011073:Polyamines / Q000494:pharmacology; D011892:Rana catesbeiana; D017661:Receptors, Presynaptic / Q000187:drug effects / Q000378:metabolism*; D020866:omega-Conotoxin GVIA
D002121:Calcium Channel Blockers; C071920:FTX, spider toxin; D018377:Neurotransmitter Agents; D010455:Peptides; D011073:Polyamines; D017661:Receptors, Presynaptic; D002104:Cadmium; D020866:omega-Conotoxin GVIA; D008274:Magnesium; D002118:Calcium