Effects of 4-aminopyridine at the frog neuromuscular junction.
Micromolar concentrations of 4-aminopyridine (4-AP) were able to increase the amplitude of the end-plate current in frog neuromuscular junction blocked either by d-tubocurarine or by low Ca++ high Mg++ medium. The end-plate potential was also increased. These effects were reversible. The changes in the end-plate current amplitude observed after 4-AP treatment had no effect on the end-plate current time course. There was no significant difference in the resting membrane potential or mean amplitude and frequency of spontaneous miniature end-plate potentials in the presence of 4-AP. The quantal content of the end-plate potential was increased in every preparation tested and the minimal synpatic delay was lengthened in a dose-related way. 4-AP did not modify the dependence of the amplitude of the end-plate current on membrane potential. In the presence of 4-AP, the time constant of the falling phase of the end-plate current remained an exponential function of the membrane potential. The end-plate current equilibrium potential was unaffected by 4-AP. The increase in the amount of acetylcholine released by nerve impulse induced by 4-AP occurs without modification in the calcium cooperativity. The authors suggest that 4-AP, by prolonging the presynaptic action potential, could increase calcium concentration in the nerve terminal and, thus, the transmitter release.
D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D001001:Anura; D066298:In Vitro Techniques; D009045:Motor Endplate / Q000187:drug effects; D009469:Neuromuscular Junction / Q000187:drug effects* / Q000378:metabolism; D018377:Neurotransmitter Agents / Q000378:metabolism; D011725:Pyridines / Q000494:pharmacology*; D011893:Rana esculenta; D009435:Synaptic Transmission / Q000187:drug effects
D018377:Neurotransmitter Agents; D011725:Pyridines