Augmentation and suppression of the excitatory and inhibitory neuromuscular transmission induced by the ionophores, X-537A and A23187, in the red muscle of carp, Cyprinus carpio.

The effects of the ionophores, X-537A and A23187, on excitatory junction potentials (ejp), inhibitory junction potentials (ijp), diphasic junction potentials (diphasic jp) composed of ejp and ijp, and miniature excitatory junction potentials (mejp) were examined in the red muscles of carp, Cyprinus carpio. When 25 microM X-537A and 5 microM A23187 were applied, the amplitude of ejp and ijp increased transiently, then decreased gradually, and finally disappeared. The duration of ejp and ijp was little affected by the ionophores. The ionophores induced a transient increase in the frequency of mejp, which then decreased gradually after the maximum increase was attained, and finally fell below the control level. The ionophores had little effect on the resting membrane potential, membrane resistance of the muscle fiber, and amplitude of the compound action potential recorded from nerve bundles innervating this muscle. The ionophores caused ACh-evoked potentials to decrease gradually and finally disappear. These results suggest that the increase in the amplitude of ejp and ijp and in the frequency of mejp might be mainly due to the augmentation of ACh release from nerve terminals caused by an elevation of the intracellular Ca2+ concentration. On the other hand, the decrease and the abolition in the amplitude of ejp and ijp and in the frequency of mejp might be mainly due to the desensitization of the postsynaptic membrane.