Properties of Spontaneous Electrical Activity in Smooth Muscle of the Guinea-Pig Renal Pelvis.

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Abstract
In the guinea-pig renal pelvis, most smooth muscle cells examined (>90%), using a conventional microelectrode, had a resting membrane potential of about -50 mV and produced spontaneous action potentials with initial fast spikes and following plateau potentials. The remainder (<10%) had a resting membrane potential of about -40 mV and produced periodical depolarization with slow rising and falling phases. Experiments were carried out to investigate the properties of spontaneous action potentials. The potentials were abolished by nifedipine, suggesting a possible contribution of voltage-gated Ca(2+) channels to the generation of these potentials. Niflumic acid and 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), inhibitors of Ca(2+)-activated Cl(-) channels, showed different effects on the spontaneous action potentials, and the former but not the latter inhibited the activities, raised the question of an involvement of Cl(-) channels in the generation of these activities. Depleting internal Ca(2+) stores directly with caffeine or indirectly by inhibiting Ca(2+)-ATPase at the internal membrane with cyclopiazonic acid (CPA) prevented the generation of spontaneous activity. Chelating intracellular Ca(2+) by 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) increased the amplitude of the spike component of spontaneous activity. Indomethacin inhibited the spontaneous activity, whereas prostaglandin F(2 alpha) enhanced it. The results indicate that in smooth muscle of the renal pelvis, the generation of spontaneous activity is causally related to the activation of voltage-gated Ca(2+) channels through which the influx of Ca(2+) may trigger the release of Ca(2+) from the internal stores to activate a set of ion channels at the membrane. Endogenous prostaglandins may be involved in the initiation of spontaneous activity.