Role of K+ channels in spontaneous electrical and mechanical activity of smooth muscle in the guinea-pig mesotubarium.

1. The spontaneous electrical and mechanical activity and the efflux rate of 86Rb+ in the guinea-pig mesotubarium were studied in the presence of agents interacting with K+ channels. 2. Tetraethylammonium (TEA, 10 mM) increased the amplitude of the action potentials while having no consistent effect on the frequency or amplitude of spontaneous contractions. 3. 4-Aminopyridine (4-AP, 1-5 mM) caused a graded increase in the duration of the contractions and of the electrical slow waves, and a decrease in the duration of the relaxed period between contractions. At 4 mM-4-AP or more the cell was unable to repolarize from the slow wave and the membrane depolarized to -26 mV from the normal resting potential of -63 mV. The rate of 86Rb+ efflux in the presence of 5 mM-4-AP was higher than that at 60 mM-K+, where the membrane potential is -24 mV. 4. 4-AP (5 mM) evoked a contracture in Ca(2+)-free solution, containing 1 mM-EGTA, both at the normal [K+]o of 5.9 mM and at 60 mM-K+, suggesting release of intracellular Ca2+. 5. Apamin (0.1-1 microM) and charybdotoxin (1-10 nM), blockers of Ca(2+)-dependent K+ channels, were without effects on the spontaneous electrical and mechanical activity. 6. The K+ channel opener pinacidil (10 microM) inhibited the spontaneous contractions and hyperpolarized the membrane by about 7 mV. The permeability to 86Rb+ was increased by a factor of 1.4. 7. It is concluded that different K+ channels are involved in the generation of spikes and slow waves: one sensitive to TEA and responsible for repolarization of the individual action potential, and another sensitive to 4-AP and responsible for repolarization of the slow wave. The duration of the relaxed period can be influenced by activation of K+ channels sensitive to pinacidil.