IONIC BASIS OF THE ACTION-POTENTIAL OF GUINEA-PIG GALLBLADDER SMOOTH-MUSCLE CELLS

Smooth muscle cells in the intact guinea pig gallbladder had a resting membrane potential of about -45 mV and had spontaneous action potenti als that consisted of a rapid depolarization, a transient repolarizati on, a plateau phase, and a complete repolarization. These action poten tials lasted approximately 570 ms and occurred at a frequency of appro ximately 0.4 Hz. Action potentials were abolished by the dihydropyridi ne (DHP)-sensitive Ca2+ channel blocker nifedipine (1.0 muM) and were enhanced by the DHP-sensitive Ca2+ channel agonist BAY K 8644 (0.5 muM ). The K+ channel blockers tetraethylammonium chloride (5.0 mM) and 4- aminopyridine (4-AP; 2.0 mM) prolonged the action potential, whereas c harybdotoxin (100 nM), a blocker of calcium-activated potassium channe ls, had no effect. Whole cell currents were characterized in enzymatic ally isolated smooth muscle cells from the same preparation. 4-AP, a b locker of voltage-dependent K+ channels, suppressed 70% of the outward current at 0 mV. Charybdotoxin (100 nM) reduced an additional 15% of the current at 0 mV. Single calcium-activated potassium channels were identified. The potential for half-activation of these channels, at a cytosolic Ca2+ concentration of 100 nM, was 66.8 mV. A fivefold increa se in cytosolic Ca2+ resulted in a shift of the activation curve by -5 3 mV. External tetraethylammonium chloride (200 muM) reduced the mean single channel current by 48% at 0 mV. The whole cell outward current was abolished by replacement of intracellular K+ for Cs+. Ca2+ current s were inhibited by nifedipine and were increased by BAY K 8644. We co nclude that DHP-sensitive voltage-dependent Ca2+ channels are responsi ble for the depolarization of the action potentials and that the repol arization is due to primarily 4-AP-sensitive K+ current.