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 potentials that consisted of a rapid depolarization, a transient repolarization, a plateau phase, and a complete repolarization. These action potentials lasted approximately 570 ms and occurred at a frequency of approximately 0.4 Hz. Action potentials were abolished by the dihydropyridine (DHP)-sensitive Ca2+ channel blocker nifedipine (1.0 microM) and were enhanced by the DHP-sensitive Ca2+ channel agonist BAY K 8644 (0.5 microM). The K+ channel blockers tetraethylammonium chloride (5.0 mM) and 4-aminopyridine (4-AP; 2.0 mM) prolonged the action potential, whereas charybdotoxin (100 nM), a blocker of calcium-activated potassium channels, had no effect. Whole cell currents were characterized in enzymatically isolated smooth muscle cells from the same preparation. 4-AP, a blocker 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 increase in cytosolic Ca2+ resulted in a shift of the activation curve by -53 mV. External tetraethylammonium chloride (200 microM) 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+ currents were inhibited by nifedipine and were increased by BAY K 8644. We conclude that DHP-sensitive voltage-dependent Ca2+ channels are responsible for the depolarization of the action potentials and that the repolarization is due to primarily 4-AP-sensitive K+ current.
Zhang
L
L
Department of Anatomy, College of Medicine, University of Vermont, Burlington 05405.
Bonev
A D
AD
Nelson
M T
MT
Mawe
G M
GM
D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester / Q000494:pharmacology; D015761:4-Aminopyridine / Q000494:pharmacology; D000200:Action Potentials / Q000187:drug effects / Q000502:physiology*; D000818:Animals; D015220:Calcium Channels / Q000187:drug effects / Q000502:physiology*; D002462:Cell Membrane / Q000502:physiology; D018999:Charybdotoxin; D004594:Electrophysiology / Q000379:methods; D005260:Female; D005704:Gallbladder / Q000502:physiology*; D006168:Guinea Pigs; D066298:In Vitro Techniques; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects; D009130:Muscle, Smooth / Q000502:physiology*; D009543:Nifedipine / Q000494:pharmacology; D026902:Potassium Channel Blockers; D015221:Potassium Channels / Q000502:physiology*; D012604:Scorpion Venoms / Q000494:pharmacology; D019789:Tetraethylammonium; D013757:Tetraethylammonium Compounds / Q000494:pharmacology; D013779:Tetrodotoxin / Q000494:pharmacology; D013997:Time Factors
D015220:Calcium Channels; D026902:Potassium Channel Blockers; D015221:Potassium Channels; D012604:Scorpion Venoms; D013757:Tetraethylammonium Compounds; D018999:Charybdotoxin; D013779:Tetrodotoxin; D019789:Tetraethylammonium; D001498:3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethyl-5-nitro-4-(2-(trifluoromethyl)phenyl)-, Methyl ester; D015761:4-Aminopyridine; D009543:Nifedipine