Characterization of membrane currents in single smooth muscle cells from the guinea-pig gastric antrum.
1. Smooth muscle cells, enzymatically isolated from the antrum of the guinea-pig stomach, were voltage clamped at room temperature using the whole-cell patch clamp technique. In physiological salt solution (PSS), step depolarization from a holding potential of -90 mV elicited inward calcium current (ICa) followed and superimposed by outward potassium current. 2. Outward current was divided into components depending on the presence of extracellular Ca2+ and others which were not activated as a result of Ca2+ influx. Ca(2+)-dependent components were (1) a fast transient component most likely representing Ca(2+)-activated K+ current (IK(Ca)) immediately triggered by the initial peak of ICa and (2) spontaneous transient outward currents (STOCs) apparently reflecting synchronized opening of IK(Ca) channels by cyclic release of Ca2+ from intracellular stores. Ca2+ influx-independent outward current could be divided into two main components: (1) a transient component (I(to)) showing voltage-dependent activation and inactivation and (2) a non-inactivating component (Ini). 3. I(to) activated with a threshold around -30 mV, was fully available at -90 mV and completely inactivated at -10 mV. The time course of both activation and inactivation of I(to) at different potentials could be described by single exponential functions. Time constants of activation decreased from 35 ms at -10 mV to 10 ms at +40 mV. The time constant of inactivation was about 2 s and only weakly voltage dependent. Time constants for exponentially developing recovery from inactivation of I(to) ranged from 0.1 s at -100 mV to 10 s at -30 mV. I(to) was insensitive to 4-aminopyridine (4-AP, 5 mmol/l), slightly sensitive to tetraethylammonium (TEA, 10 mmol/l), but substantially inhibited by caffeine (10 mmol/l) and Cd2+ (5 mmol/l). Estimates of the single-channel conductance by current fluctuation analysis indicated a small value of about 2.5 pS. 4. The action of TEA on current elicited from a holding potential of -10 mV indicated a major contribution to Ini of a distinct component (Ini,K) that was completely blocked by this substance at a concentration of 10 mmol/l. Ini was almost unaffected by 4-AP (5 mmol/l) and caffeine (10 mmol/l), but strongly suppressed by Cd2+ (5 mmol/l). Current fluctuation analysis of Ini,K gave a value for the single-channel conductance of about 60 pS. 5. Ca2+ inward current was studied in PSS ([Ca2+]o = 2.5 mmol/l) using pipette solution in which K+ was replaced by Cs+ to suppress outward K+ currents.(ABSTRACT TRUNCATED AT 400 WORDS)
Noack
T
T
Department of Physiology, Philipps University, Marburg, FRG.
Deitmer
P
P
Lammel
E
E
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; D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D006168:Guinea Pigs; D066298:In Vitro Techniques; D007700:Kinetics; D008564:Membrane Potentials / Q000187:drug effects; D009130:Muscle, Smooth / Q000187:drug effects / Q000378:metabolism*; D009543:Nifedipine / Q000494:pharmacology; D011188:Potassium / Q000378:metabolism; D011706:Pyloric Antrum / Q000187:drug effects / Q000378:metabolism*; D019789:Tetraethylammonium; D013757:Tetraethylammonium Compounds / Q000494:pharmacology
D013757:Tetraethylammonium Compounds; D002110:Caffeine; 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; D011188:Potassium; D002118:Calcium