Suppression of steady membrane currents by acetylcholine in single smooth muscle cells of the guinea‐pig gastric fundus.

1. Single smooth muscle cells from the fundus region of the guinea‐pig stomach, which showed contractile responses to acetylcholine (ACh) at concentrations greater than or equal to 10(‐7) mol/l, were obtained by enzymatic digestion using highly purified collagenase and papain. They were studied by recording membrane currents under voltage clamp with the patch pipette technique in the whole‐cell configuration at 25‐28 degrees C. 2. By applying voltage jumps from negative holding levels (‐70 to ‐60 mV) to more positive levels, we identified two major activating currents: an initial inward Ca2+ current (ICa) was followed, and partly overlapped, by an outward K+ current (IK). 3. Cholinergic effects on membrane currents were investigated in the range of negative membrane potentials by determining current‐voltage relations in the absence of ACh and during its continuous presence in the bathing fluid. 4. ACh induced a decrease in the steady‐state conductance which was reversibly blocked by atropine. At physiological external K+ concentration [( K+]o = 6 mmol/l), the reversal potential (Erev) of the current suppressed by ACh (3 x 10(‐6) mol/l) was about 20 mV more positive than the calculated K+ equilibrium potential (EK). 5. When [K+]o was increased, Erev was shifted positively; but at each [K+]o, Erev was more positive than EK. 6. Like ACh (10(‐6) mol/l), tetraethylammonium (TEA, 1 mmol/l) also suppressed a current with a reversal potential that was, at physiological [K+]o, 20 mV more positive than EK. ACh (10(‐5) mol/l) applied in the presence of 1 mmol/l TEA suppressed a pure K+ current (Erev = EK), which was also suppressed by 10 mmol/l TEA. 7. When K+ in the pipette and in the bathing solution was completely replaced by Na+, both ACh (10(‐5) mol/l) and TEA (1 mmol/l) caused a reduction of the membrane conductance that appeared to be identical. TEA added to the bathing solution in the presence of ACh did not produce a significant additional conductance decrease. These results did not depend on whether Cl‐ was present as a charge carrier or not. 8. It is concluded that in fundus muscle of the guinea‐pig stomach a major mechanism underlying muscarinic activation is a decrease of a K+ conductance. In addition the results indicate a suppression of a small Na+ conductance which is made up by a population of channels that are also blocked by TEA.