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 (I(Ca) was followed, and partly overlapped, by an outward K+ current (I(K)). 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 (E(rev)) of the current suppressed by ACh (3 x 10(-6) mol/l) was about 20 mV more positive than the calculated K+ equilibrium potential (E(K)). 5. When [K+]o was increased, E(rev) was shifted positively; but at each [K+]o, E(rev) was more positive than E(K). 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 E(K). ACh (10(-5) mol/l) applied in the presence of mmol/l TEA suppressed a pure K+ current (E(rev) = E(K)), 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.