EFFECT OF MEMBRANE-POTENTIAL ON ACETYLCHOLINE-INDUCED INWARD CURRENT IN GUINEA-PIG ILEUM

1. The whole‐cell patch clamp technique with caesium aspartate internal solution was used with single isolated cells from the longitudinal muscle layer of guinea‐pig ileum, to investigate the voltage‐dependent gating of ACh‐induced inward current. 2. In voltage clamp experiments, at holding potentials ranging from ‐80 to ‐30 mV, ACh (300 microM) produced a slow sustained inward current in physiological salt bath solution (PSS). The measurements of the reversal potentials on substituting Na+ by other monovalent and divalent cations showed that this current is through non‐selective cation channels (Ins, ACh). 3. During hyperpolarizations, Ins, ACh instantaneously increased in amplitude and then relaxed to a new steady‐state level. The I‐V relationship of the instantaneous peak was linear with a reversal potential of 0 mV, while that of the steady state was bell‐shaped. The time course of relaxation appeared to be monoexponential and its time constants were reduced by stronger hyperpolarizations. 4. These results were not affected by the organic Ca2+ antagonists D600 or nitrendipine (10 microM). Under this condition, maximal chord conductance of Ins, Ach which was observed at 0 mV was about 1.5 nS. The steady‐state activation relationship was well fitted by Boltzmann's equation with a half‐maximal activation (Vh) of ‐50 mV and a slope factor (k) of ‐15 mV at membrane potentials negative to 0 mV, but over 0 mV the degree of activation was again decreased. The time constants for relaxation also appeared to follow a sigmoid curve. 5. In current clamp experiments, superfusion of ACh (300 microM) depolarized the membrane up to ‐10 to 0 mV. Inward current injection resulting in the moderate hyperpolarization of the membrane (‐70 to ‐80 mV) attenuated ACh‐induced depolarization and stronger hyperpolarization (less than ‐80 mV) abolished it. 6. These results show that ACh‐induced depolarization is controlled by the membrane potential, which is explained by the voltage‐dependent gating of Ins, Ach. © 1990 The Physiological Society