An estimate of the proportion of the resting membrane conductance of the smooth muscle of guinea-pog taenia coli attributable to chloride.

1. The contribution of Cl and K to the steady-state membrane conductance of the smooth muscle of the guinea-pig taenia coli has been investigated by measuring the increase in membrane resistance in Cl-deficient and K-deficient solutions. A large external electrode was used for passing current and the voltage change was recorded with an intracellular electrode. The experiments were carried out in hypertonic solution in order to suppress spontaneous activity.2. When the tissue was exposed to K-deficient solution, the amplitude and time course of the electrotonic potential was increased and the membrane was hyperpolarized.3. When the tissue was exposed to Cl-deficient solutions in which Cl was replaced with an impermeant anion, benzene sulphonate, the size and time course of the electrotonic potential was increased whereas the membrane potential, after 20-30 min equilibration, was unchanged. The magnitude of the increase of the electrotonic potential depended on the substituted amount of Cl.4. When the tissue was exposed to low K the relation between the membrane conductance and the remaining external K concentration was the same in the presence of the normal Cl concentration and in Cl-deficient solution (7 mM-Cl). It was therefore concluded that any effect of benzene sulphonate on the K conductance is negligible.5. The effects of Cl or K deficiency on the electrotonic potential indicate that Cl and K ions account for most of the membrane conductance and that the contribution by Cl is about 40%.6. The ratio of P(Cl)/P(K) was calculated from the constant field theory on the assumption that the contribution of ions other than K and Cl to the membrane conductance is negligibly small. The mean value was 0.40.