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1. The effect of temperature on the membrane conductance of the smooth muscle of guinea-pig taenia coli was investigated electrophysiologically, using the double sucrose gap method, and by ion-flux determinations.

2. In the range between 19° C and 37° C, the membrane conductance increased with rising temperature and decreased with cooling (Mean Q₁₀ about 2·5).

3. When the Cl concentration in the external medium was low (Cl^- substitution with benzene-sulphonate), the effect of changing the temperature was smaller (Mean Q₁₀ about 1·5).

4. In muscle depolarized by excess K (27 mM) in a solution containing low Cl, cooling repolarized the membrane and markedly increased membrane resistance.

5. The slow phases of the ⁴²K- and ³⁶Cl-exchange were greatly slowed by cooling which did not change the rate constants of the ²⁴Na-efflux curve. Thus observations on the rate of exchange of ions agreed with the electrophysiological findings indicating a reduction of K- and Cl-conductance of the cell membrane at low temperature.

6. The depolarization produced by lowering the external Ca-concentration was greatest at high temperature (37° C). With cooling the membrane was repolarized and membrane resistance increased.

7. The sustained state of depolarization observed when the external Ca was replaced by Ba (2·5 mM) at high temperature (37° C) was terminated by cooling.

8. The observations may be interpreted by the hypothesis that Ca, located at two different membrane sites, controls, independently, at the outer layer mainly Na- (and Ba-) permeability and at the inner layer mainly K-permeability, the Ca-binding at these sites being affected by temperature in the opposite direction.

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