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1. The beta-action of catecholamines on the smooth muscle of guinea-pig taenia coli was investigated by observing the effects of isoprenaline (2 X 10(-8)-7.2 X 10(-6) M) in the presence of an alpha-blocker, phentolamine (6.3 X 10(-6) M). Electrical and mechanical activity were recorded with the double sucrose gap method. Calcium and potassium fluxes were determined using the 45Ca and 42K isotopes. 2. Isoprenaline suppressed spontaneous spike generation, reduced the size of evoked phasic contractions, and caused a small hyperpolarization of the membrane without change in membrane resistance. These effects were abolished by a beta-blocker, propranolol (6.8 X 10(-6) M). 3. The hyperpolarization induced by isoprenaline was smaller in quiescent, nonstimulated muscle than in active, frequently stimulated preparations. It occurred with the same time course as the reduction in the size of evoked phasic contractions. Both effects were dose dependent and reached a maximum at 7.2 X 10(-7) M-isoprenaline. 4. Hyperpolarization by direct current application did not reduce the size of evoked phasic contractions until excitation threshold was reached. In the presence of isoprenaline, repolarization of the membrane to its original level by depolarizing current application did not restore the reduced phasic contractions to their original size. 5. The slopes of the current-voltage relation in the absence and presence of isoprenaline were parallel, confirming the absence of a change in membrane resistance. Isoprenaline also did not affect membrane resistance when applied in the modified ionic environments used. 6. In different external K concentrations (0.60-29.5 mM) the relationship between the size of the electrotonic potential and the magnitude of the isoprenaline-induced hyperpolarization was linear. A similar, direct relation was seen between isoprenaline hyperpolarization and membrane resistance when the latter was increased by lowering external chloride to 13.3 mM. 7. Excess Cao (7.5 mM) hyperpolarized the membrane and reduced membrane resistance, but the hyperpolarization by isoprenaline was larger than in normal solution, being inversely related to the membrane resistance. The hyperpolarization was directly related to the external Ca concentration, suggesting that the magnitude of the response to the beta-action might depend on the cytoplasmic Ca concentration. 8. In low external Na (18 mM-Nao) the beta-action was scarcely affected. Complete replacement of Na with choline increased membrane resistance, muscle tone and phasic contractions; in this condition the effects of isoprenaline were abolished. 9. When the Na pump was blocked by exposure to zero K, to ouabain, or to both simultaneously, isoprenaline remained highly effective. However, prolonged exposure to ouabain abolished the beta-action. 10. Isoprenaline (1.4 X 10(-6) M) increased 45Ca efflux by about 20%, while 45Ca influx was not changed, and 42K efflux remained constant. 11...

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1. The effects of removing the external Ca ions on the spontaneous and evoked activity of the smooth muscle of the guinea-pig taenia coli were investigated with the double sucrose-gap method.2. In Ca-free Locke solution the membrane was depolarized, the membrane resistance became low, the spike amplitude became small and the mechanical response decreased. In most preparations the electrical and mechanical activity was abolished within 10 min, but in some preparations the electrical activity continued for more than 30 min.3. In Ca-free solution containing 0.1 mM-EGTA, the membrane was depolarized and the electrical and mechanical activity was abolished within 5 min in every preparation. When NaCl was replaced with sucrose, the effects of Ca removal on the spike activity and contraction appeared very slowly and the membrane potential and membrane resistance remained unchanged.4. When Ca was replaced with Mg (2 mM) the spike was blocked within 1 min without depolarization or reduction of the membrane resistance. In Na-deficient (sucrose) solution, the presence of Mg accelerated the disappearance of the spike caused by Ca removal.5. In Ca-free solution containing 0.5 mM-Mg, a spike-like activity was observed without accompanying mechanical response. This activity was blocked by increasing the Mg concentration above 2 mM. It was Na-dependent, since it was abolished by removing Na from the external solution, but it was not influenced by tetrodotoxin (2 x 10(-6) g/ml.).6. It was concluded that calcium has at least two functions, one as current carrier for the action potential and another as controller of the Na permeability of the membrane. It was also suggested that the Ca which is bound at the membrane may be utilized as a source of Ca ions to carry the current for the action potential.

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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 degrees C and 37 degrees C, the membrane conductance increased with rising temperature and decreased with cooling (Mean Q(10) 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 Q(10) 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 (42)K- and (36)Cl-exchange were greatly slowed by cooling which did not change the rate constants of the (24)Na-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 degrees 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 degrees 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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