Mechanisms of inhibitory action of phenylephrine in guinea-pig taenia coli.

The effect of phenylephrine, an alpha-agonist, on the Ca movements and the influence of removal of external Na+ on the relaxant activity of phenylephrine were examined in the taenia coli of guinea pigs. Phenylephrine (10(-7)-10(-5)M) caused dose-dependent relaxation of the taenia coli contracted by 20 mM KCl in Locke-Ringer solution. Phenylephrine (10(-5) M) suppressed the spike discharges of the taenia coli evoked by 20 mM KCl without affecting the membrane potential, and this was accompanied by the muscle relaxation. Phenylephrine also inhibited the cellular 45Ca-uptake in the taenia coli, but had no discernible effect on the 45Ca-efflux from the smooth muscle. These effects of phenylephrine were not observed in a Na-free solution or in the highly depolarized smooth muscle. These findings suggest that the inhibition of Ca-influx in the taenia coli may be involved in the phenylephrine-induced relaxation in the partly depolarized tissue. Reasons for reduction of phenylephrine action encountered under the Na-free condition were also discussed.