Mechanism of relaxant action of papaverine. II. Differences between modes of relaxant effects of papaverine and amytal in guinea-pig taenia coli.

Mode of relaxant effect of papaverine on guinea-pig taenia coli was compared with that of amytal, especially in regard to their actions on the calcium movements. The present results provided evidence that the mode of relaxant effect of papaverine differs from that of amytal. Papaverine relaxed the taenia coli even in the Na-free solution although its relaxant activity was markedly reduced, while amytal failed to reveal relaxant activity in the same condition. Reintroduction of a small amount of sodium ion to the Na-free solution restored the relaxant activities of papaverine and amytal but their sodium ion dependencies of the restoration were not equal. Amytal was more sensitive to sodium ion than papaverine. Papaverine induced the muscle relaxation with synchronous acceleration of 45Ca-efflux, while the relaxation in response to amytal was not accompanied by acceleration of 45Ca-efflux. The cellular 45Ca-uptake was inhibited by papaverine, but not by amytal. In these experiments, both papaverine and amytal were used at sufficient doses to inhibit the mitochondrial respiration. From these findings, it is concluded that papaverine relaxes the smooth muscle through different mechanism from that of amytal and thus the relaxant effect of papaverine may not be related to its inhibitory action on the mitochondrial respiration.