Comparison of the inhibitory effects of organic Ca antagonist and nitric oxide-compound on smooth muscles.

Abstract

Effects of an organic Ca antagonist (verapamil), a nitric oxide-compound (sodium nitroprusside) and a metabolic inhibitor (antimycin A) on smooth muscle contractions were compared. 1) Verapamil dose-dependently suppressed the K-induced contraction, but had little effect on the norepinephrine-induced contraction in rabbit aorta. In guinea-pig taenia coli, verapamil inhibited both the histamine- and the K-induced contractions dose-dependently. Effect of verapamil was antagonized by raising the concentration of external Ca. Verapamil inhibited the K-induced increase in 45Ca uptake at the concentration needed to inhibit the K-induced contraction. 2) Sodium nitroprusside inhibited the norepinephrine-induced contraction in aorta, but had no effect on the K-induced contraction in aorta and the histamine- and the K-induced contractions in taenia coli. The norepinephrine-induced contraction in rat aorta was also inhibited by sodium nitroprusside, and this inhibitory effect was antagonized by methylene blue-pretreatment. 3) Antimycin A did not suppress the K-induced contraction in aorta, although this inhibitor strongly inhibited the K-induced contraction in the absence of added glucose. In guinea-pig taenia coli, the K-induced contraction was inhibited by antimycin A, and the inhibitory effect was antagonized by excess glucose. Antimycin A inhibited the K-induced increase in 45Ca uptake at the concentration lower than that needed to inhibit the K-induced contraction in aorta. Antimycin A also decreased both the ATP content and the oxygen consumption of the muscles. In conclusion, we found that verapamil, sodium nitroprusside and antimycin A exert differential effects on smooth muscle contractions.