A comparison of the differential effects of nitroglycerin, nifedipine and papaverine on contractures induced in vascular and intestinal smooth muscle by potassium and lanthanum.

To elucidate the mechanisms of smooth muscle relaxant effects of nitroglycerin, the effects of this substance on the potassium contracture (K+-contracture) and the lanthanum-induced contracture (La3+-contracture) were studied using smooth muscle preparations of the canine coronary artery and colon and the findings compared with the effects of nifedipine, a representative calcium antagonistic vasodilator. The effects of papaverine, a prototype smooth muscle relaxant, were also studied. La3+-contracture was induced in a calcium-free environment with the addition of lanthanum, an effective blocker of calcium influx. In the coronary artery, nitroglycerin produced a relaxation both of the La3+-contracture not K-contracture in the colon was affected. Nifedipine did not relax the La3+-contracture in a range of doses at which K+-contracture of both types of smooth muscles was relaxed. Papaverine produced a relaxation of La3+-contracture as well as K+-contracture, in both types of smooth muscles. Unlike the mechanism related to the relaxant effects of nifedipine, which is generally admitted to be an inhibition of calcium influx, the relaxant effect of nitroglycerin was attribute to the suppression of calcium release from the intracellular store sites and/or stimulation of calcium uptake into the intracellular store-sites. Papaverine was assumed to produce a relaxation through augmentation of the calcium binding to the intercellular store sites for calcium as well as through inhibition of the calcium influx.