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

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Abstract
To elucidate the mechanisms of smooth muscle relaxant effects of nitroglycerin, the effects of this substance on the K+-contracture and the 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 Ca antagonistic vasodilator. The effects of papaverine, a prototype smooth muscle relaxant, were studied. La3+-contracture was induced in a Ca-free environment with the addition of La, an effective blocker of Ca influx. In the coronary artery, nitroglycerin produced a relaxation both of the La3+-contracture and the K+ contracture. La3+-contracture and K-contracture in the colon were not 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 and 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 Ca influx, the relaxant effect of nitroglycerin was due to the suppression of Ca release from the intracellular store sites and/or stimulation of Ca uptake into the intracellular store-sites. Papaverine may cause a relaxation through augmentation of the Ca binding to the intracellular store sites for Ca and through inhibition of the Ca influx.
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