Nitroglycerine‐induced biphasic relaxation in vascular smooth muscle of rat aorta

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Abstract
1 Nitroglycerine induced biphasic relaxation in the rat aorta, previously contracted by noradrenaline; a rapid decrease in tension was followed by a gradual increase reaching a steady level below the control contractile tension. No initial transient relaxation was induced by nitroglycerine in high K-stimulated muscle. 2 The initial transient relaxation, but not the sustained relaxation, was dependent on the concentration of external K; maximum relaxation was observed in the presence of 2.7 mm K solution and only a slight relaxation was observed in 0 mm or 10.8 mm K solution. The initial transient relaxation was also inhibited by ouabain or low Na solution. 3 On an appropriate increase in the concentration of external K, noradrenaline-induced contraction was transiently relaxed. Previous application of nitroglycerine potentiated this K-induced relaxation. 4 Pretreatment of the muscle with methylene blue, an inhibitor of guanylate cyclase, inhibited both the initial transient and the sustained relaxations induced by nitroglycerine, but not the K-induced transient relaxation. 5 It is suggested that the nitroglycerine-induced initial transient relaxation, but not the sustained relaxation, may be due to a stimulation of an electrogenic Na pump. Both relaxation phases may be mediated by cyclic GMP.