Verapamil and Nifedipine Inhibition of Contractions Induced by Potassium and Noradrenaline in Human Mesenteric Arteries and Veins

Abstract

Ring preparations of human mesenteric arteries and veins were contracted by noradrenaline (1.8×10^-5M) or potassium (127mM). Isometric tension was recorded. In the arterial preparations, the maximum response to noradrenaline was 97±8% (mean±S.E.M.) of that to potassium. In the veins, the corresponding figure was 38±4%. The calcium antagonists verapamil (2.2±10^-7-2.2± 10^-5M) and nifedipine (2.9×10^-8-2.9×10^-6M) relaxed both arteries and veins contracted by noradrenaline or potassium, and reduced the responses to these agents when added 15 min. before stimulation. The time course of relaxation of potassium contracted preparations, induced by verapamil and nifedipine, was more rapid and complete than that produced by a calcium-free, high potassium solution. In contrast to verapamil, nifedipine caused a more pronounced inhibition of the potassium than of the noradrenaline evoked contractions in both arteries and veins. After exposure to a calcium-free medium for 30 min., the arterial response to noradrenaline was significantly (P-6M) and nifedipine (2.9-10^-7M) were equi-effective in reducing the noradrenaline and potassium induced responses in calcium-free medium. The results suggest that there are differences in reactivity, not only between mesenteric arteries and veins, but also between, e.g. peripheral and mesenteric vessels. The calcium antagonists nifedipine and verapamil do not have an identical mode of action. However, both agents seem to inhibit influx of extracellular calcium, and might also have an inhibitory effect on the release of intracellular calcium.