The relationship between age and relaxation of vascular smooth muscle in the rabbit and rat

Relaxation of rabbit and rat aorta by isoproterenol decreases with increasing age, whereas such responses caused by nitroglycerin or sodium nitrite are not age-dependent. In the present study, we sought to determine whether this relationship also exists in pulmonary arteries and portal veins. As was the case with the aorta, isoproterenol-induced relaxation of pulmonary arteries decreased as the animal aged; relaxation by nitroglycerin was minimally altered. Aging did not influence responses of rabbit and rat portal veins to isoproterenol or nitroglycerin. If the responses of these blood vessels are characteristic of the responses of other vascular smooth muscles to vasodilators, then this study suggests a difference in the manner by which arteries and veins age. We also confirmed that isoproterenol-induced relaxation of rabbit and rat aorta markedly decreases with increasing age and that the responses of rat aorta to nitroglycerin are independent of age. Because of the agonist used to contract the tissues prior to drug-induced relaxation, the results of the first series of experiments with nitroglycerin on rabbit aorta were at variance with our earlier findings. When KCl was used to contract the aortas, the mean effective dose (ED50) obtained for nitroglycerin for tissues from 2-year-old rabbits was 8 to 19 times larger than that obtained from 2-month-old rabbits. This ratio dropped to 4 when the tissues were contracted with histamine. Since KCl and histamine contract rabbit aorta by different mechanisms, this finding suggests that, in addition to a specific loss in beta-receptor activity, increasing age results in an alteration in the contraction-relaxation process of rabbit aortic tissue.