Calcium antagonists: systemic and regional haemodynamic effects in conscious spontaneously hypertensive rats (SHR).

The systemic and regional (renal, mesenteric and hindquarter) haemodynamic effects of the calcium antagonists verapamil, nifedipine and PY 108-068 [diethyl 4-(2, 1, 3-benzoxadiazol-4-yl)-1, 4-dihydro-2, 6-dimethylpyridine-3, 5-dicarboxylate] were studied in conscious, chronically instrumented spontaneously hypertensive rats (SHR). All three calcium antagonists caused a dose-dependent fall in mean arterial pressure (MAP) and total peripheral resistance. In intact animals the fall in resistance was related primarily to a vasodilatation in the hindquarter (mainly muscular) vascular bed. During the early fall in blood pressure a reflex rise in cardiac output and heart rate was observed following nifedipine and PY 108-068, but not after verapamil. Following sinoaortic baroreflex denervation the dose-response curve for the calcium antagonist-induced fall in blood pressure and total peripheral resistance was shifted to the left. Moreover, the calcium antagonists then caused a comparable degree of vasodilation in all three vascular beds studied. It is concluded that calcium antagonists preferentially dilate the muscular bed in intact animals; however, sinoaortic baroreflex denervation unmasks an unselective pattern of vasodilatation.