Vasoconstrictor and vasodilator effects in normal and atherosclerotic conscious rabbits

1. Rabbits were fed a cholesterol-rich diet for 5 two-week intervals. Polyvinyl catheters were then implanted into the femoral artery and vein. Dose-response curves to acetylcholine (ACh), noradrenaline (NA), phenylephrine (Phen) and angiotensin II (AII), were obtained in 6 cholesterol-fed and 6 control rabbits before and after isradipine (code name PN200-110) 100 micrograms kg-1. After these experiments the animals were killed and aortic rings were suspended in an organ bath. ACh but not nitroprusside-induced relaxation was impaired in atherosclerotic but not in control preparations. 2. ACh decreased blood pressure dose-dependently in both groups of rabbits even though ACh did not relax the aortae of the same rabbits in vitro. 3. Blood pressure effects reflect mostly changes in resistance vessels. The pressor effects of NA, Phen and AII were enhanced in atherosclerotic compared with normal rabbits. 4. After a dose of 100 micrograms kg-1 isradipine the dose-response curves of all agents were shifted to the right. The differences between atherosclerotic and control rabbits disappeared, except for the AII-induced pressor response, which remained enhanced in atherosclerotic animals. The calcium antagonist thus only partly corrected the atherosclerosis-associated hyperresponsiveness to vasoconstrictor agents.