Vascular Response to Angiotensin II in Atherosclerosis

High-cholesterol alimentation is associated with an induction of angiotensin-converting enzyme and angiotensin II receptor expression within the vascular wall of the aorta. Despite an enhanced pressure response to angiotensin II in atherosclerotic conscious rabbits, angiotensin II-induced contraction was reduced in isolated vascular rings from the aorta and unchanged in those from the iliac artery. We, therefore, investigated whether cholesterol-induced atherosclerosis enhances overall vascular responsiveness to angiotensin II in intact animals and whether an altered arterial baroreflex sensitivity can explain the discrepancy between experiments in intact animals and isolated blood vessels. Rabbits were maintained on a high-cholesterol diet (2 g/d cholesterol plus 20 mL/d sunflower seed oil, n=11) or on a standard diet (n=12) for 12 weeks. Total serum lipids markedly increased (P<0.05). Tissue examinations 6 weeks after termination of the high-cholesterol diet revealed distinct atherosclerosis and elevated cholesterol content in the aorta (P<0.05). A high-cholesterol diet did not change baseline hemodynamic parameters. However, angiotensin II-induced increases in total peripheral resistance were larger in the atherosclerotic animals (86.3+/-13.0 versus 41.9+/-9.7 mm Hg. L(-1). min, P<0.05). In addition, the blood pressure pulse interval relationship was markedly reduced (slope: 0.80+/-0.14 versus 0. 49+/-0.06 ms/mm Hg, P<0.05), which suggested that the baroreflex blunted the angiotensin II response to a lesser extent in atherosclerotic animals. In conclusion, the overall vascular responsiveness to angiotensin II is increased in the atherosclerotic rabbit as indicated by the larger increase in total peripheral resistance. An attenuation of the arterial baroreflex sensitivity may contribute to this effect.