Progressive microvascular alterations with the development of renovascular hypertension.

Norepinephrine-induced changes in diameters of first- (1A), second- (2A), and third-order (3A) arterioles in the exposed cremaster muscles of normotensive and renovascular hypertensive rats were quantitated via television microscopy. By 2 weeks following the surgery to induce hypertension, we found that 3A sensitivity to norepinephrine had increased and the 1As had chronically constricted. By 4 weeks, the constriction had progressed to include both 1A and 2A. Sensitivity was no longer increased in 3As and, in fact, sensitivity had decreased in 1As and 2As. The 1As and 2As could not be dilated with isoproterenol or nitroprusside; thus, the vessels appeared to have undergone a structural alteration. We suggest from these results that the early increased 3A sensitivity contributes to the initial development of hypertension. The larger arterioles then constrict to protect the downstream vessels from increased luminal pressure. As the hypertension develops, the constriction progresses to smaller arterioles in an attempt to maintain normal pressure in the capillaries (site of water exchange). The constricted arterioles contribute to increased total peripheral resistance, and with the constriction, there occurs a general decrease in vessel responsiveness.