The mechanism of beta-adrenergic receptor blockade-induced elevation of arterial blood pressure in rats

Beta-adrenergic blockade increases peripheral vascular resistances acutely and paradoxically lowers arterial blood pressure in most species but not in the rat. In this study the hypothesis has been tested of a significant participation of unopposed alpha-adrenergic mediated vasoconstriction following beta-adrenergic blockade in blood pressure regulation of conscious rats. Alpha-adrenergic blockade in propranolol-pretreated rats significantly decreased mean arterial blood pressure by 22%, heart rate by 20%, and total peripheral resistance by 14% when compared to rats treated only with propranolol, whereas cardiac output was similar in the two groups. A significant 28% reduction of coronary blood flow in rats treated with alpha- and beta-adrenergic blockers, when compared to rats treated only with propranolol, is most likely related to phentolamine's indirect effects on the coronary vasculature. Cerebrovascular and renovascular resistances were similar in these groups. All changes reported were significant at the 95% confidence level. It is concluded that the mechanism of increased blood pressure following beta-adrenergic blockade in rats is related at least in part to unmasking of unopposed alpha-receptor tone, which, however, is minimal in the coronary, cerebral and renal circulations.