Effects of acute and chronic beta-adrenoceptor blockade on baroreflex sensitivity in humans

To determine whether beta-adrenoceptor blockade lowers blood pressure by potentiating arterial baroreflex sensitivity (BRS), we compared the effect of acute i.v. and chronic oral beta-blockade on the BRS (phenylephrine technique) of 51 subjects with essential hypertension. Subjects were randomly assigned in a double-blind protocol to one of atenolol, metoprolol, pindolol or propranolol. There was an increase in BRS, unrelated to changes in heart rate, after both acute and chronic beta-blockade. This effect was most evident in younger and less hypertensive subjects. Decreases in blood pressure after 5-months' treatment were unrelated to increases in BRS, indicating that the hypotensive action of these drugs is not dependent upon augmented baroreflex control of heart rate. Only propranolol, of the 4 beta-blockers, increased BRS significantly after acute and chronic treatment. The acute effect of propranolol was significantly different from that of i.v. metoprolol (P less than 0.008) but the effect of long-term treatment with propanolol was not significantly different from that of the other 3 beta-blockers. We conclude that the impaired reflex regulation of heart rate can be improved in younger and mild-to-moderate hypertensive patients by beta-adrenoceptor blockade. Further studies, involving larger numbers and perhaps fewer drugs are needed to determine the relative importance of lipophilicity and beta 1- or beta 2-receptor selectivity in mediating the increase in baroreflex sensitivity seen with treatment.