Mechanism of increased alpha adrenergic vasoconstriction in human essential hypertension.

Multiple components of vascular alpha adrenergic responsiveness were investigated in twenty-four men with mild hypertension and eighteen age- and weight-matched normotensive controls. Arterial plasma norepinephrine (paNE), an index of sympathetic drive, was increased in hypertensives compared to normotensives (mean +/- SE), 199 +/- 24 vs. 134 +/- 11 pg/ml, P less than 0.02. The effective concentration of intra-arterial (iaNE) increasing forearm vascular resistance (FAVR) 30% (NE-EC30, an index of vascular alpha-receptor sensitivity) was similar in normotensives and hypertensives, 9 +/- 1 vs. 13 +/- 3 ng/100 ml per min, respectively, P greater than 0.3. The phentolamine induced reduction in FAVR, an index of vascular alpha-tone, was greater in hypertensives, -21.3 +/- 1.8 vs. normotensives, -14.9 +/- 1.2 U, P less than 0.02. We interpret these data as evidence for normal vascular alpha-receptor sensitivity to norepinephrine in mild hypertensives. Consequently, the increased sympathetic drive in mild hypertensives explains the elevated vascular alpha-tone. Although vascular alpha-receptor sensitivity to iaNE was normal, the FAVR responses at high doses (reactivity) were greater in hypertensives to regional infusion of both NE and angiotensin II. This "nonspecific" enhancement of vascular reactivity is probably explained by structural vascular changes in hypertensives.