Increased Vascular Adrenergic Vasoconstriction and Decreased Vasodilation in Blacks

Abstract

Abstract-Blood pressure reactivity is enhanced in young black subjects through mechanisms that are poorly understood.We compared ␣-adrenergic-mediated vasoconstrictor and ␤-adrenergic vasodilator sensitivity and their relation to sympathetic activity in blacks and whites. Ten healthy black (age, 29.9Ϯ2.4 years) and 10 white (age, 28.3Ϯ1.9 years) men were studied. Forearm blood flow was measured with strain-gauge plethysmography after the intrabrachial artery administration of phenylephrine (1.25 to 20 g/min) and isoproterenol (60 and 400 ng/min) after application of lower-body negative pressure and after a cold pressor test. Forearm and systemic norepinephrine spillover were measured with a radioisotope dilution technique. ␣-Adrenergic vasoconstriction was markedly increased (ANOVA Pϭ0.008) and ␤-adrenergic vasodilation decreased (ANOVA Pϭ0.02) in blacks. Phenylephrine (10 g/min) decreased forearm blood flow by 58.0Ϯ2.5% in blacks but only by 26.6Ϯ6.0% in whites (PϽ0.001). Vasoconstrictor response to endogenous norepinephrine, stimulated by a cold pressor test, resulted in a higher forearm vascular resistance in blacks than in whites (107.3Ϯ13 versus 64.8Ϯ13 mm Hg ⅐ mL Ϫ1 ⅐ 100 mL Ϫ1 , Pϭ0.03). There were no significant ethnic differences in basal or stimulated forearm or systemic norepinephrine spillover. Increased vasoconstrictor and decreased vasodilator responses in blacks were not correlated. Increased sympathetically mediated vascular tone caused by enhanced vasoconstriction and attenuated vasodilation, effects that would be additive, and not increased sympathetic activity could enhance vascular reactivity and may play a role in the pathogenesis of hypertension in blacks.

Serum calcium revisited: associations with 24-h ambulatory blood pressure and cardiovascular reactivity in Africans

“…Not surprisingly, calcium infusion in humans also raised systolic and diastolic blood pressure and total peripheral resistance in both hypertensive and normotensive subjects with no change in cardiac output, and infusion of the calcium channel blocker, verapamil, attenuated this increase in both total peripheral resistance and blood pressure. 38,39 In addition, African men show increased vascular a1-adrenergic vasoconstriction (and therefore, a higher calcium sensitivity) when exposed to stress compared with Caucasian men and women, [17][18][19] and they also seem to respond better to calcium channel blockers compared with Caucasians. 15,40,41 This experimental evidence supports our results on associations in the younger men, indicating an increased vascular resistance and decreased stroke volume with increasing serum calcium when subjected to stress.…”

“…15,16 Cardiovascular reactivity studies show that African men have greater a-adrenergic vasoconstriction compared with African women and Caucasian men and women. [17][18][19] Serum calcium was not included in these studies. Animal studies do indeed suggest that serum calcium increases blood pressure because of an abnormally high calcium permeability of vascular smooth muscle cells when a-adrenergic receptors are activated, thereby increasing vasoconstriction.…”

Alpha 2 -Adrenergic Receptor–Induced Vascular Constriction in Blacks and Whites

“…2 Many investigators have observed that blood pressure responses to stress are higher in normotensive black than white subjects. [3][4][5] This finding suggests that ethnic differences exist, either in regulation of sympathetic activity or in vascular response to the same degree of sympathetic activation. In keeping with this possibility is the observation that the decrease in blood pressure in response to antihypertensive drugs acting through adrenergic mechanisms is smaller in blacks.…”

“…These include the following findings: attenuated ␤ 2 -adrenergic [11][12][13] and endotheliumdependent nitric oxide-mediated vasodilation 12,14 and enhanced ␣ 1 -adrenergic receptor (AR) vasoconstriction in blacks. 5 Presynaptic ␣ 2 -ARs mediate inhibition of sympathetic activity centrally by decreasing sympathetic outflow, whereas postsynaptic vascular ␣ 2 -ARs mediate vasoconstriction peripherally. Despite the considerable interest in possible ethnic differences in the regulation of adrenergic response, the contribution of the ␣ 2 -AR is not clear.…”

Job Strain, Ethnicity, and Sympathetic Nervous System Activity

“…Pharmacological studies of BP reactivity have found that hypertensive subjects tend to have blunted baroreflex sensitivity 13 and heightened pressor sensitivity in response to an ␣-agonist. 14 Thus, hypertensive subjects have increased vascular sensitivity to sympathetic output 15,16 and impaired reflexes that might return BP back toward normal.…”