Baroreflex sensitivity modulates vasodepressor response to nitroprusside

Baroreflex activity is a determinant of the homeostatic response to alteration in blood pressure. We examined the factors that determine the magnitude of the vasodepressor response to sequential incremental intravenous infusions of sodium nitroprusside (NP), 0.05 to 6.4 μg/kg/min, in eight male patients with essential hypertension. Each infusion level was of 10 minutes’ duration. Change from control values of mean arterial pressure (ΔMAP), heart rate (ΔHR) and plasma norepinephrine (ΔNE) were obtained at the end of each infusion level. Significant correlations were found between ΔMAP vs log dose NP, ΔHR vs ΔMAP and ΔNE vs ΔMAP for each patient (p < 0.05). However, the slopes of these relationships varied widely between subjects and were significantly correlated with the control blood pressure of each patient. In addition, the sympathetic responsiveness, as measured by ΔNE vs ΔMAP, was inversely correlated with the degree of vasodepressor response seen. Thus, the magnitude of the vasodepressor response was determined by two major factors: 1) the predrug blood pressure, possibly reflecting altered vascular geometry with hypertension; 2) the degree of sympathetic response, which probably acts by mediating the degree of reflex alpha-adrenergic-mediated arteriolar vasoconstriction.