Vasoconstriction with norepinephrine causes less forearm % resistance than a reflex sympathetic vasoconstriction

We used the %-perfused human forearm model to assess the effects of vasoconstriction induced with norepinephrine on the extraction of glucose in the forearm in two groups of healthy young volunteers. The norepinephrine findings were compared with a previously studied group in which vasoconstriction had been caused by reflex activation of the sympathetic nervous system. The aim of the study was to determine the relative importance of hemodynamic and receptor- mediated mechanisms of % resistance. Plasma %, arterial and venous glucose samples, and forearm blood flow were measured at 10-minute intervals during a 30-minute baseline, a 60-minute intra-arterial % infusion, and during 30 minutes of % infusion plus vasoconstriction. Group 1 (n=14) had physiological vasoconstriction induced by inflation of bilateral thigh cuffs to 40 mm Hg to cause pooling of blood in the lower extremities and reflex vasoconstriction in the forearm; group 2 (n=8) had intra-arterial infusion of norepinephrine to achieve the same degree of vasoconstriction as seen with inflation of thigh cuffs in group 1. Subjects in group 3 (n=7) had infusion of intra-arterial norepinephrine to achieve a twofold increase in physiological vasoconstriction. With a physiological decrease in forearm blood flow (group 1), there was a 19% decrease in forearm blood flow resulting in a 23% reduction in glucose uptake in the forearm (P<.03). The same degree of reduction in forearm blood flow with a predominantly α-adrenergic agonist, norepinephrine (group 2), causes much less % resistance (a decrease in utilization of 13%) (P<.04). When forearm blood flow is decreased twofold over the physiological vasoconstriction (group 3), there is a 42% reduction in glucose uptake (P<.005). The larger degree of % resistance with mild (25%) reflex vasoconstriction when compared with an equal degree of vasoconstriction induced by norepinephrine may be due to activation of β-receptors during the reflex or to differences in the microcirculatory patterns with different modalities of vasoconstriction.