BIPHASIC PRESSOR-RESPONSES TO NOREPINEPHRINE IN HUMANS

We investigated presser responses to intravenous bolus infusion of nor epinephrine in seven healthy volunteers. Norepinephrine (1, 2, and 4 m u g/kg) elevated blood pressure in a concentration-dependent manner an d decreased heart rate. The presser response to norepinephrine was bip hasic (early and late). Intravenous administration of phentolamine (10 mg) completely abolished the presser response to norepinephrine, and prazosin (5 mg, given orally) inhibited the early and late responses t o a same extent. Continuous intravenous infusion of nicardipine (2 mu g/kg/min) inhibited the late presser response but not the early one. T hese data suggest that bolus infusion of norepinephrine evoked the bip hasic pressor response and that the late response depends on vasoconst riction by calcium influx through nicardipine-sensitive calcium channe ls.