Postsynaptic alpha 1- and alpha 2-adrenoceptors in human blood vessels: interactions with exogenous and endogenous catecholamines.

The relative efficacy of epinephrine and norepinephrine on vascular alpha 1- and alpha 2-adrenoceptors and also the difference in vasoconstriction induced by exogenous norepinephrine as opposed to neuronally released norepinephrine were studied in the forearm of healthy volunteers. Intra-arterial cumulative dose infusions of epinephrine and norepinephrine (0.6, 1.6 and 4.0 ng kg-1 min-1) were given in the presence of saline, the selective alpha 1-antagonist doxazosin (0.1 microgram kg min-1) the selective alpha 2-antagonist yohimbine (1.0 microgram kg min-1) and the combination of both antagonists. beta-Adrenoceptor-mediated effects were prevented by a concomitant i.a. infusion of propranolol (1.0 microgram kg-1 min-1). Forearm blood flow (FBF) was measured before each infusion and at the end of each dose step. Neuronal norepinephrine was released by i.a. infusion of tyramine in three cumulative doses (0.25, 0.50 and 1.25 micrograms kg-1 min-1) and by lower body negative pressure (LBNP, -40 mmHg for 5 min). Changes in FBF were measured without and with concomitant i.a. infusions of the aforementioned doses of doxazosin and yohimbine. In the LBNP experiment the opposite arm was used as a control. Forearm blood flow was measured by plethysmography. Epinephrine and norepinephrine induced an equal and dose-dependent vasoconstriction, which was significantly inhibited by doxazosin as well as yohimbine and to a greater extent by the combination of the antagonists. No differences were found between epinephrine and norepinephrine in this respect.(ABSTRACT TRUNCATED AT 250 WORDS)

RCT Entities:   Population Interventions Outcomes