Beta-receptor stimulation by adrenaline elevates plasma noradrenaline and enhances the pressor responses to cold exposure and isometric exercise.

Isoprenaline 3.5-35 ng/kg/min, salbutamol 17.5-175 ng/kg/min and prenalterol 1-32 micrograms/kg were given intravenously (i.v.) to 10, 12 and nine subjects with borderline hypertension respectively. For a given chronotropic effect the effects on plasma noradrenaline were in the order: salbutamol greater than isoprenaline greater than prenalterol, indicating that the increase in noradrenaline was a beta 2-receptor-mediated response. This was supported by the antagonistic effects of atenolol and propranolol on this response. Adrenaline 30 ng/kg/min i.v. raised plasma adrenaline by a factor of 10 in 17 subjects with borderline hypertension. Plasma noradrenaline rose from 218 +/- 29 to 264 +/- 35 pg/ml (mean +/- s.e.m., P less than 0.05). The increments in blood pressure and plasma noradrenaline in response to cold exposure and isometric exercise were greater during the adrenaline infusion than during saline. This effect of adrenaline was antagonized by propranolol 0.5 mg i.v. These in vivo results lend support to the view that adrenaline may act as a physiological modulator of sympathetic neurotransmission by facilitating the release of noradrenaline through activation of presynaptic beta 2-type receptors.