Effect of Age on the Responsiveness of Vascular α-Adrenoceptors in Man

Abstract

Aging has been reported to alter the responsiveness of β-receptors, but there have been few comparable studies of α-receptors. This study compares in six young and six healthy elderly subjects the haemodynamic effects of the α₁-antagonist prazosin and the pressor responses to the α₁-agonist phenylephrine. Prazosin orally lowered erect (but not supine) blood pressure in both groups by a similar amount between 2 and 6 h after dosing. Maximal falls in systolic pressure were 19.5 ± 15.7 and 29.3 ± 11.4 mm Hg (mean ± SD) in young and old, respectively. There was a significant difference in the associated heart rate response: in the young group mean heart rate increased to 103 beats/min, but there was no corresponding increase in the elderly group, which had a mean heart rate of 80 beats/min. Following the intravenous infusion of increasing doses of phenylephrine, log dose—response curves were derived, and the dose required to raise mean arterial pressure by 20 mm Hg (PD₂₀) was compared. The mean PD₂₀, was significantly different in the two groups: 2.5 ± 1.6 in the young, compared to 4.6 ± 2.3 μg/kg/min in the elderly, indicating reduced pressor responsiveness in the elderly. However, no significant difference was apparent when pressor responsiveness was determined following the administration of prazosin. Thus, while there is no evidence of an age-related increase in the sensitivity of α-adrenoceptor-mediated vaso-constriction, the data are not inconsistent with an age-related reduction in responsiveness to α-adrenoceptor activation.