Effect of Age on the Responsiveness of Vascular Alpha-adrenoceptors in Man

Aging has been reported to alter the responsiveness of beta-receptors, but there have been few comparable studies of alpha-receptors. This study compares in six young and six healthy elderly subjects the haemodynamic effects of the alpha 1 -antagonist prazosin and the pressor responses to the alpha 1-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 has a mean heart rate of 80 beats/min. Following the intravenous infusion of increasing doses of phenylephrine, log dose-response curves were deprived, 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 micrograms/kg/min 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 alpha-adrenoceptor-mediated vasoconstriction, the data are not inconsistent with an age-related reduction in responsiveness to alpha-adrenoceptor activation.