CIRCADIAN VARIATION OF ALPHA(1)-ADRENOCEPTOR-MEDIATED PRESSOR RESPONSE TO PHENYLEPHRINE IN MAN

The variability in the presser effects of the alpha(1)-adrenoceptor ag onist phenylephrine was observed under placebo conditions in ten healt hy subjects in a double blind randomized study. Phenylephrine infusion s were administered before administration of placebo (baseline) and 2, 4, 8, 12, 24 and 48 h later. The doses of phenylephrine required to i ncrease systolic blood pressure by 20 mmHg after 8 and 12 h (5.30 and 9.30 pm, 81.4 +/- 15.3 and 71.1 +/- 16.0 mu g min(-1), respectively) w ere significantly (P < 0.01) less than the baseline values (8.30 am, 1 08.0 +/- 27.6 g min(-1)). These results might indicate a circadian var iation in the phenylephrine-induced alpha-adrenoceptor-mediated vascul ar response in healthy subjects. These observations lend further insig ht into circadian variations of vascular tone that might contribute to circadian rhythms in cardiovascular disease.