Circadian Variation of α1-Adrenoceptor-mediated Pressor Response to Phenylephrine in Man

The variability in the pressor effects of the α1-adrenoceptor agonist phenylephrine was observed under placebo conditions in ten healthy subjects in a double blind randomized study. Phenylephrine infusions were administered before administration of placebo (baseline) and 2, 4, 8, 12, 24 and 48 h later. The doses of phenylephrine required to increase 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 μg min−1, respectively) were significantly (P < 0.01) less than the baseline values (8.30 am, 108.0 ± 27.6 g min−1). These results might indicate a circadian variation in the phenylephrine-induced α-adrenoceptor-mediated vascular response in healthy subjects. These observations lend further insight into circadian variations of vascular tone that might contribute to circadian rhythms in cardiovascular disease.