Five healthy normotensive volunteers (mean ± SEM age 29.6 ± 4.6 years) were infused with l-noradrenaline for a 15 min period every 3 hours for 24 hours from 0900. The concentrations of l-noradrenaline used were 0.02. 0.03, 0.05 [μg·kg⁻¹·min⁻¹, preceded by a saline infusion, each step lasting 5 min. The lowest preinfusion blood pressures and heart rates were at 0300 and 0600, the range being 1.45 kPa (11.6 mmHg) systolic, 1.33 kPa (10.0 mmHg) diastolic pressure, and 12.2 beats·min⁻¹, heart rate.

The slope of the log-dose response curve relating systolic pressure to infused noradrenaline was significantly reduced at 0300 compared with 0900 (P <0.05) and at 1200 compared with 1500 (P < 0.05). Baroreflex sensitivity as measured by the regression of mean RR interval on systolic pressure, was maximal at 1200 and 0300. The circadian variation to tilt was studied in the same subjects on a separate occasion. Mean tilted minus mean lying systolic pressure was least at 1200 and 0300. Supine plasma noradrenaline did not differ with clock time, but on tilt to 70° head up, plasma noradrenaline was significantly lower at 1200 and 0300 than at 0900 and 1500 respectively (P <0.05). The slope of RR interval on systolic pressure in response to tilt is significantly increased at 1200 and 0300 compared with 0900 and 2100 (P <0.05). These results suggest a decline in adrenergic vascular reactivity and an increase in the gain of the baroreflex at 1200 and 0300.