Diurnal rhythms of plasma renin activity, atrial natriuretic peptide and arterial pressure during head-down bed rest in humans.

The effects of prolonged head-down bed rest on the rhythms of several parameters (blood pressure, heart rate, haematocrit, plasma renin activity (PRA), atrial natriuretic peptide (ANP) were assessed in six healthy men, aged 33 (SEM 2) years, who were submitted to bed rest for 28 days (D1-28). Systolic and diastolic blood pressure (BPs and BPd) and heart rate were measured at 0700 and 1900 hours; circulating PRA and ANP were determined from blood samples drawn at 0800, 1000, 1200, 1500, 1800 and 2200 hours before bed rest (D - 5), D1, 2, 7, 20, 27 during bed rest and post bed rest (D + 2). The BPs was the lowest at 0700 hours and increased at 1900 hours. There was a significant difference between values during all the measurements. The BPd and heart rate were lower at 0700 hours before and after bed rest and no significant difference appeared between these two values during the bed rest. The PRA and ANP concentrations were more stable during bed rest, and had not returned to original rhythmicity 2 days after bed rest. The mean daily concentration of ANP decreased during bed rest. It would seem from this study that changes occur in those rhythms during bed rest.