Night-day differences in the ultradian rhythmicity of plasma renin activity

During night-sleep, plasma renin activity displays periodic oscillations which are closely related to the alternation of REM-NREM sleep. To see whether this nocturnal rhythm persisted during the day-time, plasma renin activity was measured every 10 min for 24 hours in 4 human volunteers and in 4 others over a 10-h day-time period. To avoid the influence of repeated food intake which stimulates renin release, the subjects were on continuous enteral nutrition. Spectral analyses of the data revealed clear differences between the nocturnal and the diurnal PRA patterns. In subjects on enteral nutrition, sustained 100-min oscillations with strong spectral densities persisted during the night. They were closely related to REM-NREM cycles. During the day-time, however, the fluctuations were damped and less regular, and power spectra were split into 2 or 3 peaks without any predominant period. These results demonstrate that regular 100-min PRA oscillations could only be detected during night-sleep. The clear night-day differences suggest that the underlying oscillatory mechanisms may be weaker during day-time or may be counteracted by other physiological processes.