Ultradian rhythms in renal excretion in dogs

Urine flow, osmolality, concentrations of sodium and potassium were measured every 10 min for 24 hours in 6 female dogs, whose sleep-wake stages were monitored electrographically. Experiments were conducted under hydration and progressive dehydration conditions. Visual examination and spectral analysis revealed significant diurnal ultradian rhythms of about 200 min/cycle in urine flow, which were out of phase with similar rhythms in osmolality in different hydratory conditions. Diurnal rhythms with similar characteristics were found in urinary potassium only in the hydration condition. Sodium excretions did not show consistent rhythmic patterns. No consistent rhythms were found in nocturnal urine parameters in different hydratory conditions. The variations in renal activities were related to the dogs' diurnal ultradian sleep-wake cycle, urine volume increased and osmolality decreased during diurnal waking episodes, and conversely volume decreased and osmolality increased during the diurnal sleep episodes.