Rhythmic fluctuations in the urine composition of sheep: separation of feed-dependent from other rhythms.

The relative contribution of feed-induced and feed-independent changes to the pattern of electrolyte excretion have been determined in sheep maintained for extended schedules providing feeds at intervals of 24, 19, 4 and 1 hr. Power spectral analysis indicated that the pattern of electrolyte excretion was predominantly fixed by the feeding cycle. Frequent small feeds reduced the peak levels of excretion. However, feed-independent periodicities on hourly feeding regimes included a 24 hrs rhythm in urinary total solids, K, Cl and HCO3 and a 32 (or 64) hrs periodicity in a wide range of components. None of these occurred in all animals. It would appear that the sheep studied were not as closely influenced by normal 24 hrs environmental cycles as are human subjects. Furthermore, the sheep electrolyte rhythms were not correlated with the 24 hrs variation in adrenal steroid production.