Are ultradian and circadian rhythms in renal potassium excretion related?

Spectral and band-pass analyses of renal potassium excretion time series demonstrated the presence of significant ultradian periodicities. These rhythms and their relationships to circadian rhythms were examined under varying conditions, with and without external (LD cycle) or internal (plasma cortisol) Zeitgebers. The results showed that ultradian rhythms appear to be independent endogenous oscillations. They seem related to circadian periodicities in that they demonstrated several characteristics of subharmonic synchronization in the entrained state, and underwent detuning and amplitude diminution with the removal of controlling Zeitgebers. Ultradian rhythms play a more prominent role in the absence of strong external or internal time cues. They may be important in the formation and stabilization of circadian timekeeping or may represent some dissociated state of elements in the circadian system.