Circadian and ultradian rhythms of body temperature and peripheral concentrations of insulin and nitrogen in lactating dairy cows

To investigate possible circadian and ultradian periodicities for periphera l insulin and urea in lactating dairy cows, integrated 15-min blood samples were taken sequentially over 48 hr from six cows. In addition, radioteleme try measurements of body temperature were averaged over the same 15-min per iods. Cows were housed in an environmental chamber at 19 degrees C with lig hts on 0700 to 2300 hr; fed daily at 0900 hr; and milked at 0800 and 2000 h r. For five of the six cows, body temperature showed a circadian rhythm pea king at 2323 hr with an amplitude of 0.34 degrees C. For the sixth-cow, bod y temperature was 180 degrees out-of-phase, peaking at 1230 hr with an ampl itude of 0.12 degrees C. Circadian rhythms for insulin and urea were consis tent for all six cows peaking at 1743 hr with an amplitude of 0.74 ng/ml fo r insulin and at 1034 hr with an amplitude of 3.83 mM for urea. Body temper ature and insulin also displayed episodic increases that often exceeded the amplitudes of circadian rhythms. For body temperature, a broad increase in spectral power was seen for periods between 100 and 175 min; time interval s between peaks averaged around 100 min. For insulin, power spectra for ind ividual cows universally indicated rhythms with periods of approximately 45 and 80 min; time intervals between peaks averaged approximately 65 min. Fo r urea, almost all spectral energy was confined to the 24-hr rhythm, althou gh there was evidence of a low-amplitude, 60-min rhythm. In conclusion, whe n animals are acclimated to a rigidly controlled environment and frequent b lood sampling is accomplished with minimal intervention, it is possible to detect rhythms inherent in the regulation of metabolic variables. (C) Elsev ier Science Inc. 1999.