CIRCADIAN AND ULTRADIAN RHYTHMS OF PERIPHERAL CORTISOL CONCENTRATIONSIN LACTATING DAIRY-COWS

To investigate possible circadian and ultradian periodicities for plas ma cortisol in lactating dairy cows, integrated 15-min blood samples t aken sequentially over 48 h from six cows were analyzed by radioimmuno assay. The cows were housed in an environmental chamber at about 19-de grees-C, 50% relative humidity, and 16 h of light and 8 h of darkness (lights on at 0700 h); fed daily at 0900 h; and milked at 0830 and 200 0 h. Peripheral concentrations of cortisol for all six cows exhibited weak circadian rhythms - average minima were 3.1 ng/ml at 1800 h, maxi ma were 4.5 ng/ml at 0530 h - and strong ultradian rhythms with period s around 120 min. Peak to trough amplitudes of ultradian rhythms, 1 to 17 ng/ml, varied among and within cows over time. No direct relation between ultradian peaks and milking or feeding was apparent. Spectral analysis indicated that most power (variation) was centered around 1 c ycle every 2 h, i.e., around a period of 120 min. This very strong, ea sily observable, ultradian rhythm indicates that an oscillation with a period around 120 min is probably intrinsic to mechanisms regulating peripheral glucocorticoid concentrations.