Circadian and ultradian rhythms of peripheral prolactin concentrations in lactating dairy cows

To investigate possible circadian and ultradian periodicities for peripheral prolactin in lactating dairy cows, integrated 15-min blood samples taken sequentially over 48 h from six cows were analyzed by radioimmunoassay. Cows were housed in an environmental chamber at 19 degrees C with lights on 0700 to 2300, fed daily at 0900, and milked at 0800 and 2000. Peripheral concentrations exhibited sinusoidal circadian rhythms. For two cows, a nadir of 15 ng/ml occurred around 1900 and a zenith of 29 ng/ml around 0730. For four cows, circadian rhythms were 180 degrees out of phase, with a nadir of 28 ng/ml around 0840 and a zenith of 40 mg/ml around 1920. Ultradian peaks, which varied in amplitude within and among animals, showed no consistent relationship with either milking or feeding. Spectral analyses indicated rhythms around 100 min for the first two animals and 150 min for the four animals. In conclusion, there were no simple relations between prolactin rhythms and lights on or lights off. Further, results suggest that changes in the phase of circadian rhythms may be associated with changes in the periods of ultradian rhythms.