CIRCADIAN AND ULTRADIAN RHYTHMS OF PERIPHERAL GROWTH-HORMONE CONCENTRATIONS IN LACTATING DAIRY-COWS

To investigate possible circadian and ultradian periodicities for plas ma growth hormone in lactating dairy cows, integrated 15-min blood sam ples taken sequentially over 48 hr from six cows were analyzed by radi oimmunoassay. The cows were housed in an environmental chamber at 19 /- 0.5 degrees C, 50% relative humidity, and 16 hr of light and 8 hr o f darkness (lights on at 0700 hr); fed daily at 0900 hr; and milked at 0800 and 2000 hr. Peripheral concentrations of growth hormone for all six cows exhibited sinusoidal circadian rhythms with average minima o f 4.1 ng/ml at 1820 hr and maxima of 5.3 ng/ml at 0630 hr. Estimated p eriods of ultradian rhythms for individual cows by spectral analysis, peak identification, and fitting cosine functions using least squares were 71 to 83 min for all cows. No direct relationship between ultradi an peaks and milking or feeding was apparent. In conclusion, a circadi an rhythm and an ultradian rhythm with a period around 80 min are prob ably intrinsic to mechanisms regulating peripheral growth hormone conc entrations in the lactating dairy cow.