EFFECT OF TWO-MONTH TREATMENT WITH GROWTH HORMONE-RELEASING FACTOR ON GROWTH HORMONE RELEASE IN DAIRY COWS

Two experiments were conducted to examine the effect of a human growth hormone-releasing factor fragment (hGRF(1–29)NH2) on growth hormone (GH) secretion in lactating dairy cows. An initial study was conducted on 12 lactating dairy cows to determine the dose effect (5, 10 and 20 μg kg−1) of a subcutaneous (s.c.) injection of hGRF(1–29)NH2 on GH secretion. The three doses induced similar GH response, with peaks and areas under the curve (AUC) of 38.5, 41.2 and 38.0 ng mL−1, and 3496, 4112 and 4079 ng mL−1 min at 5, 10 and 20 μg kg−1, respectively. In a second study, the GH response after daily s.c. injections of saline (n = 8) or 10 μg kg−1 of hGRF(1–29)NH2 (n = 9) for 57 d was evaluated in dairy cows averaging 252 d in lactation. Blood samples were collected on d 1, 15, 29, 43 and 57 of treatment. On the last day of treatment, the GRF-induced GH peak was higher than on the first day, 46.8 vs. 25.2 ng mL−1, while the AUC was not different, 4010 vs. 2684 ng mL−1 min. Finally, to determine if the pituitary's ability to release GH was modified after 57 d of GRF treatment, the 17 cows were challenged with an intravenous injection of hGRF(1–29)NH2 (1 μg kg−1). Control cows had a lower peak (21.8 vs. 32.2 ng mL−1) but a similar AUC (2190 vs. 2579 ng mL−1 min) than GRF-treated cows. Hence, in lactating dairy cows, (1) s.c. administration of hGRF(1–29)NH2 induced a similar GH release with doses ranging from 5 to 20 μg kg−1 and (2) the GRF-induced GH peak was higher after 57 d of GRF treatment while the AUC was not different. Therefore, 2-mo GRF treatment of dairy cows during their two last month of lactation does not result in a decreased GH response to GRF. Key words: Growth hormone-releasing factor, growth hormone, cows (dairy)