Growth hormone, thyrotropin and prolactin responses to simultaneous administration of human growth hormone-releasing factor and thyrotropin releasing hormone in the bovine.

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Abstract
The effects of intravenous injection of synthetic human pancreatic growth hormone-releasing factor-44-NH2 (hpGRF-44) and synthetic thyrotropin releasing hormone (TRH), or hpGRF-44 in combination with TRH on growth hormone (GH), thyrotropin (TSH), and prolactin (PRL) release in dairy female calves (6- and 12-month-old) were studied. When 0.25μg of hpGRF-44 per kg of body weight (bw) was injected in combination with TRH (1.0μg per kg of bw), the mean plasma GH concentration of the 12-month-old calves rose to a maximum level of 191.5ng/ml (P<0.001) at 15 min from the value of 6.8ng/ml before injection at 0 min. The maximum level was 3.1 and 6.1 times as high as the peak values obtained after injection of hpGRF-44 (0.25μg per kg of bw) and TRH (1.0μg per kg of bw), respectively (P<0.001). The area under the GH response curve for the 12-month-old calves for 3 hr after injection of hpGRF-44 in combination with TRH was 2.5 times as large as the sum of the areas obtained by hpGRF-44 and TRH injections. In contrast, the mean plasma GH level was unchanged in saline injected calves. The magnitudes of the first and the second plasma GH responses in the 6-month-old calves to two consecutive injections of hpGRF-44 in combination with TRH at a 3-hr interval were very similar. The peak values of plasma GH in the calves after hpGRF-44 injection were 2-4 times as high as those after TRH injection. When hpGRF-44 was given to the 12-month-old calves, no elevation of plasma TSH or PRL level was observed. These results indicate that when hpGRF-44 and TRH are administered together a synergistic effect on bovine GH release occurs and hpGRF-44 is a much more potent and specific stimulator of GH release than TRH in the bovine.