Effect of human growth hormone-releasing factor and(or) thyrotropin-releasing factor on hormone concentrations in dairy calves

To determine the effect of chronic treatment with human growth hormone-releasing factor (1-29)NH2 (GRF) and(or) thyrotropin-releasing factor (TRF), 20 calves averaging 70.2 kg BW were divided into four groups (n = 5) according to a 2 X 2 factorial design. For 86 d, calves in each group received twice daily s.c. injections of either .9% NaCl, GRF (5 micrograms/kg BW), TRF (1 microgram/kg BW) or GRF (5 micrograms/kg BW) plus TRF (1 microgram/kg BW). On d 87, all calves received a s.c. injection of GRF (5 micrograms/kg BW) plus TRF (1 microgram/kg BW). Blood samples were collected every 20 min for 18 h on d 1, 29, 57 and 85, and for 8 h on d 87. Hormone responses were measured as area under the hormone concentration curve over time. GRF and TRF acted in synergy (P less than .10) on GH release throughout the treatment period. Growth hormone responsiveness to GRF and(or) TRF decreased (P less than .01) with days of treatment, but this decrease was due to aging rather than to chronic treatment, because GH response to GRF plus TRF was similar (P greater than .10) between control and treated calves on d 87. TRF increased prolactin (Prl) concentration until the end of the treatment period (P less than .01). The response of thyroid-stimulating hormone (TSH) to TRF disappeared (P greater than .10) after 1 mo of treatment, whereas the thyroxine (T4) response decreased (P less than .01) throughout the treatment period. GRF did not induce nor did it interact with TRF on TSH and T4 release.(ABSTRACT TRUNCATED AT 250 WORDS)