Growth rate and secretion of pituitary hormones in relation to age and chronic treatment with thyrotropin-releasing hormone in prepubertal dairy heifers

Effect of exogenous thyrotropin-releasing hormone (TRH) administration on plasma levels of triiodothyronine (T3), thyroxine (T4) and growth hormone (GH) in chronically catheterized suckling piglets

The purpose of the present study was to determine experimental conditions to stimulate secretion of thyroid hormones (T3 and T4) with thyrotropin-releasing hormone (TRH) injections in suckling piglets during the first weeks of postnatal life. Three consecutive experiments were conducted. Four 10-20 d old piglets were i.m. injected with 0, 20, 100, 500 micrograms (experiment 1) or 0, 4, 20, 100 micrograms TRH/kg BW (experiment 2) according to a 4 X 4 latin square design involving different litters in each experiment. Blood samples were taken -15, -1, 15, 30, 45, 60, 90, 120 180 and 300 min after TRH injection in experiment 1, and -.25, -.08, .25, .5, 1, 2, 4, 6, 8, 12, 24, 30, 36, 48, 60 and 72 hr after TRH injection in experiment 2. T3 and T4 levels were significantly (P less than .01) increased as soon as 30 and 45 min after TRH injection, respectively. Maximal levels of T3 and T4 were obtained 2 and 4 hr after the injection of 100 micrograms TRH. T3 and T4 returned to basal levels within 6 and 8 hr post injection, respectively. Plasma pGH levels were significantly (P less than .001) increased 15 min after TRH injection in piglets injected with 500 micrograms. In experiment 3, 100 micrograms TRH/kg BW were injected i.m. either daily or every other day from 10 to 23 days of age. Results showed that T4 response to TRH did not decrease after repeated injections. These results indicate that daily i.m. injections of 100 micrograms TRH/kg BW can be used to increase thyroid hormone levels for at least 13 d in the young suckling piglet.

Effect of long-term administration of human growth hormone-releasing factor and (or) thyrotropin-releasing factor on hormone concentrations in lactating dairy cows

Fifteen cows (87 +/- 8 d in lactation; 641 +/- 33 kg BW) were randomly assigned to treatment and then subjected for 182 d to daily sc injection (1000 hr), in the cervical area, of saline (control), thyrotropin-releasing factor (TRF: 1 micrograms/kg BW), growth hormone-releasing factor (1-29)NH2 (GRF; 10 micrograms/kg BW) or GRF plus TRF (10 and 1 micrograms/kg BW, respectively) according to a 2 x 2 factorial design. On days 1, 31, 88 and 179, jugular blood samples were collected from 2 hr before to 6 hr after injection. Samples were also collected for 5 consecutive days after cessation of treatment. GRF always induced growth hormone (GH) release (600 vs 7925 ng.min/ml) with augmentation of response with time (interaction GRF * day; P less than .001). TRF did not affect (P greater than .25) GH release; there was no interaction (P greater than .25) with time. There was no significant interaction (P greater than .25) between GRF and TRF on GH release. However, the amount of GH release with GRF plus TRF was always greater than with GRF alone (9419 vs 6431 ng.min/ml). TRF induced a significant release of prolactin (23769 vs 42175 ng.min/ml) but GRF reduced the amount of prolactin release on the last day of sampling. TRF induced thyroid stimulating hormone (TSH) release only on the first day of injection while triiodothyronine (T3) and thyroxine (T4) continued to respond to TRF throughout the treatment period. Concentrations of T3 and T4 fell below control levels after cessation of TRF injection. In conclusion, GRF-induced GH release and TRF-induced Prl and thyroid hormone release were maintained over a 6-mo treatment period. TRF induced TSH release only on the first day of injection. Overall, these results raised the possibility of a direct effect of TRF on the thyroid gland.

Dose effect of human growth hormone-releasing factor and thyrotropin-releasing factor on hormone concentrations in lactating dairy cows

Two experiments were conducted to study the effects of growth hormone-releasing factor (GRF) and thyrotropin-releasing factor (TRF) administration on hormone concentrations in dairy cows. In the first trial, 12 cows were used on 5 consecutive days to determine the effect of four sc doses of GRF (0, 1.1, 3.3 and 10 micrograms.kg-1 BW) and three sc doses of TRF (0, 1.1 and 3.3 micrograms.kg-1 BW) combined in a factorial arrangement. GRF and TRF acted in synergy (P = .02) on serum growth hormone (GH) concentration even at the lowest dose tested and GH response to the two releasing factors was higher than the maximal response observed with each factor alone. TRF increased (P less than .01) prolactin (Prl), thyrotropin (TSH), triiodothyronine (T3) and thyroxine (T4) concentrations similarly at the 1.1 and 3.3 micrograms.kg-1 doses and GRF did not interact (P greater than .40) with TRF on the release of these hormones. In the second trial, the effect of GRF (3.3 micrograms.kg-1 BW, sc) and TRF (1.1 micrograms.kg-1 BW, sc) was tested at three stages (18, 72 and 210 days) of lactation on serum Prl and TSH concentrations. Eighteen cows (n = 6 per stage of lactation) were used in two replicates of a 3 X 3 latin square. The TRF and GRF-TRF treatments were equipotent (P greater than .05) in increasing Prl and TSH concentrations. Prl and TSH responses were similar (P greater than .40) throughout lactation. In summary, GRF at doses ranging from 1.1 to 10.0 micrograms.kg-1 and TRF at doses ranging from 1.1 to 3.3 micrograms.kg-1 act in synergy on GH release and do not interact on Prl, TSH, T3 and T4 concentrations in dairy cows. Furthermore, Prl and TSH response to TRF are not affected by stage of lactation.