INFLUENCE OF GROWTH-HORMONE-RELEASING FACTOR AND (OR) THYROTROPIN-RELEASING FACTOR ON SOMATOTROPIN, PROLACTIN, TRIIODOTHYRONINE AND THYROXINE RELEASE IN GROWING PIGS

Abstract

Three experiments were conducted to examine the influence of growth-hormone-releasing factor (GRF) and thyrotropin-releasing factor (TRF) on somatotropin (ST), triiodothyronine (T3), thyroxine (T4) and prolactin (PRL) release in gilts. Two experiments designed as 5 × 5 Latin squares were conducted to determine first the dose effect of human GRF (1–44)NH2 at 0, 3, 9, 27 and 81 μg kg−1 on serum GRF and ST concentrations, and second the effect of TRF at 0, 1,3,9 and 27 μg kg−1 on serum ST, T3, T4 and PRL. Areas under the curve (AUC) calculated for GRF increased (P < 0.001) proportionately to the dose: however, the ST response was maximized at an intermediary dose around 27 μg kg−1 (linear effect (P < 0.05) and quadratic effect (P < 0.01)). Injections of TRF did not influence ST AUC; however, there were significant (P < 0.05) linear and quadratic effects of TRF on T3, T4 and PRL AUC. In the third trial, to verify any interactions between the simultaneous administration of GRF and TRF, 12 Yorkshire purebred gilts averaging 137 ± 13 kg were randomly allotted, one by one, to six different treatments daily and this procedure was repeated for 3 consecutive days: A, control saline; B, porcine (p)GRF(1–29)NH2 (9 μg kg−); C, TRF (3 μg kg−1); D, TRF (9 μg kg−1); E, pGRF (9 μg kg−1) + TRF (3 μg kg−1); F, pGRF (9 μg kg−1) + TRF (9 μg kg−1)) according to a 2 × 3 factorial design. Each hypothalamic factor produced its own specific effect similar to those above without any evidence of interaction when injected simultaneously at 24-h intervals between each stimulation. In summary, a maximum ST response to s.c. GRF injection was seen at a dose close to 27 μg kg−1 and maximal PRL, T3 and T4 responses to s.c. TRF injection were obtained at a dose of 9 μg kg−1; TRF does not stimulate ST release and GRF and TRF injected simultaneously induced ST, PRL, T3 and T4 release without interacting with each other. Key words: Somatotropin, prolactin, thyroid hormones, growth hormone and thyrotropin-releasing factors, pigs