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-mu-g (experiment 1) or 0, 4, 20, 100-mu-g 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 < .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-mu-g TRH. T3 and T4 returned to basal levels within 6 and 8 hr post injection, respectively. Plasma pGH levels were significantly (P < .001) increased 15 min after TRH injection in piglets injected with 500-mu-g. In experiment 3, 100-mu-g 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-mu-g TRH/kg BW can be used to increase thyroid hormone levels for at least 13 d in the young suckling piglet.