General and Comparative Endocrinology vol:48 issue:4 pages:469-475
The vena abdominalis of pentobarbital anesthetized frogs (Rana ridibunda) was cannulated for injection and blood sampling. After acetone and petroleum ether extraction, serum was concentrated 4 and 20 times and T3 and T4 were determined by radioimmunoassay. In 8 frogs, an injection of 10 μg synthetic TRH (pGlu-His-ProNH2) increased control levels of 0.35 ± 0.08 ng T4/ml after 1, 2, 4, and 8 hr, with a maximal increase up to 2.65 ± 0.48 ng/ml after 4 hr. In 7 frogs, 1 μg TRH raised control levels of 0.52 ± 0.22 ng/ml after 1, 2, and 4 hr, with a peak value of 1.74 ± 0.45 ng/ml after 2 hr. Control injections (Ringer), 0.01 and 0.1 μg TRH were without effect. Control levels of T3 (0.029 ± 0.005 ng/ml) were also raised after 4 hr (0.051 ± 0.007 ng/ml) in 7 animals, but only with the 10-μg TRH dose. Two days after removal of the pars distalis, circulating levels of T4 (but not T3) in frog serum were significantly depressed, whereas T3 and T4 content of thyroids remained unchanged. An intravenous injection of 10 μg TRH did not alter circulating levels of T3 and T4. These results indicate that the activity of the thyroid gland of R. ridibunda is stimulated by intravenous injections of TRH and that this effect is mediated through the hypophysis.