Increased plasma levels of thyroid hormones in a frog Rana ridibunda following intravenous administration of TRH

Abstract

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 T 3 and T 4 were determined by radioimmunoassay. In 8 frogs, an injection of 10 μg synthetic TRH (pGlu-His-ProNH 2) increased control levels of 0.35 ± 0.08 ng T 4/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 T 3 (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 T 4 (but not T 3) in frog serum were significantly depressed, whereas T 3 and T 4 content of thyroids remained unchanged. An intravenous injection of 10 μg TRH did not alter circulating levels of T 3 and T 4. 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.