In vivo release of prolactin in Rana ridibunda following an intravenous injection of thyrotropin-releasing hormone

All experiments were performed on female Rana ridibunda during the months of April and May. Injections were made into the canulated vena abdominalis and blood samples were taken up to 8 hr thereafter and checked for prolactin concentration using a heterologous frog radioimmunoassay. An injection of 10 μg TRH (thyrotropin releasing hormone; pGlu-His-Pro NH₂) will elevate control serum levels of 28.54 ± 5.40 ng/ml prolactin to 59.56 ± 8.78 ng/ml (n = 6) and this within half an hour. A smaller increase was present with 1 μg of TRH and none with 0.1 or 0.01 μg. Prolactin remains elevated up to 2 hr after the injection but reaches control values again after 4 hr. Two days after removal of the pars distalis serum levels have declined to 14.48 ± 1.67 ng/ml (n = 15) and no influence of TRH injections, tested at several doses, on this is seen. Previous thyroxine (T₄) and tri-iodothyronine (T₃) assays using the same blood have revealed that 1 and 10 μg TRH also increases T₄ in a dose-related way after 1 hr, with a maximal increase at 2 hr, and T₃ after 4 hr. It is concluded that TRH is able to induce the release of both prolactin and TSH from the hypophysis in the frog R. ridibunda as it does in mammals.