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.
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