Effects of TRH, bovine TSH, and pituitary extracts on thyroidal T4 release in Ambystoma mexicanum

Circulating levels of T4 are measured by radioimmunoassay after intravenous injection of TRH, bovine (b) TSH, and pituitary extracts in the Mexican axolotl (Ambystoma mexicanum). Very low control levels of T4 are found (53 +/- 3 pg/ml (n = 27), but they are increased sevenfold following injection of 1/2 pars distalis extract or 1/2-1/10 IU b-TSH. Increased levels following these injections are found in plasma up to 48 hr after the injection. An in vitro assay also indicates that a 1/2 pars distalis of the axolotl is able to release T4 from the thyroid of R. ridibunda somewhat less effectively than a 1/50 pars distalis of the same size of R. ridibunda itself. TRH (50 and 100 micrograms) is unable to stimulate the release of T4 when injected intravenously into the axolotl. It is concluded that both the hypophysis and the thyroid gland of A. mexicanum may release optimal amounts of hormones necessary for metamorphosis following proper stimulation, but that TRH cannot function as a releasing hormone in this respect.