Difference of the in vivo responsiveness to thyrotropin stimulation between the neotenic and metamorphosed axolotl, Ambystoma mexicanum : Failure of prolactin to block the thyrotropin-induced thyroxine release
General and Comparative Endocrinology vol:56 issue:2 pages:321-325
Basal and TSH-induced plasma concentrations of T4 have been measured by radioimmunoassay in the neotenic and metamorphosed male axolotl Ambystoma mexicanum both before and after an ovine prolactin pretreatment. All injections are made into the vena abdominalis. Basal levels of T4 are low in neotenes (85 ± 19 pg/ml) and somewhat higher in metamorphosed Ambystoma (171 ± 39 pg/ml), but are increased during metamorphosis (1094 ± 138 pg/ml). Following injection of 5 mU bovine TSH circulating levels of T4 are raised about 4 times in neotenes, but more than 50 times in metamorphosed animals. Three intravenous injections, each of 640 mU prolactin and given, respectively, 24 and 13 hr before and simultaneously with 5 mU TSH, do not inhibit the TSH-induced release in both experimental groups. In the metamorphosed Ambystoma again a more than 50-fold T4 increase is present, whereas in neotenes a 10-fold TSH-induced T4 release is seen, which is more pronounced than before the prolactin treatment. It is concluded that in A. mexicanum ovine prolactin does not block a TSH-induced T4 release and that any antagonistic action with thyroid hormones is not mediated through the thyroid gland.