Development of radioimmunoassay for bullfrog thyroid-stimulating hormone (TSH): Effects of hypothalamic releasing hormones on the release of TSH from the pituitary in vitro

A bullfrog (Rana catesbeiana) thyroid-stimulating hormone (TSH) β-subunit (TSHβ) antiserum was produced by employing a C-terminal peptide synthesized on the basis of the amino acid sequence deduced from bullfrog TSHβ cDNA. Immunohistochemical studies revealed that the bullfrog adenohypophyseal cells that immunologically reacted with the anti-bullfrog TSHβ corresponded to those positively stained with an antiserum against human (h) TSHβ. The antiserum was used for the development of a specific and sensitive radioimmunoassay (RIA) for the measurement of bullfrog TSH. The sensitivity of the RIA was 0.75±0.07ng TSH/100μl assay buffer. The interassay and intraassay coefficients of variation were 7.6 and 5.3%, respectively. Several dilutions of pituitary homogenates of larval and adult bullfrogs, or medium in which bullfrog pituitary cells were cultured, yielded dose-response curves that were parallel to the standard curve. Bullfrog prolactin, growth hormone, luteinizing hormone, follicle-stimulating hormone, and α-subunit derived from glycoprotein hormones did not react in this assay. Immunoassayable TSH in the pituitary culture medium was confirmed to exist in the form of TSHβ coupled with the α-subunit by an immunoprecipitation experiment using the TSHβ antiserum and an α-subunit antiserum. TSH released from pituitary cells into the medium was also confirmed to possess a considerable activity in stimulating the release of thyroxine from the thyroid glands of larval bullfrogs in vitro. The effects of hypothalamic hormones such as mammalian gonadotropin-releasing hormone (mGnRH), ovine corticotropin-releasing hormone (oCRH), and thyrotropin-releasing hormone (TRH) on the release of TSH by dispersed anterior pituitary cells of the bullfrog larvae and adults were also studied. CRH markedly stimulated the release of TSH from both adult and larval pituitary cells. Both TRH and GnRH moderately stimulated the release of TSH from adult pituitary cells but not from the larval cells. This is the first report on the development of an RIA for amphibian TSH, which has provided the direct evidence that the release of TSH from the amphibian pituitary is enhanced by the hypothalamic releasing hormones such as CRH, TRH, and GnRH.