Thyrotropin from amphibian ( Rana catesbeiana) pituitaries and evidence for heterothyrotropic activity of bullfrog luteinizing hormone in reptiles

Abstract

Evidence is presented for the isolation of a third glycoprotein hormone from the pituitary of the bullfrog ( Rana catesbeiana) identified as thyrotropin (TSH) on the basis of its ability to stimulate various aspects of thyroid function in the homologous species (e.g., thyroxin release, thyroid epithelial cell height, and metamorphic stimulation). In contrast, bullfrog luteinizing hormone (LH) and follicle-stimulating hormone (FSH) had relatively little effect on the bullfrog thyroid. This bullfrog TSH was also compared with the two bullfrog gonadotropins in several reptilian thyrotropin bioassays (e.g., stimulation of thyroxine release, radioiodine uptake, and thyroid epithelial cell height in a turtle, Pseudemys, and radioiodine uptake in a lizard, Anolis). In these heterologous species, bullfrog LH was the most potent thyrotropic factor of the three frog hormones; whereas, bullfrog TSH and FSH had little activity. Bullfrog LH, therefore, functions as a “heterothyrotropin” in a heterologous species. These results provide evidence that the thyrotropin-receptor complex in the Reptilia has undergone extensive independent evolution such that its thyroid receptor preferentially reponds to the LH rather than to the TSH from another class.