Pituitary hormones that stimulate the thyroidal system in teleost fishes.

Abstract

The pituitaries of American plaice, winter flounder, salmon, and carp contain fractions, unadsorbed (Con AI) or adsorbed (Con AII) on concanavalin A-Sepharose, which stimulate the thyroidal system of hypophysectomized (hypex) winter flounder. Chum salmon Con AII thyrotropic fraction (Con AII TF) stimulates the accumulation of thyroxine (T4) and some triiodothyronine (T3). The appropriate fraction has a molecular weight of approximately 40,000, is retained on DEAE Bio-Gel A and on CM Bio-Gel A, and is designated Con AII 40K DEII CMII. This fraction was previously shown not to stimulate the synthesis of 11-ketotestosterone at a concentration where the corresponding CMI-gonadotropin (GtH) increased the level 45-fold. The salmon thyrotropic fraction not adsorbed on Con A, Con AI 45K DEIII CMII, stimulates T3 accumulation in a dose-dependent manner and only has a transient effect on plasma T4. The CMII thyrotropic fraction failed to stimulate vitellogenesis in hypex flounder at a dose where the appropriate CMI-GtH was active. Flounder and plaice Con AI TF also mainly stimulate plasma triiodothyronine (T3), resulting in low plasma ratios; Con AII TF behaves like its presumed bovine TSH homolog in elevating plasma thyroxine (T4) with small changes in T3, resulting in high plasma ratios. That Con AI TF stimulates primarily plasma T3 is supported by the significant reduction in plasma T3 and elevation of ratio after administering antibodies specific for Con AI TF; that Con AII TF stimulates primarily plasma T4 is supported by the significant reduction in plasma T4 and drop in ratio after administering antibodies specific for Con AII TF. Carp Con AI TF increased plasma T3; it had the same behavior on ion exchangers as those from the other species. The findings are supported by comparative studies using salmon Con AII hormone on rainbow trout, and antibodies to the salmon hormones on trout and Atlantic salmon. As expected, antibodies to Con AI TF of salmon decreased plasma T3 while antibodies to salmon Con AII TF diminished plasma T4.