Thyrotrophin-releasing hormone (TRH)-induced growth hormone secretion in fowl: binding of TRH to pituitary membranes

Abstract

ABSTRACT Heat-labile specific binding sites for [3H]3-methylhistidine2-TRH ([3H]Me-TRH) were demonstrated on chicken adenohypophysial membranes. These binding sites were of both high affinity (dissociation constant, Kd = 15·53 nm) and low capacity (maximum binding capacity, Bmax = 8·73 pmol/g tissue) and of low affinity (Kd = 104·5 nm) and high capacity (Bmax = 32·41 pmol/g). Binding of [3H]Me-TRH to the pituitary membranes was greater at 4 °C than at 39 °C and occurred with rate constants (k1) of 1·6× 106 and 3·39×106m-1 min-1 respectively. Dissociation of [3H]Me-TRH binding at 4 °C occurred with a rate constant (k-1) of 0·125 min-1. Binding sites for [3H]Me-TRH were found in the cephalic lobe of the pituitary gland (the location of most lactotrophs and thyrotrophs) and in the caudal lobe (the location of most somatotrophs). The number of binding sites was greater in the caudal lobe than in the cephalic lobe, although the affinity of [3H]Me-TRH binding did not differ. The binding of [3H]Me-TRH to caudal lobe membranes was displaced by Me-TRH, TRH, pGlu-His-Pro-Gly-NH2 and [Glu1]-TRH, with half-maximal effective doses of 33 nm, 70·7 nm, 1·23 μm and 22 μm respectively, but not by [Phe2]-TRH, TRH free acid or His-Pro-diketopiperazine. The number of caudal lobe binding sites for [3H]Me-TRH in old birds was less than that in young ones, and the number of binding sites was increased in birds deprived of food for 48 h. TRH-induced GH secretion in birds would thus appear to be mediated by specific receptors on caudal lobe somatotrophs, and these results suggest that physiological changes in GH secretion (during growth and periods of fasting) are causally related to the abundance of TRH-binding sites.