HOMOLOGOUS AND HETEROLOGOUS REGULATION OF SOMATOSTATIN-BINDING SITES ON CHICKEN ADENOHYPOPHYSEAL MEMBRANES

Binding of I-125-labelled [Tyr1]-somatostatin (I-125-]Tyr1]-SRIF) to pituitary caudal lobe membranes was suppressed in immature chickens 1 and 2 h after i.v. administration of unlabelled SRIF at concentrations of 1-100-mu-g/kg. In-vitro preincubation of chicken pituitary glands for 0.5-4.0 h with 0.1-mu-mol SRIF/1 similarly reduced the binding of I-125-[Tyr1]-SRIF to caudal lobe membrane preparations. After a 4-h incubation in 0.1 mmol SRIF/1, the withdrawal of SRIF from the incubation media was accompanied 4 h later by a partial recovery in the binding of I-125-[Tyr1]-SRIF to pituitary membranes. Passive immunoneutralization of endogenous SRIF resulted in a prompt (within 1 h) and sustained (for at least 24 h) suppression of I-125-[Tyr1]-SRIF binding to pituitary membranes. The i.m. administration of cysteamine (300 mg/kg) to 12-week-old birds depleted hypothalamic SRIF stores and decreased the density of I-125-[Tyr1]-SRIF-binding sites in the caudal and cephalic lobes of the chicken pituitary gland. The reduction in SRIF content and in SRIF-binding sites occurred within 1 h of cysteamine administration and was maintained for at least 24 h. In 6-week-old birds, cysteamine (300 mg/kg) administration suppressed pituitary binding of I-125-[Tyr1]-SRIF for at least 5 days. Circulating concentrations of GH were markedly decreased 1 and 4 h after cysteamine injection, but not after 24 h. Pituitary binding sites for I-125-[Tyr1]-SRIF were not affected by pretreatment of pituitary glands for 2-12 h in vitro with thyroxine or oestradiol-17-beta-(1 nmol/1-10-mu-mol/l) or with ovine GH or recombinant DNA-derived chicken GH (1-100-mu-g/ml in vitro and 100-1000-mu-g/kg in vivo). Ovine prolactin, at concentrations of 1-100-mu-g/ml was also without effect on I-125-[Tyr1]-SRIF binding to pituitary membranes following a 2- or 4-h incubation with pituitary glands. Pituitary binding sites for I-125-[Tyr1]-SRIF were, however, increased after a 24-h incubation with 1-mu-mol tri-iodothyronine (T3)/l in vitro and 4 and 24 h after the administration of T3 (100-1000-mu-g/kg) in vivo. Although T3 had no direct inhibitory effect on I-125-[Tyr1]-SRIF binding to pituitary membranes, binding was suppressed 1 and 2 h after the in-vivo administration of T3 at concentrations of 100-1000-mu-g/kg. These results therefore demonstrate homologous and heterologous regulation of SRIF-binding sites in the chicken pituitary gland.