Homologous and heterologous regulation of somatostatin-binding sites on chicken adenohypophysial membranes

Abstract

Binding of 125I-labelled [Tyr1]-somatostatin (125I-[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 micrograms/kg. In-vitro preincubation of chicken pituitary glands for 0.5-4.0 h with 0.1 mumol SRIF/l similarly reduced the binding of 125I-[Tyr1]-SRIF to caudal lobe membrane preparations. After a 4-h incubation in 0.1 mmol SRIF/l, the withdrawal of SRIF from the incubation media was accompanied 4 h later by a partial recovery in the binding of 125I-[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 125I-[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 125I-[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 125I-[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.(ABSTRACT TRUNCATED AT 250 WORDS)