Somatostatin binding to chicken adenohypophysial membranes

[125I-Tyr1]-Somatostatin (SRIF)-binding sites were demonstrated on crude plasma membrane preparations from chicken pituitary glands. These binding sites were saturable and of high affinity (dissociation constant less than 1.0 nM) and low capacity (maximal binding capacity less than 200 fmol/mg protein) and were specific for SRIF moieties. The number and affinity of these binding sites in the caudal lobe of the pituitary, in which somatotrophs predominate, were similar to those in the cephalic lobe, in which lactotrophs and thyrotrophs are confined. Gonadotrophs are present in the caudal lobe, but whereas exogenous SRIF inhibited secretagogue-induced GH release from incubated pituitary glands, it had no effect on basal or secretagogue-induced LH release. The half-maximal binding of SRIF to the caudal lobe membranes (3 nM) was similar to that required for half-maximal suppression of TRH-induced GH release, suggesting a role for these binding sites in the regulation of GH secretion in birds.