Pituitary somatostatin receptors: dissociation at the pituitary level of receptor affinity and biological activity for selective somatostatin analogs.
High affinity and saturable binding of [125I-Tyr11]somatostatin (SS) is described in membrane homogenates from a pituitary transplantable tumor (GH4C1) rich in somatotrophs (KD for SS = 0.67 nM; Bmax = 30 fmol/mg protein). Binding characteristics and pharmacology are similar to those measured on normal pituitary membranes. The potency of various SS analogs highly correlates with that measured in in vitro bioassay for growth hormone. This suggests that those GH4C1 membranes are a good model for SS receptors on somatotrophs. Interestingly however, analogs in which the Asn5 is deleted (Des-Asn5) or D-Ser replaces Ser13 show dissociated potencies between the various assays: [D-Ser13] analogs are more potent in pituitary than in GH4C1 membranes binding assay. Des-Asn5-modified analogs are much more potent in both pituitary binding assays than in the bioassay. This could reflect a multiplicity of SS receptor subtypes in pituitary.
D000818:Animals; D013006:Growth Hormone / Q000378:metabolism; D066298:In Vitro Techniques; D008297:Male; D009374:Neoplasms, Experimental / Q000378:metabolism; D010903:Pituitary Gland, Anterior / Q000378:metabolism*; D010911:Pituitary Neoplasms / Q000378:metabolism; D051381:Rats; D011919:Rats, Inbred Strains; D011956:Receptors, Cell Surface / Q000378:metabolism*; D017481:Receptors, Somatostatin; D013004:Somatostatin / Q000031:analogs & derivatives* / Q000378:metabolism*
D011956:Receptors, Cell Surface; D017481:Receptors, Somatostatin; D013004:Somatostatin; C027163:somatostatin, des-Asn(5)-; C027165:somatostatin, Ser(13)-; D013006:Growth Hormone