Secretin receptor activity in rat gastric glands. Binding studies, cAMP generation and pharmacology

Abstract

We measured 125I-secretin binding to memebranes prepared from rat fundic glands and compared the abilities of natural and synthetic secretin (SN) analogs to inhibit 125I-secretin binding and to activate the cAMP generating system in glandular and subcellular preparations from the fundus and antrum. The natural peptides structurally related to porcine secretin (pSN) included: chicken secretin (cSN), vasoactive intestinal peptide (VIP), porcine peptide with N-terminal histidine and C-terminal isoleucine amide (PHI), helodermin, growth hormone releasing factors isolated from the rat hypothalamus (rhGRF-43, rhGRF-29) or from a human pancreatic tumour (hpGRF-40). These peptides inhibited the binding of 125I-secretin to rat fundic membranes: pSN>cSN>PHI, VIP and activated the cAMP generating system in fundic glands, according to the following order of potency; pSN>cSN>PHI, VIP>rhGRF-29>rhGRF-43. Porcine peptide with N-terminal tyrosine and C-terminal tyrosine (PYY), GIP, SOM and hpGRF-40 were inactive. Structural requirements for secretin receptor activity were evaluated with four synthetic secretin analogs corresponding to porcine secretin substituted at the N-terminal end by sequence portion of VIP, GIP, GLU and SOM: Ala 4-Val 5-SN(VIP-SN); Tyr 1-Ala 2-Glu 3-SN (GIP-SN); Gln 3-SN (GLU-SN) and Phe 1-Phe 1-Trp 3-Lys 4-SN (SOM-SN). The relative potencies of the analogs in fundic and antral preparations were: pSN>VIP-SN>VIP, GIP-SN>GLU-SN>SOM-SN for 125I-secretin displacement and cAMP production (glandular cAMP generation and adenylate cyclase activation). We conclude that secretin receptors in rat fundic and antral mucosae are pharmacologically identical and are preferentially localized in antral glands. The 1–8 N-terminal and 9–27 C-terminal parts of secretin sequence are of importance in determining secretin receptor interaction and activation in the rat gastric mucosae.