The antisecretory effects of somatostatin and analogues in rat descending colon mucosa.

Somatostatin-14 (SS-14) and somatostatin-28 (SS-28) produce concentration dependent reductions in short-circuit current in rat colonic mucosa. EC50 values of 15.0 and 13.3 nM were obtained for SS-14 and SS-28 respectively while the N-terminal fragments of SS-28, namely somatostatin-(1-12) (SS1-12) and somatostatin-(1-14) (SS1-14) were inactive. Cyclo(Pro-Phe-D-Trp-Lys-Thr-Phe) and cyclo(Pro-Tyr-D-Trp-Lys-Thr-Phe) were potent antisecretory peptides, like SS-14 and SS-28; while the putative somatostatin antagonist, cyclo(7-aminoheptanoyl-Phe-D-Trp-Lys-Thr[Bzl]) exhibited neither agonist nor antagonist effects. Responses to SS-14 could be regulated by agents which affected the secretory state of the epithelium. Antisecretory effects of SS-14 were markedly attenuated by piroxicam and were restored following piroxicam plus either forskolin or vasoactive intestinal polypeptide (VIP). SS-14 also attenuated secretory responses produced by carbachol, substance P (SP), VIP and alpha- and beta-calcitonin gene related peptide (alpha-, beta-CGRP). Therefore, SS-14 exhibits broad spectrum antisecretory effects in rat descending colon mucosa.