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Mediation by SRIF1 receptors of the contractile action of somatostatin in rat isolated distal colon; studies using some novel SRIF analogues

1 The motor effects of somatostatin‐14 (SRIF), and several SRIF peptide analogues were investigated on the rat isolated distal colon. The objective of these studies was to characterize the receptor mediating the contractile action of SRIF by comparing the relative agonist potencies of a range of SRIF analogues. 2 SRIF (1 nm‐1 μm) produced concentration‐dependent contractions with an EC50 value of approximately 10 nm. Contractile responses induced by SRIF were insensitive to atropine (1 μm) or naloxone (1 μm) but abolished by tetrodotoxin (1 μm). Somatostatin‐28 (SRIF28), also induced concentration‐dependent contractions and was equipotent with SRIF. Phosphoramidon (1 μm) and amastatin (10 μm) did not increase the potency of either SRIF or SRIF28. 3 The SRIF peptide analogues, octreotide, SRIF25, seglitide, angiopeptin and CGP23996 (1 nm‐1 μm) produced contractile responses in the rat distal colon, each having similar potency and maximal activity relative to SRIF. The SSTR2 receptor‐selective hexapeptide, BIM23027 (0.1 nm‐1 μm), and the SRIF stereoisomer, D‐Trp8‐SRIF (0.1 nm‐1 μm), were the most potent agonists examined being approximately 12 and 7 times more potent than SRIF, respectively. In contrast, the SSTR5 receptor‐selective analogue, L362,855, was approximately 120 times weaker than SRIF, whilst the SSTR3 receptor‐selective analogue, BIM23056, was inactive at concentrations up to 3 μm. 4 The putative SRIF receptor antagonist, (cyclo(7‐aminoheptanoyl Phe‐D‐Trp‐Lys‐Thr[Bzl]))(CPP) (1 μm), had no agonist activity and had no effect on contractions induced by SRIF. 5 The contractile actions of BIM23027 and seglitide were subject to pronounced desensitization. Desensitization of preparations by BIM23027 (0.3 μm) abolished the contractile action of SRIF and SRIF28 but had no effect on contractions produced by acetylcholine (0.1 nm‐1 μm), suggesting that BIM23027, SRIF and SRIF28 act via a common receptor mechanism. 6 In conclusion, the rat isolated distal colon contracts in response to SRIF and a number of SRIF analogues. Seglitide and octreotide exhibited similar potency and maximal activity relative to SRIF, suggesting that in the rat colon the receptor mediating contraction belongs to the SRIF1‐receptor group, of which the recombinant SSTR2, SSTR3 and SSTR5 receptors appear to be subtypes. The high potency of BIM23027, the weak agonist activity of L362,855 and the lack of activity exhibited by BIM23056 suggests that the SRIF receptor mediating contraction in the rat distal colon is similar to the recombinant SSTR2 receptor.