Somatostatin receptors mediating inhibition of basal and stimulated electrogenic ion transport in rat isolated distal colonic mucosa

Abstract

The aim of this study was to examine the potencies of several recently identified selective somatostatin (SRIF)-receptor ligands as inhibitors of electrogenic ion transport in the rat distal colonic mucosa with the view to identifying the SRIF receptor type involved. Under basal conditions, cumulative administration of SRIF and SRIF2g decreased short circuit current (SCC), a measure of electrogenic ion transport, with EC₅₀ values of 4 nM and 9 nM respectively. The peptidase inhibitors, phosphoramidon (1 μM) and amastatin (10 μM), had no effect on the potencies of either SRIF or SRIF₂₈. The inhibitory action of SRIF on basal SCC was suppressed by piretanide and diphenylamine-2-carboxylate, compatible with the assumption that the Na⁺K⁺2Cl⁻ co-transporter and Cl⁻ channels, respectively, may be involved in this antisecretory action of SRIF. Tetrodotoxin (1 μM) had no effect on the antisecretory action of SRIF, suggesting that the process was not neuronally mediated.

All of the SRIF analogues examined, with the exception of BIM-23056, maximally inhibited basal SCC to a similar extent as SRIF. Seglitide and octreotide were both more potent antisecretory agents than SRIF (respective EC₅₀ values, 0.4 nM and 1.5 nM) suggesting that this effect was mediated by a receptor belonging to the SRIF₁ receptor group. The most distinguishing feature of the rank order of agonist potencies was the high potency of the selective sst₂ receptor ligand, BIM-23027 (EC₅₀, value 0.32 nM), the weaker potency exhibited by the selective sst₅ receptor ligand, L-362855 (EC₅₀ value 21 nM), and the lack of agonist activity displayed by the selective sst₃ receptor ligand, BIM-23056 (EC₅₀ value > 1000 nM). This profile is comparable with that observed in binding studies on the recombinant sst₂ receptor.

Forskolin-stimulated secretion was suppressed by SRIF analogues with the rank order of agonist potencies BIM-23027 > SRIF > L-362855 > BIM-23056 which resembled that exibited under basal conditions. However, the absolute potencies of these agonists were lower (respective EC₅₀ values 2 nM, 14 nM, 38 nM and > 1000 nM) whilst the magnitude of inhibition was about three fold greater. BIM-23027 and SRIF (both 30 nM) also inhibited carbachol-stimulated increases in basal SCC by 60–70%, while a similar concentration of L-362855 inhibited these responses by 11 %. BIM-23056 (1 μM) had no effect on carbachol-simulated secretion. Radioligand binding studies on rat colonic mucosal membranes using [¹²⁵I]-Tyr¹¹-SRIF suggested heterogeneity of SRIF binding sites. Thus, SRIF and SRIF₂₈ competed for binding (IC₅₀ values, 0.32 and 0.63 nM, respectively) with Hill slopes less than unity; while seglitide and BIM-23027 both maximally displaced only 30–40% of specific binding with apparent high affinity (respective pIC₅₀ values, 10.1 nM and 10.0).

In conclusion, SRIF decreases basal as well as both cAMP and Ca²⁺-dependent Cl⁻ secretion in rat colonic mucosa. The rank order of agonist potencies suggests that receptors resembling the recombinant sst₂ receptor mediate inhibition of basal and forskolin-stimulated secretion. Radioligand binding studies suggest that BIM-23027 interacts with a sub-population of [¹²⁵I]Tyr¹¹-SRIF binding sites in rat colonic mucosal membranes which probably correspond to the receptors mediating the antisecretory effects described here.