Anti-secretory properties of non-peptide somatostatin receptor agonists in isolated rat colon: luminal activity and possible interaction with P-glycoprotein

Abstract

1. The diverse physiological actions of somatostatin are mediated by a family of G-protein coupled receptors (SSTRs). Several peptide analogues of somatostatin such as octreotide have been developed for therapeutic use, including treatment of gastrointestinal disorders such as secretory diarrhoea. However, their development as anti-diarrhoeal agents has been limited by poor oral bioavailability, necessitating parenteral administration. This in vitro study investigated the anti-secretory potential of a group of novel, non-peptide, somatostatin-receptor agonists that selectively activate specific SSTR subtypes to assess their potential for oral administration. 2. The ability of the agonists to inhibit forskolin-stimulated chloride secretion was measured using a sensitive bioassay system in isolated rat colonic mucosa. 3. The SSTR-2 selective agonist, L-779,976 was 10-times more potent than octreotide as an inhibitor of secretion when added to the basolateral surface of rat colon. Non-peptide agonists selective for SSTR1 (L-797,591), SSTR3 (L-796,778), SSTR4 (L-803,087) or SSTR5 (L-817,818) showed little or no anti-secretory activity in this preparation. 4. L-779,976 was able to inhibit secretion when applied to the luminal surface at sub-micromolar concentrations suggesting that it can cross the colonic epithelium. The anti-secretory potency of luminal L-779,976 was increased 3 fold in the presence of GF120918, a known inhibitor of P-glycoprotein. 5. Non-peptide somatostatin receptor agonists may provide a basis for the development of new, orally available anti-diarrhoeal therapies.

Figure 1

Inhibition of forskolin (FSK)-stimulated ion secretion in rat colon in vitro by serosal addition of somatostatin and analogues. Tissues were stimulated by addition of FSK to the serosal surface to a final concentration of 0.25 μM. The mean basal I_sc and the mean FSK-stimulated increase in I_sc were 16.7±4.2 μA.cm⁻² and 30.4±2.3 μA.cm⁻² respectively. At the peak of the short circuit current (I_sc) response (shown as t=0), somatostatin, octreotide, L-779,976 (all at 1 nM) or control buffer were added serosally. Results show per cent decrease in the FSK-stimulated I_sc and represent mean±s.e.mean for n=6 – 12 tissues in each group.

Figure 2

Dose response inhibition of FSK-stimulated secretion by non-peptide somatostatin analogues. Rat colonic mucosa stimulated by 0.25 μM FSK (final conc.) were exposed serosally to several concentrations of L-797,591 (SSTR1), L-779,976 (SSTR2), L-796,778 (SSTR3), L-803,087 (SSTR4), L-817,818 (SSTR5), octreotide and SST-14. Results show the decrease in I_sc plotted as a percentage of the FSK-stimulated response measured 10 min after addition of the analogues. Data are shown as mean±s.e.mean for n=5 – 7 tissues in each group.

Figure 3

Anti-secretory action of L-779,976 added to the mucosal surface of FSK-stimulated rat colon. L-779,976 (1 – 1000 nM) or SST-14 (1000 nM) was added to the mucosal surface of FSK-stimulated tissue at the peak of the FSK response and the change in I_sc monitored over 60 min. Data are shown as the decrease in I_sc plotted as a percentage of the FSK-stimulated response. Values are mean±s.e.mean for 6 – 8 tissues in each group.

Figure 4

Comparison of the dose response curves for inhibition of FSK-stimulated I_sc by serosal and mucosal L-779,976 in rat colon. Values were measured 10 min after addition of the agonist to the serosal surface or 30 min after addition to the mucosal (luminal) surface of the tissue. Data are shown as the decrease in I_sc plotted as a percentage of the FSK-stimulated response. Values are mean±s.e.mean for 6 – 8 tissues in each group.

Figure 5

Anti-secretory effect of luminal L-779,976 is potentiated by GF120918 in distal colon. Inhibition by L-779,976 of FSK-stimulated I_sc was measured 30 min after addition of the agonist at a concentration of 30 nM to the luminal surface of proximal or distal colonic mucosa in the presence or absence of 30 μM GF120918. GF120918 was added to both luminal and serosal compartments 15 min prior to addition of L-779,976. Data are shown as the decrease in I_sc plotted as a percentage of the FSK-stimulated response. Values are mean±s.e.mean for n=6 tissues in each group. *P<0.025.

Figure 6

Molecular structures of the SSTR2-selective non-peptide agonist L-779,976 and the peptide agonist octreotide.