GALANIN INHIBITS GLUCAGON-LIKE PEPTIDE-1 SECRETION THROUGH PERTUSSIS-TOXIN-SENSITIVE G-PROTEIN AND ATP-DEPENDENT POTASSIUM CHANNELS IN RAT ILEAL L-CELLS

The neuropeptide galanin is widely distributed in the gastrointestinal tract and exerts several inhibitory effects, especially on intestinal motility and on insulin release from pancreatic beta-cells. The prese nce of galanin fibres not only in the myenteric and submucosal plexus but also in the mucosa, prompted us to investigate the regulatory role of galanin, and its mechanism of action, on the secretion of the insu linotropic hormone glucagon-like peptide-1 (GLP-1). Rat ileal cells we re dispersed through mechanical vibration followed by moderate exposur e to hyaluronidase, DNase I and EDTA, and enriched for L-cells by coun terflow elutriation. A 6- to 7-fold enrichment in GLP-1 cell content w as registered after elutriation, as compared with the crude cell prepa ration (929 +/- 81 vs 138 +/- 14 fmol/10(6) cells). L-cells then accou nted for 4-5% of the total cell population. Bombesin induced a time-(1 5-240 min) and dose- (0.1 nM-1 mu M) dependent release of GLP-1. Gluco se-dependent insulinotropic peptide (GIP, 100 nM), forskolin (10 mu M) and the phorbol ester 12-0-tetradecanoylphorbol-13-acetate (TPA, 1 mu M) each stimulated GLP-1 secretion over a l-h incubation pelted. Gala nin (0.01-100 nM) induced a dose-dependent inhibition of bombesin-and of GIP-stimulated GLP-1 release (mean inhibition of 90% with 100 nM ga lanin). Galanin also dose-dependently inhibited forskolin-induced GLP- 1 secretion (74% of inhibition with 100 nM galanin), but not TPA-stimu lated hormone release. Pre-treatment of cells with 200 ng/ml pertussis toxin for 3 h, or incubation with the ATP-sensitive K+ channel blocke r disopyramide (200 mu M), prevented the inhibition by galanin of bomb esin-and GIP-stimulated GLP-1 secretion. These studies indicate that i ntestinal secretion of GLP-1 is negatively controlled by galanin, that acts through receptors coupled to pertussis toxin-sensitive G protein and involves ATP-dependent K+ channels.