Chronic regulation of colonic epithelial secretory function by activation of G protein-coupled receptors

Enteric neurotransmitters that act at G protein-coupled receptors (GPCRs) are well known to acutely promote epithelial Cl(-) and fluid secretion. Here we examined if acute GPCR activation might have more long-term consequences for epithelial secretory function.

Cl(-) secretion was measured as changes in short-circuit current across voltage-clamped T(84) colonic epithelial cells. Protein expression was measured by western blotting and intracellular Ca(2+) levels by Fura-2 fluorescence.

While acute (15 min) treatment of T(84) cells with a cholinergic G(q) PCR agonist, carbachol (CCh), rapidly stimulated Cl(-) secretion, subsequent CCh-induced responses were attenuated in a biphasic manner. The first phase was transient and resolved within 6 h but this was followed by a chronic phase of attenuated responsiveness that was sustained up to 48 h. CCh-pretreatment did not chronically alter responses to another G(q)PCR agonist, histamine, or to thapsigargin or forskolin which elevate intracellular Ca(2+) and cAMP, respectively. This chronically acting antisecretory mechanism is not shared by neurotransmitters that activate G(s)PCRs. Conditioned medium from CCh-pretreated cells mimicked its chronic antisecretory actions, suggesting involvement of an epithelial-derived soluble factor but further experimentation ruled out the involvement of epidermal growth factor receptor ligands. Acute CCh exposure did not chronically alter surface expression of muscarinic M(3) receptors but inhibited intracellular Ca(2+) mobilization upon subsequent agonist challenge.

These data reveal a novel, chronically acting, antisecretory mechanism that downregulates epithelial secretory capacity upon repeated G(q)PCR agonist exposure. This mechanism involves release of a soluble factor that uncouples receptor activation from downstream prosecretory signals.