Extracellular polyamines regulate fluid secretion in rat colonic crypts via the extracellular calcium-sensing receptor

Background & Aims: Polyamines are essential for the normal postnatal development, maintenance, and function of gastrointestinal epithelia. The extracellular Ca2+ (Ca-o(2+)/nutrient)-sensing receptor is expressed on both luminal and basolateral membranes of colonocytes, and, in other cell systems, this receptor has been shown to respond to polyamines. Thus, the Ca2+-sensing receptor could provide a mechanism for modulation of colonocyte function by dietary and systemic extracellular polyamines. In the present study, we investigated the interaction of polyamines, particularly spermine, and extracellular Ca2+ on second messenger generation by, and on function of, rat distal colonic crypts. Methods: Calcium-sensing receptor activation was assessed in colonic epithelial cells and intact crypts freshly isolated from distal colon by monitoring intracellular IP3 and Ca2+ accumulation using radioimmunoassay and Fluo-3 fluorometry, respectively. Interactions of extracellular Ca2+ and spermine on regulation of both basal and forskolin-stimulated fluid transport were measured in crypts microperfused in vitro. Results: Polyamine (spermine > spermidine > putrescine)-mediated enhancement of intracellular D-myo-inositol 1,4,5-trisphosphate (IP3) and Ca2+ accumulation required extracellular Ca2+, and the EC50 for extracellular Ca2+-mediated activation of the calcium-sensing receptor was reduced by polyamines. Extracellular spermine modulated both basal and forskolin-stimulated fluid secretion in perfused colonic crypts, and the EC50 for spermine-induced reduction in forskolin-stimulated fluid secretion was inversely dependent on extracellular Ca2+ (Ca-o(2+)). Conclusions: The interactions of extracellular Ca2+ and polyamines on second messenger accumulation and fluid secretion support a role for the luminal and basolateral calcium-sensing receptors in mediating some of the effects of polyamines on distal colonic epithelial cells.