Ursodeoxycholic Acid Stimulates CA 2+ Influx and Exerts Antisecretory Actions in Colonic Epithelial Cells

Abstract

Background Previously, we have shown that ursodeoxycholic acid (UDCA) exerts antisecretory effects In Vitro and thus may represent a novel therapeutic target in treating intestinal disorders associated with diarrhea. These antisecretory effects are mediated by inhibition of basolateral Na+/K+-ATPase pumps and K+ currents, key components of the Clsecretory pathway. Here, we aimed to further elucidate molecular mechanisms underlying the antisecretory actions of UDCA. Methods Clsecretion was measured as changes in short-circuit current (Isc) across voltage-clamped T84 cell monolayers. Intracellular Ca2+ was measured by fluorescence imaging and cAMP using a commercially available assay. Results As previously reported, UDCA inhibited secretory responses to both Ca2+and cAMP-dependent agonists in T84cells. Antisecretory effects of UDCA were very rapid in onset being apparent within 1 minute of addition. Apical addition of TUDCA, the taurine conjugated, membrane-impermeable derivative of UDCA, reduced CCh-induced secretory responses to 60 ± 10.9% of controls, suggesting the bile acid exerts its effects through interactions with the membrane, rather than intracellularly. Even though it did not stimulate secretory responses, UDCA (50 μM) rapidly elevated intracellular levels of Ca2+ (ΔF340/380 = 0.2995 ± 0.04) in T84 cells (n = 12). UDCA also inhibited subsequent Ca2+ mobilization responses to CCh by 63 ± 0.573% compared to controls (n = 10, p < 0.01). Removal of Ca2+ from the extracellular bathing solution abolished UDCA-induced Ca2+ elevations (n = 5, p < 0.01) but had minimal effects on CCh-induced peak Ca2+ responses. UDCA did not alter FSK-induced elevations in intracellular cAMP levels. Conclusion: These data suggest that UDCA elevates intracellular Ca2+ in colonic epithelial cells through an interaction with the cell membrane and stimulation of Ca2+influx. In contrast to the well-described prosecretory actions of Ca2+ released from intracellular stores, influx of extracellular Ca2+ induced by UDCA appears to be antisecretory. These data provide intriguing new insights into the mechanisms by which UDCA exerts antisecretory actions in the colon.