Molecular mechanisms of calcium signaling in the modulation of small intestinal ion transports and bicarbonate secretion

Abstract
Background and Purpose: While calcium (Ca2+) signaling is known to influence ion secretion in the small intestine, its exact role and the underlying molecular mechanisms of Ca2+-mediated biological actions remain poorly understood.

Key Results: Stimulation of muscarinic receptors by carbachol (CCh) enhanced the short-circuit current (Isc) in mouse duodenum, an effect that was notably reduced in a Ca2+-free serosal solution and by various selective store-operated Ca2+ channel (SOC) blockers applied to the serosal side of duodenal tissues. Additionally, our findings suggest that CRAC/Orai channels are likely candidates for SOC in the intestinal epithelium. CCh elevated intracellular Ca2+ levels without affecting cAMP, and Ca2+ signaling was crucial for duodenal chloride (Cl-) and bicarbonate (HCO3-) secretion in wild-type mice, but not in CFTR knockout mice. CCh also triggered duodenal ion secretion and activated PI3K/Akt pathways in the duodenal epithelium, with these effects being inhibited by various selective PI3K inhibitors. Furthermore, CCh-induced Ca2+ signaling led to phosphorylation of CFTR proteins and their relocation to the plasma membrane of duodenal epithelial cells, processes that were similarly blocked by PI3K inhibitors.

Materials and Methods: We conducted functional, biochemical, and morphological analyses to investigate ion secretion, PI3K/Akt activity, and CFTR functionality in mouse duodenal epithelium. Additionally, Ca2+ imaging was performed using HT-29 cells.

Conclusions and Implications: Ca2+ signaling is essential for intestinal ion secretion through the CRAC/Orai-mediated store-operated Ca2+ entry (SOCE) mechanism on the serosal side of the epithelium. Our study also elucidates the molecular mechanisms by which Ca2+ signaling regulates CFTR-mediated secretion via a novel PI3K/Akt pathway. These insights offer new avenues for developing therapeutic targets aimed at protecting the upper gastrointestinal tract and managing fluid balance in the small Galicaftor intestine.