The calcium-sensing receptor (CaSR) is activated by extracellular calcium (Ca2+(o)) and mediates many of the known effects of extracellular divalent minerals on body cells. Both surface and crypt cells express CaSR transcripts and protein on both apical and basolateral surfaces. Raising Ca2+(o) elicited increases in intracellular calcium (Ca2+(o)) in both surface and crypt cells with an EC50 of 2 mM. The Ca-induced increase in Ca2+(i) was associated with increases in inositol 1,4,5-trisphosphate and eliminated by U-73129, an inhibitor of phosphatidylinositol-phospholipase C, as well as by thapsigargin. Other CaSR agonists, Gd3+ and neomycin, mimicked these Ca2+(o)-induced responses. Both luminal and bath Ca2+(o), Gd3+, and neomycin induced increases in Ca2+(i) in isolated perfused crypts. The stimulatory effect of forskolin on net fluid secretion in perfused crypts was abolished by increasing Ca2+(o) in either luminal or bath perfusates. Thus both apical and basolateral CaSR on crypt cells are functional and provide pathways modulating net intestinal fluid transport that may have important implications for the prevention and treatment of certain diarrheal diseases associated with elevated cAMP.
Homeostasis of intravascular volume, Na ؉ , Cl ؊ , and K ؉ is interdependent and determined by the coordinated activities of structurally diverse mediators in the distal nephron and the distal colon. The behavior of these flux pathways is regulated by the reninangiotensin-aldosterone system; however, the mechanisms that allow independent modulation of individual elements have been obscure. Previous work has shown that mutations in WNK4 cause pseudohypoaldosteronism type II (PHAII), a disease featuring hypertension with hyperkalemia, due to altered activity of specific Na-Cl cotransporters, K ؉ channels, and paracellular Cl ؊ flux mediators of the distal nephron. By coexpression studies in Xenopus oocytes, we now demonstrate that WNK4 also inhibits the epithelial Na ؉ channel (ENaC), the major mediator of aldosterone-sensitive Na ؉ (re)absorption, via a mechanism that is independent of WNK4's kinase activity. This inhibition requires intact C termini in ENaC -and ␥-subunits, which contain PY motifs used to target ENaC for clearance from the plasma membrane. Importantly, PHAIIcausing mutations eliminate WNK4's inhibition of ENaC, thereby paralleling other effects of PHAII to increase sodium balance. The relevance of these findings in vivo was studied in mice harboring PHAII-mutant WNK4. The colonic epithelium of these mice demonstrates markedly increased amiloride-sensitive Na ؉ flux compared with wild-type littermates. These studies identify ENaC as a previously unrecognized downstream target of WNK4 and demonstrate a functional role for WNK4 in the regulation of colonic Na ؉ absorption. These findings support a key role for WNK4 in coordinating the activities of diverse flux pathways to achieve integrated fluid and electrolyte homeostasis.aldosterone ͉ hyperkalemia ͉ hypertension ͉ pseudohypoaldosteronism ͉ renin-angiotensin system
The intestinal epithelium is equipped with sensing receptor mechanisms that interact with luminal microorganisms and nutrients to regulate barrier function and gut immune responses, thereby maintaining intestinal homeostasis. Herein, we clarify the role of the extracellular calcium-sensing receptor (CaSR) using intestinal epithelium-specific Casr−/− mice. Epithelial CaSR deficiency diminished intestinal barrier function, altered microbiota composition, and skewed immune responses towards proinflammatory. Consequently, Casr−/− mice were significantly more prone to chemically induced intestinal inflammation resulting in colitis. Accordingly, CaSR represents a potential therapeutic target for autoinflammatory disorders, including inflammatory bowel diseases.
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