Cholera is a global health problem with no targeted therapies and oral rehydration solution (ORS) is the mainstay of treatment. Ca²⁺-sensing receptor (CaSR) is regulator of intestinal ion transport and therapeutic target for diarrhea, and Ca²⁺ is considered its main agonist. Here we found that increasing extracellular Ca²⁺ up to 20 mM had minimal effect on forskolin-induced Cl^- secretion in human intestinal epithelial T84 cells. However, extracellular Mg²⁺, an often-neglected CaSR agonist, suppressed forskolin-induced Cl^- secretion in T84 cells by 65% at physiological levels seen in stool (10-20 mM). Mg²⁺ effect was via CaSR-Gq signaling that leads to intracellular Ca²⁺ elevation and cAMP hydrolysis. Mg²⁺ (10 mM) also suppressed Cl^- secretion induced by clinically relevant secretagogues (cholera toxin, heat-stable E. coli enterotoxin and vasoactive intestinal peptide) by 50%. In wild-type mice, 10 mM Mg²⁺ reduced forskolin-induced jejunal Cl^- secretion by 40% with no effect in intestinal epithelia-specific CaSR knockout mice (Vil1-cre;Casr-flox). In mouse intestinal closed-loops, luminal Mg²⁺ treatment (20 mM) inhibited cholera toxin-induced fluid accumulation by 40%. In mouse intestinal perfusion model of cholera, adding 10 mM Mg²⁺ to the perfusate reversed the net fluid transport from secretion to absorption. These results suggest that Mg²⁺ is the key CaSR activator in mouse and human intestinal epithelia at physiological levels seen in stool. Although Mg²⁺ causes osmotic diarrhea at high stool concentrations (>100 mM), here we found that Mg²⁺ largely inhibits cyclic nucleotide-induced Cl^- secretion at physiological levels (10-20 mM) seen in stool. Since stool Mg²⁺ concentrations in cholera patients are essentially zero, oral Mg²⁺ supplementation (alone or in ORS) can be a new therapy for cholera and other cyclic nucleotide-mediated secretory diarrheas including Traveler's diarrhea, VIPoma and GUCY2C mutations.

Qi Gao

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