Cholera is a global health problem with no targeted therapies. Ca2+-sensing receptor (CaSR) is regulator of intestinal ion transport and therapeutic target for diarrhea, and Ca2+ is considered its main agonist. We found that increasing extracellular Ca2+ 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 mM). Mg2+ effect was via CaSR-Gq signaling that leads to cAMP hydrolysis. Mg2+ (10 mM) also suppressed Cl- secretion induced by cholera toxin, heat-stable E. coli enterotoxin and vasoactive intestinal peptide by 50%. 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. Since stool Mg2+ concentrations in cholera patients are essentially zero, oral Mg2+ supplementation, alone or in oral rehydration solution (ORS), can be a potential therapy for cholera and other cyclic nucleotide-mediated secretory diarrheas.
Livia de Souza Goncalves, Qi Tifany Chu, Riya Master, Parth D. Chhetri, Qi Gao, Onur Cil