Calcium transport across intestinal epithelia depends on voltage-gated sodium channels and endocannabinoid system

The intestinal wall consists of a number of cell types, e.g., enterocytes, neurons, endocrine cells and muscle cells. While it is well established that intestinal calcium absorption is tightly regulated by circulating hormones, particularly 1,25-dihydroxyvitamin D₃, little is known about whether factors from enteric neurons can modulate transepithelial calcium transport. Herein, we aimed to use a broad-spectrum inhibitor of voltage-gated sodium channels (Na_v), namely tetrodotoxin (TTX), which blocks almost all Na_v channels in enteric neurons, to elucidate the contribution of Na_v channels to calcium transport. Our in silico molecular docking indicated that TTX could bind to glutamate and aspartate residues in the pore of several Na_v channels (e.g., Na_v1.2), thereby occluding the Na⁺-permeable pathway. By using radioactive ⁴⁵Ca in Ussing chamber, 0.1 μM TTX was shown to markedly diminish the leucine-induced calcium transport in the rat duodenum. However, its inhibitory action was absent in the intestinal epithelium-like Caco-2 monolayer, which had no Na_v-expressing cells. TTX did not have any effect on the duodenal permeability to Na⁺ or Cl^-, or transepithelial resistance, suggesting that it did not cause tight junction leakage. Since endocannabinoids from enteric neurons normally modulate intestinal functions, we demonstrated that arachidonyl-2-chloroethylamide (CB1 agonist) significantly enhanced the duodenal calcium transport, which could be diminished by TTX. A carbon monoxide-releasing molecule (CORM-2), which reportedly blocked Na_v1.5, was without any effect on calcium transport. In conclusion, Na_v and CB1 contributed to calcium transport across the rat duodenum, thus corroborating the existence of neural control of intestinal calcium absorption.