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

Abstract

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 (Nav), namely tetrodotoxin (TTX), which blocks almost all Nav channels in enteric neurons, to elucidate the contribution of Nav channels to calcium transport. Our in silico molecular docking indicated that TTX could bind to glutamate and aspartate residues in the pore of several Nav channels (e.g., Nav1.2), 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 Nav-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 Nav1.5, was without any effect on calcium transport. In conclusion, Nav and CB1 contributed to calcium transport across the rat duodenum, thus corroborating the existence of neural control of intestinal calcium absorption.