Enhancement of intestinal calcium transport by short-chain fatty acids: roles of Na+/H+ exchanger 3 and transient receptor potential vanilloid subfamily 6

Small organic molecules in the intestinal lumen, particularly short-chain fatty acids (SCFAs) and glucose, have long been postulated to enhance calcium absorption. Here, we used 45Ca radioactive tracer to determine calcium fluxes across the rat intestine after exposure to glucose and SCFAs. Confirming previous reports, glucose was found to increase the apical-to-basolateral calcium flux in the cecum. Under apical glucose-free conditions, SCFAs (e.g., butyrate) stimulated the cecal calcium fluxes by approximately twofold, while having no effect on proximal colon. Since SCFAs could be absorbed into the circulation, we further determined whether basolateral SCFA exposure rendered some positive actions. It was found that exposure of duodenum and cecum on the basolateral side to acetate or butyrate increased calcium fluxes. Under butyrate-rich conditions, cecal calcium transport was partially diminished by Na+/H+ exchanger 3 (NHE3) inhibitor (tenapanor) and nonselective transient receptor potential vanilloid subfamily 6 (TRPV6) inhibitor (miconazole). To confirm the contribution of TRPV6 to SCFA-stimulated calcium transport, we synthesized another TRPV6 inhibitor that was demonstrated by in silico molecular docking and molecular dynamics to occlude TRPV6 pore and diminish the glucose- and butyrate-induced calcium fluxes. Therefore, besides corroborating the importance of luminal molecules in calcium absorption, our findings provided foundation for development of more effective calcium-rich nutraceuticals in combination with various absorptive enhancers, e.g., glucose and SCFAs.NEW & NOTEWORTHY Organic molecules in the intestinal lumen, e.g., glucose and short-chain fatty acids (SCFAs), the latter of which are normally produced by microfloral fermentation, can stimulate calcium absorption dependent on transient receptor potential vanilloid subfamily 6 (TRPV6) and Na+/H+ exchanger 3 (NHE3). A selective TRPV6 inhibitor synthesized and demonstrated by in silico docking and molecular dynamics to specifically bind to the pore domain of TRPV6 was used to confirm a significant contribution of this channel. Our findings corroborate physiological significance of nutrients and SCFAs in enhancing calcium absorption.

Vasoactive intestinal peptide and cystic fibrosis transmembrane conductance regulator contribute to the transepithelial calcium transport across intestinal epithelium-like Caco-2 monolayer

Vasoactive intestinal peptide (VIP) as a neurocrine factor released by enteric neurons has been postulated to participate in the regulation of transcellular active calcium transport across intestinal epithelium, but the preceding evidence is scant and inconclusive. Herein, transepithelial calcium flux and epithelial electrical parameters were determined by Ussing chamber technique with radioactive tracer in the intestinal epithelium-like Caco-2 monolayer grown on Snapwell. After 3-day culture, Caco-2 cells expressed mRNA of calcium transporters, i.e., TRPV6, calbindin-D_9k, PMCA_1b and NCX1, and exhibited transepithelial resistance of ~200 Ω cm², a characteristic of leaky epithelium similar to the small intestine. VIP receptor agonist was able to enhance transcellular calcium flux, whereas VIP receptor antagonist totally abolished calcium fluxes induced by 1,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃]. Since the intestinal cystic fibrosis transmembrane conductance regulator (CFTR) could be activated by VIP and calciotropic hormones, particularly parathyroid hormone, we sought to determine whether CFTR also contributed to the 1,25(OH)₂D₃-induced calcium transport. A selective CFTR inhibitor (20–200 μM CFTR_inh-172) appeared to diminish calcium fluxes as well as transepithelial potential difference and short-circuit current, both of which indicated a decrease in electrogenic ion transport. On the other hand, 50 μM genistein—a molecule that could rapidly activate CFTR—was found to increase calcium transport. Our in silico molecular docking analysis confirmed direct binding of CFTR_inh-172 and genistein to CFTR channels. In conclusion, VIP and CFTR apparently contributed to the intestinal calcium transport, especially in the presence of 1,25(OH)₂D₃, thereby supporting the existence of the neurocrine control of intestinal calcium absorption.

Na+/H+ exchanger 3 inhibitor diminishes the amino-acid-enhanced transepithelial calcium transport across the rat duodenum

Na⁺/H⁺ exchanger (NHE)-3 is important for intestinal absorption of nutrients and minerals, including calcium. The previous investigations have shown that the intestinal calcium absorption is also dependent on luminal nutrients, but whether aliphatic amino acids and glucose, which are abundant in the luminal fluid during a meal, similarly enhance calcium transport remains elusive. Herein, we used the in vitro Ussing chamber technique to determine epithelial electrical parameters, i.e., potential difference (PD), short-circuit current (Isc), and transepithelial resistance, as well as ⁴⁵Ca flux in the rat duodenum directly exposed on the mucosal side to glucose or various amino acids. We found that mucosal glucose exposure led to the enhanced calcium transport, PD, and Isc, all of which were insensitive to NHE3 inhibitor (100 nM tenapanor). In the absence of mucosal glucose, several amino acids (12 mM in the mucosal side), i.e., alanine, isoleucine, leucine, proline, and hydroxyproline, markedly increased the duodenal calcium transport. An inhibitor for NHE3 exposure on the mucosal side completely abolished proline- and leucine-enhanced calcium transport, but not transepithelial transport of both amino acids themselves. In conclusion, glucose and certain amino acids in the mucosal side were potent stimulators of the duodenal calcium absorption, but only amino-acid-enhanced calcium transport was NHE3-dependent.