TRPV3 and TRPV4 as candidate proteins for intestinal ammonium absorption

Aim: Absorption of ammonia from the gut has consequences that range from encephalitis in hepatic disease to global climate change induced by nitrogenous excretions from livestock. Since patch clamp data show that certain members of the transient receptor potential (TRP) family are permeable to NH4+, participation in ammonium efflux was investigated. Methods: Digesta, mucosa and muscular samples from stomach, duodenum, jejunum, ileum, caecum and colon of pigs were analysed via colourimetry, qPCR, Western blot, immunohistochemistry and Ussing chambers. Results: qPCR data show high duodenal expression of TRPV6. TRPM6 was highest in jejunum and colon, with expression of TRPM7 ubiquitous. TRPM8 and TRPV1 were below detection. TRPV2 was highest in the jejunum but almost non-detectable in the colon. TRPV4 was ubiquitously expressed by mucosal and muscular layers. TRPV3 mRNA was only found in the mucosa of the caecum and colon, organs in which NH4+ was highest (>7 mmol·L−1). Immunohistochemically, an apical expression of TRPV3 and TRPV4 could be detected in all tissues, with effects of 2-APB and GSK106790A supporting functional expression. In symmetrical NaCl Ringer, removal of mucosal Ca2+ and Mg2+ increased colonic short circuit current (Isc) and conductance (Gt) by 0.18 ± 0.06 µeq·cm−2·h−1 and 4.70 ± 0.85 mS·cm−2 (P < .05, N/n = 4/17). Application of mucosal NH4Cl led to dose-dependent and divalent-sensitive increases in Gt and Isc, with effects highest in the caecum and colon. Conclusion: We propose that TRP channels contribute to the intestinal transport of ammonium, with TRPV3 and TRPV4 promising candidate proteins. Pharmacological regulation may be possible.