A comparative study of ammonia transport across ruminal epithelia from Bos indicus crossbreds versus Bos taurus

Abstract

Absorption of ammonia from the rumen of cattle decreases nitrogen availability for fermentational protein synthesis, leading to increased competition of cattle with humans for protein and enhancing the release of toxic nitrogenous compounds into the environment. Given that differences in feeding and breeding might induce differences in ruminal ammonia transport, we compared electrophysiological, histological, and molecular biological characteristics of ruminal epithelia of Bos indicus crossbreds (Sahiwal‐Mix, SWM) with those of Bos taurus (Holstein‐Friesian, HF). As in HF, the stratified cornified epithelium of SWM expressed claudin 1 and 4. Measurements of ammonia flux (HF) and serosal pH (both breeds) suggested that at a mucosal pH of 6.4, net transport primarily occurred as NH4+. As shown previously for HF, NH4+ induced a concentration‐dependent rise in short circuit current (Isc) in SWM that could be further stimulated by the TRP channel agonist menthol. Relative mRNA expression levels for TRPV3, TRPV4, TRPM6, and TRPM7 were significantly lower in SWM than in HF, with TRPA1 expression near the limit of detection. We conclude that uptake of ammonia from the rumen of both breeds occurs electrogenically as NH4+ with functional and molecular biological evidence pointing towards involvement of TRPV3 and TRPV4.

The TRPV3 channel of the bovine rumen: localization and functional characterization of a protein relevant for ruminal ammonia transport

“…We confirmed the permeability of this channel to NH 4 + , Na + , and K + in Ringer solutions with physiological concentrations of Ca 2+ and Mg 2+ . In conjunction with previous studies of our group [8,40,45,48], we conclude that bTRPV3 is involved in the ruminal uptake of NH 4 + , Na + , and Ca 2+ and contributes to the apical conductance of K + [29] ( Fig. 11).…”

“…In summary, we provide clear evidence for the expression of bTRPV3 by the ruminal epithelium. In conjunction with previous functional studies of the ruminal epithelium [3,40,45], a role of this channel in mediating the ruminal transfer of NH 4…”

“…However this may be, there is convincing evidence to suggest that bTRPV3 mediates transport of NH 4 + across the ruminal epithelium. In Ussing chamber experiments, TRPV3 channel agonists stimulate currents carried by Na + and NH 4 + and enhance Ca 2+ flux [40,45] (Fig. 11a).…”

“…Systematic studies of ruminal transport of ammonia have established that electroneutral transport of NH 3 is predominant at an alkaline pH of 7.4. At a physiological pH of 6.4, transport primarily involves electrogenic uptake of NH 4 + [3,4,7,22,40]. Thus, studies with microelectrodes and in Ussing chambers have shown that exposure to NH 4 + depolarizes both the apical membrane and the intact ruminal epithelium in toto with acidification of the cytosolic space [32,45], resulting in stimulation of NHE [3,4].…”

“…Thus, studies with microelectrodes and in Ussing chambers have shown that exposure to NH 4 + depolarizes both the apical membrane and the intact ruminal epithelium in toto with acidification of the cytosolic space [32,45], resulting in stimulation of NHE [3,4]. Based on both functional evidence and mRNA data, the bovine homologue of the non-selective transient receptor potential cation channel, subfamily V, member 3 (bTRPV3) has recently emerged as a candidate for uptake of cations from the rumen, including NH 4 + and Ca 2+ [40,45]. Like most other members of the large family of TRP channels [39], both the human and the bovine homologue of TRPV3 are known to be permeable to a number of monovalent and divalent cations, including Na + and Ca 2+ [48,63].…”