Absorption of ammonia from the gastrointestinal tract results in problems that
range from hepatic encephalopathy in humans to poor nitrogen efficiency of
cattle with consequences for the global climate. Previous studies on epithelia
and cells from the native ruminal epithelium suggest functional involvement of
the bovine homologue of TRPV3 (bTRPV3) in ruminal NH4+ transport. Since the
conductance of TRP channels to NH4+ has never been studied, bTRPV3 was
overexpressed in HEK-293 cells and investigated using the patch-clamp
technique and intracellular calcium imaging. Control cells contained the empty
construct. Divalent cations blocked the conductance for monovalent cations in
both cell types, with effects higher in cells expressing bTRPV3. In bTRPV3
cells, but not in controls, menthol, thymol, carvacrol, or 2-APB stimulated
whole cell currents mediated by Na+, Cs+, NH4+, and K+, with a rise in
intracellular Ca2+ observed in response to menthol. While only 25% of control
patches showed single-channel events (with a conductance of 40.8 ± 11.9 pS for
NH4+ and 25.0 ± 5.8 pS for Na+), 90% of bTRPV3 patches showed much larger
conductances of 127.8 ± 4.2 pS for Na+, 240.1 ± 3.6 pS for NH4+, 34.0 ± 1.7 pS
for Ca2+, and ~ 36 pS for NMDG+. Open probability, but not conductance, rose
with time after patch excision. In conjunction with previous research, we
suggest that bTRPV3 channels may play a role in the transport of Na+, K+, Ca2+
and NH4+ across the rumen with possible repercussions for understanding the
function of TRPV3 in other epithelia