Introduction:

In the fermentative parts of the gut, large quantities of ammonia are produced and absorbed via poorly understood pathways. Previous studies have shown that mechanisms for the efflux of ammonia from the sheep rumen include a conductance for the cationic form (NH₄⁺). No information is available for the bovine species.

Methods:

Ion-selective microelectrodes, Ussing chambers, and the patch clamp technique were used to study intact epithelium and epithelial cells isolated from the bovine rumen.

Results:

As in sheep rumen, replacement of 40 mmol·l^-1 NMDG⁺ by an equimolar amount of NH₄⁺ induced a depolarization of the apical membrane and an acidification of the cytosolic space in conjunction with a significant rise in transepithelial short circuit current (n = 12). This current could be further enhanced by removal of divalent cations (P < 0.05, n = 6). Stimulatory effects were also seen after mucosal application of the TRP channel agonist menthol (200 µmol·l^-1 (n = 9) and 1 mmol·l^-1 (n = 12), P < 0.05), effects decreased with time. Patch clamp experiments showed an induction of inward current by NH₄⁺ which was strongly enhanced by removal of divalent cations. Outward currents (Na⁺ or K⁺) were less markedly stimulated.

Conclusion:

Transport of NH₄⁺ across the bovine ruminal epithelium involves non-selective cation channels that may belong to the TRP group. Further studies are needed to determine which specific type of TRP channel is involved, what other conductances are affected, and if other TRP modulators have significant effects on ammonia absorption across the rumen.