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1. Participation of NH4+ in total ammonia transport across sheep rumen wall was studied in vitro. 2. In Ussing-type chambers addition of NH4+ or K+ to the incubation solution on the mucosal side of the epithelium evoked positive short-circuit currents (ISC). 3. The K+ induced ISC but not the NH4+ induced ISC could be blocked by Ba2+ whereas both, K+ or NH4+ induced ISC were blocked by quinidine. 4. Transepithelial fluxes of ammonia were significantly reduced when quinidine was present in the incubation solution. 5. When the transepithelial potential difference was clamped to -25 mV (polarity on the serosal side) ammonia fluxes from the mucosal to the serosal side were significantly larger than fluxes of ammonia measured as a potential difference of +25 mV. 6. It is suggested that NH4+ takes part in ammonia transport across the rumen epithelium, most probably by using a K(+)-transport system.

Discs of stripped mucosa from the proximal ventral colon were prepared immediately after slaughter of 8 Shetland cross-breed ponies. The mucosae were fixed in incubation chambers and used in incubation experiments to study the transmucosal fluxes of the amino acids lysine, histidine and arginine (150 min) and of ammonia (90 min). When the amino acid concentrations in the mucosal solution were in the physiological range (2.8-3.0 mmol/l) no transport to the serosal side of the tissue was found. When the concentrations were raised 10-fold, less than 2% of the mucosal amino acid pool was recovered in the serosal solution. Ammonia was transported across the mucosa at significant rates although only 63% of the ammonia that disappeared from the mucosal solution was found in the serosal solution. The findings show that the equine proximal colon is virtually impermeable to luminal free amino acids whereas ammonia is transported at rates equal to, or higher than those observed with rumen mucosa from sheep.

Carbonic anhydrase activity in mucosa of the sheep rumen wall was completely inhibited by acetazolamide and ethoxyzolamide. This inhibition significantly reduced ammonia flux across mucosal discs in vitro but was ineffective if short-chain fatty acids were present in the bathing solution. The findings are discussed in relation to the mechanism of ruminal ammonia absorption.

The effects of short-chain fatty acids (SCFA) on ammonia net absorption from the sheep rumen in vivo and on ammonia transport across rumen wall mucosa in vitro were studied. Ammonia net absorption was directly, though in a non-linear manner, correlated with the SCFA concentration in the artificial rumen fluid. Almost 70% of total ammonia absorption was dependent upon the presence of SCFA when 12 mmol l-1 ammonia and 67.5 mmol l-1 SCFA were present. Lactic acid was ineffective. Incubation experiments showed that mucosal disappearance and serosal appearance of ammonia were reduced by 38% and 32%, respectively, when SCFA (63 mmol l-1) were replaced by lactic acid. The SCFA effect was independent of the type of SCFA used. In part of the experiments up to 54% of the ammonia taken up by the tissue was not recovered in the serosal incubation solution and must have been metabolized in the mucosa.

In fistulated sheep (50-60 kg live weight) the absorption of ammonia from the reticulo-rumen in vivo was studied applying the technique of the temporarily isolated and washed reticulo-rumen. It was found that, at ammonia concentrations between 3 and 18 mM, ammonia efflux and ammonia net absorption were linearly related to the ammonia concentration in the artificial rumen fluid, whereas influx of ammonia nitrogen from endogenous sources remained almost constant. When the concentration of unionized NH3 was changed at the ratio 1:10:76 by varying the pH from 5.8 to 6.8 and 7.7, ammonia net absorption did not reflect the concentration ratio of unionized NH3, indicating either flux of NH4+ ions or titration of NH4+ to NH3 at the absorptive surface. In the experiments with buffer solutions without ammonium salts and extended over 2 h, ammonia concentrations in the artificial rumen fluid increased due to endogenous nitrogen influx and reached levels far beyond the expected plateau concentration of about 2 mM. Labelling of the N pool in the isolated organ by 15N showed that ammonia efflux had almost ceased in these experiments. It is argued that as yet unidentified changes have taken place in the artificial rumen fluid during the experiment, but there is some reason to believe that volatile fatty acid (VFA) absorption was affecting ammonia absorption.