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Urea recycling in ruminants exhibits large variations (1) and is effectively inhibited by the luminal concentration of ammonia in vitro (2) and in vivo (3), which is in marked contrast to the stimulating effects of SCFA and CO2 on urea transport (4). Because all three fermentation products are simultaneously present in the rumen fluid, interactions between the inhibitory effects of ammonia on the one hand and the stimulatory effects of SCFA and CO2 on the other hand can be anticipated, with consequences for urea recycling. It was therefore the aim of this study to describe possible interactions between SCFA and ammonia on urea transport.
This study was conducted under short circuit conditions using the conventional Ussing chamber technique with isolated epithelial tissues from sheep rumen. All buffer solutions contained a mixture of antibiotics and the urease inhibitor phenylphosphordiamidate (PPD; 1 mmol/l).
Results: Increasing the luminal SCFA concentration from 0 to 40 or 80 mmol/l (pH 6.4) caused a linear and significant increase of urea flux in the serosal to mucosal direction (Jsm) from 34.1 ± 8.8 (0 nmol/l SCFA) to 99.8 ± 19.1 and 161.9 ± 26.5 nmol?cm-2?h-1 (0, 40 or 80 mmol/l SCFA, respectively). Addition of ammonia (5 mmol/l) did not change urea transport at 0 mmol/l SCFA (29.5 ± 7.9 nmol?cm-2?h-1), but reduced it significantly to 52.3 ± 14.4 and 65.9 ± 20.6 nmol?cm-2?h-1 in a buffer solution with 40 or 80 mmol/l luminal SCFA, respectively. The effect of ammonia on urea transport depended on the luminal pH (table 1), with decreasing luminal pH causing a bell-shape change of transport rates.
The present study confirms the well known inhibitory effect of ruminal ammonia on urea recycling (1) and urea transport in vitro (2). However, the obtained results clearly show that luminal SCFA and pH modulate urea transport rates in a marked fashion both in the presence and in the absence of ammonia. It is worth mentioning that at pH > 7.0, the observed urea transport rates were low and not significantly decreased by ammonia. This pH range (>7.0) is normally observed at low feed intake or starvation and hence, ammonia requirement for microbial protein synthesis is low and there is no necessity for urea recycling. The inhibition of urea transport by low pH and ammonia may explain the shift of urea recycling to urinary urea excretion in high producing ruminants (1).