The effects of intraruminal infusions of sodium bicarbonate, ammonium chloride and sodium butyrate on urea metabolism in sheep.

1. Three sheep fitted with rumen cannulas were fed hourly a daily ration of 1000 g pelleted-grass cubes, and during four successive 2-week periods were intraruminally infused (0.45 l/d) with solutions containing sodium chloride (0.47 mol/d), sodium bicarbonate (0.47 mol/d), .ammonium chloride (0.47 mol/d) and sodium butyrate (0.47 mol/d). Each solution, except that for NaHCO,, was adjusted to pH 7 before infusion, and provided equal sodium intakes for sheep in all periods. 2. In the final week of each infusion period, a balance trial was conducted and on separate days each sheep was continuously infused with [14C]urea and NaHI4CO, intravenously and NaHI4CO, intraruminally. Carbon transfer rates between blood urea, blood bicarbonate and rumen fluid bicarbonate were calculated from the specific radioactivity of urea and bicarbonate samples and isotope infusion rates during each experimental period. 3. There was no significant effect of intraruminal infusions on N balance, and with the exception of sheep infused with NH,CI, all sheep utilized apparently digested N with similar efficiency for N retention. Sheep infused with NH,CI (6.2 g N/d) excreted the equivalent of 93% of the infused N as urea in urine. 4. Infusion of NaHCO,, NH,CI and sodium butyrate significantly ( P < 0.05) increased the rumen fluid concentrations of bicarbonate, ammonia and butyric acid respectively, and all infusions significantly ( P < 0.05) increased total volatile fatty acid concentrations. Both NaHCO, and sodium butyrate significantly ( P < 0.05) increased the pH of rumen fluid. There was no significant effect of infusion on the proportions of propionic acid or the osmolality of rumen fluid. 5. Intraruminal infusions of NH,CI significantly ( P < 0.05) increased and infusion of sodium butyrate significantly ( P < 0.05) decreased plasma urea concentrations. Sheep infused with NH,CI had higher rates of urea synthesis and urinary urea excretion compared with sheep on the other treatments, and a significantly ( P < 0.05) lower proportion of urea synthesized by these sheep was degraded in the digestive tract. Sheep infused with sodium butyrate degraded a significantly ( P < 0.05) greater amount (3.2 g N/d) and proportion (0.24) of total urea synthesis in the rumen than did shcep infused with NaCI. Corresponding values for the control (NaCI) sheep were 1-5 g N/d and 0.13 respectively. There was no significant effect of other infusions on the amount of urea recycled to the rumen or on the distribution of total urea degradation between the rumen and lower digestive tract. Plasma urea clearance to the rumen was significantly ( P < 0.05) increased during sodium butyrate infusion, and the clearance of urea to the lower digestive tract was significantly ( P < 0.05) decreased during NH,CI infusion. 6. The mechanism by which urea entry into the rumen is regulated by rumen metabolite levels is discussed.