No abstract

S U M M A R YTwo experiments were carried out to determine endogenous losses and the response of urinary purine derivatives to increased duodenal inputs of purine bases. Four ewes each fitted with a re-entrant cannula at the proximal duodenum, and conventionally fed, were subjected to full replacement of duodenal digesta followed by the administration of a solution either free of purines (Expt 1) or enriched with increasing amounts of purines, to supply 0-48-21-27 mmol/animal per day (Expt 2). Basal daily urinary excretions of allantoin, uric acid, hypoxanthine and xanthine were 11-5 + 0-94, 9-9 ±0-67, 6-9 ±0-46 and 1-2 ±016 mg/kg W 075 . Allantoin was the only purine derivative which increased in response to incremental inputs of duodenal purines. The relationship between allantoin excretion and infused purines showed a urinary recovery of 0-8 for purines infused at > 220 umol/kg W 076 . Lower rates of infusion did not alter allantoin excretion. The results show urinary allantoin to be a useful index to estimate duodenal input of purines when animals are fed close to or above their energy maintenance requirements.

An experiment was conducted with dairy cows to study the partitioning of excreted purine derivatives between urine and milk and to quantify the endogenous contribution following the isotopic labeling of microbial purine bases. Three lactating cows in their second lactation that had been cannulated in the rumen and the duodenum were fed a mixed diet (48:52, roughage/concentrate ratio) distributed in equal fractions every 2 h, and duodenal flow of purine bases was determined by the dual-phase marker system. Nitrogen-15 was infused continuously into the rumen to label microbial purine bases, and the endogenous fraction was determined from the isotopic dilution in urinary purine derivatives. Urinary and milk recovery of duodenal purine bases were estimated at early (wk 10) and late (wk 33) lactation by the duodenal infusion of incremental doses (75 and 150 mmol purine bases/d) of RNA from Torula yeast. Each period was 6 d, with RNA being infused during the last 4 d, followed by measurement of the flow of purine bases to the duodenum. The isotope dilution of purine derivatives in urine samples confirmed the presence of an endogenous fraction (512 +/- 36.43 micromol/W0.75 or 56.86 mmol/d) amounting to 26 +/- 3.8% of total renal excretion. Total excretion of purine derivatives in urine plus milk was linearly related to the duodenal input of purine bases, but the slopes differed (P < 0.005) between lactation stages resulting in a lower equimolar recovery in early (y = 58.86 (+/-3.89) +0.56 (+/-0.0164) x; r = 0.90) than late lactation (y = 58.86 (+/-3.89) + 0.70 (+/-0.046) x; r = 0.80). Excretion of purine derivatives through milk represented a minimum fraction of total excretion but responded significantly to the duodenal input of purine bases. No differences between lactation stages were detected, and variations in milk yield did modify significantly the amount of purine derivatives excreted through the milk.

Two dry cows fitted with simple cannula in the rumen and duodenum, and fed with a mixed diet (straw:barley, 50:50), were used to determine endogenous losses and response of urinary purine derivatives (PDs) to duodenally infused yeast-RNA. Duodenal flow of purine bases (PBs) was determined by a dual marker system, and 15N was infused continuously into the rumen to label microbial PBs. The isotope dilution of urinary PDs in relation to duodenal PBs confirmed the presence of an endogenous fraction (236 μmol/kg LW0.75) bigger than in sheep and lower than values estimated in cows with impaired rumen fermentation. Excretion of PDs increased linearly in response to incremental supply of PBs with an equimolar recovery of 0.84 mol/mol. However, net recovery of duodenal PBs was 0.67 for the basal diet and 0.65, 0.90, 0.79 and 0.82 for the four levels of PB infusion. It is suggested that differences in digestibility between yeast-RNA and duodenal PBs might explain differences in recovery estimations.

The aim of this study was to investigate the effect of presence or absence of protozoa on rumen fermentation and efficiency of microbial protein synthesis under different diets. Of 20 twin paired lambs, 1 lamb of each pair was isolated from the ewe within 24 h after birth and reared in a protozoa-free environment (n = 10), whereas their respective twin-siblings remained with the ewe (faunated, n = 10). When lambs reached 6 mo of age, 5 animals of each group were randomly allocated to 1 of 2 experimental diets consisting of either alfalfa hay as the sole diet, or 50:50 mixed with ground barley grain according to a 2 × 2 factorial arrangement of treatments. After 15 d of adaptation to the diet, the animals were euthanized and total rumen and abomasal contents were sampled to estimate rumen microbial synthesis using C(31) alkane as flow marker. Different ((15)N and purine bases) and a novel (recombinant DNA sequences) microbial markers, combined with several microbial reference extracts (rumen protozoa, liquid and solid associated bacteria) were evaluated. Absence of rumen protozoa modified the rumen fermentation pattern and decreased total tract OM and NDF digestibility in 2.0 and 5.1 percentage points, respectively. The effect of defaunation on microbial N flow was weak, however, and was dependent on the microbial marker and microbial reference extract considered. Faunated lambs fed with mixed diet showed the greatest rumen protozoal concentration and the least efficient microbial protein synthesis (29% less than the other treatments), whereas protozoa-free lambs fed with mixed diet presented the smallest ammonia concentration and 34% greater efficiency of N utilization than the other treatments. Although (15)N gave the most precise estimates of microbial synthesis, the use of recombinant DNA sequences represents an alternative that allows separate quantification of the bacteria and protozoa contributions. This marker showed that presence of protozoa decrease the bacterial-N flow through the abomasum by 33%, whereas the protozoa-N contribution to the microbial N flow increased from 1.9 to 14.1% when barley grain was added to the alfalfa hay. Absolute data related to intestinal flow must be treated with caution because the limitations of the sampling and maker system employed.