EVALUATION OF THE N-15-ISOTOPE DILUTION TECHNIQUE FOR DETERMINING THERECOVERY OF ENDOGENOUS PROTEIN IN ILEAL DIGESTION OF PIGS - EFFECT OFDILUTION IN THE PRECURSOR POOL FOR ENDOGENOUS NITROGEN SECRETION

Three barrows, average initial BW of 35 kg, were fitted with a re-entr ant cannula at the distal ileum and catheters in each of the external jugular veins. The barrows were fed twice daily a diet containing barl ey as the sole source of protein. A 1-d intravenous infusion of saline was followed by an 8-d continuous infusion of [N-15]leucine at a rate of 30 mg/(kg BW x d). Blood samples, taken hourly for 9 d, were poole d over the 12-h period between feeding. Ileal digesta were collected h ourly on d 3 and 8 of the [N-15]leucine infusion and pooled over 12 an d 24 h. Enrichments (atom percentage excess N-15) in nitrogen (N), leu cine, isoleucine, valine, and alanine were measured in the trichloroac etic acid (TCA)-soluble and -insoluble fractions of plasma and in ilea l digesta. Although the enrichments in N were similar in all pools, en richments in amino acids in the TCA-soluble fraction of plasma were at least twice that in the TCA-insoluble fraction and in ileal digesta. The estimated contributions of endogenous N and amino acids to total N and amino acids in ileal digesta were 82.3 +/- 9.38, 39.0 +/- 3.83, 4 3.8 +/- 6.50, 51.9 +/- 3.24, and 51.9 +/- 9.80 for N, leucine, isoleuc ine, valine, and alanine, respectively. The results from this study in dicate that enrichments in total N in the TCA-soluble fraction of plas ma may not accurately reflect enrichments in endogenous N secreted int o the intestinal lumen. This factor should be considered when using th e N-15-isotope dilution technique to estimate the recovery of endogeno us protein in ileal digesta of pigs fed protein-containing diets.