TRACER STUDIES OF UREA KINETICS IN GROWING PIGS .1. THE EFFECT OF INTRAVENOUS-INFUSION OF UREA ON UREA RECYCLING AND THE SITE OF UREA SECRETION INTO THE GASTROINTESTINAL-TRACT

Four gilts (average BW 80 kg) were used in the first experiment to study the effect of i.v. infusion of urea on urea kinetics by means of a radioisotope dilution technique. The pigs were fed twice daily 600 g of a cornstarch-based diet formulated to contain 16% CP by supplementation with isolated soy protein. Infusion of urea, compared with saline, increased (P < .05) plasma urea concentration, urea pool size, urea entry, urea excretion, and urea degradation rates; urea turnover rate and urea space were not affected (P > .05). Expressed as a percentage of the total entry rate, a lower (P < .05) percentage of urea was recycled in pigs infused with urea. The urea infused was almost completely excreted in urine, so there were no differences (P > .05) in N balance. In the second experiment, four gilts (average BW 40 kg), fitted with ileocecal reentrant cannulas, were used to determine whether the upper or the lower digestive tract represents the preferential site of urea secretion in pigs. Two pigs were fed twice daily 600 g of a cornstarch-based diet, formulated to contain 16% CP from soybean meal. The other two pigs were fed the same diet in which 15% cornstarch was replaced by beet pulp. After labeling the body urea pool of one pig on each treatment with [N-15]urea, the reentrant cannulas were disconnected to prevent the flow of digesta from the small into the large intestine. Ileal digesta, collected simultaneously from the unlabeled pig on the same dietary treatment, were infused via the distal cannula into the large intestine of the other [N-15]-labeled pig, thus maintaining fermentation. The luminal secretion of urea from the circulation into the large intestine was negligible and not affected by diet composition; the upper digestive tract (stomach and small intestine) represented the main site of urea secretion in pigs.