title: Estimation of allantoin flux using continuous infusion of [14C]allantoin: sensitivity to plasma loading with unlabelled allantoin.
abstract: Allantoin net flux through the plasma allantoin compartment was determined in sheep given a roughage diet by means of a continuous infusion of [4,5-14C]allantoin for 17 h. Unlabelled allantoin was infused intravenously during the last 7 h of the tracer infusion to increase the allantoin flux by approximately 75 %. When unlabelled allantoin was infused, the specific radioactivity of allantoin in plasma and urine declined exponentially to approach a lower plateau some 2-3 h later. The estimate of net flux during the infusion of unlabelled allantoin, estimated from blood plasma and urine, was on average 79 and 90 % of expected values. Expected values of allantoin net flux during infusion of unlabelled allantoin were calculated as the sum of allantoin net flux pre-loading plus the known rate of infusion of the allantoin load. It is probable that endogenous allantoin synthesis was decreased by the infusion of allantoin: allosteric inhibition of uricase appears a plausible explanation for this observation, and for lower estimates of net flux. Appearance of labelled allantoin-C in ruminal or blood bicarbonate was negligible. Our results indicate that net flux of allantoin through blood plasma is a good predictor of the entry rate of allantoin into the primary compartment and should be a better predictor of rumen microbial outflow than urinary allantoin excretion. However, measurements of allantoin-specific radioactivity, during continuous infusion, should be taken after a period of 24 h, at which time the true plateau specific radioactivity value of allantoin in plasma would be attained.
authors.affiliation: School of Rural Science and Natural Resources, University of New England, Armidale, NSW 2351, Australia. lkahn2@metz.une.edu.au
authors.forename: L P
authors.initials: LP
authors.lastname: Kahn
authors.forename: D
authors.initials: D
authors.lastname: Ouellet
authors.forename: J V
authors.initials: JV
authors.lastname: Nolan
mesh_terms: D000481:Allantoin / Q000097:blood / Q000493:pharmacokinetics* / Q000652:urine; D000818:Animals; D002250:Carbon Radioisotopes; D005260:Female; D007262:Infusions, Intravenous; D012756:Sheep / Q000378:metabolism*
chemical_list: D002250:Carbon Radioisotopes; D000481:Allantoin