Reassessment of primed constant-infusion tracer method to measure urea kinetics.

The validity of the primed constant-infusion tracer technique to make short-term measurements of urea production rates (Ra) in humans in a physiological steady state and during disruption of steady state was evaluated. Four subjects received a primed constant infusion (P/I = 560 min) of [13C]urea for 8 h. A plateau in urea enrichment was reached after 2 h and maintained throughout. When [13C]- and [18O]urea were simultaneously infused into four subjects at P/I ratios of 560:1 and 360:1, respectively, both tracers reached plateau enrichment at the same time (2-4 h). The enrichment at plateau was a function of the infusion rate rather than the priming dose, and calculated urea Ra was the same with either prime. In five additional experiments the technique responded acutely to a physiological perturbation (alanine infusion) in a dose-dependent manner. The results confirm that this technique is appropriate for short-term measurements of urea Ra, and the requirement for accuracy in estimating the priming dose is not impractically stringent.