AN INTERPRETATION OF C-14 UREA AND C-14 PRIMIDONE EXTRACTION IN ISOLATED RABBIT LUNGS

We measured the venous concentration versus time curves of C-14-urea a nd C-14-primidone after rapid bolus injections of a vascular reference indicator, fluorescein isothiocyanate dextran, and one of the two C-1 4-labeled indicators in isolated rabbit lungs perfused with Krebs-Ring er bicarbonate solution containing 4.5% bovine serum albumin at flow r ates (F) of 6.67, 3.33, 1.67, and 0.83 ml/sec and with nearly constant microvascular pressure and total lung vascular volume. When we calcul ated the permeability-surface area product, PS, from the C-14-urea and C-14-primidone outflow curves using the Crone model, the estimates of the PS product were directly proportional to F. However, the fraction al change in the PS with flow was different for the two indicators. We also estimated the PS from the same C-14-urea and C-14-primidone data using an alternative model that includes perfusion heterogeneity, est imated in a previous study, and flow-limited and barrier-limited extra vascular volumes accessible to both urea and primidone. This model was able to fit the outflow curves of either C-14-urea or C-14-primidone at all four flows studied with one flow-independent PS for each indica tor. The ability of the new model to explain the C-14-urea and C-14-pr imidone data with no flow-dependent change in PS suggests that a chang e in PS with F estimated using other models such as the Crone model is not sufficient evidence for capillary surface area recruitment.