THETA VALUES FOR (COO)-O-16-O-18 AND (CO2)-O-18 RELATED TO RESPECTIVEPULMONARY DIFFUSING CAPACITIES

The single-breath diffusing capacities for singly and doubly O-18-labe led CO2, DLC16O18O and DLC18O2, as well as for NO, were determined in seven anesthetized rabbits to investigate whether the theoretically pr edicted ratio of specific blood uptake rates of both isotopic CO2 spec ies, theta(C18O2)/theta(C16O18O) = 2.0, can be derived from the measur ed values of DLC16O18O and DLC18O2. Data of DL were obtained by inflat ing the lungs with gas mixtures containing 0.35% (COO)-O-16-O-18 Or 0. 8% (CO2)-O-18 or 0.05% NO in nitrogen, with breath-holding periods of 0.05-0.5 s and 2-12 s for the CO2 and NO tests, respectively. theta(C1 8O2)/theta(C16O18O) was calculated by applying the double-reciprocal R oughton-Forster equation to DL values obtained in each animal and by a ssuming that NO diffusing capacity represents the gas conductance of t he alveolar-capillary membrane. The measured ratio was theta(C18O2)/th eta(C16O18O) = 1.9 +/- 0.2 (mean ir SD), thus comparing reasonably wit h the predicted one. Therefore, our findings provide evidence that the greater value of DLC18O2 is mainly due to the twofold higher probabil ity (or theta value) for (CO2)-O-18 than for (COO)-O-16-O-18 to disapp ear within red blood cells via isotopic exchange reactions.