Theta values for C16O18O and C18O2 related to respective pulmonary diffusing capacities.

The single-breath diffusing capacities for singly and doubly 18O-labeled CO2, DLC16O18O and DLC18O2, as well as for NO, were determined in seven anesthetized rabbits to investigate whether the theoretically predicted ratio of specific blood uptake rates of both isotopic CO2 species, theta C18O2/theta C16O18O = 2.0, can be derived from the measured values of DLC16O18O and DLC18O2. Data of DL were obtained by inflating the lungs with gas mixtures containing 0.35% C16O18O or 0.8% C18O2 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 C18O2/theta C16O18O was calculated by applying the double-reciprocal Roughton-Forster equation to DL values obtained in each animal and by assuming that NO diffusing capacity represents the gas conductance of the alveolar-capillary membrane. The measured ratio was theta C18O2/theta C16O18O = 1.9 +/- 0.2 (mean +/- SD), thus comparing reasonably with the predicted one. Therefore, our findings provide evidence that the greater value of DLC18O2 is mainly due to the twofold higher probability (or theta value) for C18O2 than for C16O18O to disappear within red blood cells via isotopic exchange reactions.