Plasma protein binding and toluidine blue O (TBO) reduction in the pulmonary circulation

TBO is reduced during passage through the lung. The reduced colorless form, TBOH, is taken up by the lung. To determine the impact of plasma protein binding on the TBO and TBOH disposition, TBO and a reference tracer indocyanine green (ICG) were injected into the pulmonary artery of isolated rabbit lungs perfused with a 5% serum albumin solution. On-line optical densitometry was used to measure venous effluent ICG and oxidized TBO (TBO⁺) prior to re-oxidation of TBOH. Total outflow TBO-TBO⁺ plus TBOH-was measured by collecting effluent flow samples and re-oxidizing TBOH before measuring absorbance. TBOH concentration profiles were obtained by subtracting the TBO⁺ profiles, which were corrected for differences between sampling methods, from total TBO curves. A mathematical model that includes the kinetics of TBO reduction, rapidly equilibrating TBO plasma albumin binding and rapidly equilibrating partitioning of TBOH between plasma albumin and tissue fit the TBO⁺ and TBOH concentration curves over a wide range of flows with a flow independent reduction rate. We conclude that albumin does not influence the rate of unbound TBO⁺ reduction, but it is the means by which TBOH, which would otherwise be virtually entirely taken up by the lung, reaches the venous effluent.