Toluidine blue O and methylene blue as endothelial redox probes in the intact lung

There is increasing evidence that the redox activities of the pulmonary end othelial surface may have important implications for the function of both l ungs and blood. Because of the inherent complexity of intact, organs, it ca n be difficult to study these activities in situ; Given:the availability of appropriate indicator probes, the multiple-indicator dilution (MID) method is one approach for dealing with some aspects of this complexity. Therefor e, the objectives,:of the present study were to 1) evaluate the potential u tility of two thiazine redox indicators, methylene blue (NIB) and toluidine blue O (TBO), as MID electron acceptor probes for in situ pulmonary endoth elium and 2) develop a-mathematical model of the pulmonary disposition of t hese indicators as a tool for quantifying their reduction on passage throug h the lungs. Experiments were carried out using isolated rabbit lungs perfu sed with physiological salt solution with or without plasma albumin over a range of flow rates. A-large:fraction of the injected TBO disappeared from the perfusate on passage through the lungs. The reduction of its oxidized, strongly polar, relatively hydrophilic blue form to its Colorless, highly l ipophilic reduced form was revealed by the presence of the reduced form in the venous effluent when plasma albumin was included in the perfusate. MB w as also lost:from: the perfusate, but the fraction was considerably smaller than for TBO. A distributed-in-space-and-time model was: developed to esti mate the reduction rate parameter, which:was similar to 29 and 1.0 ml/s for TBO and MB, respectively, and almost flow rate independent for both indica tors. The results suggest the utility particularly of TBO as an electron ac ceptor probe for MID studies of in situ pulmonary endothelium and of the mo del for quantitative evaluation of the data.