Lung microvascular permeability determinations by two‐photon microscopy of single microvessels

The lung microvascular barrier regulates transvascular macromolecular permeability ( P ) across the vascular surface area of filtration ( S ). The difficulty of P determinations lies in quantifying S. Hence, we addressed P in lung microvascular segments of fixed S . In the isolated, blood‐perfused rat lung held at constant left atrial pressure ( Pla ) of 3 cmH 2 O, we infused fluorescent dextran (20 kDa) in single venular capillaries. Since this Pla causes microvascular absorption of interstitial fluid (AJP 1993, 265:H198–204), we defined (1) diffusive capillary‐interstitial dextran flux, j , as j ( t ) = P [ Cv ( t )− Ci ( t )] across constant S , where Cv and Ci are the luminal and interstitial dextran concentrations, respectively, and (2) Ci ( t )=∫ j ( t ) Sdt/V , where V is interstitial volume that receives transported dextran. To determine Cv and Ci, we quantified capillary and interstitial dextran fluorescence by two‐photon microscopy. Combining the above relations, we determined P for the capillary 2 min after the onset of dextran infusion. At baseline, P was 1.9±0.6 × 10 −6 cm/sec (mean±SE, n=3). Following infusion of the permeability enhancer oleic acid (40 mg/ml, 10 min), P increased 121±26% (p<0.05). We conclude, our new method sensitively defines lung microvascular permeability at baseline and under barrier deteriorating conditions. (Support: HL80878 , HL36024 ).