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

Abstract

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 cmH2O, 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).