REMOVAL OF ALBUMIN MICROINJECTED IN RAT LUNG PERIMICROVASCULAR SPACE

We used a microinjection approach to assess hydraulic properties of lu ng perimicrovascular adventitia (interstitial cuff surrounding microve ssels). Isolated blood-perfused rat lungs held at constant airway pres sure were microscopically viewed to identify subpleural venules (20 mu m diam). Venular adventitia were microinjected with 20 nl of fluoresc ent albumin (4 g/dl), and then adventitial fluorescence was quantified at the injection site by either photometery or imaging. Nonlinear dec ay of adventitial fluorescence indicated liquid flux from the injectio n site into normal interstitium. In some experiments, we determined th at the adventitial fluorescence flowed longitudinally along the venule length and filled single lymphatics. The fluorescence decay at the in jection site was best described by equations of convective but not dif fusive transport. The decay time constant (time to 37% initial), which relates inversely to hydraulic conductivity, increased 10-fold above baseline on lung expansion with airway pressure from 5 to 15 cmH(2)O ( P < 0.05). However, presence or absence of blood flow, increase in fil tration pressure, and tissue edema were all without effect on the time constant. Our estimate of the lower limit of baseline adventitial hyd raulic conductivity was 5 X 10(-6) ml.cm(-2).s(-1).cmH(2)O(-1). We con clude that hydraulic conductivity of perimicrovascular adventitia is n ot augmented by edema but that it is decreased by lung expansion.