HIGH MICROVASCULAR ENDOTHELIAL WATER PERMEABILITY IN MOUSE LUNG MEASURED BY A PLEURAL SURFACE FLUORESCENCE METHOD

Transport of water between the capillary and airspace compartments in lung encounters serial barriers: the alveolar epithelium, interstitium , and capillary endothelium. We previously reported a pleural surface fluorescence method to measure net capillary-to-airspace water transpo rt. To measure the osmotic water permeability across the microvascular endothelial barrier in intact lung, the airspace was filled with a wa ter-immiscible fluorocarbon. The capillaries were perfused via the pul monary artery with solutions of specified osmolaiites containing a hig h-molecular-weight fluorescent dextran, An increase in perfusate osmol ality produced a prompt decrease in surface fluorescence due to dye di lution in the capillaries, followed by a slower return to initial fluo rescence as capillary and lung interstitial osmolality equilibrate. A mathematical model was developed to determine the osmotic water permea bility coefficient (P-f) of lung microvessels from the time course of pleural surface fluorescence. As predicted, the magnitude of the promp t change in surface fluorescence increased with decreased pulmonary ar tery perfusion rate and increased osmotic gradient size. With raffinos e used to induce the osmotic gradient, P-f was 0.03 cm/s at 23 degrees C and was reduced 54% by 0.5 mM HgCl2. Temperature dependence measure ments gave an Arrhenius activation energy (E-a) of 5.4 kcal/mol (12-37 degrees C). The apparent P-f induced by the smaller osmolytes mannito l and glycine was 0.021 and 0.011 cm/s (23 degrees C). Immunoblot anal ysis showed similar to 1.4 x 10(12) aquaporin-1 water channels/cm(2) o f capillary surface, which accounted quantitatively for the high P-f. These results establish a novel method for measuring osmotically drive n water permeability across microvessels in intact lung. The high P-f, low E-a, and mercurial inhibition indicate the involvement of molecul ar water channels in water transport across the lung endothelium.