RESPONSES OF RAT PLEURAL MESOTHELIA TO INCREASED INTRATHORACIC PRESSURE

Direct measurements of pleural fluid hydrostatic and colloid osmotic p ressures after infusion of saline, bovine serum albumin, or silicone i nto the pleural space were coupled with ultrastructural morphometric a nalyses to assess the response of pleural mesothelial cells to hydroth orax Increases of hydrostatic pressure, either independent or in combi nation with decreases of osmotic pressure, served to increase the numb er of plasmalemmal vesicles in mesothelial cells of both the visceral and parietal pleurae. These results support the hypothesis that an inc rease in vesicle numerical density represents a response to elevations of extracellular fluid pressures. Fluid resorption from the pleural s pace with subsequent accumulation within the visceral pleural intersti tium was also associated with the formation of invaginations of the me sothelial basal plasmalemma. That the invaginations were not observed in the absence of interstitial fluid accumulation supports the concept that basal surface invaginations represent distortions of mesothelial cell membranes in response to pressure differentials across the plasm alemma. The results of this study are most consistent with the interpr etation that increased numbers of plasmalemmal vesicles and invaginati ons of the basal plasmalemma represent adaptive conformational mechani sms of pleural mesothelial cells to prevent monolayer disruption by el evated extracellular fluid pressures.