Modulation of transalveolar fluid absorption by endogenous aldosterone in adult

We examined whether transalveolar fluid transport is modulated by aldostero ne in adult rats. Because colocalization of mineralocorticoid receptors (MR ) with 11 beta -hydroxysteroid dehydrogenase type 2 (11 beta HSD2) is impor tant for aldosterone specific action, we first determined the immunohistoch emical distribution of MR and 11 beta HSD2 in the lung. We found that alveo lar epithelial cells express both MR and 11 beta HSD2. Reverse transcriptas e polymerase chain reaction (RT-PCR) demonstrated that rat alveolar type II epithelial cells express both MR and 11 beta HSD2. We then measured alveol ar fluid clearance in rats treated with chronic low-sodium diet. A low-sodi um diet (0.1% NaCl for 12 to 14 days) caused hyperaldosteronism accompanied by hypokalemia, whereas serum corticosterone and adrenaline levels remaine d normal. We found that hyperaldosteronism was associated with significantl y higher alveolar fluid clearance and that this increase was related to the amiloride-sensitive component. In addition, the increase in alveolar fluid clearance was inhibited by spironolactone. Our results show that aldostero ne is able to stimulate Na+ channels of alveolar epithelial cells. We concl ude that alveolar epithelium is a physiological target tissue for aldostero ne and transalveolar fluid adsorption could in part be modulated by endogen ous aldosterone acting via MR.