CONTRIBUTION OF ACTIVE NA+ AND CL- FLUXES TO NET ION-TRANSPORT BY ALVEOLAR EPITHELIUM

Changes in bioelectric properties of alveolar epithelial cell monolayers due to pharmacological agents such as beta-agonists, amiloride and ouabain have recently been reported. In order to determine specifically which ionic species contribute to these changes, fluxes of Na+ and Cl- across primary cultured monolayers of rat type II pneumocytes were directly measured. Monolayers were mounted in modified flux chambers and short-circuited. Unidirectional fluxes of Na-22 (or Cl-36) and [C-14]-mannitol were measured simultaneously. Experimental maneuvers included apical (A) exposure to 10-mu-M amiloride, basolateral (B) exposure to 1 mM ouabain, or basolateral exposure to 20-mu-M terbutaline. Results show that baseline monolayers actively reabsorb Na+ (about 0.14-mu-Eq.cm-2.h-1) from the apical fluid, while mannitol and Cl- appear to traverse the alveolar epithelium passively. Active Na+ reabsorption was abolished by amiloride or ouabain, while Cl- and mannitol fluxes were unaffected. Terbutaline, on the other hand, markedly increased net absorption of Na+ and caused active transport of Cl- in the A to B direction. Passive mannitol flow was somewhat increased with terbutaline. These data indicate that active Na+ reabsorption across alveolar epithelial monolayers is dependent on intact Na+, K+-ATPase activity and cell Na+ entry (probably via Na+ channels), and can be stimulated by beta-agonists. Beta-agonists also cause active reabsorption of Cl- (passive under other conditions).