Modulation of lung liquid clearance by isoproterenol in rat lungs

β-Adrenergic agonists have been reported to increase lung liquid clearance by stimulating active Na⁺ transport across the alveolar epithelium. We studied mechanisms by which β-adrenergic isoproterenol (Iso) increases lung liquid clearance in isolated perfused fluid-filled rat lungs. Iso perfused through the pulmonary circulation at concentrations of 10^-4 to 10^-8 M increased lung liquid clearance compared with that of control lungs (P < 0.01). The increase in lung liquid clearance was inhibited by the β- antagonist propranolol (10^-5 M), the Na⁺-channel blocker amiloride (10^{- 4} M), and the antagonist of Na-K-ATPase, ouabain (5 x 10^-4 M). Colchicine, which inhibits cell microtubular transport of ion-transporting proteins to the plasma membrane, blocked the stimulatory effects of Iso on active Na⁺ transport, whereas the isomer lumicolchicine, which does not affect cell microtubular transport, did not inhibit Na⁺ transport. In parallel with these changes, the Na-K-ATPase α₁-subunit protein abundance and activity increased in alveolar type II cells stimulated by 10^-6 M Iso. Colchicine blocked the stimulatory effect of Iso and the recruitment of Na-K-ATPase α₁-protein to the basolateral membrane of alveolar type II cells. Accordingly, Iso increased active Na⁺ transport and lung liquid clearance by stimulation of β-adrenergic receptors and probably by upregulation of apical Na⁺ channels and basolateral Na-K-ATPase mechanisms. Recruitment from intracellular pools and microtubular transport of Na⁺ pumps to the plasma membrane participate in β-adrenergic stimulation of lung liquid clearance in rat lungs.