β 2-Adrenergic Stimulation Blunts Inhibition of Epithelial Ion Transport by Hypoxia of Rat Alveolar Epithelial Cells

摘要

Hypoxia impairs alveolar fluid clearance by inhibition of Nasup+/sup reabsorption, and also impairs β sub2/sub adrenergic signaling in alveolar epithelium. Since both are major rescue mechanisms preventing pulmonary edema, we studied whether acute and prolonged treatment with terbutaline would prevent hypoxic inhibition of ion transport. Short circuit currents (ISC) were measured on normoxic and hypoxic (1.5% Osub2/sub; 24h) primary rat alveolar type II (ATII) cells in absence and presence of terbutaline (1 to 100 μM, 24h). Control and pre-treated cells were stimulated acutely with terbutaline. Transepithelial transport was measured as short circuit current (ISC) in Ussing chambers. Terbutaline induced a rapid decrease ISC (-20%) followed by a slow raise. The transient change in ISC was not inhibited by amiloride but was prevented after Clsup-/sup depletion indicating a Clsup-/sup current. The slow increase after this transient was amiloride-sensitive indicating a Nasup+/sup current. Total ISC, its amiloride-sensitive component, and the transient decrease upon terbutaline stimulation were decreased by hypoxia. 24h treatment with terbutaline stimulated these currents in normoxia and hypoxia, although stimulation was less in the latter. 24h treatment with terbutaline increased the capacity of Nasup+/sup/Ksup+/sup-ATPase and ENaC as measured after permeabilization with amphotericin. These changes were not paralleled by altered mRNA expression. Acutely applied terbutaline induced a 4-fold increase in cAMP formation in normoxia; terbutaline-induced cAMP-formation was impaired by hypoxia (-20%). Pre-treatment with terbutaline for 24h decreased terbutaline-induced cAMP formation by 85%. Despite this desensitization, addition of terbutaline to terbutaline pre-treated cells caused a larger increase in Clsup-/sup and Nasup+/sup transport both in normoxia and hypoxia than in non pre-treated cells. These results indicate that β sub2/sub adrenergic stimulation increased Nasup+/sup- and Clsup-/sup transport in ATII cells even in hypoxia thus restoring normal reabsorption.