β 2 -Adrenergic Receptor Overexpression Increases Alveolar Fluid Clearance and Responsiveness to Endogenous Catecholamines in Rats

Abstract

β-Adrenergic agonists accelerate the clearance of alveolar fluid by increasing the expression and activity of epithelial solute transport proteins such as amiloride-sensitive epithelial Na + channels (ENaC) and Na,K-ATPases. Here we report that adenoviral-mediated overexpression of a human β 2 -adrenergic receptor (β 2 AR) cDNA increases β 2 AR mRNA, membrane-bound receptor protein expression, and receptor function (procaterol-induced cAMP production) in human lung epithelial cells (A549). Receptor overexpression was associated with increased catecholamine (procaterol)-responsive active Na + transport and increased abundance of Na,K-ATPases in the basolateral cell membrane. β 2 AR gene transfer to the alveolar epithelium of normal rats improved membrane-bound β 2 AR expression and function and increased levels of ENaC (α subunit) abundance and Na,K-ATPases activity in apical and basolateral cell membrane fractions isolated from the peripheral lung, respectively. Alveolar fluid clearance (AFC), an index of active Na + transport, in β 2 AR overexpressing rats was up to 100% greater than sham-infected controls and rats infected with an adenovirus that expresses no cDNA. The addition of the β 2 AR-specific agonist procaterol to β 2 AR overexpressing lungs did not increase AFC further. AFC in β 2 AR overexpressing lungs from adrenalectomized or propranolol-treated rats revealed clearance rates that were the same or less than normal, untreated, sham-infected controls. These experiments indicate that alveolar β 2 AR overexpression improves β 2 AR function and maximally upregulates β-agonist–responsive active Na + transport by improving responsiveness to endogenous catecholamines. These studies suggest that upregulation of β 2 AR function may someday prove useful for the treatment of pulmonary edema.