Aldosterone-mediated Na/K-ATPase expression is α₁ isoform specific in the renal cortical collecting duct

In the renal cortical collecting duct (CCD), mineralocorticoid hormones, like aldosterone, augment the abundance of Na/K-ATPase molecules. It has been postulated that this response involves an isoform switch of the Na/K-ATPase catalytic subunit, α, as the molecular basis for the differential regulation of mineralocorticoid-induced and constitutively expressed Na/K-ATPase pools. In opposition to this attractive hypothesis, three lines of independent evidence are presented which demonstrate that the CCD exclusively expresses the α₁ form despite mineralocorticoid-mediated changes in functional Na/K pump density. First, aldosterone increased [³H]ouabain binding in CCD 2.5- fold without changing the ouabain dissociation constant. Second, an electrophysiological assay for pump activity revealed that aldosterone increased maximum Na/K pump current in parallel with the change in ouabain binding without altering the apparent sodium affinity. Third, Western blot analysis with α isoform-specific, antipeptide antibodies demonstrated that aldosterone exclusively increased the total chemical pool of the α₁ form of the pump without inducing other α subunit isoforms. In summary, aldosterone increases the abundance of Na/K-ATPase molecules in the CCD which are pharmacologically, physiologically, and chemically indistinguishable from those that are normally expressed.