Regulation of Na<sup>+</sup>/K<sup>+</sup>-ATPase by Corticosteroids in Cultured Renal Medullary Collecting Duct

Abstract

Distinct subsegments of the inner medullary collecting duct (IMCD) have previously been delineated by morphological studies; specifically, basal plasma membrane area (mm²/mm³) decreases from the initial to the terminal segment. The present work measures Na⁺/K⁺-ATPase activity in monolayer cultures of defined IMCD subsegmental cell populations to directly evaluate the regulation by corticosteroids of the primary active ion transport enzyme. Na⁺/K⁺-ATPase activity was determined by enzymatic coupling of ATP hydrolysis, and the subsequent increase in ADP, to the appearance of the highly fluorescent form of NAD with alkali treatment. Basal, noninduced Na⁺/K⁺-ATPase activities (pmol/min/µg of protein) were: initial IMCD (IMCD_i; 0-50% of total length) 10.3 ± 1.00; middle IMCD (IMCD_m; 50–75%) 7.2 ± 0.55; terminal IMCD (IMCD_t; 75–100%) 6.4 ± 0.53. Aldosterone increased Na⁺/K⁺-ATPase activity in IMCD_i and IMCD_m monolayer cultures by 90 and 65 %, respectively, after 48 h of incubation. Dexamethasone increased the enzyme activity in IMCD_i and IMCD_m by 78 and 85%, respectively. These data indicate that IMCD subsegmental cell populations in monolayer culture express Na⁺/K⁺-ATPase activities corresponding to the in vivo profile of surface density of the basal plasma membrane area; they demonstrate that the enzyme activity is regulated in vitro by aldosterone and dexamethasone, particularly in the initial and upper terminal segments of the IMCD.