ADRENOCORTICOID REGULATION OF NA+,K+-ATPASE IN ADULT-RAT KIDNEY - EFFECTS ON POSTTRANSLATIONAL PROCESSING AND MESSENGER-RNA ABUNDANCE

The mechanisms by which adreno-corticoid hormones regulate Na+,K+-ATPase in adult kidney were studied in adrenalectomized (Adx) rats. Five days after adrenalectomy, Na+,K+-ATPase activity was significantly reduced in the renal cortex homogenate (C = 13.0 +/- 0.8 vs. Adx = 7.1 +/- 0.7-mu-mol Pi mg-1 protein h-1) and in renal microsomes (C = 30.3 +/- 1.9 vs Adx = 14.6 +/- 1.3-mu-mol Pi mg-1 protein h-1). Glucocorticoid replacement treatment of adrenalectomized rats with betamethasone (20-mu-g kg-1 body wt twice daily for 5 days) effectively counteracted the observed reduction in Na+,K+-ATPase activity. In cortical homogenate the protein level of alpha-1 and beta-1 subunits measured in immunoblots was not significantly different in Adx and control rats, indicating that 5 days after adrenalectomy the alpha-1 and beta-1 subunits were present in renal cortical cells to almost normal extent but could not be assembled into a transmembrane functional unit. In support of this conclusion we found that the protein level of both the alpha-1 and beta-1 subunits was significantly lower (P < 0.001 for both subunits) in microsomes from Adx than in control rats. The mRNA abundance for alpha-1 and beta-1 subunits were not lower in Adx as compared to control rats 1 and 5 days after surgery. However, if Adx rats were given a single dose of betamethasone (600-mu-g kg-1 body wt), a significant 2-fold increase in both alpha-1 and beta-1 mRNAs was observed (P < 0.05 for both subunits). These data suggest that glucocorticoids can upregulate the mRNA of both Na+,K+-ATPase subunits but that the low renal Na+,K+-ATPase activity in adult Adx rats is mainly due to loss of glucocorticoid regulation of the post-translational processing of the enzyme.