Glucocorticoid inhibition of Na-Pi cotransport in renal epithelial cells is mediated by protein kinase C.

The effect and mechanism of action of glucocorticoids (GC) on Na-Pi cotransport were evaluated in opossum kidney cells. Dexamethasone (1-1000 nM) inhibited sodium-dependent Pi uptake in a time- and concentration-dependent manner. Inhibition was maximal after a 6-h incubation with dexamethasone and was prevented by cycloheximide and actinomycin D. The effect was related to a 37% decrease of the Vmax value after incubation with 100 nM dexamethasone. The effect of dexamethasone was mimicked by cortisol and blocked by GC receptor antagonists RU38486 and progesterone. GC affected neither glucose or alanine uptake nor Na/H exchange activity. Inhibition of Pi uptake persisted when Na/H was blocked by amiloride or dimethylamiloride. GC had no effect on basal or parathyroid hormone- and forskolin-stimulated intracellular cAMP content. Dexamethasone and extracellular cAMP, parathyroid hormone, or 3-isobutyl-1-methylxanthine had additive inhibitory effects on Pi uptake. Staurosporine, GF109203X, or calphostin C (three dissimilar inhibitors of protein kinase C (PKC)) and PKC down-regulation blunted the inhibitory effect of glucocorticoids on Pi uptake. GC increased both membrane-bound PKC activity and the membrane/cytosol PKC activity ratio. This is the first report of GC activation of PKC in renal cells, which appears to mediate the steroid inhibitory effect on Pi transport.