Protein kinase C-mediated inhibition of renal Ca2+ ATPase by physiological concentrations of angiotensin II is reversed by AT1- and AT2-receptor antagonists.

Abstract Angiotensin II (Ang II) increases the cytosolic Ca 2+ concentration in different cell types. In this study, we investigate the effect of Ang II on the Ca 2+ ATPase of purified basolateral membranes of kidney proximal tubules. This enzyme pumps Ca 2+ out of the cytosol in a reaction coupled to ATP hydrolysis, and it is responsible for the fine-tuned regulation of cytosolic Ca 2+ activity. Ca 2+ -ATPase activity is inhibited by picomolar concentrations of Ang II, with maximal inhibition being attained at ≈50% of the control values. The presence of raising concentrations (10 −11 to 10 −7 M) of losartan (an AT 1 -receptor antagonist) or PD123319 (an AT 2 -receptor antagonist) gradually reverts inhibition by Ang II. Both the phospholipase C (PLC) inhibitor U-73122 (10 −6 M) and the inhibitor of protein kinase C (PKC) staurosporine (10 −7 M) prevent inhibition of the Ca 2+ pump by Ang II. Incubation of the previously isolated membranes with a PKC activator—the phorbol ester 12- O -tetradecanoylphorbol-13-acetate (10 −8 M)—mimics the inhibition found with Ang II, and the effects of the compounds are not additive. Taken as a whole, these results indicate the Ang II inhibits Ca 2+ -ATPase by activation of a PKC system present in primed state in these membranes after binding of the hormone to losartan- and PD123319-sensitive receptors coupled to a PLC. Therefore, inhibition of the basolateral membrane Ca 2+ -ATPase by kinase-mediated phosphorylation appears to be one of the pathways by which Ang II promotes an increase in the cytosolic Ca 2+ concentration of proximal tubule cells.