Biochemical and Immunological Characterization of Protein Kinase C Isozymes in Rat Kidney Proximal Tubule: Effect of Angiotensin II

Abstract

The rat proximal tubule is the site for reabsorption of 75% of the normal filtered load of bicarbonate. This luminal acidification results from the following cascade of events: filtered HCO₃- binds a H⁺ to give H₂CO₃. Proton issued from hydration of cellular CO₂ is secreted into the tubular fluid (mainly via the apical Na/H antiporter) and combines to filtered HCO₃- to give H₂CO₃. H₂CO₃ is converted into CO₂ and H₂O as rapidly as it is formed, because of the presence of apical carbonic anhydrase in contact with the tubular fluid. Bicarbonate generated in the cell also from hydration of CO₂ is transported into fluid via the basolateral Na⁺-HCO₃- cotransporter. Peptide hormones are potent regulators of bicarbonate reabsorption in this nephron segment. AngiotensiniII (AII) stimulates it by increasing both Na⁺/H⁺ antiporter and Na⁺-HCO₃- cotransporter activities. We have recently shown that AII receptors were coupled to the inositol phosphate-Ca⁺⁺ signaling pathway (Poggioli et al., 1992) and to well known cAMP pathway (Woodcock and Johnson 1982). Thus, our results indicated that AII was able to activate the Ca⁺⁺-phospholipid -dependent protein kinase C (PKC, Nishizuka, 1988). Both functional and biochemical approaches suggested that the effect of AII on transepithelial transport was mediated, at least in part, through PKC activation. In vivo microperfusion experiments have shown that this AII effect was reproduced by phorbol esters and inhibited by PKC inhibitor H7 (Liu and Cogan,1990; Wang and Chan,1991). Weinman et al. (1989) demonstrated on brush border membrane vesicles that an increase in the activity of the luminal Na⁺/ H⁺ antiporter resulted from a PKC-mediated phosphorylation. Ruiz and Arruda (1992) reported on basolateral membrane vesicles that an increase in Na⁺-HCO₃- cotransporter activity resulted from a PKC-mediated phosphorylation too.