Overexpression of PKC-βI and -δ contributes to higher PKC activity in the proximal tubules of old Fischer 344 rats

Abstract

Previously, we reported that natriuretic and diuretic response to dopamine is diminished in old Fischer 344 rats, which is due to higher basal protein kinase C (PKC) activity and hyperphosphorylation of Na-K-ATPase in the proximal tubules (PTs) of old rats. The present study was conducted to determine whether higher PKC activity could be due to altered expression of some of the PKC isoforms in the superficial cortex (rich in PTs) of old rats. Fluorimetric measurement showed almost twofold increase in the PKC activities in homogenates and membranes of old (24 mo) compared with adult (6 mo) rats. Interestingly, in the basal state PKC-βI was overexpressed in the membranes, whereas PKC-δ expression was increased in the cytosol of old compared with adult rats. Treatment of the cortical slices with either SKF-38393, a D1-like agonist, or PDBu, a direct activator of PKC, caused translocation of PKC-βI from cytosol to membranes in adult but not in old rats. Both of these drugs caused translocation of PKC-δ from membranes to cytosol in adult but not in old rats. These drugs had no effect on translocation of PKC-ζ in both adult and old rats. Both PKC-βI and -δ coimmunoprecipiated with α1-subunit of Na-K-ATPase in adult and old rats. These observations suggest that both SKF-38393 and PDBu differentially regulate PKC-βI and -δ in adult but not in old rats. Also, PKC-βI and -δ seem to interact with Na-K-ATPase in these animals. The overexpression of both PKC-βI and -δ in old rats could be responsible for a higher basal PKC activity, which causes the hyperphosphorylation of Na-K-ATPase and contributes to the diminished inhibition of Na-K-ATPase activity by dopamine in old rats.