Angiotensin II (Ang II) Increases Phosphorylation of Rat Kidney Na, K‐Pump at S938 in Proximal Tubule (PCT) Cells through a Phosphatidylinositol 3′‐Kinase (PI3K)/Akt Pathway Facilitated via Epac

Abstract

To understand how Ang II increases sodium reabsorption in PCT cells and thereby increases blood pressure, we earlier demonstrated that Ang II directly stimulates rat kidney Na, K‐pump activity by increasing phosphorylation of the Na, K‐pump at S938. Phosphorylation at S938, a site conserved in human kidney Na, K‐pump, was initially discovered under in vitro conditions to be mediated by protein kinase A (PKA). Nevertheless, when rat Na, K‐pump is stably co‐expressed with the AT1a receptor in opossum kidney (OK) cells, a PCT cell line, Ang II‐dependent increases in S938 phosphorylation were not blocked by H‐89, an inhibitor of PKA. Ang II‐dependent increases in phosphorylation at S938, as measured by a new antibody to phosphorylated S938, were instead blocked by wortmannin, a selective inhibitor of PI3K. We also found that Ang II activates Akt, likely via PI3K. In addition, we discovered that phosphorylation at S938 was increased by 8‐CPT‐2Me‐cAMP, an analog of cAMP that selectively activates Epac, but not PKA. We also showed that Akt is activated by Epac. Thus, we propose that Ang II acutely regulates Na, K‐pump activity in PCT cells via S938 and a novel Akt signaling pathway.