Clinical and experimental data show an increase in sodium reabsorption on the proximal tubule (PT) in essential hypertension. It is well known that there is a link between essential hypertension and renal angiotensin II (Ang II). The present study was designed to examine ouabain-insensitive Na(+)-ATPase activity and its regulation by Ang II in spontaneously hypertensive rats (SHR). We observed that Na(+)-ATPase activity was enhanced in 14-week-old but not in 6-week-old SHR. The addition of Ang II from 10(-12) to 10(-6) mol/L decreased the enzyme activity in SHR to a level similar to that obtained in WKY. The Ang II inhibitory effect was completely reversed by a specific antagonist of AT(2) receptor, PD123319 (10(-8) mol/L) indicating that a system leading to activation of the enzyme in SHR is inhibited by AT(2)-mediated Ang II. Treatment of SHR with losartan for 10 weeks (weeks 4-14) prevents the increase in Na(+)-ATPase activity observed in 14-week-old SHR. These results indicate a correlation between AT(1) receptor activation in SHR and increased ouabain-insensitive Na(+)-ATPase activity. Our results open new possibilities towards our understanding of the pathophysiological mechanisms involved in the increased sodium reabsorption in PT found in essential hypertension.

Angiotensin II (Ang II) stimulates the proximal tubule Na(+)-ATPase through the AT(1) receptor/phosphoinositide phospholipase Cbeta (PI-PLCbeta)/protein kinase C (PKC) pathway. However, this pathway alone does not explain the sustained effect of Ang II on Na(+)-ATPase activity for 30 min. The aim of the present work was to elucidate the molecular mechanisms involved in the sustained effect of Ang II on Na(+)-ATPase activity. Ang II induced fast and correlated activation of Na(+)-ATPase and PKC activities with the maximal effect (115%) observed at 1 min and sustained for 30 min, indicating a pivotal role of PKC in the modulation of Na(+)-ATPase by Ang II. We observed that the sustained activation of PKC by Ang II depended on the sequential activation of phospholipase D and Ca(2+)-insensitive phospholipase A(2), forming phosphatidic acid and lysophosphatidic acid, respectively. The results indicate that PKC could be the final target and an integrator molecule of different signaling pathways triggered by Ang II, which could explain the sustained activation of Na(+)-ATPase by Ang II.

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Previous data showed that prostaglandin E₂ (PGE₂) mediates the inhibitory effect of bradykinin (BK) on proximal tubule (PT) Na+-ATPase activity. The aim of this work was to investigate the molecular mechanisms involved in the effect of PGE₂ on PT Na+-ATPase. We used isolated basolateral membrane (BLM) from pig PT, which expresses several components of different signaling pathways. The inhibitory effect of PGE₂ on PT Na+-ATPase activity involves G-protein and the activation of protein kinase A (PKA) because: (1) PGE₂ increased [³⁵S]GTPγS binding; (2) GDPβS abolished the inhibitory effect of PGE₂; (3) PGE₂ increased PKA activity; (4) the inhibitory effect of PGE₂ was abolished by PKA inhibitor peptide. We observed that the PKA-mediated inhibitory effect of PGE₂ on PT Na+-ATPase activity requires previous activation of protein kinase C. In addition, we observed that PGE₂ stimulates Ca²+-independent phospholipase A₂ activity representing an important positive feedback to maintain the inhibition of the enzyme. These results open new perspectives to understanding the mechanism involved in the effect of PGE₂ on proximal tubule sodium reabsorption.