Loss of Biphasic Effect on Na/K-ATPase Activity by Angiotensin II Involves Defective Angiotensin Type 1 Receptor-Nitric Oxide Signaling

Oxidative stress causes changes in angiotensin (Ang) type 1 receptor (AT1R) function, which contributes to hypertension. Ang II affects blood pressure via maintenance of sodium homeostasis by regulating renal Na+ absorption through its effects on Na/K-ATPase (NKA). At low concentrations, Ang II stimulates NKA; higher concentrations inhibit the enzyme. We examined the effect of oxidative stress on renal AT1R function involved in biphasic regulation of NKA. Male Sprague-Dawley rats received tap water (control) and 30 mmol/L of L-buthionine sulfoximine (BSO), an oxidant, with and without 1 mmol/L of Tempol (antioxidant) for 2 weeks. BSO-treated rats exhibited increased oxidative stress, AT1R upregulation, and hypertension. In proximal tubules from control rats, Ang II exerted a biphasic effect on NKA activity, causing stimulation of the enzyme at picomolar and inhibition at micromolar concentrations. However, in BSO-treated rats, Ang II caused stimulation of NKA at both of the concentrations. The effect of Ang II was abolished by the AT1R antagonist candesartan and the mitogen-activated protein kinase inhibitor UO126, whereas the Ang type 2 receptor antagonist PD-123319 and NO synthase inhibitor NG-nitro-L-arginine methyl ester had no effect. The inhibitory effect of Ang II was sensitive to candesartan and NG-nitro-L-arginine methyl ester, whereas PD-123319 and UO126 had no effect. In BSO-treated rats, Ang II showed exaggerated stimulation of NKA, mitogen-activated protein kinase, proline-rich-tyrosine kinase 2, and NADPH oxidase but failed to activate NO signaling. Tempol reduced oxidative stress, normalized AT1R signaling, unmasked the biphasic effect on NKA, and reduced blood pressure in BSO-treated rats. In conclusion, oxidative stress-mediated AT1R upregulation caused a loss of NKA biphasic response and hypertension. Tempol normalized AT1R signaling and blood pressure. (Hypertension. 2008; 52: 1099-1105.)