Tempol reduces oxidative stress and restores renal dopamine D₁-like receptor- G protein coupling and function in hyperglycemic rats

Abstract

Dopamine via activation of renal D1-like receptors inhibits the activities of Na-K-ATPase and Na/H exchanger and subsequently increases sodium excretion. Decreased renal dopamine production and sodium excretion are associated with hyperglycemic conditions. We have earlier reported D1-like receptor-G protein uncoupling and reduced response to D1-like receptor activation in streptozotocin (STZ)-treated hyperglycemic rats (Marwaha A, Banday AA, and Lokhandwala MF. Am J Physiol Renal Physiol 286: F451-F457, 2004). The present study was designed to test the hypothesis that oxidative stress associated with hyperglycemia increases basal D1-like receptor serine phosphorylation via activation of the PKC-G protein receptor kinase (GRK) pathway, resulting in loss of D1-like receptor-G protein coupling and function. We observed that STZ-treated rats exhibited oxidative stress as evidenced by increased lipid peroxidation. Furthermore, PKC activity and expression of PKC-betaI- and -delta-isoforms were increased in STZ-treated rats. In addition, in STZ-treated rats there was increased GRK2 translocation to proximal tubular membrane and increased basal serine D1-like receptor phosphorylation. Supplementation with the antioxidant tempol lowered oxidative stress in STZ-treated rats, led to normalization of PKC activity, and prevented GRK2 translocation. Furthermore, tempol supplementation in STZ-treated rats restored D1-like receptor-G protein coupling and inhibition of Na-K-ATPase activity on D1-like receptor agonist stimulation. The functional consequence was the restoration of the natriuretic response to D1-like receptor activation. We conclude that oxidative stress associated with hyperglycemia causes an increase in activity and expression of PKC. This leads to translocation of GRK2, subsequent phosphorylation of the D1-like receptor, its uncoupling from G proteins and loss of responsiveness to agonist stimulation.

Oxidative stress reduces renal dopamine D1 receptor-Gq/11α G protein-phospholipase C signaling involving G protein-coupled receptor kinase 2

“…In addition, Rankin et al (31) have shown that GRKs can phosphorylate D1R in absence of agonist activation, resulting in constitutive desensitization. Our own studies involving animal models associated with oxidative stress such as obese Zucker rats, streptozotocin-treated rats, and Fisher 344 old rats as well as renal proximal tubular cultures exposed to insulin or H 2 O 2 showed that D1R were constitutively phosphorylated and uncoupled from G proteins (3,4,7,27). In addition, we found that treatment of these animals with antioxidants, while reducing oxidative stress, normalized GRK2 sequestration and receptor phosphorylation and restored D1R signaling (7,12,27).…”

“…In addition, we found that treatment of these animals with antioxidants, while reducing oxidative stress, normalized GRK2 sequestration and receptor phosphorylation and restored D1R signaling (7,12,27). Although these reports provide substantial evidence related to the role of GRKs in D1R hyperphosphorylation and subsequent desensitization, most of these studies were focused on D1R coupling to G s ␣ and AC activation (6,7,12,27). Numerous studies have demonstrated that renal D1R also mediates its functional effects through activation of the G q/11 ␣ protein and its downstream effector, phospholipase C (PLC) (13,14,40,48).…”

“…There is a remarkable parallel in the abnormal dopamine signaling, via D1R, and oxidative stress in hypertension and diabetes (6,7,26,27). White and Sidhu (42) have shown that in spontaneously hypertensive rats (SHR), the renal D1R-G protein coupling defect is contributed by increased oxidative stress.…”

“…Our own studies involving animal models associated with oxidative stress such as obese Zucker rats, streptozotocin-treated rats, and Fisher 344 old rats as well as renal proximal tubular cultures exposed to insulin or H 2 O 2 showed that D1R were constitutively phosphorylated and uncoupled from G proteins (3,4,7,27). In addition, we found that treatment of these animals with antioxidants, while reducing oxidative stress, normalized GRK2 sequestration and receptor phosphorylation and restored D1R signaling (7,12,27). Although these reports provide substantial evidence related to the role of GRKs in D1R hyperphosphorylation and subsequent desensitization, most of these studies were focused on D1R coupling to G s ␣ and AC activation (6,7,12,27).…”

Renal Redox Balance and Na+, K+-ATPase Regulation: Role in Physiology and Pathophysiology

“…A reactive oxygen species-associated defect in renal dopamine type 1 receptor function has been observed not only in experimental models of hypertension (Banday et al, 2008;Hussain et al, 1999;Hussain & Lokhandwala, 1997a;Hussain & Lokhandwala, 1997b) but also in diabetes (Banday et al, 2005;Marwaha & Lokhandwala, 2006) and ageing (Fardoun et al, 2006;Hussain et al, 1999;Vieira-Coelho et al, 1999). Failure of dopamine to modulate sodium reabsorption results in diminished natriuresis and blood pressure elevation.…”