Dopamine, which is synthesized in the kidney, independent of renal nerves, plays an important role in the regulation of fluid and electrolyte balance and systemic blood pressure. Lack of any of the five dopamine receptor subtypes (D1R, D2R, D3R, D4R, and D5R) results in hypertension. D1R, D2R, and D5R have been reported to be important in the maintenance of a normal redox balance. In the kidney, the antioxidant effects of these receptors are caused by direct and indirect inhibition of pro-oxidant enzymes, specifically, nicotinamide adenine dinucleotide phosphate, reduced form (NADPH) oxidase, and stimulation of anti-oxidant enzymes, which can also indirectly inhibit NADPH oxidase activity. Thus, stimulation of the D2R increases the expression of endogenous anti-oxidants, such as Parkinson protein 7 (PARK7 or DJ-1), paraoxonase 2 (PON2), and heme oxygenase 2 (HO-2), all of which can inhibit NADPH oxidase activity. The D5R decreases NADPH oxidase activity, via the inhibition of phospholipase D2, and increases the expression of HO-1, another antioxidant. D1R inhibits NADPH oxidase activity via protein kinase A and protein kinase C cross-talk. In this review, we provide an overview of the protective roles of a specific dopamine receptor subtype on renal oxidative stress, the different mechanisms involved in this effect, and the role of oxidative stress and impairment of dopamine receptor function in the hypertension that arises from the genetic ablation of a specific dopamine receptor gene in mice.

The D ₁ dopamine receptor (D ₁ R) is widely expressed in the kidney and plays a crucial role in blood pressure regulation. Although much is known about D ₁ R desensitization, especially through G‐protein‐coupled receptor kinase 4 (GRK4), comparatively little is known about other aspects of D ₁ R trafficking and the proteins involved in the process. We now report the discovery of a dynamic interaction between sorting nexin 5 (SNX5), a component of the mammalian retromer, and D ₁ R in human renal epithelial cells. We show that internalization of agonist‐activated D ₁ R is regulated by both SNX5 and GRK4, and that SNX5 is critical to the recycling of the receptor to the plasma membrane. SNX5 depletion increases agonist‐activated D ₁ R phosphorylation (>50% at basal condition), prevents D ₁ R internalization and cAMP response, and delays receptor recycling compared to mock siRNA‐transfected controls. Moreover, renal restricted subcapsular infusion of Snx5 ‐specific siRNA ( vs. mock siRNA) decreases sodium excretion (Δ= –0.2±0.005 mEq/mg creatinine) and further elevates the systolic blood pressure (Δ=48±5 mm Hg) in spontaneously hypertensive rats, indicating that SNX5 depletion impairs renal D ₁ R function. These studies demonstrate an essential role for SNX5 in regulating D ₁ R function, which may have important diagnostic, prognostic, and therapeutic implications in the management of essential hypertension.—Villar, V. A. M., Armando, I., Sanada, H., Frazer, L. C., Russo, C. M., Notario, P. M., Lee, H., Comisky, L., Russell, H. A., Yang, Y., Jurgens, J. A., Jose, P. A., Jones, J. E. Novel role of sorting nexin 5 in renal D ₁ dopamine receptor trafficking and function: implications for hypertension. FASEB J. 27, 1808–1819 (2013). www.fasebj.org

The peripheral dopaminergic system plays a crucial role in blood pressure regulation through its actions on renal hemodynamics and epithelial ion transport. The dopamine D5 receptor (D₅R) interacts with sorting nexin 1 (SNX1), a protein involved in receptor retrieval from the trans-Golgi network. In this report, we elucidated the spatial, temporal, and functional significance of this interaction in human renal proximal tubule cells and HEK293 cells stably expressing human D₅R and in mice. Silencing of SNX1 expression via RNAi resulted in the failure of D₅R to internalize and bind GTP, blunting of the agonist-induced increase in cAMP production and decrease in sodium transport, and up-regulation of angiotensin II receptor expression, of which expression was previously shown to be negatively regulated by D₅R. Moreover, siRNA-mediated depletion of renal SNX1 in C57BL/6J and BALB/cJ mice resulted in increased blood pressure and blunted natriuretic response to agonist in salt-loaded BALB/cJ mice. These data demonstrate a crucial role for SNX1 in D₅R trafficking and that SNX1 depletion results in D₅R dysfunction and thus may represent a novel mechanism for the pathogenesis of essential hypertension.

The dopamine D ₃ receptor (D ₃ R) is crucial in the regulation of blood pressure and sodium balance, in that Drd3 gene ablation in mice results in hypertension and failure to excrete a dietary salt load. The mechanism responsible for the renal sodium retention in these mice is largely unknown. We now offer and describe a novel mechanism by which D ₃ R decreases sodium transport in the long term by inhibiting the deubiquitinylating activity of ubiquitin‐specific peptidase 48 (USP48), thereby promoting Na ⁺ ‐H ⁺ exchanger (NHE)‐3 degradation. We found that stimulation with the D ₃ R‐specific agonist PD128907 (1 μM, 30 min) promoted the interaction and colocalization among D ₃ R, NHE3, and USP48; inhibited USP48 activity (–35±6%, vs. vehicle), resulting in increased ubiquitinylated NHE3 (+140±10%); and decreased NHE3 expression (–50 ±9%) in human renal proximal tubule cells (hRPTCs). USP48 silencing decreased NHE3's half‐life ( USP48 siRNA t _1/2 =6.1 h vs. vehicle t _1/2 =12.9 h), whereas overexpression of USP48 increased NHE3 half‐life ( t _1/2 =21.8 h), indicating that USP48 protects NHE3 from degradation via deubiquitinylation. USP48 accounted for ~30% of the total deubiquitinylating activity in these cells. Extending our studies in vivo,we found that pharmacologic blockade of D ₃ R via the D ₃ R‐specific antagonist GR103691 (1 μg/kg/min, 4 d) in C57Bl/6J mice increased renal NHE3 expression (+310±15%, vs. vehicle), whereas an innovative kidney‐restricted Usp48 silencing via siRNA (3 μg/d, 7 d) increased ubiquitinylated NHE3 (+250±30%, vs. controls), decreased total NHE3 (–23±2%), and lowered blood pressure (–24± 2 mm Hg), compared with that in control mice that received either the vehicle or nonsilencing siRNA. Our data demonstrate a crucial role for the dynamic interaction between D ₃ R and USP48 in the regulation of NHE3 expression and function.—Armando, I., Villar, V. A. M., Jones J. E., Lee, H., Wang, X., Asico L. D., Yu, P., Yang, J., Escano, C. S. Jr., Pascua‐Crusan, A. M., Felder, R. A., Jose, P. A. Dopamine D3 receptor inhibits the ubiquitin‐specific peptidase 48 to promote NHE3 degradation. FASEB J. 28, 1422–1434 (2014). www.fasebj.org

The homeostatic control of blood pressure hinges upon the delicate balance between prohypertensinogenic and antihypertensinogenic systems. D₁-like dopamine receptors [dopamine D₁and D₅receptors (D₁Rs and D₅Rs, respectively)] and the α_1A-adrenergic receptor (α_1A-AR) are expressed in the renal proximal tubule and engender opposing effects on Na⁺transport, i.e., natriuresis (via D₁Rs and D₅Rs) or antinatriuresis (via α_1A-ARs). We tested the hypothesis that the D₁R/D₅R regulates the α_1A-AR. D₁-like dopamine receptors coimmunoprecipitated, colocalized, and cofractionated with α_1A-ARs in lipid rafts in immortalized human renal proximal tubule cells. Long-term treatment with the D₁R/D₅R agonist fenoldopam resulted in decreased D₁R and D₅R expression but increased α_1A-AR abundance in the plasma membrane. Short-term fenoldopam treatment stimulated the translocation of Na⁺-K⁺-ATPase from the plasma membrane to the cytosol that was partially reversed by an α_1A-AR agonist, which by itself induced Na⁺-K⁺-ATPase translocation from the cytosol to the plasma membrane. The α_1A-AR-specific agonist A610603 also minimized the ability of fenoldopam to inhibit Na⁺-K⁺-ATPase activity. To determine the interaction among D₁Rs, D₅Rs, and α_1A-ARs in vivo, we used phenylephrine and A610603 to decrease Na⁺excretion in several D₁-like dopamine receptor knockout mouse strains. Phenylephrine and A61603 treatment resulted in a partial reduction of urinary Na⁺excretion in wild-type mice and its abolition in D₁R knockout, D₅R knockout, and D1R-D5R double-knockout mice. Our results demonstrate the ability of the D₁-like dopamine receptors to regulate the expression and activity of α_1A-AR. Elucidating the intricacies of the interaction among these receptors is crucial for a better understanding of the crosstalk between anti- and pro-hypertensive systems.