Nicolás Martín Kouyoumdzian

Renal dopaminergic system: Pathophysiological implications and clinical perspectives

Fluid homeostasis, blood pressure and redox balance in the kidney are regulated by an intricate interaction between local and systemic anti-natriuretic and natriuretic systems. Intrarenal dopamine plays a central role on this interactive network. By activating specific receptors, dopamine promotes sodium excretion and stimulates anti-oxidant and anti-inflammatory pathways. Different pathological scenarios where renal sodium excretion is dysregulated, as in nephrotic syndrome, hypertension and renal inflammation, can be associated with impaired action of renal dopamine including alteration in biosynthesis, dopamine receptor expression and signal transduction. Given its properties on the regulation of renal blood flow and sodium excretion, exogenous dopamine has been postulated as a potential therapeutic strategy to prevent renal failure in critically ill patients. The aim of this review is to update and discuss on the most recent findings about renal dopaminergic system and its role in several diseases involving the kidneys and the potential use of dopamine as a nephroprotective agent.

Effects of chronic fructose overload on renal dopaminergic system: alteration of urinary L-dopa/dopamine index correlates to hypertension and precedes kidney structural damage

Insulin resistance induced by a high-fructose diet has been associated to hypertension and renal damage. The aim of this work was to assess alterations in the urinary L-dopa/dopamine ratio over three time periods in rats with insulin resistance induced by fructose overload and its correlation with blood pressure levels and the presence of microalbuminuria and reduced nephrin expression as markers of renal structural damage. Male Sprague-Dawley rats were randomly divided into six groups: control (C) (C4, C8 and C12) with tap water to drink and fructose-overloaded (FO) rats (FO4, FO8 and FO12) with a fructose solution (10% w/v) to drink for 4, 8 and 12 weeks. A significant increase of the urinary L-dopa/dopamine ratio was found in FO rats since week 4, which positively correlated to the development of hypertension and preceded in time the onset of microalbuminuria and reduced nephrin expression observed on week 12 of treatment. The alteration of this ratio was associated to an impairment of the renal dopaminergic system, evidenced by a reduction in renal dopamine transporters and dopamine D1 receptor expression, leading to an overexpression and overactivation of the enzyme Na+, K+-ATPase with sodium retention. In conclusion, urinary L-dopa/dopamine ratio alteration in rats with fructose overload positively correlated to the development of hypertension and preceded in time the onset of renal structural damage. This is the first study to propose the use of the urinary L-dopa/dopamine index as marker of renal dysfunction that temporarily precedes kidney structural damage induced by fructose overload.

Atrial Natriuretic Peptide Stimulates Dopamine Tubular Transport by Organic Cation Transporters: A Novel Mechanism to Enhance Renal Sodium Excretion.

The aim of this study was to demonstrate the effects of atrial natriuretic peptide (ANP) on organic cation transporters (OCTs) expression and activity, and its consequences on dopamine urinary levels, Na+, K+-ATPase activity and renal function. Male Sprague Dawley rats were infused with isotonic saline solution during 120 minutes and randomized in nine different groups: control, pargyline plus tolcapone (P+T), ANP, dopamine (DA), D-22, DA+D-22, ANP+D-22, ANP+DA and ANP+DA+D-22. Renal functional parameters were determined and urinary dopamine concentration was quantified by HPLC. Expression of OCTs and D1-receptor in membrane preparations from renal cortex tissues were determined by western blot and Na+, K+-ATPase activity was determined using in vitro enzyme assay. 3H-DA renal uptake was determined in vitro. Compared to P+T group, ANP and dopamine infusion increased diuresis, urinary sodium and dopamine excretion significantly. These effects were more pronounced in ANP+DA group and reversed by OCTs blockade by D-22, demonstrating that OCTs are implied in ANP stimulated-DA uptake and transport in renal tissues. The activity of Na+, K+-ATPase exhibited a similar fashion when it was measured in the same experimental groups. Although OCTs and D1-receptor protein expression were not modified by ANP, OCTs-dependent-dopamine tubular uptake was increased by ANP through activation of NPR-A receptor and protein kinase G as signaling pathway. This effect was reflected by an increase in urinary dopamine excretion, natriuresis, diuresis and decreased Na+, K+-ATPase activity. OCTs represent a novel target that links the activity of ANP and dopamine together in a common mechanism to enhance their natriuretic and diuretic effects.

Signaling pathways involved in renal oxidative injury: role of the vasoactive peptides and the renal dopaminergic system.

The physiological hydroelectrolytic balance and the redox steady state in the kidney are accomplished by an intricate interaction between signals from extrarenal and intrarenal sources and between antinatriuretic and natriuretic factors. Angiotensin II, atrial natriuretic peptide and intrarenal dopamine play a pivotal role in this interactive network. The balance between endogenous antioxidant agents like the renal dopaminergic system and atrial natriuretic peptide, by one side, and the prooxidant effect of the renin angiotensin system, by the other side, contributes to ensuring the normal function of the kidney. Different pathological scenarios, as nephrotic syndrome and hypertension, where renal sodium excretion is altered, are associated with an impaired interaction between two natriuretic systems as the renal dopaminergic system and atrial natriuretic peptide that may be involved in the pathogenesis of renal diseases. The aim of this review is to update and comment the most recent evidences about the intracellular pathways involved in the relationship between endogenous antioxidant agents like the renal dopaminergic system and atrial natriuretic peptide and the prooxidant effect of the renin angiotensin system in the pathogenesis of renal inflammation.

Losartan prevents the imbalance between renal dopaminergic and renin angiotensin systems induced by fructose overload. l-Dopa/dopamine index as new potential biomarker of renal dysfunction

BACKGROUND The renin angiotensin system (RAS) and the renal dopaminergic system (RDS) act as autocrine and paracrine systems to regulate renal sodium management and inflammation and their alterations have been associated to hypertension and renal damage. Nearly 30-50% of hypertensive patients have insulin resistance (IR), with a strong correlation between hyperinsulinemia and microalbuminuria. OBJECTIVE The aim of this study was to demonstrate the existence of an imbalance between RAS and RDS associated to IR, hypertension and kidney damage induced by fructose overload (FO), as well as to establish their prevention, by pharmacological inhibition of RAS with losartan. MATERIALS/METHODS Ninety-six male Sprague-Dawley rats were randomly divided into four groups and studied at 4, 8 and 12 weeks: control group (C4, C8 and C12; tap water to drink); fructose-overloaded group (F4, F8 and F12; 10% w/v fructose solution to drink); losartan-treated control (L) group (L4, L8 and L12; losartan 30 mg/kg/day, in drinking water); and fructose-overloaded plus losartan group (F + L4, F + L8 and F + L12, in fructose solution). RESULTS FO induced metabolic and hemodynamic alterations as well as an imbalance between RAS and RDS, characterized by increased renal angiotensin II levels and AT1R overexpression, reduced urinary excretion of dopamine, increased excretion of l-dopa (increased l-dopa/dopamine index) and down-regulation of D1R and tubular dopamine transporters OCT-2, OCT-N1 and total OCTNs. This imbalance was accompanied by an overexpression of renal tubular Na+, K+-ATPase, pro-inflammatory (NF-kB, TNF-α, IL-6) and pro-fibrotic (TGF-β1 and collagen) markers and by renal damage (microalbuminuria and reduced nephrin expression). Losartan prevented the metabolic and hemodynamic alterations induced by FO from week 4. Increased urinary l-dopa/dopamine index and decreased D1R renal expression associated to FO were also prevented by losartan since week 4. The same pattern was observed for renal expression of OCTs/OCTNs, Na+, K+-ATPase, pro-inflammatory and pro-fibrotic markers from week 8. The appearance of microalbuminuria and reduced nephrin expression was prevented by losartan at week 12. CONCLUSION The results of this study provide new insight regarding the mechanisms by which a pro-hypertensive and pro-inflammatory system, such as RAS, downregulates another anti-hypertensive and anti-inflammatory system such as RDS. Additionally, we propose the use of l-dopa/dopamine index as a biochemical marker of renal dysfunction in conditions characterized by sodium retention, IR and/or hypertension, and as a predictor of response to treatment and follow-up of these processes.

Immunohistochemical expression of intrarenal renin angiotensin system components in response to tempol in rats fed a high salt diet

AIM To determine the effect of tempol in normal rats fed high salt on arterial pressure and the balance between antagonist components of the renal renin-angiotensin system. METHODS Sprague-Dawley rats were fed with 8% NaCl high-salt (HS) or 0.4% NaCl (normal-salt, NS) diet for 3 wk, with or without tempol (T) (1 mmol/L, administered in drinking water). Mean arterial pressure (MAP), glomerular filtration rate (GFR), and urinary sodium excretion (UVNa) were measured. We evaluated angiotensin II (Ang II), angiotensin 1-7 (Ang 1-7), angiotensin converting enzyme 2 (ACE2), mas receptor (MasR), angiotensin type 1 receptor (AT1R) and angiotensin type 2 receptor (AT2R) in renal tissues by immunohistochemistry. RESULTS The intake of high sodium produced a slight but significant increase in MAP and differentially regulated components of the renal renin-angiotensin system (RAS). This included an increase in Ang II and AT1R, and decrease in ACE-2 staining intensity using immunohistochemistry. Antioxidant supplementation with tempol increased natriuresis and GFR, prevented changes in blood pressure and reversed the imbalance of renal RAS components. This includes a decrease in Ang II and AT1R, as increase in AT2, ACE2, Ang (1-7) and MasR staining intensity using immunohistochemistry. In addition, the natriuretic effects of tempol were observed in NS-T group, which showed an increased staining intensity of AT2, ACE2, Ang (1-7) and MasR. CONCLUSION These findings suggest that a high salt diet leads to changes in the homeostasis and balance between opposing components of the renal RAS in hypertension to favour an increase in Ang II. Chronic antioxidant supplementation can modulate the balance between the natriuretic and antinatriuretic components of the renal RAS.

Urodilatin regulates renal dopamine metabolism

BACKGROUND Sodium and water transport across renal proximal tubules is regulated by diverse hormones such as dopamine and urodilatin. We have previously reported that urodilatin stimulates extraneuronal dopamine uptake in external renal cortex by activation of the type A natriuretic peptide receptor, coupled to cyclic guanylate monophosphate signaling and protein kinase G. Moreover, urodilatin enhances dopamine-induced inhibition of Na+, K+-ATPase activity in renal tubules. The aim of the present study was to evaluate whether urodilatin could also alter renal dopamine synthesis, release, catabolism and turnover. METHODS The effects of urodilatin on dopamine synthesis, release, catabolism and turnover were measured in samples of renal cortex from Sprague Dawley rats. RESULTS The results indicate that urodilatin increases L-DOPA decarboxylase activity and decreases catechol-o-methyl transferase and monoamine oxidase activity. Moreover, urodilatin does not affect either dopamine basal secretion or potassium chloride-induced dopamine release in external renal cortex, and reduces amine turnover. CONCLUSIONS Both the present results and previous findings show that urodilatin modifies dopamine metabolism in external renal cortex of rats by enhancing dopamine uptake and synthesis and by decreasing catechol-o-methyl transferase and monoamine oxidase activity and dopamine turnover. Those effects taken together may favor dopamine accumulation in renal cells and increase its endogenous content and availability. This would permit D1 receptor recruitment and stimulation and, in turn, overinhibition of Na+, K+-ATPase activity, which results in decreased sodium reabsorption. Therefore, urodilatin and dopamine enhance natriuresis and diuresis through a common pathway.

LOSARTAN NORMALIZES BLOOD PRESSURE AND PREVENTS RENAL DAMAGE AND INFLAMMATION INDUCED BY FRUCTOSE OVERLOAD. L-DOPA/DOPAMINE INDEX AS A NEW POTENTIAL BIOMARKER OF RENAL DAMAGE

Objective: The renin angiotensin system (RAS) and the renal dopaminergic system (RDS) act as autocrine and paracrine systems to regulate renal sodium management and inflammation, and their alterations have been associated to hypertension and renal damage. Nearly 30–50% of hypertensive patients have insulin resistance (IR), which has a strong correlation to microalbuminuria. The aim of this study was to evaluate the effects of RAS blockade with losartan on blood pressure and renal damage in a model of IR produced by fructose overload (FO), and its association to changes in the RDS. Finally, we studied the urinary L-dopa/dopamine index as a potential biomarker of renal dysfunction. Design and method: Male Sprague Dawley rats were divided into: Control (C, tap water), FO (10% w/v of fructose solution), Losartan (L, 30 mg/kg/day in tap water), FO + L (30 mg/kg/day in fructose solution) groups for 4, 8 and 12 weeks. Systolic blood pressure (SBP) and metabolic parameters were measured. Urinary L-dopa and dopamine, diuresis, natriuresis and microalbuminuria were determined. Renal expression of D1 receptor (D1R), pro-inflammatory markers (IL-6, TNF-alpha, TGF-beta1, angiotensin II [Ang II]) and Na + ,K + -ATPase expression and activity were measured. Results: Losartan prevented the increase in SBP and Na + ,K + -ATPase activity and the reduction in natriuresis induced by FO from week 4 (p < 0.05). Increased L-dopa/dopamine index and decreased D1R expression in FO rats were prevented by losartan since week 4 (p < 0.05). The same pattern was observed for renal expression of Na + ,K + -ATPase, IL-6, TNF-alpha and TGF-beta1 since week 8 (p < 0.05), with no changes in Ang II. FO was associated with the appearance of microalbuminuria at week 12, effect prevented by losartan (p < 0.001). Conclusions: These results provide the mechanisms by which a prohypertensive and proinflammatory system, such as RAS, downregulates another antihypertensive and antiinflammatory system such as RDS, establishing a positive feedback loop that leads to hypertension and kidney inflammation due to FO. Furthermore, we demonstrated the potential usefulness of the L-dopa/dopamine index as a biochemical marker of renal dysfunction, earlier than microalbuminuria, and as a predictor of treatment response and follow-up of hypertension and kidney damage.

Regulation of Dopamine Uptake by Vasoactive Peptides in the Kidney

Considering the key role of renal dopamine in tubular sodium handling, we hypothesized that c-type natriuretic peptide (CNP) and Ang-(1-7) may regulate renal dopamine availability in tubular cells, contributing to Na+, K+-ATPase inhibition. Present results show that CNP did not affect either 3H-dopamine uptake in renal tissue or Na+, K+-ATPase activity; meanwhile, Ang-(1-7) was able to increase 3H-dopamine uptake and decreased Na+, K+-ATPase activity in renal cortex. Ang-(1-7) and dopamine together decreased further Na+, K+-ATPase activity showing an additive effect on the sodium pump. In addition, hydrocortisone reversed Ang-(1-7)-dopamine overinhibition on the enzyme, suggesting that this inhibition is closely related to Ang-(1-7) stimulation on renal dopamine uptake. Both anantin and cANP (4-23-amide) did not modify CNP effects on 3H-dopamine uptake by tubular cells. The Mas receptor antagonist, A-779, blocked the increase elicited by Ang-(1-7) on 3H-dopamine uptake. The stimulatory uptake induced by Ang-(1-7) was even more pronounced in the presence of losartan, suggesting an inhibitory effect of Ang-(1-7) on AT1 receptors on 3H-dopamine uptake. By increasing dopamine bioavailability in tubular cells, Ang-(1-7) enhances Na+, K+-ATPase activity inhibition, contributing to its natriuretic and diuretic effects.

Adverse effects of tempol on hidrosaline balance in rats with acute sodium overload

We studied the effects of tempol, an oxygen radical scavenger, on hydrosaline balance in rats with acute sodium overload. Male rats with free access to water were injected with isotonic (control group) or hypertonic saline solution (0.80 mol/l NaCl) either alone (Na group) or with tempol (Na-T group). Hydrosaline balance was determined during a 90 min experimental period. Protein expressions of aquaporin 1 (AQP1), aquaporin 2 (AQP2), angiotensin II (Ang II) and endothelial nitric oxide synthase (eNOS) were measured in renal tissue. Water intake, creatinine clearance, diuresis and natriuresis increased in the Na group. Under conditions of sodium overload, tempol increased plasma sodium and protein levels and increased diuresis, natriuresis and sodium excretion. Tempol also decreased water intake without affecting creatinine clearance. AQP1 and eNOS were increased and Ang II decreased in the renal cortex of the Na group, whereas AQP2 was increased in the renal medulla. Nonglycosylated AQP1 and eNOS were increased further in the renal cortex of the Na-T group, whereas AQP2 was decreased in the renal medulla and was localized mainly in the cell membrane. Moreover, p47-phox immunostaining was increased in the hypothalamus of Na group, and this increase was prevented by tempol. Our findings suggest that tempol causes hypernatremia after acute sodium overload by inhibiting the thirst mechanism and facilitating diuresis, despite increasing renal eNOS expression and natriuresis.