Decreased urinary dopamine excretion and disturbed dopamine/sodium relationship in type 1 diabetes mellitus

Dopamine and renal function and blood pressure regulation

Dopamine is an important regulator of systemic blood pressure via multiple mechanisms. It affects fluid and electrolyte balance by its actions on renal hemodynamics and epithelial ion and water transport and by regulation of hormones and humoral agents. The kidney synthesizes dopamine from circulating or filtered L-DOPA independently from innervation. The major determinants of the renal tubular synthesis/release of dopamine are probably sodium intake and intracellular sodium. Dopamine exerts its actions via two families of cell surface receptors, D1-like receptors comprising D1R and D5R, and D2-like receptors comprising D2R, D3R, and D4R, and by interactions with other G protein-coupled receptors. D1-like receptors are linked to vasodilation, while the effect of D2-like receptors on the vasculature is variable and probably dependent upon the state of nerve activity. Dopamine secreted into the tubular lumen acts mainly via D1-like receptors in an autocrine/paracrine manner to regulate ion transport in the proximal and distal nephron. These effects are mediated mainly by tubular mechanisms and augmented by hemodynamic mechanisms. The natriuretic effect of D1-like receptors is caused by inhibition of ion transport in the apical and basolateral membranes. D2-like receptors participate in the inhibition of ion transport during conditions of euvolemia and moderate volume expansion. Dopamine also controls ion transport and blood pressure by regulating the production of reactive oxygen species and the inflammatory response. Essential hypertension is associated with abnormalities in dopamine production, receptor number, and/or posttranslational modification.

Glycaemic control with insulin prevents the reduced renal dopamine D1 receptor expression and function in streptozotocin-induced diabetes

BACKGROUND

It was demonstrated in streptozotocin (STZ)-induced diabetic rats that the D(1) receptor agonist failed to promote sodium excretion as a result of reduced renal D(1) receptor expression and decreased receptor G protein coupling. The present study examined the influence of glycaemic control with insulin on the renal D(1) receptor dysfunction in STZ-induced type 1 diabetes.

METHODS

Renal function, blood pressure, the natriuretic response to 5% volume expansion (VE) and the effects of the D(1) receptor agonist fenoldopam on natriuresis and on Na(+)/K(+)-ATPase activity in renal tubules were evaluated in uninephrectomized and sham-operated Wistar rats treated with STZ and compared with controls and STZ-treated rats made euglycaemic with insulin. D(1) receptor immunohistochemistry and protein abundance by western blot were also determined in all groups.

RESULTS

Treatment of sham and uninephrectomized rats with STZ caused a 4-fold increase in glucose plasma levels compared to controls and euglycaemic diabetic rats. A blunted natriuretic response to VE was observed in both sham and uninephrectomized hyperglycaemic diabetic rats, and this was accompanied by failure of fenoldopam to increase natriuresis and to inhibit renal Na(+)/K(+)-ATPase activity. In contrast, in both sham and uninephrectomized euglycaemic diabetic rats, the natriuretic response to VE, the fenoldopam-induced natriuresis and the accompanied inhibition of Na(+)/K(+)-ATPase activity were similar to those of the corresponding controls. D(1) receptor immunodetection and protein abundance were reduced in hyperglycaemic diabetic rats, but not in euglycaemic diabetic animals.

CONCLUSIONS

We conclude that the renal expression and natriuretic response to D(1) receptor activation is compromised in both sham and uninephrectomized rats with STZ-induced diabetes. These abnormalities were prevented by lowering glucose blood levels with insulin, thus providing evidence for the involvement of hyperglycaemia in the disturbances that underlie the compromised dopamine-sensitive natriuresis and increase of blood pressure in type 1 diabetes.

Fenoldopam treatment improves peripheral insulin sensitivity and renal function in STZ-induced type 2 diabetic rats

Dopamine and diabetes mellitus are reported to have close link between them. We have studied the effect of six-week treatment with D1 receptor agonist fenoldopam (1 mg/kg, i.p., daily) on glucose, lipid, and renal profile in streptozotocin (STZ)-induced (non-insulin dependent) type 2 diabetic rats. Streptozotocin (90 mg/kg, i.p.) was injected to two day old Sprague-Dawley pups. Streptozotocin produced hyperglycemia, hyperinsulinemia, hyperlipidemia, hypertension, increase in serum urea and creatinine by the time animals were 10 week old. Treatment with fenoldopam significantly decreased serum glucose, insulin, cholesterol, triglyceride, urea, creatinine, and blood pressure. During oral glucose tolerance test (OGTT), diabetic rats showed increase in AUC(glucose) and AUC(insulin). Fenoldopam significantly decreased AUC(glucose) in diabetic rats. Diabetic rats showed lower insulin sensitivity index (K(TTT)) that was significantly increased by treatment with fenoldopam in diabetic rats. Diabetic rats showed decrease in urinary sodium. Fenoldopam treatment significantly increased urine output as well as urinary sodium indicating reduced sodium retention. Our data indicates fenoldopam treatment improves peripheral insulin sensitivity and renal function in STZ-induced type 2 diabetic rats.

The therapeutic potential of dopamine modulators on the cardiovascular and renal systems

In the periphery, physiological dopamine increases renal blood flow, decreases renal resistance and acts on the kidney tubule to enhance natriuresis and diuresis. The loss of dopamine function may be involoved in the deterioration in kidney function associated with ageing and may have a role in the pathogenesis of hypertension and diabetes. Intravenous dopamine is used as a positive inotrope in the treatment of acute heart failure and cardiogenic shock and as a diuretic in renal failure. The clinical uses of dopamine are limited, as it must be given intravenously, and also has widespread effects. The levels of peripheral dopamine can be increased by the administration of L-dopa to increase synthesis, prodrugs to release dopamine (docarpamine, glu-dopa) or by inhibiting the breakdown of dopamine (nitecapone). Preliminary clinical trials suggest that docarpamine may be useful in patients with low cardiac output syndrome after cardiac surgery and in refractory cirrhotic ascites. Ibopamine is an agonist at dopamine D1 and D2 receptors, which may retard the progression of chronic renal failure. Glu-dopa is selective for the kidney, thus avoiding widespread side effects. The early clinical studies with ibopamine as a diuretic in heart failure were favourable but the subsequent large mortality study showed that ibopamine increased mortality. Fenoldopam is a selective dopamine D1 receptor agonist. Intravenous fenoldopam may be useful in the treatment of hypertension associated with coronary artery bypass surgery or in hypertensive emergencies. Although our understanding of physiological and pathological roles of peripheral dopamine has been increasing rapidly in recent times, we still need more information to allow the design of clinically useful drugs that modify these roles. One priority is an orally-active selective dopamine D1 receptor agonist.

Beneficial effects of fenoldopam treatment on renal function in streptozotocin-induced diabetic rats

We have studied the effect of chronic treatment with dopamine D1 receptor agonist fenoldopam (1 mg/kg, i.p. daily for 6 weeks) on renal function and metabolic parameters in streptozotocin (STZ)-diabetic rats. Diabetes was induced by a single tail vein injection of STZ (45 mg/kg). STZ produced severe hyperglycemia, hypoinsulinemia, hypercholesterolemia, hypertriglyceridemia, hypertension and bradycardia. Fenoldopam treatment significantly reduced fasting but not fed blood glucose levels and lowered the blood pressure in diabetic animals. Significant change was not observed in insulin, cholesterol, triglyceride levels. Diabetic animals showed increase in AUCglucose and decrease in AUCinsulin during oral glucose tolerance test. Fenoldopam treatment did not significantly change these values in diabetic animals. STZ produced increase in serum urea, creatinine and blood urea nitrogen. Diuresis and urinary sodium retention was observed in diabetic animals. Renal hypertrophy was observed as seen from increased kidney weight/body weight ratio and increased total RNA content as well as decreased total DNA content. Fenoldopam treatment significantly lowered serum urea, creatinine and blood urea nitrogen. Urinary sodium retention was significantly reduced and renal hypertrophy was prevented with fenoldopam treatment as seen from the improved kidney weight/body weight ratio. Fenoldopam treatment significantly prevented reduction in total DNA content and increase in total RNA content further substantiating reduced renal hypertrophy. Our data suggest that STZ induced diabetes is associated with renal dysfunctions and fenoldopam treatment could be beneficial in a condition where diabetes mellitus co-exists with hypertension and compromised renal function.

Changes of urinary dopamine excretion early after balloon mitral commissurotomy in mitral stenosis

1. In order to investigate the changes of reduced urinary free dopamine excretion (uDA) in heart failure, 15 patients with symptomatic mitral stenosis were investigated on their uDA, endogenous creatinine (Cr) clearance, urinary excretion of sodium (UNaV), fractional excretion of sodium (FENa), plasma noradrenaline (pNA) and plasma L-dopa concentration before and early after percutaneous transvenous mitral commissurotomy (PTMC) by the clearance study. The delivery of L-dopa to renal proximal tubules (plasma L-dopa x Cr clearance), and the conversion ratio of plasma L-dopa to urinary dopamine in the kidney [uDA/(plasma L-dopa x Cr clearance)] were also estimated. 2. After successful PTMC, uDA, UNaV and FENa showed a significant but incomplete improvement and the changes of uDA were correlated positively with those of cardiac index (CI) (r = 0.665, P < 0.01), not with changes of pulmonary wedge pressure. While plasma L-dopa and plasma L-dopa x Cr clearance improved, uDA/(plasma L-dopa x Cr clearance) was not significantly changed early after PTMC. 3. From these results, it was suggested that reduced uDA tended to increase incompletely in relation with functional recovery of heart, and that increased plasma L-dopa and a delivery of L-dopa to renal proximal tubules have some positive role on urinary dopamine excretion, at least, early after PTMC.

Dopamine-sodium relationship in type 2 diabetic patients

Diabetes mellitus is known to be associated with sodium retention. The aim of the present paper was to investigate the possible role of the renal dopaminergic system in the disturbed sodium homeostasis of Type 2 diabetic patients. The urinary dopamine excretion, which represents the local kidney production, was lower in Type 2 diabetic patients as compared to controls and decreased in insulin treated patients as compared to patients treated without insulin. Urinary dopamine excretion correlated positively with sodium excretion in non-insulin treated patients and in controls, but not in insulin treated patients. In contrast to findings in healthy volunteers, an intravenous sodium load failed to increase the dopamine excretion in Type 2 diabetic patients, despite similar increments in sodium excretion. A low-dose dopamine infusion caused significantly lower natriuretic responses in insulin treated Type 2 diabetic patients as compared to controls, but not in non-insulin treated patients. These findings suggest that Type 2 diabetic patients display a derangement of the renal dopaminergic system, which is accentuated by insulin treatment.