Effect of dopamine on ecto-5'-nucleotidase expression in human glomerular mesangial cells

Preconditioning offers cardioprotection in hyperlipidemic rat hearts: possible role of Dopamine (D2) signaling

BACKGROUND

Ischemic preconditioning (IPC) induced cardioprotection has been reported to be blunted in hyperlipidemic subjects. Dopamine, via its D2 receptor signaling, appears to mimic the signaling cascade involved in myocardial preconditioning and is also involved in the inhibition of hyperlipidemia induced mediators. The present study was designed to investigate the possible involvement of D2 receptors in IPC and to see whether dopamine preconditioning can offer cardioprotection in hyperlipidemic rat hearts.

METHODS

Wistar albino rats were divided into 8 groups and fed on normal or high fat diet for 4 weeks. Hyperlipidemia was confirmed after 4 weeks by serum lipid estimations. Isolated perfused hearts were subjected to ischemic preconditioning or dopamine induced pharmacological preconditioning followed by 30-min ischemic insult and 60-min reperfusion. Clozapine was administered as D2 antagonist. Coronary perfusate (basal and post-ischemic) was collected for the estimations of LDH (Lactate dehydrogenase) and CKMB (Creatine kinase MB). Hearts were then removed and frozen for infarct size measurement.

RESULTS

A significant increase body weight, serum lipids except HDL was noted in high fat diet fed rats, as compared to normal rats. The level of LDH, CKMB in coronary effluent and infarct size were found to be decreased in preconditioned normal hearts, as compared to hearts treated with ischemia reperfusion. This effect was found to be blunted in hyperlipidemic animals. Dopamine (10 μM) alone and in combination with ischemic preconditioning significantly reduced the levels of LDH, CKMB and infarct size in hyperlipidemic hearts, as compared to preconditioned and non-preconditioned hyperlipidemic hearts. This effect was abolished significantly by Clozapine (D2 antagonist).

CONCLUSION

The present study reveals possible involvement of D2 receptors in ischemic preconditioning and suggests that dopamine preconditioning may offer significant cardioprotection in hyperlipidemic rat hearts.

Imbalance of renal production between 5-hydroxytryptamine and dopamine in patients with essential hypertension complicated by microalbuminuria

BACKGROUND

In the kidney, 5-hydroxytryptamine (5-HT) and dopamine (DA) are formed by the same enzyme, l-aromatic amino acid decarboxylase, but act on renal function and glomerular structure in an opposite direction. The present study was designed to explore whether rates of renal production of 5-HT relative to that of DA are altered in patients with essential hypertension and microalbuminuria.

METHODS

We measured urinary levels of 5-HT and DA, reflecting renal production of 5-HT and DA as well as 24-hour ambulatory blood pressure and urinary albumin excretion in 82 consecutive untreated, essential hypertensives without overt proteinuria.

RESULTS

Urinary 5-HT excretion and the ratio of urinary 5-HT to DA were significantly higher in 22 patients with microalbuminuria than in the remaining patients with normoalbuminuria, although urinary DA levels did not differ between the groups. The 24-hour systolic and diastolic blood pressures were also higher in the microalbuminuric group than in the normoalbuminuric group. Multiple regression analysis revealed that urinary 5-HT excretion and 24-hour systolic blood pressure were independently associated with urinary albumin excretion. Furthermore, urinary 5-HT excretion was positively correlated with creatinine clearance as well as blood pressure but tended to be negatively correlated with fractional excretion of sodium.

CONCLUSIONS

Renal production of 5-HT is enhanced compared with that of DA in essential hypertensives with microalbuminuria. This imbalance may contribute to the genesis of hypertensive glomerular damage.

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.

Are diabetic neuropathy, retinopathy and nephropathy caused by hyperglycemic exclusion of dehydroascorbate uptake by glucose transporters?

Vitamin C exists in two major forms. The charged form, ascorbic acid (AA), is taken up into cells via sodium-dependent facilitated transport. The uncharged form, dehydroascorbate (DHA), enters cells via glucose transporters (GLUT) and is then converted back to AA within these cells. Cell types such as certain endothelial and epithelial cells as well as neurons that are particularly prone to damage during diabetes tend to be those that appear to be dependent on GLUT transport of DHA rather than sodium-dependent AA uptake. We hypothesize that diabetic neuropathies, nephropathies and retinopathies develop in part by exclusion of DHA uptake by GLUT transporters when blood glucose levels rise above normal. AA plays a central role in the antioxidant defense system. Exclusion of DHA from cells by hyperglycemia would deprive the cells of the central antioxidant, worsening the hyperglycemia-induced oxidative stress level. Moreover, AA participates in many cellular oxidation-reduction reactions including hydroxylation of polypeptide lysine and proline residues and dopamine that are required for collagen production and metabolism and storage of catecholamines in neurons. Increase in the oxidative stress level and metabolic perturbations can be expected in any tissue or cell type that relies exclusively or mainly on GLUT for co-transport of glucose and DHA including neurons, epithelial cells, and vascular tissues. On the other hand, since DHA represents a significant proportion of total serum ascorbate, by increasing total plasma ascorbate concentrations during hyperglycemia, it should be possible to correct the increase in the oxidative stress level and metabolic perturbations, thereby sparing diabetic patients many of their complications.