Renal response to amino acid infusion in rats: effect of dopamine receptor antagonists and benserazide

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.

Effect of L-Dopa Decarboxylase Inhibitor Benserazide on Renal Function in Streptozotocin-Diabetic Rats

BACKGROUND/AIMS

Benserazide (BZD), an inhibitor of the dopamine synthesis, abolished the increase in glomerular filtration rate (GFR) following the infusion of a mixed amino acid solution. These results reveal endogenous dopamine as a mediator in the renal response to amino acids. The aim of the present study was to evaluate whether dopamine is also involved in the regulation of glomerular hyperfiltration during the early state of diabetes mellitus (DM).

METHODS

Male Sprague-Dawley rats were injected with a single dose of streptozotocin (60 mg/kg i.p.) for induction of experimental DM (n = 7-8/group). Age-matched non-diabetic animals, injected with citrate buffer, served as controls (CON, n = 8/group). Clearance experiments were performed 2 weeks after induction of DM in thiopental-anesthetized rats (80 mg/kg i.p.), which were continuously infused either with BZD (30 microg/min/kg) or vehicle (VHC).

RESULTS

Mean arterial blood pressure was around 110 mm Hg and did not significantly differ among the groups. GFR was 0.95 +/- 0.02 ml/min/100 g b.w. in VHC-treated CON. BZD treatment did not significantly change GFR in the CON group (0.92 +/- 0.06 ml/min/100 g b.w.). As expected, glomerular hyperfiltration was observed in diabetic rats infused with VHC (1.24 +/- 0.08 ml/min/100 g b.w.). Treatment with BZD significantly reduced the diabetes-induced increase in GFR to control levels (0.95 +/- 0.05 ml/min/100 g b.w.).

CONCLUSION

Our results show that the inhibition of dopamine synthesis prevented the increase in GFR due to diabetic conditions, indicating that endogenous dopamine is involved in the regulation of DM-induced changes in renal hemodynamics.

Absence of amino acid-induced glomerular hyperfiltration in dopamine D3 receptor knockout mice

Pharmacological inhibition of receptors of the dopamine D2-like family has been shown to abolish the glomerular hyperfiltration in response to amino acids. To further discriminate between the receptor subtypes within the D2-like family, we investigated the effects of amino acid infusion on renal function in dopamine D3 receptor knockout (-/-) mice. In clearance experiments pentobarbital-anesthetized D3 receptor (-/-) and wild-type (+/+) mice were infused with Ringer solution at baseline, followed by a continuous infusion of mixed amino acids (10%). Baseline glomerular filtration rate (GFR), assessed by renal clearance of [3H]-inulin, was the same in D3 receptor (-/-) mice (0.56+/-0.08 ml/min per g kidney weight) and wild-type animals (0.56+/-0.04 ml/min per g kw). During infusion of amino acids, GFR was significantly elevated by 50% in D3 receptor (+/+) mice. In contrast, this amino acid-induced response of GFR was abolished in D3 receptor (-/-) mice. Baseline urinary water and sodium excretion was not significantly different in both groups of mice. As observed in GFR, these renal excretory parameters were significantly elevated during amino acid infusion in D3 receptor (+/+) but not in D3 receptor (-/-) mice. Time controls, constantly infused with Ringer solution, did not show significant changes in GFR, renal water or sodium excretion during the entire experiment, indicating stable experimental conditions. Taken together, the data underline the involvement of dopamine D2-like receptors in the renal response to amino acid infusion and, in addition, attribute this effect to the dopamine D3 receptor subtype.

Effect of dopamine D3 receptor blockade on renal function and glomerular size in diabetic rats

Dopamine D2-like receptors, including D2, D3, and D4 receptors, are involved in the regulation of glomerular hyperfiltration due to diabetes mellitus. These hemodynamic alterations represent a risk factor for the later development of diabetic nephropathy. The aim of the present study was to determine whether the D3 receptor subtype modulates the diabetes-induced increase in glomerular filtration rate (GFR) in rats. Renal function was studied in Sprague-Dawley rats 14 days after induction of a moderate diabetes mellitus (DM) by streptozotocin and in non-diabetic controls (CON). Rats were orally treated either with the peripherally acting, selective dopamine D3 receptor antagonist BSF 135170 (BSF, 10 mg/kg per day for 2 weeks) or with vehicle (VHC). Perfusion-fixed kidneys were used for estimation of glomerular volume. In conscious rats, which were treated with BSF, the DM-induced increase in fluid intake, urinary output, and renal sodium excretion was significantly less pronounced than in the vehicle group (DM-VHC). In the clearance experiments, GFR in CON was about 0.84+/-0.04 ml/min per 100 g body weight. The DM-VHC group presented a significant glomerular hyperfiltration (1.09+/-0.04 ml/min per 100 g body weight). Treatment with BSF significantly lowered GFR towards levels of CON. The estimated glomerular volume was 0.73+/-0.03 x 10(6) microm3 in the CON-VHC group and 0.86+/-0.04 x 10(6) microm3 in the DM-VHC animals. Interestingly, treatment with BSF decreased the glomerular volume in both groups. Irrespective of BSF treatment, kidney wet weight related to body weight was about 36% higher in DM animals compared with CON animals. We conclude that dopamine D3 receptors represent a target for the modulation of diabetes-induced glomerular hyperfiltration. Therefore, the results encourage the testing of the possible beneficial effects of long-term D3 receptor blockade on the development of diabetic nephropathy.

Reduction of glomerular hyperfiltration by dopamine D(2)-like receptor blockade in experimental diabetes mellitus

BACKGROUND

Dopamine D(2)-like receptors are involved in the physiological response of renal haemodynamics to amino-acid infusion. The present study was performed to investigate whether domperidone, a D(2)-like receptor antagonist, modulates the pathological hyperfiltration in experimental diabetes mellitus.

METHODS

Renal function was studied in anaesthetized rats 2 weeks after induction of moderate diabetes mellitus by streptozotocin, and in non-diabetic controls. Rats in both groups continuously received domperidone or vehicle via drinking water. Following infusion of Ringer's saline for measurement of baseline values, an i.v. amino-acid load was applied to investigate the renal functional reserve.

RESULTS

In vehicle-treated diabetic rats baseline glomerular filtration rate and renal plasma flow were significantly higher compared with controls (1.10+/- 0.04 vs. 0.83+/-0.02 (P<0.004) and 4.83+/-0.26 vs 3.32+/-0.24 ml/min/100 g body weight (bw) (P<0.001) respectively). Domperidone completely normalized glomerular filtration rate and renal plasma flow in diabetic rats to values observed in vehicle-treated controls (0.81+/-0.07 (P=0.740) and 3.35+/- 0.30 ml/min/100 g bw (P=0.889) respectively). In the clearance experiments, arterial blood pressure, urinary flow rate and sodium excretion did not significantly differ when comparing the four groups. However, in conscious rats, urinary flow rate and sodium excretion were significantly higher in diabetic rats compared with non-diabetic controls. In both non-diabetic groups, amino-acid infusion induced a significant glomerular hyperfiltration that was completely absent in diabetic rats, and restored by domperidone treatment. In conscious vehicle-treated diabetic rats urinary albumin excretion was enhanced (449.0+/-47.7 vs. 185.7+/- 18.1 microg/24 h in non-diabetic rats (P<0.001)) and significantly lowered in diabetic rats by domperidone treatment (109.8+/-15.4 microg/24 h (P<0.001)).

CONCLUSION

The data suggest that dopaminergic mechanisms are involved in the changes in renal haemodynamics during early experimental diabetes mellitus in rats.

Dopamine D2-like receptors and amino acid-induced glomerular hyperfiltration in humans

AIMS

In rodents, blockade of dopamine D2-like receptors abolishes both the physiological increase in glomerular filtration rate (GFR) induced by amino acids and the pathological hyperfiltration in experimental diabetes mellitus. This study addressed the contribution of dopamine D2-like receptors to changes in renal haemodynamics after amino acid infusion in humans.

METHODS

Twelve healthy volunteers participated in this double-blind, randomized, cross-over study. GFR and renal blood flow (RPF) were assessed by renal clearance of inulin and p-aminohippuric acid (PAH), respectively. Following infusion of 0.45% saline at baseline, an electrolyte-balanced solution of mixed amino acids (10%) was infused. Prior to the experiments, the subjects received orally either placebo, or sulpiride (10 mg kg-1), a centrally and peripherally acting D2-like receptor antagonist, or domperidone (1 mg kg-1) which affects only peripheral D2-like receptors.

RESULTS

In the placebo series, amino acid infusion significantly increased GFR and RPF by up to 15.8 +/- 5.3% and 14.4 +/- 6.1%, respectively, while mean blood pressure and heart rate remained unchanged. Pretreatment with domperidone only marginally altered the renal response to amino acids (maximal increase by 13.2 +/- 5.6 and 11.9 +/- 4.0% in GFR and RPF, respectively), while sulpiride completely abolished the renal haemodynamic changes induced by amino acids. Total and fractional urinary sodium excretion as well as urinary osmolality were similar at baseline and increased in response to amino acids, to the same extent, in all series. No changes in renal dopamine excretion occurred.

CONCLUSION

The results indicate that in man dopamine D2-like receptors are involved in the renal haemodynamic response to amino acid infusion. Whether dopamine D2-like receptor blockade diminishes glomerular hyperfiltration in pathological states requires clinical investigations.

Renal dopamine receptors in health and hypertension

During the past decade, it has become evident that dopamine plays an important role in the regulation of renal function and blood pressure. Dopamine exerts its actions via a class of cell-surface receptors coupled to G-proteins that belong to the rhodopsin family. Dopamine receptors have been classified into two families based on pharmacologic and molecular cloning studies. In mammals, two D1-like receptors that have been cloned, the D1 and D5 receptors (known as D1A and D1B, respectively, in rodents), are linked to stimulation of adenylyl cyclase. Three D2-like receptors that have been cloned (D2, D3, and D4) are linked to inhibition of adenylyl cyclase and Ca2+ channels and stimulation of K+ channels. All the mammalian dopamine receptors, initially cloned from the brain, have been found to be expressed outside the central nervous system, in such sites as the adrenal gland, blood vessels, carotid body, intestines, heart, parathyroid gland, and the kidney and urinary tract. Dopamine receptor subtypes are differentially expressed along the nephron, where they regulate renal hemodynamics and electrolyte and water transport, as well as renin secretion. The ability of renal proximal tubules to produce dopamine and the presence of receptors in these tubules suggest that dopamine can act in an autocrine or paracrine fashion; this action becomes most evident during extracellular fluid volume expansion. This renal autocrine/paracrine function is lost in essential hypertension and in some animal models of genetic hypertension; disruption of the D1 or D3 receptor produces hypertension in mice. In humans with essential hypertension, renal dopamine production in response to sodium loading is often impaired and may contribute to the hypertension. The molecular basis for the dopaminergic dysfunction in hypertension is not known, but may involve an abnormal post-translational modification of the dopamine receptor.