Impaired intrarenal dopamine production following intravenous sodium chloride infusion in type 1 (insulin-dependent) diabetes mellitus

A mathematical model of rat proximal tubule and loop of Henle

Proximal tubule and loop of Henle function are coupled, with proximal transport determining loop fluid composition, and loop transport modulating glomerular filtration via tubuloglomerular feedback (TGF). To examine this interaction, we begin with published models of the superficial rat proximal convoluted tubule (PCT; including flow-dependent transport in a compliant tubule), and the rat thick ascending Henle limb (AHL). Transport parameters for this PCT are scaled down to represent the proximal straight tubule (PST), which is connected to the thick AHL via a short descending limb. Transport parameters for superficial PCT and PST are scaled up for a juxtamedullary nephron, and connected to AHL via outer and inner medullary descending limbs, and inner medullary thin AHL. Medullary interstitial solute concentrations are specified. End-AHL hydrostatic pressure is determined by distal nephron flow resistance, and the TGF signal is represented as a linear function of end-AHL cytosolic Cl concentration. These two distal conditions required iterative solution of the model. Model calculations capture inner medullary countercurrent flux of urea, and also suggest the presence of an outer medullary countercurrent flux of ammonia, with reabsorption in AHL and secretion in PST. For a realistically strong TGF signal, there is the expected homeostatic impact on distal flows, and in addition, a homeostatic effect on proximal tubule pressure. The model glycosuria threshold is compatible with rat data, and predicted glucose excretion with selective 1Na(+):1glucose cotransporter (SGLT2) inhibition comports with observations in the mouse. Model calculations suggest that enhanced proximal tubule Na(+) reabsorption during hyperglycemia is sufficient to activate TGF and contribute to diabetic hyperfiltration.

Intrarenal dopamine inhibits progression of diabetic nephropathy

The kidney has a local intrarenal dopaminergic system, and in the kidney, dopamine modulates renal hemodynamics, inhibits salt and fluid reabsorption, antagonizes the renin-angiotensin system, and inhibits oxidative stress. The current study examined the effects of alterations in the intrarenal dopaminergic system on kidney structure and function in models of type 1 diabetes. We studied catechol-O-methyl-transferase (COMT)(-/-) mice, which have increased renal dopamine production due to decreased dopamine metabolism, and renal transplantation was used to determine whether the effects seen with COMT deficiency were kidney-specific. To determine the effects of selective inhibition of intrarenal dopamine production, we used mice with proximal tubule deletion of aromatic amino acid decarboxylase (ptAADC(-/-)). Compared with wild-type diabetic mice, COMT(-/-) mice had decreased hyperfiltration, decreased macula densa cyclooxygenase-2 expression, decreased albuminuria, decreased glomerulopathy, and inhibition of expression of markers of inflammation, oxidative stress, and fibrosis. These differences were also seen in diabetic mice with a transplanted kidney from COMT(-/-) mice. In contrast, diabetic ptAADC(-/-) mice had increased nephropathy. Our study demonstrates an important role of the intrarenal dopaminergic system to modulate the development and progression of diabetic kidney injury and indicate that the decreased renal dopamine production may have important consequences in the underlying pathogenesis of diabetic nephropathy.

Regulation of glomerulotubular balance. I. Impact of dopamine on flow-dependent transport

In response to volume expansion, locally generated dopamine decreases proximal tubule reabsorption by reducing both Na/H-exchanger 3 (NHE3) and Na-K-ATPase activity. We have previously demonstrated that mouse proximal tubules in vitro respond to changes in luminal flow with proportional changes in Na(+) and HCO(3)(-) reabsorption and have suggested that this observation underlies glomerulotubular balance. In the present work, we investigate the impact of dopamine on the sensitivity of reabsorptive fluxes to changes in luminal flow. Mouse proximal tubules were microperfused in vitro at low and high flow rates, and volume and HCO(3)(-) reabsorption (J(v) and J(HCO3)) were measured, while Na(+) and Cl(-) reabsorption (J(Na) and J(Cl)) were estimated. Raising luminal flow increased J(v), J(Na), and J(HCO3) but did not change J(Cl). Luminal dopamine did not change J(v), J(Na), and J(HCO3) at low flow rates but completely abolished the increments of Na(+) absorption by flow and partially inhibited the flow-stimulated HCO(3)(-) absorption. The remaining flow-stimulated HCO(3)(-) absorption was completely abolished by bafilomycin. The DA1 receptor blocker SCH23390 and the PKA inhibitor H89 blocked the effect of exogenous dopamine and produced a two to threefold increase in the sensitivity of proximal Na(+) reabsorption to luminal flow rate. Under the variety of perfusion conditions, changes in cell volume were small and did not always parallel changes in Na(+) transport. We conclude that 1) dopamine inhibits flow-stimulated NHE3 activity by activation of the DA1 receptor via a PKA-mediated mechanism; 2) dopamine has no effect on flow-stimulated H-ATPase activity; 3) there is no evidence of flow stimulation of Cl(-) reabsorption; and 4) the impact of dopamine is a coordinated modulation of both luminal and peritubular Na(+) transporters.

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.

Renoprotective effect of a dopamine D3 receptor antagonist in experimental type II diabetes

Diabetic nephropathy is the leading cause of end-stage renal disease. Dopamine receptors are involved in the regulation of renal hemodynamics and may play a role in diabetes-induced hyperfiltration. To test this hypothesis, we investigated the renal effect of a dopamine D3 receptor antagonist (D3-RA) in hypertensive type II diabetic SHR/N-cp rats. Lean and obese SHR/N-cp rats were randomly assigned to D3-RA, angiotensin-converting enzyme inhibitor (ACE-i), or D3-RA+ACE-i treatment or control conditions. Treated animals were given the D3-RA A-437203 (10 mg/kg/body weight (BW)/day) or the ACE-i trandolapril (0.3 mg/kg BW/day) or a combination of both. At 6 months following perfusion, fixed kidneys were analyzed by morphological and stereological methods. Indices of renal damage (glomerulosclerosis, glomerulosclerosis damage index (GSI), tubulointerstitial and vascular damage), glomerular geometry and functional variables such as urinary albumin excretion, glomerular filtration rate, blood pressure, blood chemistry and BW were determined. The GSI (score 0-4) was significantly higher (P<0.05) in untreated diabetic animals (1.62+/-0.3) compared to nondiabetic controls (0.4+/-0.2) and the treatment groups (D3-RA: 0.31+/-0.12; ACE-i: 0.29+/-0.1; combination treatment: 0.12+/-0.01). Urinary albumin excretion (mg/24 h) was higher in untreated diabetic controls (102+/-19) compared to nondiabetic controls (31+/-12) and the treatment groups (D3-RA: 44+/-15; ACE-i: 41+/-13; combination treatment: 15+/-8). Mean glomerular volume was higher in untreated diabetic animals compared to nondiabetic controls and to the treatment groups. Desmin expression, a marker of podocyte damage, was elevated in untreated diabetic controls and diminished in all treatment groups. These data suggest that in a model of type II diabetes, the dopamine D3-RA had a beneficial effect on renal morphology and albuminuria, which was comparable in magnitude to that of ACE-i treatment.

Reduced renal dopamine D1 receptor function in streptozotocin-induced diabetic rats

Dopamine, via activation of renal D(1) receptors, inhibits the activities of Na-K-ATPase and Na/H exchanger and subsequently increases sodium excretion. Decreased renal dopamine production and sodium excretion are associated with type I diabetes. However, it is not known whether the response to D(1) receptor activation is altered in type I diabetes. The present study was designed to examine the effect of streptozotocin-induced type I diabetes on renal D(1) receptor expression and function. Streptozotocin treatment of Sprague-Dawley rats caused a fourfold increase in plasma levels of glucose along with a significant decrease in insulin levels compared with control rats. Intravenous administration of SKF-38393, a D(1) receptor agonist, caused a threefold increase in sodium excretion in control rats. However, SKF-38393 failed to produce natriuresis in diabetic rats. SKF-38393 caused a concentration-dependent inhibition of Na-K-ATPase activity in renal proximal tubules of control rats. However, the ability of SKF-38393 to inhibit Na-K-ATPase activity was markedly diminished in diabetic rats. D(1) receptor numbers and protein abundance as determined by [(3)H]SCH-23390 ligand binding and Western blot analysis were markedly reduced in diabetic rats compared with control rats. Moreover, SKF-38393 failed to stimulate GTP gamma S binding in proximal tubular membranes from diabetic rats compared with control rats. We conclude that the natriuretic response to D(1) receptor activation is reduced in type I diabetes as a result of a decrease in D(1) receptor expression and defective receptor G protein coupling. These abnormalities may contribute to the sodium retention associated with type I diabetes.

Renal kallikrein-kinin system, but not renal dopamine system, mediates the natriuretic response to intravenous saline infusion in healthy Chinese subjects

1. To assess the role of renal dopamine (DA), sympathetic nervous system (SNS) activity and the renal kallikrein-kinin system in sodium excretion in Chinese subjects, we studied the effects of intravenous saline infusion on the urinary excretions of sodium, free DA, free noradrenaline (NA) and kallikrein in eight healthy males aged 23-25 years. 2. After a baseline period of 1 h (hour 0), these subjects received 11 of 0.9% saline over 2 h (hours 1 and 2), followed by a 4-h recovery period (hours 3-6). From hours 0-4, subjects remained in the supine position, except to void urine. Distilled water was given orally throughout the study to ensure an adequate diuresis. 3. A 31-39% increase in sodium excretion (P < 0.05) was seen during hours 2 and 3. Urinary DA did not change throughout the study period. Urinary free NA showed no changes while the subjects remained supine, but an increase of 91-105% (P < 0.02) was seen after the subjects became ambulatory. However, there was a 103-140% increase in urinary kallikrein excretion (P < 0.05) during the saline infusion. Urinary kallikrein was still much higher (by 74%) than the basal level 1 h after the completion of the saline infusion. 4. There is no evidence from the present study that renal DA or SNS play any role in the natriuretic response to saline infusion in Chinese subjects. The brisk urinary kallikrein response, despite a relatively small salt load, suggests that the renal kallikrein-kinin system may play an important role in extracellular fluid volume and sodium homeostasis in Chinese subjects.

Effects of carbidopa and of intravenous saline infusion into normal and hypertensive subjects on urinary free and conjugated dopamine

OBJECTIVE

To investigate the possible role played by endogenous dopamine as a modulator of renal sodium (Na+) reabsorption after a combined Na+ and volume load.

DESIGN

A randomized placebo-controlled study.

METHODS

Ten healthy volunteers and four hypertensive patients were subjected to intravenous infusions of 21 0.9% saline (308 mmol Na+) administered from 1000 to 1300 h after oral administration of placebo or of carbidopa, a dopamine decarboxylase inhibitor.

RESULTS

Studies on control subjects after placebo showed that natriuresis occurred during the 6 h after commencement of the saline infusion, with falls in plasma albumin concentration, plasma renin activity and plasma aldosterone concentration; in comparison with results of mock infusion (6 mmol Na+) there was no change in the urinary excretion of dopamine and noradrenaline (In their free or conjugated forms). There was, however, a marked surge in excretion of urinary conjugated dopamine and in the dopamine: noradrenaline ratio from 1300 to 1600 h, after either type of infusion. Administration of carbidopa before the saline infusion resulted in a marked decrease in excretion of urinary free dopamine, but had no effect on the surge in excretion of urinary conjugated dopamine. Saline infusion decreased proximal fractional Na+ reabsorption. Administration of carbidopa delayed but did not prevent this decrease. The effects of saline infusion and of carbidopa on the urinary excretion of dopamine and noradrenaline from hypertensive patients were similar to those observed with the healthy volunteers.

CONCLUSIONS

These findings indicate that volume expansion by intravenous saline infusion has no appreciable effect on the urinary free dopamine excretion from normal or hypertensive humans; with any apparent increase, it is important to exclude the possibility of conversion of conjugates to free dopamine in vitro. Furthermore, that carbidopa administration did not inhibit the afternoon surge of conjugated dopamine suggests that administration of carbidopa is deficient as a tool to investigate the functional role of the renal dopamine system.

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.

Effect of insulin on renal sodium handling and renal haemodynamics in insulin-dependent (type 1) diabetes mellitus patients

The effects of insulin on renal haemodynamics and renal sodium handling were studied in eight insulin-dependent (type 1) diabetic patients (aged 30 +/- 3 years). Seven healthy men (aged 38 +/- 4 years) served as controls. The type 1 diabetic patients were resistant to insulin-stimulated glucose disposal as estimated by a 45% lower metabolic (P < 0.01) clearance of glucose as compared with controls. However, type 1 diabetic patients were still sensitive to the distal tubular antinatriuretic effect of insulin, as indicated by an increase in distal sodium reabsorption (95.5 +/- 0.5% to 96.9% +/- 0.4%; P < 0.05) during insulin infusion compared with controls (95.5% +/- 0.6% to 97.4% +/- 0.3%; P < 0.05). In control subjects insulin infusion was associated with 9% increases (P < 0.05) in lithium clearance and in renal plasma flow, whereas no significant increases in lithium clearance and in renal plasma flow were observed in the type 1 diabetic patients. In both groups, the changes in renal plasma flow in response to insulin infusion were positively correlated with that in lithium clearance (r = 0.80 and r = 0.90, respectively; P < 0.05-0.01). In conclusion, the present result demonstrates an intact distal tubular sodium retaining effect in conjunction with a blunted decrease in proximal tubular sodium reabsorption following insulin infusion, which could be the result of an impaired renal vasodilation in type 1 diabetes mellitus.

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

In Type 1 diabetes an increased total body sodium and an impaired ability to excrete a sodium load have been described. A possible involvement of the renal dopaminergic system in this abnormal sodium handling was evaluated through measurements of the urinary output of dopamine, sodium, the dopamine/sodium correlation, and through examining the effect of a dopamine infusion on urinary sodium excretion. Twenty-four hour urinary dopamine excretion was significantly lower in Type 1 diabetic patients as compared to normal controls. A significant correlation between urinary dopamine and sodium excretion was present in normoalbuminuric Type 1 diabetic patients and in normal controls. However, no such correlation could be found in microalbuminuric patients. The increase in fractional excretion of sodium during a 1 h low-dose dopamine (3 micrograms kg-1 min-1) infusion in Type 1 diabetic patients was negatively correlated with diabetes duration. Patients with short duration of diabetes (less than 15 years) had a comparable dopamine-induced increase in fractional excretion of sodium as normal controls. However, patients with longer duration of diabetes (more than 15 years) and microalbuminuric patients displayed no significant changes in sodium output during dopamine infusion. These findings suggest that in Type 1 diabetes mellitus a deficiency of renal dopamine production could be responsible for the impaired sodium handling. Longer duration of the disease and microalbuminuria seem to be associated with an uncoupling of the urinary dopamine/sodium relationship.

Effect of hydrochlorothiazide and indomethacin treatment on renal function in nephrogenic diabetes insipidus

The purpose of this study was to investigate the effects of treatment with hydrochlorothiazide and hydrochlorothiazide and indomethacin combined on renal function in four boys, two with nephrogenic diabetes insipidus and two with partial nephrogenic diabetes insipidus using the clearances of inulin and para-aminohippuric acid under water diuresis and lithium clearance. Hydrochlorothiazide reduced urine flow and lithium clearance. These effects were further potentiated by addition of indomethacin. No consistent effects on renal plasma flow or glomerular filtration rate were found. It is concluded that treatment with hydrochlorothiazide alone and hydrochlorothiazide and indomethacin combined reduces urine flow in nephrogenic diabetes insipidus by increasing proximal tubular reabsorption of sodium.

Effect of dietary protein on the renal response to fenoldopam in diabetic rats

The effect of the dopamine D1 receptor agonist, fenoldopam, was studied in streptozotocin-induced diabetic rats treated with insulin to maintain a moderate hyperglycemia and fed a low, normal or high protein diet. Fenoldopam at 1 microgram/kg per min i.v. resulted in a significant increase in both glomerular filtration rate (GFR) and renal blood flow (RBF) in diabetic rats fed a normal protein diet. In rats fed a low protein diet, fenoldopam failed to alter either parameter, however, there was a very significant increase in both GFR and RBF in diabetic rats fed a high protein diet. Since diabetes is associated with a decrease in both urinary dopamine excretion as well as the hyperemic response to protein ingestion, it is possible that stimulation of dopamine D1 receptors by fenoldopam restores renal functional reserve in diabetic animals. The observation that the dopamine D1 receptor antagonist, SCH 23390, at a dose (1 microgram/kg per min) that abolished the renal vasodilator effects of fenoldopam, failed to alter renal hemodynamics in diabetic rats suggests that endogenous dopamine has little effect.

Dopamine and renal blood flow in radiocontrast-induced nephropathy in humans

Previous studies suggest a role for renal vasoconstriction in the pathogenesis of radiocontrast-induced nephropathy (RCIN). A renal vasodilator such as dopamine may be protective. However, the effect of dopamine on renal blood flow (RBF) in patients with chronic renal failure (CRF) is controversial. Patients with CRF of diabetic (DM) or nondiabetic (NDM) origin were hydrated with 0.45% NaCl intravenously at 100 mL/h for 12 h and then randomized to either 0.45% NaCl IV at 100 mL/h (Group 1) or dopamine IV at 2 micrograms/kg/min in 0.45% NaCl at 100 mL/h for 2 h during and after cardiac catheterization. Mean arterial pressure (MAP), cardiac output (CO), and RBF were measured at baseline (t = 0), after 5 min of vehicle (Group 1) or dopamine (Group 2) but before ionic radiocontrast (t = 5 min), after ventriculogram (t = 15 min), and after coronary angiography (t = 65 min). Serum creatinine (SCr) was measured at baseline and 24 and 48 h after cardiac catheterization. RCIN was defined as a 25% increase of SCr above baseline 48 h after cardiac catheterization. Baseline characteristics demonstrated the groups to be equivalent in age, SCr, creatinine clearance, CO, MAP, RBF, and radiocontrast dose administered. The incidence of RCIN was not different between Group 1 and Group 2 (Group 1, 6 of 15 patients; Group 2, 5 of 15 patients). Dopamine infusion was associated with a significant increase in RBF at 5 min (Group 1, 110 +/- 13%; Group 2, 193 +/- 40% at t = 5, p < .05). RBF remained elevated throughout the catheterization in Group 2.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of insulin on renal haemodynamics and the proximal and distal tubular sodium handling in healthy subjects

The effects of insulin on renal haemodynamics and renal sodium handling were studied in 10 healthy males. Using the euglycaemic insulin clamp technique, insulin was infused on separate days resulting in two levels of hyperinsulinaemia (41 +/- 3 and 90 +/- 7 mU/l, respectively). Renal haemodynamics and the proximal and distal tubular sodium handling were studied using inulin, para-amino-hippuric acid, sodium and lithium clearances. Low- and high-dose insulin infusions were followed by a fall in sodium clearance from 1.6 +/- 0.1 ml/min to 1.2 +/- 0.1 and 1.0 +/- 0.1 ml/min, respectively. Both levels of hyperinsulinaemia resulted in increased distal tubular sodium reabsorption. The distal antinatriuretic effect of insulin was associated with dose- and time-dependent decline in proximal tubular sodium reabsorption. The changes in proximal tubular sodium handling occurred without any significant changes in natriuretic factors, such as renal dopamine and plasma atrial natriuretic peptide levels. However, hyperinsulinaemia resulted in time- and dose-dependent increases in renal plasma flow, and renal vasodilatation could, possibly via changes in renal interstitial pressure, have contributed to the fall in the proximal tubular sodium reabsorption. The results also suggest that decreased proximal sodium reabsorption may be a compensatory mechanism counteracting the insulin-induced sodium retention.

Effects of hyperglycaemia on kidney function, atrial natriuretic factor and plasma renin in patients with insulin-dependent diabetes mellitus

In normoalbuminuric patients with insulin-dependent diabetes mellitus, plasma atrial natriuretic factor (ANF), cyclic GMP and active renin and the renal clearances of [99Tcm]-diethylenetriaminepentaacetic acid (DTPA) lithium and sodium were studied on a hyperglycaemia day and a euglycaemia day. Baseline euglycaemia was achieved by an overnight variable insulin infusion, which during study days was fixed at the rate necessary to maintain euglycaemia in the morning. After a baseline euglycaemic clearance period of 90 min, measurements were repeated in a new 90-min period beginning 150 min later. On the hyperglycaemia day i.v. infusion of 20% glucose was started at the end of the euglycaemic baseline period, increasing blood glucose (5.3 +/- 1.3 vs 12.1 +/- 1.2 mmol l-1, p less than 0.01). On the euglycaemia day blood glucose declined (5.1 +/- 1.0 vs 4.2 +/- 1.0 mmol l-1, p less than 0.02). Glomerular filtration rate (GFR) was unchanged by acute hyperglycaemia (127 +/- 16 vs 129 +/- 24 ml min-1, NS), but nearly normalized during maintained euglycaemia on the euglycaemia day (124 +/- 17 vs 105 +/- 16 ml min-1, p less than 0.01). When comparing the hyperglycaemic study period with the similarly timed period on the euglycaemia day, GFR was elevated by hyperglycaemia (129 +/- 24 vs 105 +/- 16 ml min-1, p less than 0.01), while the renal clearances of lithium and sodium were similar. Consequently, the calculated absolute proximal reabsorption rate of sodium and water was elevated during hyperglycaemia. Hyperglycaemia reduced the slight decline in plasma concentrations of ANF and cyclic GMP observed on the euglycaemia day. Active renin, glucagon and plasma osmolality were unchanged. In conclusion, marked changes in glomerular filtration rate are induced by changes in blood glucose concentration, but the effect is delayed and thus not directly related to renal tubular transport of glucose. Hyperglycaemia does not affect renal clearances of lithium and sodium, while proximal tubular reabsorption is markedly stimulated. These changes are not related to changes in ANF, renin, glucagon or plasma osmolality.

Peripheral dopamine in pathophysiology of hypertension. Interaction with aging and lifestyle

Dopamine, an ancestral catecholamine, is physiologically natriuretic and vasodilating, thus essentially protecting against hypertension. Its actions are overshadowed by the opposite effects of its main biological partner, norepinephrine, and this is accentuated with aging. Clinical observations combined with molecular biology approaches to catecholamine-synthesizing and catecholamine-metabolizing enzymes and receptors permit the identification of some inborn defects. Subtle changes in the dopamine-norepinephrine balance may account for the enhanced peripheral noradrenergic activity seen in the setting of decreased dopaminergic activity in advanced age. These changes may contribute to the diminished ability of the aged kidney to excrete a salt load, as well as to the finding that systolic blood pressure increases with age in populations with a high, but not in those with a low, intake of salt. The attainment of advanced age in Western societies with adverse lifestyle changes (mental rather than physical stress, excess salt intake, overeating, sedentarism) appears to facilitate the development of hypertension. The adaptation to all the preceding lifestyle changes necessitates an increased dopamine generation, which may initially compensate to maintain appropriate natriuresis and vasodilation since many patients with initial borderline essential hypertension express their sympathetic hyperfunction, in addition to increased norepinephrine release, by excessive dopamine release. However, the progression of hypertension is accompanied by a peripheral dopaminergic deficiency and diminished ability to excrete salt. This may represent an eventual inadequacy of a phylogenetically redundant system resulting in decreased natriuresis and vasodilation and may account for the responsiveness of established chronic hypertension to salt restriction, diuretics, and dopaminomimetic medication.