Impact of hyperglycemia on the renin angiotensin system in early human type 1 diabetes mellitus

Plasma aldosterone concentration in the patient with diabetes mellitus

BACKGROUND

Vascular injury at the microvascular and macrovascular levels plays a crucial role in the patient with diabetes mellitus. Evidence for renin-system activation in many patients with type 1 diabetes mellitus has raised the possibility that aldosterone-widely recognized as a contributor to vascular injury-could play a role.

METHODS

We examined the state of the renin-angiotensin-aldosterone system (RAAS) in 58 subjects with type 1 diabetes mellitus and 64 age-matched normal control subjects. All studies were performed on a fixed sodium (200 mmol/day) and potassium (100 mmol/day intake), and samples were drawn at 8:00 a.m. to avoid the influence of circadian rhythms.

RESULTS

The patient with diabetes mellitus showed an increase in plasma renin activity (PRA) (P < 0.01), plasma angiotensin II concentration (P < 0.01), and plasma aldosterone concentration (P < 0.001). A striking influence of the angiotensin receptor blocker, candesartan, on plasma aldosterone concentration in the patients with diabetes mellitus suggested strongly that renin-system activation is responsible for the elevated plasma aldosterone concentration.

CONCLUSION

Pharmacologic interruption of the effects of aldosterone at the tissue level could be especially useful in patients with diabetes mellitus. The dose of agents that block the renin-angiotensin system (RAS) should be adjusted to maximize the fall in plasma aldosterone concentration.

Renal nitric oxide production is decreased in diabetic rats and improved by AT1 receptor blockade

OBJECTIVE

Diabetes mellitus is associated with increased incidence of cardiovascular complications. Lack of nitric oxide production may exacerbate these complications. We hypothesized that diabetes decreases renal nitric oxide (NO) production, an effect that is reversed via inhibition of angiotensin subtype-1 receptor.

METHODS

We monitored changes in renal interstitial fluid nitric oxide by a microdialysis technique in the renal cortex of conscious Sprague-Dawley rats. Rats (n = 8 each group) were given streptozotocin 30 mg/kg intravenously to induce diabetes. Changes in renal interstitial fluid angiotensin II and NO were evaluated at baseline before and over 12 weeks during the development of diabetes and at 4 and 8 h after oral administration of the angiotensin subtype-1 (AT1) receptor blockers, losartan (30 mg/kg) or valsartan (10 mg/kg).

RESULTS

Renal interstitial fluid angiotensin II significantly increased after development of diabetes. In contrast, basal renal interstitial fluid nitric oxide decreased significantly over 12 weeks after development of diabetes. Both losartan and valsartan caused a further increase in renal angiotensin II levels. Some 4 h after administration, there was significantly greater increase in renal nitric oxide after administration of valsartan than of losartan. At 8 h post- treatment, only valsartan caused a significant increase in renal nitric oxide levels.

CONCLUSION

These results demonstrate that diabetes mellitus is associated with an increase in renal production of angiotensin II, while renal production of nitric oxide is reduced. The decrease in renal NO is reversed by AT1 receptor blockade.

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.

Effect of short-term hyperglycaemia on haemodynamics in type 1 diabetic patients

OBJECTIVES

Mechanisms underlying glucose-mediated development and progression of diabetic complications are incompletely understood. We tested the impact of short-term hyperglycaemia on systemic blood pressure and regulatory hormones in type 1 diabetic patients.

DESIGN AND METHODS

We included 18 patients [13 men, mean (SEM) diabetes duration 10 (1) years] without signs of autonomic neuropathy or renal complications in a randomized single-blinded cross-over trial using insulin-glucose clamp technique. Patients were clamped for 90 min to blood glucose of 5 mmol L(-1) (euglycaemia) and 15 mmol L(-1) (hyperglycaemia) in random order. Blood pressure was measured noninvasively every 5 min (Takeda TM2421 device). Regulatory hormones were determined at the end of each clamp period.

RESULTS

Systolic blood pressure increased [mean (95% CI)] 3 (1, 5) mmHg during hyperglycaemia from 123 (SEM 2) during euglycaemia, P=0.01. Diastolic blood pressure remained unchanged at 78 (2) mmHg. Hyperglycaemia reduced plasma concentrations of: renin [14 (4, 23)%, P=0.02], angiotensin II [17 (8, 25)%, P<0.01] and adrenaline [20 (10, 29)%, P<0.01]. Plasma concentration of atrial natriuretic peptide increased by 11 (6, 17) pg mL(-1) (P<0.01) from 43 (2) pg mL(-1). We calculated a median (range) increase in extracellular volume and plasma volume (PV) of 2.6 (0.7-5.3)% and 5.0 (-4.7 to 8.6)%, respectively.

CONCLUSIONS

In type 1 diabetic patients without signs of autonomic neuropathy short-term hyperglycaemia induced a modest increase in systolic blood pressure and suppression of the renin-angiotensin system, possibly caused by PV expansion because of fluid shift from intra- to extracellular compartment.

Low plasma aldosterone despite normal plasma renin activity in uncomplicated type 1 diabetes mellitus: effects of RAAS stimulation

BACKGROUND

Data on levels and responsiveness of PRA and aldosterone in type 1 diabetes mellitus are conflicting. Earlier studies were not standardized with respect to the type of diabetes mellitus, the presence of diabetic complications or sodium intake. Therefore, we studied plasma renin activity and plasma aldosterone in uncomplicated type 1 diabetes mellitus by evaluating the effects of endogenous (sodium restriction) and exogenous (angiotensin I infusion) stimulation.

DESIGN

Twenty-four type 1 diabetic patients and 24 matched healthy subjects were studied after 1 week of liberal sodium diet (200 mmol 24 h-1) and 1 week of low sodium diet (50 mmol 24 h-1). Angiotensin (Ang)I was infused at 4 and 8 ng kg-1 min-1 during both study days.

RESULTS

During liberal and low sodium intake, plasma aldosterone was lower in type 1 diabetic patients compared with healthy subjects both at 08:00 h (P < 0.05) and after a 2-h euglycaemic clamp (P < 0.05), despite similar PRA levels. The correlations between changes in PRA and changes in plasma aldosterone when shifting sodium intake were similar in both groups. During liberal sodium intake, the aldosterone levels after AngI infusion were lower in type 1 diabetic patients, whereas during low sodium they were not different.

CONCLUSIONS

Plasma aldosterone was deceased relative to PRA in uncomplicated type 1 diabetic patients, irrespective sodium intake. The responsiveness to sodium restriction was adequate and sodium restriction was able to overcome the decreased plasma aldosterone response to exogenous AngI, which was observed during liberal sodium in diabetic patients. The lower aldosterone is not secondary to diabetic complications and does not depend on the level of sodium intake.

Relationship between diurnal blood pressure, renal hemodynamic function, and the renin-angiotensin system in type 1 diabetes

In patients with diabetes, altered diurnal blood pressure (BP) regulation (high night-to-day [N/D] ratio, or "nondipping") is associated with increases in albumin excretion and a decline in the glomerular filtration rate (GFR) by an unknown mechanism. Because it is known that renin angiotensin system (RAS) activation and defective glucose control contribute to adverse renal outcomes, we examined renal responses to high glucose and to manipulation of the RAS in adolescents (mean age 14 +/- 2 years) with uncomplicated type 1 diabetes, segregated into two groups on the basis of the presence or absence of normal N/D BP ratio. In the first experiment, renal hemodynamic comparisons were made during euglycemia (4-6 mmol/l) and hyperglycemia (9-11 mmol/l), maintained by modified clamp techniques. The induction of hyperglycemia resulted in a significant increase in GFR and filtration fraction (FF) in the high N/D ratio group. In the second experiment, we examined the renal response to graded angiotensin II (Ang II) infusion while subjects were euglycemic and salt replete. High N/D ratio was associated with an enhanced FF response to Ang II. In the third experiment, the N/D ratio and GFR were assessed after 3 weeks of ACE inhibition. This maneuver corrected the high N/D ratio, but it had no effect on glomerular hyperfiltration. These results suggest that RAS activation does not explain the hyperfiltration state, nor can it explain the poor outcomes, at least in this population. However, the observed deleterious hemodynamic responses to high glucose and Ang II and the insensitivity to ACE inhibition may, taken together, provide an explanation for the adverse renal outcomes in patients with type 1 diabetes and high N/D ratio.

Outcome studies in diabetic nephropathy

Outcome studies in diabetic nephropathy have focused on strategies to prevent progression of diabetic nephropathy, the leading cause of ESRD in the United States. Once diabetics develop overt nephropathy, prognosis is poor. Risk factors for diabetic nephropathy are discussed, and include hyperglycemia, hypertension, angiotensin II, proteinuria, dyslipidemia, smoking, and anemia. Major outcomes as well as outcome studies in diabetic nephropathy for patients with microalbuminuria and macroalbuminuria are reviewed. Furthermore, the role of therapy with angiotensin converting enzyme inhibitors, angiotensin II receptor blockers, calcium channel blockers, and mineralocorticoid receptor antagonists as well as selected combination therapy are discussed. Recommendations for therapy with ace inhibitors and angiotensin II receptor blockers are made based on this evidence.

Angiotensin II and the glomerulus: focus on diabetic kidney disease

The renin-angiotensin system plays a key role in the progression of kidney disease, in addition to its well-described role in the maintenance of extracellular fluid volume and blood pressure. Recent studies have shown that blockade of the renin-angiotensin system at the level of the angiotensin II type 1 receptor can have important effects on proteinuria and the rate of progression of kidney disease in patients with type 2 diabetes mellitus. This review first discusses recent experimental studies relating angiotensin II to kidney function in diabetes mellitus and changes in glomerular permselectivity, and then focuses on recent clinical trials with angiotensin II receptor blockers in patients with type 2 diabetes mellitus.

Intrarenal hemodynamic changes after captopril test in patients with type 2 diabetes: a duplex Doppler sonography study

OBJECTIVE

ACE inhibitors are known to be effective in preventing the progression of diabetic nephropathy. Activation of the renin-angiotensin system (RAS) is reported to contribute to intrarenal hemodynamic abnormality in diabetic patients. We examined whether RAS blockade by captopril induces intrarenal hemodynamic changes in normotensive patients with type 2 diabetes.

RESEARCH DESIGN AND METHODS

The patients ranged in age from 40 to 65 years (20 men and 20 women). A total of 15 age- and sex-matched healthy individuals served as control subjects. Resistive index (RI) of interlobar arteries was examined by duplex Doppler sonography before and after the oral captopril (25 mg) test.

RESULTS

At baseline, no significant differences in RI values or plasma renin activity (PRA) were seen between the patients and healthy subjects. In healthy subjects, the RI values after the captopril test were significantly higher than baseline values (P < 0.01). However, in patients with type 2 diabetes, both with normoalbuminuria and microalbuminuria, RI values after the test were significantly lower than baseline values (P < 0.001). There were significant negative correlations between DeltaRI value and HbA1c (r = -0.458, P < 0.005) and between DeltaRI value and baseline PRA in diabetic patients (r = -0.339, P < 0.05). Multiple regression analysis showed that HbA1c and baseline PRA significantly and independently affected the magnitude of decrease in RI values after captopril administration in diabetic patients (R2 = 0.391, P < 0.0001).

CONCLUSIONS

These results indicate that the intrarenal RAS may be activated in diabetic patients, that such activation may be affected by poor glycemic control, and that blockade of RAS activation by ACE inhibitor reduces intrarenal vascular resistance in diabetic patients. The results emphasize the beneficial effects of ACE inhibition in improving intrarenal hemodynamics in diabetic patients.

Glomerular hemodynamics and the renin-angiotensin system in patients with type 1 diabetes mellitus

BACKGROUND

Many studies have reported that blocking the renin-angiotensin-system (RAS) with an angiotensin-converting enzyme (ACE) inhibitor or an angiotensin receptor blocker in the patient with diabetes mellitus leads to an increase in renal plasma flow (RPF), no change in glomerular filtration rate (GFR), and a fall in filtration fraction. This constellation is generally attributed to predominant efferent arteriolar dilation.

METHODS

This study examined the renal hemodynamic response to blocking the RAS with both captopril and candesartan on separate days in 31 patients with type 1 diabetes mellitus.

RESULTS

There was a wide range of changes in RPF and GFR in response to the two agents, each administered at the top of its dose-response range. The RPF response to the two agents was strongly concordant (r = 0.65; P < 0.001), as was the GFR response (r = 0.81; P < 0.001). Moreover, there was a strong correlation between the RPF response and the change in GFR with each agent (r = 0.83 and 0.66; P < 0.01). A significant rise in RPF was followed by a rise in GFR. The RPF dependency of GFR in the type 1 diabetics suggests strongly that glomerular filtration equilibrium exists in the glomeruli of the diabetic kidney: Simple notions of local control based on afferent:efferent arteriolar resistance ratios are too simplistic.

CONCLUSION

Our data suggest that the intrarenal RAS is activated in over 80% of patients with type 1 diabetes mellitus. Abundant evidence suggests that this activation predisposes to diabetic nephropathy.

Effect of enalapril and losartan on the events that precede diabetic nephropathy in rats

BACKGROUND

Mesangial cell proliferation, phenotype change, and increased transforming growth factor-beta (TGF-beta) precede mesangial expansion in diabetic rats. Experiments using mesangial cell culture have shown that angiotensin II increases TGF-beta production by these cells. The aim of the present study was to investigate the effect of enalapril and losartan on the events that precede diabetic nephropathy in rats. It was also analyzed if the determination of urinary TGF-beta could be a mean for the evaluation of therapeutic efficacy in this disease.

METHODS

Eighty-two female Wistar rats were made diabetic by intravenous injection of streptozotocin diluted in citrate buffer, and citrate buffer alone was injected into the control group (N = 34). Ten days later, the right kidney was removed. Thirty diabetic rats were treated with enalapril, DMN + E, in drinking water (20 mg/L) and 24 with losartan, DMN + L (50 mg/L). Urinary TGF-beta was determined 90 days after STZ or buffer injection, the animals were killed, and the kidneys were removed for histological and immunohistochemical studies.

RESULTS

The immunostaining for TGF-beta and fibronectin in the cortical tubulointerstitium and glomeruli was higher in untreated diabetic rats (p < 0.001). Treatment with enalapril or losartan reduced this increase. The urinary TGF-beta excretion (pg/mg urinary creatinine) was 48.6 +/- 5.9 in control animals, 603.9 +/- 80.41 in untreated diabetic rats, 279.3 +/- 47.0 in diabetic rats treated with enalapril, and 243.7 +/- 40.0 in rats treated with losartan.

CONCLUSIONS

We concluded that enalapril or losartan treatment can modify events that precede diabetic nephropathy by reducing TGF-beta and fibronectin expression in glomeruli and tubulointerstitium as well as urinary TGF-beta content.

Appropriate drug therapy for improving outcomes in diabetic nephropathy

Diabetic nephropathy is the leading cause of end-stage kidney disease in the United States. The majority of these cases are attributed to those with type 2 diabetes. Elevated blood pressure, proteinuria, and increased activity of the renin-angiotensin-aldosterone system (RAAS) play a major role in the development and progression of chronic kidney disease attributed to diabetes mellitus. Moreover, drugs that inhibit angiotensin II synthesis or block the angiotensin II type I receptor lower blood pressure, reduce proteinuria, and improve outcomes in patients with chronic kidney disease caused by diabetes. This article highlights improvements in the current management of diabetic nephropathy afforded by agents that inhibit the RAAS, discusses their limitations, and considers novel strategies to prevent onset and progression of diabetic nephropathy. Current opinions concerning combination drug therapy with agents that block the RAAS at multiple sites, as well as combining calcium channel blockers with either angiotensin-converting enzyme inhibitors or angiotensin II receptor antagonists, are also discussed.

Kidney and hypertension

There is a unique relationship between the kidney and blood pressure (BP): on the one hand, renal dysfunction and particularly renal disease cause an increase in BP, while on the other hand, high BP accelerates loss of function of the diseased kidney. Transplantation studies, both in experimental animals and humans, documented that "blood pressure goes with the kidney," a normotensive recipient of a kidney genetically programmed for hypertension (HT) will develop HT, while conversely hypertensive patients with renal failure receiving the kidney of a normotensive donor may develop normotension. Family studies showed higher BP values and more frequent HT in first degree relatives of patients with primary glomerulonephritis or diabetic nephropathy, both type 1 and type 2. The notion that HT accelerates the loss of renal function has been proposed at the turn of the century, but definite evidence by observational and interventional studies has only been provided in the last two decades. The issue has been much confounded by the mistaken believe that damaged kidneys require higher BP values in order to function properly. The mechanisms of BP increase in renal disease comprise: salt retention, inappropriate activity of the renin-angiotensin system (RAS) and of the sympathetic nerve system as well as impaired endothelial cell-mediated vasodilatation. There is ample evidence both in primary renal disease (AIPRI and REIN trials) and in nephropathy of type 1 and type 2 diabetes (IDNT, RENAAL) that pharmacological blockade of the RAS by angiotensin-converting enzyme inhibitors or angiotensin II type 1 receptor blockers has BP-independent renoprotective effects. More recently, it has also been shown that blockade of the sympathetic nerve system has BP-independent effects on albuminuria and on glomerulosclerosis.

Losartan titration versus diuretic combination in type 2 diabetic patients

METHODS

We compared the effects of Losartan dose titration to 100 mg versus the addition of 12.5 mg of hydrochlorothiazide, in 90 type 2 diabetic patients with microalbuminuria and blood pressure > 130/85 mmHg, receiving losartan 50 mg as initial treatment during 4 weeks.

RESULTS

With the first dose of losartan, systolic (SBP) and diastolic blood pressure (DBP) decreased from 154.5 (152.1-157.5) to 144.4 (141.3-147.5) mmHg (P < 0.001) and from 91.1 (89.4-92.8) to 84.6 (82.8-86.4) mmHg (P < 0.001), with 20 patients attaining the expected goal blood pressure (< 130/85 mmHg); albuminuria decreased from 109.8 (90.5-133.3) to 83.5 (63.6-109.5) mg per 24 h (P = 0.006). Patients not attaining the target blood pressure were randomly allocated to titration or to the combination arm. After an additional 4 weeks, patients titrated exhibited a fall in SBP and DBP from 157.1 (152.7-161.5) to 142.1 (136.4-147.8) mmHg (P < 0.001) and from 92.4 (89.5-95.3) to 83.6 (81.1-86.1) mmHg (P < 0.001); albuminuria decreased from 136.3 (97.8-189.9) to 99.7 (69.3-143.4) mg per 24 h (P = 0.002). In the combination arm, there were similar reductions in SBP and DBP from 155.3 (151.5-159.1) to 139.1 (132.1-146.1) mmHg (P < 0.001) and from 92.1 (89.3-94.9) to 80.9 (77.4-84.4) mmHg (P < 0.001); while albuminuria fell from 107.7 (82.2-141.0) to 64.2 (45.9-89.9) mg per 24 h (P = 0.001).

CONCLUSIONS

Losartan 50 mg was effective in reducing blood pressure and albuminuria in type 2 diabetic patients. When the blood pressure target was not reached, the two strategies tested seem to contribute similarly to further reductions in blood pressure and albuminuria.

Hyperglycaemia-induced intrarenal RAS activation: the contribution of metabolic pathways

Hyperglycaemia-induced activation of the renin-angiotensin system (RAS) has been observed in normal and diabetic humans. Our main objective was to determine whether the mechanism involved a physical or metabolic effect of glucose. First, Sprague-Dawley rats of the CD strain were given sequential intravenous (i.v.) doses of 0.01, 0.1, 1.0, and 3.0 mg/kg candesartan 30 minutes apart, in the presence of a continuous i.v. infusion of dextrose 20% in water (D20W). The 0.1 mg/kg dose produced a maximal renal blood flow (RBF) response and was used thereafter. Another set of animals then received an infusion of either normal saline (NS), dextrose 5% in water (D5W) or dextrose 20% in water (D20W) for 2 hours, followed by candesartan 0.1 mg/kg i.v. Finally, the response to candesartan 0.1 mg/kg i.v. during D20W infusion was compared with that during infusion of 2-deoxyglucose (2DG), a glucose analogue that competitively inhibits the glycolytic enzyme, hexokinase. RBF (electromagnetic flowmeter), blood pressure (BP), blood glucose, and urine glucose were monitored. There was no significant RBF response to candesartan on either NS (6.01 to 0.48 to 6.20 to 0.49 ml/minute/g kidney; p=0.216) or D5W (7.63 to 1.20 to 7.58 to 1.39 ml/minute/g kidney; p=0.965), whereas there was a significant response to D20W (6.64 to 0.59 to 7.46 to 0.67 ml/minute/g kidney; p=0.002). The RBF response was significantly enhanced by D20W compared with 2DG (change in RBF: 0.82 to 0.22 vs. -0.04 to 0.26; p=0.05), despite similar BP, blood glucose, and urine glucose. Glucose acts, at least in part, through intracellular utilisation to induce RAS activation, as manifested by an enhanced renal vascular response to an angiotensin II antagonist.

Variation in the renin angiotensin system throughout the normal menstrual cycle

It has been demonstrated elsewhere that circulating renin angiotensin system (RAS) components peak when plasma estrogen levels are highest, during the luteal phase of the normal menstrual cycle. This phenomenon has been attributed to "activation" of the RAS. The end-organ vasoconstrictive response to this phenomenon has not been well established. In two related experiments, the RAS was studied in healthy, premenopausal women during predefined phases of the normal menstrual cycle. In the first experiment, the circulating components of the RAS and the systemic hemodynamic response to incremental lower body negative pressure (LBNP) during the follicular and luteal phases of the menstrual cycle were examined. Response variables included mean arterial pressure (MAP), renin, plasma renin activity (PRA), angiotensin II (AngII), and aldosterone. Baseline levels of renin, PRA, and aldosterone were significantly higher in the luteal phase. In response to LBNP, there were significant increases in all variables in both phases; however, the humoral response to this stimulus was significantly augmented in the luteal phase compared with the follicular phase. Despite these elevations in circulating components of the RAS during the luteal phase, subjects were unable to maintain MAP in response to LBNP, exhibiting a dramatic depressor response that did not occur during the follicular phase. In the second experiment, renal and peripheral hemodynamic function at baseline, and in response to AngII blockade with losartan, were examined in women during these high and low estrogen phases of the menstrual cycle. The renal and peripheral hemodynamic responses were similar in the luteal phase and the follicular phase. These results demonstrate that, despite an increase in circulating RAS components during the luteal phase of the menstrual cycle, the system is blunted rather than "activated," at least at a tissue level. Further studies are needed to clarify this mechanism.

Effects of amino acids and glucagon on renal hemodynamics in type 1 diabetes

Increased dietary protein and circulating amino acids raise glomerular filtration rate (GFR) and pressure. In diabetes, this glomerular hyperfiltration response is augmented. The purpose of this study was to determine whether glucagon mediates the augmented GFR response to amino acids in diabetes and whether the responses to amino acids and glucagon depend on prostaglandins. Patients with type 1 diabetes mellitus (n = 12) and normal control subjects (n = 12) were studied in a series of six experiments, each on different occasions. Baseline GFR was not significantly increased, but filtration fraction was higher in diabetes. In response to amino acid infusion, GFR increased more and filtration fraction was greater among those with diabetes. Their augmented GFR response to amino acids was not inhibited by octreotide or indomethacin. Participants with diabetes also had enhanced GFR and renal plasma flow responses to glucagon infusion, both of which were inhibited by indomethacin. Glomerular hyperfiltration responses induced by amino acids or glucagon occur by divergent pathways in diabetes; only the response to glucagon is prostaglandin dependent.

Blood pressure control early in diabetes: a balance between angiotensin II and nitric oxide

1. Hyperglycaemia can lead to hypertension in long- standing diabetes through cumulative effects to cause progressive glomerular injury. However, it is not known how hyperglycaemia directly affects blood pressure control. Moreover, it has been difficult to isolate the actions of hyperglycaemia from other factors that can influence blood pressure. Recent studies at the earliest stages of type I diabetes have started to address these issues. 2. In humans and in animal models of type I diabetes, there is evidence that onset of hyperglycaemia can increase blood pressure. The increase is mild and has been linked to angiotensin (Ang) II, possibly due to slight overstimulation of the system in response to hyperglycaemia. 3. We have reported recently that onset of hyperglycaemia causes severe hypertension in rats that are treated chronically with N(G)-nitro-L-arginine methyl ester and the hypertension is accompanied by a progressive increase in AngII and absence of hyperfiltration. This suggests that nitric oxide (NO), at the early stages of diabetes, is important in mediating the increase in glomerular filtration rate and in preventing hypertension. 4. We propose a hypothesis based on these results that hyperglycaemia at the earliest stages of diabetes induces hyperfiltration and natriuresis and also activates a compensatory hypertensive stimulus (likely AngII) in an attempt to maintain normal blood pressure. A critical balance exists between these forces and NO plays a pivotal role by attenuating AngII- mediated renal vasoconstriction and facilitating the increase in glomerular filtration rate.

Angiotensin receptor blockers in diabetic nephropathy

Where shall we place angiotensin receptor blockers in the scheme of the prevention of diabetic nephropathy? Only the results of a large, randomized double-blind trial with a comparable and appropriate alternative would prove therapeutic efficacy. The results of several trials with angiotensin-converting enzyme (ACE) inhibitors have proven them to be the standard of care for diabetics and their kidneys. As reviewed in this article, the results of three large such clinical trials have recently been completed with angiotensin receptor blockers in patients with type 2 diabetes mellitus. Initial results appear favorable. However, whether angiotensin blockers have more to offer than ACE inhibitors is still speculative. The renin-angiotensin system plays an important role in the pathogenesis of diabetic nephropathy. Since alternative pathways to ACE have been uncovered in the formation of angiotensin II, inhibition at the final end point would provide favored blockade. Because angiotensin receptor blockers do provide this specific blockade, they offer far more promise than ACE inhibitors.

The relationship between active renin concentration and plasma renin activity in Type 1 diabetes

AIMS

Circulating activity of the renin-angiotensin-aldosterone system (RAAS) can be assessed by measuring plasma active renin concentration (ARE), as well as by measuring plasma renin activity (PRA). We aimed to assess the relationships between ARE and PRA in Type 1 diabetic compared with non-diabetic control subjects. We also assessed concentrations of the active renin precursor, prorenin.

PATIENTS AND METHODS

Thirty-five Type 1 diabetic subjects and 34 non-diabetic control subjects were assessed. Groups had similar ages, sex distributions, body mass indices, systolic and diastolic blood pressures. PRA was measured by radioimmunoassay of angiotensin I generation from endogenous substrate. ARE and total renin concentration (TRE) were measured by immunoradiometric assay (Nichols Institute Diagnostics, USA). Prorenin concentration was calculated as the difference between ARE and TRE.

RESULTS

PRA was significantly lower in Type 1 diabetic than in control subjects (0.8 (0.4-1.1) vs. 1.1 (0.9-1.9) pmol/ml per h; P < 0.005), while ARE was similar (17 (9-33) vs. 18 (15-25) mU/l; P = 0.548). PRA (loge transformed) correlated strongly with ARE in diabetic (r = 0.49; P = 0.003) and control subjects (r = 0.59; P = 0.0002), but there was significant vertical separation of the regression lines for the two groups (P < 0.0001). Prorenin concentrations were significantly higher in Type 1 diabetic subjects (249 (170-339) vs. 171 (153-219) mU/l; P = 0.005). Diabetic subjects with high prorenin concentrations (> 400 mU/l (control mean + 3 SD)) were more likely to have microalbuminuria (P = 0.027) and peripheral neuropathy (P = 0.049).

CONCLUSIONS

Type 1 diabetes is associated with an altered relationship between ARE and PRA, such that ARE is higher for a given PRA compared with non-diabetic control subjects. Both ARE and PRA are used to assess circulating RAAS activity. The altered relationship between the two in Type 1 diabetic subjects suggests that neither parameter alone is necessarily an adequate and reliable index of such activity. Higher prorenin concentrations, particularly in association with microvascular complications, were confirmed in the Type 1 diabetic subjects. Diabet. Med. 18, 451-458 (2001)

Renal vascular responses to captopril and to candesartan in patients with type 1 diabetes mellitus

BACKGROUND

Enhanced renal vasodilator responses to angiotensin-converting enzyme (ACE) inhibition in diabetes mellitus despite a normal or low plasma renin activity level have suggested intrarenal activation of the renin-angiotensin system in this disease. There is, however, a continuing debate as to the mediators of the renal hemodynamic response to ACE inhibition-reduced angiotensin II formation or pathways involving kinins, prostaglandins, and nitric oxide.

METHODS

Twelve patients with type 1 diabetes mellitus of 18 +/- 3.2 (SEM) years of duration (7 females and 5 males, ages 17 to 50, 32 +/- 4.0 years) who were free of sustained microalbuminuria and on a high-salt diet were given the ACE inhibitor captopril (25 mg orally) on one day and the AT1 receptor blocker candesartan (16 mg orally) on another day. Renal plasma flow (RPF) and glomerular filtration rate were measured before and for four hours after administration.

RESULTS

Both drugs caused a significant increase in RPF (captopril 574 +/- 26 to 625 +/- 37 mL/min/1.73 m2, P = 0.008; candesartan 577 +/- 26 to 643 +/- 37, P = 0.004). There was a highly significant correlation between the responses to captopril and to candesartan (r = 0.86, P < 0.001). Seven subjects had an RPF response to captopril that was accentuated (90 +/- 13 mL/min/1.73 m2), while five had a response that was normal (-4 +/- 9). There was no significant change in glomerular filtration rate on either drug.

CONCLUSION

The remarkable rise in RPF in response to captopril and candesartan despite high-salt balance suggests the intrarenal activation of the renin-angiotensin system in diabetes that is not reflected in plasma renin levels. The high correlation between the renal hemodynamic response to captopril and to candesartan indicates that reduced angiotensin II formation is the main mechanism of action of the ACE inhibitor.

Blood pressure in diabetic nephropathy--current controversies

Hypertension has been recognized as an early and constant feature of diabetic nephropathy, but recent studies also suggest that a genetic predisposition to hypertension is an important risk factor for diabetic nephropathy. Antihypertensive treatment attenuates progression in diabetic nephropathy, but there is increasing evidence that very early treatment and very low target blood pressures should be implemented. There is also evidence for local activation of the renin system in the kidney as a result of hyperglycaemia. Apart from blood pressure, proteinuria should be monitored and dosing of ACE inhibitors should be guided, also by reduction of protein excretion.

Effect of oral contraceptives on the renin angiotensin system and renal function

We examined the effect of oral contraceptive (OC) usage on the renin angiotensin system (RAS) in two related experiments. In the first experiment, subjects were 34 healthy, normotensive, premenopausal women, 15 OC users and 19 OC nonusers, mean age 25 +/- 1 yr, ingesting a controlled sodium diet. We assessed arterial pressure, glomerular filtration rate, effective renal plasma flow, renal vascular resistance (RVR), and filtration fraction (FF) using inulin and p-aminohippurate clearance techniques, both at baseline and in response to the ANG II receptor blocker losartan. In the second experiment, in similar subjects, 10 OC users and 10 nonusers, we examined circulating RAS components [angiotensinogen, ANG II, aldosterone, plasma renin activity (PRA), and active renin] in response to incremental lower body negative pressure (LBNP), to determine whether renin secretion is suppressed by OC usage. OC users exhibited elevations in systolic blood pressure, RVR, and FF compared with nonusers, which were partially corrected by losartan. In the LBNP phase of the study, baseline measures of PRA, angiotensinogen, ANG II, and aldosterone were all increased in the OC group compared with the control group. Active renin levels did not differ between groups. Incremental LBNP resulted in increased circulating levels of RAS components in both groups. We conclude that the RAS is activated in women using OCs. There was no evidence that decreases in renin secretion result in normalization of the RAS as a whole.

Early streptozotocin-diabetes mellitus downregulates rat kidney AT2 receptors

The interaction of ANG II with intrarenal AT1 receptors has been implicated in the progression of diabetic nephropathy, but the role of intrarenal AT2 receptors is unknown. The present studies determined the effect of early diabetes on components of the glomerular renin-angiotensin system and on expression of kidney AT2 receptors. Three groups of rats were studied after 2 wk: 1) control (C), 2) streptozotocin (STZ)-induced diabetic (D), and 3) STZ-induced diabetic with insulin implant (D+I), to maintain normoglycemia. By competitive RT-PCR, early diabetes had no significant effect on glomerular mRNA expression for renin, angiotensinogen, or angiotensin-converting enzyme (ACE). In isolated glomeruli, nonglycosylated (41-kDa) AT1 receptor protein expression (AT1A and AT1B) was increased in D rats, with no change in glycosylated (53-kDa) AT1 receptor protein or in AT1 receptor mRNA. By contrast, STZ diabetes caused a significant decrease in glomerular AT2 receptor protein expression (47.0 +/- 6.5% of C; P < 0.001; n = 6), with partial reversal in D+I rats. In normal rat kidney, AT2 receptor immunostaining was localized to glomerular endothelial cells and tubular epithelial cells in the cortex, interstitial, and tubular cells in the outer medulla, and inner medullary collecting duct cells. STZ diabetes caused a significant decrease in AT2 receptor immunostaining in all kidney regions, an effect partially reversed in D+I rats. In summary, early diabetes has no effect on glomerular mRNA expression for renin, angiotensinogen, or ACE. AT2 receptors are present in glomeruli and are downregulated in early diabetes, as are all kidney AT2 receptors. Our data suggest that alterations in the balance of kidney AT1 and AT2 receptor expression may contribute to ANG II-mediated glomerular injury in progressive diabetic nephropathy.

Nitric oxide may be required to prevent hypertension at the onset of diabetes

Nitric oxide (NO) plays an important role in the regulation of vascular tone, and evidence suggests that endothelial-dependent relaxation, possibly mediated via NO, is impaired in diabetes. However, the role of the endothelium in arterial pressure control early in diabetes, before dysfunction develops, is not known. This was evaluated in the present study by comparing the responses to induction of diabetes in vehicle-treated rats (D, n = 7) vs. rats chronically treated with N(G)-nitro-L-arginine methyl ester (L-NAME; D+L, n = 8). A nondiabetic group also was treated with L-NAME (L, n = 7) to control for L-NAME effects over time, independent of diabetes. After baseline measurements, rats were given either vehicle or L-NAME (10 microg. kg(-1). min(-1) iv) infusion throughout the experiment. Six days later, streptozotocin (60 mg/kg iv) was administered, followed by a 3-wk diabetic study period. Induction of diabetes in the D+L rats caused a marked and progressive increase in mean arterial pressure throughout the diabetic period, averaging approximately 70 mmHg greater than in the D rats and approximately 20 mmHg greater than in the L rats. Glomerular filtration rate and renal plasma flow tended to increase during diabetes, but this trend was reversed in the D+L rats. In addition, plasma renin activity increased in the D and D+L rats during week 1 of diabetes but then returned to control in the D rats, while continuing to increase in the D+L rats. These results suggest that, in the early stages of diabetes, NO synthesis is important to prevent hypertension from developing, possibly through actions to maintain glomerular filtration and suppress renin secretion.

Angiotensin II and its receptors in the diabetic kidney

Studies using either angiotensin-converting enzyme inhibitors or type 1 (AT(1)) angiotensin II (ANG II)-receptor blockers indicate that ANG II is a mediator of progressive injury in diabetic nephropathy. However, suppression of the systemic renin-angiotensin system (RAS) generally has been shown in diabetes mellitus. Evidence suggests that intrarenal RASs within glomeruli and proximal tubules may be activated with hyperglycemia, leading to stimulation of local ANG II production, which may exert feedback inhibition of systemic renin release. Once formed, intrarenal ANG II exerts most of its well-described effects through binding to AT(1) receptors that are abundantly present in cells of the glomeruli, tubules, vasculature, and interstitium. Thus, AT(1)-receptor activation increases vascular resistance, reduces renal blood flow, and stimulates production of extracellular matrix in the mesangium and tubulointerstitium. Recent studies suggest that the adult kidney also expresses type 2 (AT(2)) ANG II receptors in glomeruli, tubular segments, and vasculature. AT(2)-receptor activation is associated with increased intrarenal nitric oxide production, stimulation of natriuresis, and inhibition of cell growth and matrix synthesis, effects that oppose those of kidney AT(1) receptors. A number of studies have shown a reduction in kidney AT(1)-receptor expression in diabetic nephropathy, suggesting that the balance between AT(1)- and AT(2)-receptor-mediated cell-signaling events may be a determinant of progression rate in diabetic nephropathy and that unopposed stimulation of AT(2) receptors by ANG II with use of AT(1)-receptor blockers may contribute to the beneficial properties of these agents. Determination of the expression pattern of AT(2) receptors in diabetes and further definition of the role of AT(2) receptors in opposing the detrimental effects of AT(1) receptors may lead to more selective targeting of the RAS in diabetic nephropathy.

Effect of angiotensin II antagonist eprosartan on hyperglycemia-induced activation of intrarenal renin-angiotensin system in healthy humans

We have previously reported that hyperglycemia in healthy human subjects increased the renal vasodilator response to the angiotensin-converting enzyme inhibitor captopril. This observation raised intriguing possibilities relevant to the pathogenesis of nephropathy in patients with diabetes mellitus. To ascertain whether the effect of captopril was indeed mediated by a reduction in angiotensin II (Ang II) formation, we performed another study in which an Ang II antagonist, eprosartan, was used in place of captopril. Nine healthy subjects were studied in high sodium balance (ie, sodium intake 200 mmol/d). On the first day, the subjects received 600 mg eprosartan orally, and renal plasma flow (RPF) and glomerular filtration rate (GFR) were measured. Glucose was infused intravenously on the second and third study days to increase plasma glucose to a level below the threshold for glycosuria ( approximately 8.8 mmol/L). Eprosartan at a dose of 600 mg or placebo was administered randomly on the second or third study day 1 hour after initiation of glucose infusion. RPF increased (by 76+/-7 mL. min(-1). 1.73 m(-2), P<0.01) in response to sustained moderate hyperglycemia and then increased further (by 147+/-15 mL. min(-1). 1. 73 m(-2), P<0.01) when eprosartan was administered during hyperglycemia. Eprosartan, conversely, did not affect RPF and GFR in normoglycemic subjects. GFR was not affected by either hyperglycemia or eprosartan. Neither plasma renin activity nor plasma Ang II concentration changed during hyperglycemia, suggesting that the hormonal responses responsible for the enhanced renal vasodilator response to eprosartan occurred within the kidney. The enhancement of the renal vasodilator effect of eprosartan during hyperglycemia is consistent with activation of the intrarenal renin-angiotensin system.