Effect of insulin on renal sodium handling in hyperinsulinaemic type 2 (non-insulin-dependent) diabetic patients with peripheral insulin resistance

Renal tubule insulin receptor modestly promotes elevated blood pressure and markedly stimulates glucose reabsorption

Although the cause of hypertension among individuals with obesity and insulin resistance is unknown, increased plasma insulin, acting in the kidney to increase sodium reabsorption, has been proposed as a potential mechanism. Insulin may also stimulate glucose uptake, but the contributions of tubular insulin signaling to sodium or glucose transport in the setting of insulin resistance is unknown. To directly study the role of insulin signaling in the kidney, we generated inducible renal tubule-specific insulin receptor-KO mice and used high-fat feeding and mineralocorticoids to model obesity and insulin resistance. Insulin receptor deletion did not alter blood pressure or sodium excretion in mice on a high-fat diet alone, but it mildly attenuated the increase in blood pressure with mineralocorticoid supplementation. Under these conditions, KO mice developed profound glucosuria. Insulin receptor deletion significantly reduced SGLT2 expression and increased urinary glucose excretion and urine flow. These data demonstrate a direct role for insulin receptor-stimulated sodium and glucose transport and a functional interaction of insulin signaling with mineralocorticoids in vivo. These studies uncover a potential mechanistic link between preserved insulin sensitivity and renal glucose handling in obesity and insulin resistance.

The impact of insulin resistance on the kidney and vasculature

Insulin resistance is a systemic disorder that affects many organs and insulin-regulated pathways. The disorder is characterized by a reduced action of insulin despite increased insulin concentrations (hyperinsulinaemia). The effects of insulin on the kidney and vasculature differ in part from the effects on classical insulin target organs. Insulin causes vasodilation by enhancing endothelial nitric oxide production through activation of the phosphatidylinositol 3-kinase pathway. In insulin-resistant states, this pathway is impaired and the mitogen-activated protein kinase pathway stimulates vasoconstriction. The action of insulin on perivascular fat tissue and the subsequent effects on the vascular wall are not fully understood, but the hepatokine fetuin-A, which is released by fatty liver, might promote the proinflammatory effects of perivascular fat. The strong association of salt-sensitive arterial hypertension with insulin resistance indicates an involvement of the kidney in the insulin resistance syndrome. The insulin receptor is expressed on renal tubular cells and podocytes and insulin signalling has important roles in podocyte viability and tubular function. Renal sodium transport is preserved in insulin resistance and contributes to the salt-sensitivity of blood pressure in hyperinsulinaemia. Therapeutically, renal and vascular insulin resistance can be improved by an integrated holistic approach aimed at restoring overall insulin sensitivity and improving insulin signalling.

Na+-sensitive elevation in blood pressure is ENaC independent in diet-induced obesity and insulin resistance

The majority of patients with obesity, insulin resistance, and metabolic syndrome have hypertension, but the mechanisms of hypertension are poorly understood. In these patients, impaired sodium excretion is critical for the genesis of Na(+)-sensitive hypertension, and prior studies have proposed a role for the epithelial Na(+) channel (ENaC) in this syndrome. We characterized high fat-fed mice as a model in which to study the contribution of ENaC-mediated Na(+) reabsorption in obesity and insulin resistance. High fat-fed mice demonstrated impaired Na(+) excretion and elevated blood pressure, which was significantly higher on a high-Na(+) diet compared with low fat-fed control mice. However, high fat-fed mice had no increase in ENaC activity as measured by Na(+) transport across microperfused cortical collecting ducts, electrolyte excretion, or blood pressure. In addition, we found no difference in endogenous urinary aldosterone excretion between groups on a normal or high-Na(+) diet. High fat-fed mice provide a model of metabolic syndrome, recapitulating obesity, insulin resistance, impaired natriuresis, and a Na(+)-sensitive elevation in blood pressure. Surprisingly, in contrast to previous studies, our data demonstrate that high fat feeding of mice impairs natriuresis and produces elevated blood pressure that is independent of ENaC activity and likely caused by increased Na(+) reabsorption upstream of the aldosterone-sensitive distal nephron.

The clinical picture of metabolic syndrome

Metabolic syndrome has piqued the interest and concern of physicians and patients alike. The syndrome represents a commingling of several conditions and risk factors common in the United States and links accelerated cardiovascular disease with insulin resistance. As more details are uncovered about metabolic syndrome, new questions arise. Here, Dr Doelle reviews what is known and not yet known about metabolic syndrome, its etiology, and its treatment.

Preserved Na/HCO3 cotransporter sensitivity to insulin may promote hypertension in metabolic syndrome

Hyperinsulinemia can contribute to hypertension through effects on sodium transport. To test whether the stimulatory effect of insulin on renal proximal tubule sodium transport is preserved in insulin resistance, we compared the effects of insulin on abdominal adipocytes and proximal tubules in rats and humans. Insulin markedly stimulated the sodium-bicarbonate cotransporter (NBCe1) activity in isolated proximal tubules through the phosphoinositide 3-kinase (PI3-K) pathway. Gene silencing in rats showed that while insulin receptor substrate (IRS)1 mediates the insulin effect on glucose uptake into adipocytes, IRS2 mediates the insulin effect on proximal tubule transport. The stimulatory effect of insulin on glucose uptake into adipocytes was severely reduced, but its stimulatory effect on NBCe1 activity was completely preserved in insulin-resistant Otsuka Long-Evans Tokushima Fatty (OLETF) rats and patients with insulin resistance. Despite widespread reduction of IRS1 and IRS2 expression in insulin-sensitive tissues, IRS2 expression in the kidney cortex was exceptionally preserved in both OLETF rats and patients with insulin resistance. Unlike liver, acute insulin injection failed to change the expression levels of IRS2 and sterol regulatory element-binding protein 1 in rat kidney cortex, indicating that regulatory mechanisms of IRS2 expression are distinct in liver and kidney. Thus, preserved stimulation of proximal tubule transport through the insulin/IRS2/PI3-K pathway may play an important role in the pathogenesis of hypertension associated with metabolic syndrome.

Roles of renal proximal tubule transport in acid/base balance and blood pressure regulation

Sodium-coupled bicarbonate absorption from renal proximal tubules (PTs) plays a pivotal role in the maintenance of systemic acid/base balance. Indeed, mutations in the Na⁺-HCO₃⁻ cotransporter NBCe1, which mediates a majority of bicarbonate exit from PTs, cause severe proximal renal tubular acidosis associated with ocular and other extrarenal abnormalities. Sodium transport in PTs also plays an important role in the regulation of blood pressure. For example, PT transport stimulation by insulin may be involved in the pathogenesis of hypertension associated with insulin resistance. Type 1 angiotensin (Ang) II receptors in PT are critical for blood pressure homeostasis. Paradoxically, the effects of Ang II on PT transport are known to be biphasic. Unlike in other species, however, Ang II is recently shown to dose-dependently stimulate human PT transport via nitric oxide/cGMP/ERK pathway, which may represent a novel therapeutic target in human hypertension. In this paper, we will review the physiological and pathophysiological roles of PT transport.

Pathophysiology of the diabetic kidney

Diabetes mellitus contributes greatly to morbidity, mortality, and overall health care costs. In major part, these outcomes derive from the high incidence of progressive kidney dysfunction in patients with diabetes making diabetic nephropathy a leading cause of end-stage renal disease. A better understanding of the molecular mechanism involved and of the early dysfunctions observed in the diabetic kidney may permit the development of new strategies to prevent diabetic nephropathy. Here we review the pathophysiological changes that occur in the kidney in response to hyperglycemia, including the cellular responses to high glucose and the responses in vascular, glomerular, podocyte, and tubular function. The molecular basis, characteristics, and consequences of the unique growth phenotypes observed in the diabetic kidney, including glomerular structures and tubular segments, are outlined. We delineate mechanisms of early diabetic glomerular hyperfiltration including primary vascular events as well as the primary role of tubular growth, hyperreabsorption, and tubuloglomerular communication as part of a "tubulocentric" concept of early diabetic kidney function. The latter also explains the "salt paradox" of the early diabetic kidney, that is, a unique and inverse relationship between glomerular filtration rate and dietary salt intake. The mechanisms and consequences of the intrarenal activation of the renin-angiotensin system and of diabetes-induced tubular glycogen accumulation are discussed. Moreover, we aim to link the changes that occur early in the diabetic kidney including the growth phenotype, oxidative stress, hypoxia, and formation of advanced glycation end products to mechanisms involved in progressive kidney disease.

Salt sensitivity is associated with insulin resistance, sympathetic overactivity, and decreased suppression of circulating renin activity in lean patients with essential hypertension

BACKGROUND

The mechanisms by which a derangement of glucose metabolism causes high blood pressure are not fully understood.

OBJECTIVES

This study aimed to clarify the relation between salt sensitivity of blood pressure and insulin resistance, which are important subcharacteristics of hypertension and impaired glucose metabolism, respectively. Effects on the renin-angiotensin and sympathetic nervous systems were also studied.

DESIGN

The state of glucose metabolism was assessed by a hyperinsulinemic euglycemic glucose clamp technique and a 75-g oral-glucose-tolerance test in 24 essential hypertensive patients who were lean and without diabetes or chronic kidney disease. The subjects were classified as salt-sensitive or salt-resistant on the basis of the difference (Delta mean blood pressure > or =5%) between 24-h ambulatory blood pressure monitoring results on the seventh day of low-salt (34 mmol/d) and high-salt (252 mmol/d) diets. Urine and blood samples were collected for analyses.

RESULTS

There was a robust inverse relation between the glucose infusion rate (GIR) and the salt sensitivity index. The GIR correlated directly with the change in urinary sodium excretion and was inversely related to the change in hematocrit when the salt diet was changed from low to high, which is indicative of salt and fluid retention in salt-sensitive subjects. The GIR also showed an inverse correlation compared with the changes in urinary norepinephrine excretion, plasma renin activity, and plasma aldosterone concentration.

CONCLUSIONS

Salt sensitivity of blood pressure is strongly associated with insulin resistance in lean, essential hypertensive patients. Hyperinsulinemia, sympathetic overactivation, and reduced suppression of the renin-angiotensin system may play a role in this relation.

Effects of intensive insulin therapy alone and with added pioglitazone on renal salt/water balance and fluid compartment shifts in type 2 diabetes

OBJECTIVE

To evaluate the effects of intensive insulin therapy alone or with added pioglitazone on renal salt/water balance and body fluid compartment shifts in type 2 diabetes.

METHODS

A total of 25 insulin-treated, obese patients with type 2 diabetes were randomized to pioglitazone 45 mg (n = 12) or placebo (n = 13) and treated intensively for 12-16 weeks to achieve equivalent glycaemic control. We measured total body water (TBW) and extracellular/intracellular fluid by bioimpedance analysis; plasma/RBC volume with I(131)albumin; sodium handling by fractional excretion of sodium/lithium (FeNa/FeLi) and other renal/hormonal parameters.

RESULTS

Intensification of insulin therapy and the addition of pioglitazone significantly improved glycaemia (HbA1C 7.8-7.2% and 7.6-7.1%) and increased body weight (1.7 and 4.9 kg) respectively. TBW increased 1.7 l with insulin alone (65% intracellular) and 1.6 l with added pioglitazone (75% extracellular) (p = 0.06 and 0.09 respectively). Plasma volume increased 0.2 +/- 0.1 l with insulin alone (p = 0.05) and 0.4 +/- 0.1 l with added pioglitazone (p < 0.05). Extravascular, extracellular (interstitial) fluid increased significantly and more with added pioglitazone (0.8 +/- 0.2 l, p < 0.01) than with insulin alone (0.4 +/- 0.2 l, p = ns). At steady-state, FeLi (marker of proximal-tubular sodium delivery to the distal nephron) increased significantly with added pioglitazone (12.4 +/- 1.3 to 18.0 +/- 3.2%) vs. no significant change with insulin alone (15.4 +/- 1.2 to 14.5 +/- 2.3%). There were no significant changes in the other parameters.

CONCLUSION

In intensively insulin-treated obese type 2 diabetic patients, at equivalent glycaemic control, the addition of pioglitazone causes greater weight gain, but a similar increase in body water that is mainly extracellular and interstitial compared with intracellular increase with insulin therapy alone. Pioglitazone also increases the filtered load of sodium reabsorbed at the distal nephron with no net change in FeNa.

Increased urinary orosomucoid excretion is not related to impaired renal function in patients with type 2 diabetes

OBJECTIVES

Increased urinary orosomucoid excretion rate (UOER) independently predicted cardiovascular mortality in patients with type 2 diabetes at 5-years of follow-up. To further explore UOER in relation to local renal physiological phenomena, we studied renal glomerular and tubular functions in patients with type 2 diabetes and normal or increased UOER.

METHODS

We performed a cross-sectional study of 40 patients with type 2 diabetes (normal UOER, n=16; increased UOER, n=24) who displayed no signs of cardiovascular disease and 21 healthy control persons. The renal clearance values of [(51)Cr]ethylenediaminetetraacetic acid ([(51)Cr]EDTA), lithium, orosomucoid, albumin, and sodium were measured.

RESULTS

Patients with type 2 diabetes had normal glomerular filtration rate (GFR) measured by [(51)Cr]EDTA clearance. The clearance value of orosomucoid was highly increased in patients with increased UOER. The clearance values of albumin were similar in patients with increased UOER and in healthy controls. Investigations of renal tubular function revealed normal and similar levels of lithium clearance and proximal and distal reabsorption of sodium and water. Serum values of orosomucoid were higher in patients with increased UOER than in healthy controls (P<.001), but were still within reference limits, suggesting chronic low-grade inflammation. UOER was associated with increasing values of orosomucoid clearance (P<.0001) independently of serum orosomucoid.

CONCLUSIONS

Patients with type 2 diabetes and increased UOER had normal GFR and showed no signs of renal glomerular or tubular dysfunction. We therefore hypothesize that increased levels of UOER may be caused by local renal production of orosomucoid due to chronic low-grade inflammation.

The metabolic syndrome: metabolic changes with vascular consequences

Despite criticism regarding its clinical relevance, the concept of the metabolic syndrome improves our understanding of both the pathophysiology of insulin resistance and its associated metabolic changes and vascular consequences. Free fatty acids (FFA) and tumour necrosis factor-alpha (TNF-alpha) play prominent roles in the development of insulin resistance by impairing the intracellular insulin signalling transduction pathway. Obesity is an independent risk factor for cardiovascular disease and strongly related to insulin resistance. In case of obesity, FFAs and TNF-alpha are produced in abundance by adipocytes, whereas the production of adiponectin, an anti-inflammatory adipokine, is reduced. This imbalanced production of pro- and anti-inflammatory adipokines, as observed in adipocyte dysfunction, is thought to be the driving force behind insulin resistance. The role of several recently discovered adipokines such as resistin, visfatin and retinol-binding protein (RBP)-4 in the pathogenesis of insulin resistance is increasingly understood. Insulin resistance induces several metabolic changes, including hyperglycaemia, dyslipidaemia and hypertension, all leading to increased cardiovascular risk. In addition, the dysfunctional adipocyte, reflected largely by low adiponectin levels and a high TNF-alpha concentration, directly influences the vascular endothelium, causing endothelial dysfunction and atherosclerosis. Adipocyte dysfunction could therefore be regarded as the common antecedent of both insulin resistance and atherosclerosis and functions as the link between obesity and cardiovascular disease. Targeting the dysfunctional adipocyte may reduce the risk for both cardiovascular disease and the development of type 2 diabetes. Although lifestyle intervention remains the cornerstone of therapy in improving insulin sensitivity and its associated metabolic changes, medical treatment might prove to be important as well.

Carbohydrate metabolism in uraemia

PURPOSE OF REVIEW

Most uraemic patients are insulin resistant. This review focuses on the occurrence, mechanisms and consequences of this insulin resistance. Hypoglycaemia is also possible in a minority of uraemic patients; its causes are discussed at the end of the review.

RECENT FINDINGS

Insulin resistance is detectable when the glomerular filtration rate is below 50 ml/min per 1.73 m in non-diabetic uraemic individuals. Uraemia can alter insulin sensitivity even in diabetic patients; familial insulin resistance may favour the occurrence of diabetic nephropathy. Although reduced glucose non-oxidative disposal is the most evident defect of carbohydrate metabolism, abnormal glucose oxidation, endogenous glucose production and insulin secretion are also contributors. The accumulation of nitrogenous compounds is the most important mechanism of a specific state of insulin resistance in uraemia. Their identification is progressing, particularly in the field of carbamoylated amino acids. The consequences of chronic renal failure such as anaemia, metabolic acidosis and secondary hyperparathyroidism also indirectly play a role.

SUMMARY

The treatment of uraemia by renal replacement therapies or low-protein diets improves insulin sensitivity. However, patients still have a high cardiovascular risk. The identification of the accumulating molecular species that specifically alter insulin sensitivity is therefore of great interest. The favourable effect of non-specific insulin sensitizers such as glitazone may also help to reduce this risk.

The metabolic syndrome or the insulin resistance syndrome? Different names, different concepts, and different goals

The goal of this article is to differentiate the insulin resistance syndrome from the metabolic syndrome. In the case of the insulin resistance syndrome, the abnormalities and clinical syndromes that are increased in prevalence in insulin-resistant individuals have been summarized, with a brief discussion of the causal relationship between insulin resistance, compensatory hyperinsulinemia, and associated consequences. Discussion of the metabolic syndrome has focused on the five criteria chosen by the adult treatment panel III to diagnose the syndrome, evaluating them in terms of their relationship to insulin resistance and their role as cardiovascular disease risk factors.

Metabolic syndrome and renal sodium handling in three ethnic groups living in England

AIM/HYPOTHESIS

Increased proximal renal sodium re-absorption is associated with central adiposity and insulin resistance in white men. Our study examined whether this association also exists in other ethnic groups with different prevalences of insulin resistance and associated metabolic abnormalities.

METHODS

We studied the association between fractional renal excretion of endogenous lithium (FELi) and metabolic syndrome in a population study of 1190 randomly selected men and women who where 40 to 59 years of age (426 white, 397 of African and 367 of South Asian origin). Anthropometric values, blood pressure, biochemical values, questionnaire data and timed urine collections were obtained with standardised techniques. Endogenous lithium in serum and urine was measured by absorption spectrophotometry. Metabolic markers were the homeostasis model assessment (HOMA) index, waist circumference, serum triglycerides, serum HDL cholesterol and metabolic syndrome as defined by Adult Treatment Panel III criteria.

RESULTS

In white men and women a higher rate of proximal sodium re-absorption was inversely associated with higher waist circumference, serum triglycerides and HOMA index, and with lower serum HDL cholesterol (all p< or =0.001). No associations were found in people of African or South Asian origin. The former had lower FELi than the other groups. White people with the metabolic syndrome had a lower FELi than those without (15.9% vs 19.0%; p=0.003). No difference was found in people of African or South Asian origin.

CONCLUSIONS/INTERPRETATION

Increased proximal sodium re-absorption is associated with the metabolic syndrome in white men and women. This relationship is not seen in people of African or South Asian origin, despite a greater degree of insulin resistance.

Insulin activates Na(+)/H(+) exchanger 3: biphasic response and glucocorticoid dependence

Insulin is an important regulator of renal salt and water excretion, and hyperinsulinemia has been implicated to play a role in hypertension. One of the target proteins of insulin action in the kidney is Na(+)/H(+) exchanger 3 (NHE3), a principal Na(+) transporter responsible for salt absorption in the mammalian proximal tubule. The molecular mechanisms involved in activation of NHE3 by insulin have not been studied so far. In opossum kidney (OK) cells, insulin increased Na(+)/H(+) exchange activity in a time- and concentration-dependent manner. This effect is due to activation of NHE3 as it persisted after pharmacological inhibition of NHE1 and NHE2. In the early phase of stimulation (2-12 h), NHE3 activity was increased without changes in NHE3 protein and mRNA. At 24 h, enhanced NHE3 activity was accompanied by an increase in total and cell surface NHE3 protein and NHE3 mRNA abundance. All the effects of insulin on NHE3 activity, protein, and mRNA were amplified in the presence of hydrocortisone. These results suggest that insulin stimulates renal tubular NHE3 activity via a biphasic mechanism involving posttranslational factors and an increase in NHE3 gene expression and the effects are dependent on the permissive action of hydrocortisone.

Renal hemodynamic effects of somatostatin are not related to inhibition of endogenous insulin release

BACKGROUND

Somatostatin inhibits endocrine and exocrine secretions and exerts renal vasoconstriction. The mechanism underlying somatostatin's vascular effects is unknown. Since insulin can cause vasodilation, we hypothesized that removal of basal insulin release by somatostatin may contribute to somatostatin-induced renal vasoconstriction.

METHODS

The study was conducted in different protocols comprising forty-six healthy male volunteers. Randomized studies were performed to compare the effects of somatostatin alone (0.1 microg/kg/min) to the effects of somatostatin + low dose insulin (0.1 mU/kg/min), the effects of somatostatin + low dose insulin to the effects of somatostatin + high dose insulin (1.5 mU/kg/min), and the effects of insulin (1.5 mU/kg/min) + somatostatin. Renal plasma flow (RPF) and glomerular filtration rate (GFR) were measured with the para-aminohippurate (PAH) and the inulin clearance technique, respectively. Blood pressure and pulse rate were measured non-invasively.

RESULTS

Somatostatin alone decreased GFR (-14 +/- 6%, P < 0.001) and RPF (-16 +/- 7%, P < 0.001) whereas systemic hemodynamics were unchanged. Preceding or concomitant infusion of insulin at high doses (insulin plasma concentration of 127 +/- 25 or 144 +/- 17 microU/mL) but not co-infusion with low dose insulin (insulin plasma concentration of 11 +/- 3 microU/mL) mitigated or reversed the vasoconstrictive actions of somatostatin on GFR and RPF.

CONCLUSIONS

Somatostatin induces marked renal vasoconstriction and exogenous restoration of fasting insulin concentrations does not influence the renal vascular effects. Therefore, it is unlikely that somatostatin-induced vasoconstriction is due to removal of basal insulin. Plasma insulin concentrations in the high postprandial range can reverse somatostatin-induced renal vasoconstriction, suggesting functional antagonism.

Impaired saline-stimulated diuresis and natriuresis in the conscious hypertensive glucose-intolerant rat

BACKGROUND

A major complication of type 2 diabetes is renal dysfunction, a condition that worsens with coexistence of systemic hypertension. However, less is known regarding the renal impact of the prediabetic condition characterized by glucose intolerance. Therefore, we tested the hypothesis that chronic glucose intolerance leads to abnormal renal function, a condition also exacerbated by concurrent systemic hypertension.

METHODS

A rat model of concomitant NaCl-sensitive hypertension and glucose intolerance was used to study renal function. Glucose intolerance was produced by administering streptozotocin to neonatal Wistar-Kyoto rats, and systemic hypertension was induced by combining dietary NaCl excess with unilateral nephrectomy. Hemodynamic and renal excretory responses were determined in conscious animals before and after administration of a 30-min isotonic saline challenge (5% of body weight, intravenously).

RESULTS

Nine-month-old glucose-intolerant rats fed a normal NaCl diet did not manifest impairment in saline-stimulated diuresis and natriuresis. By contrast, 3-month-old glucose-intolerant rats fed a high NaCl diet displayed an exaggerated diuretic and natriuretic response to a saline challenge. Although saline-stimulated diuresis and natriuresis were enhanced in the hypertensive rats, they were reduced in the hypertensive glucose-intolerant animals. The reduction in renal excretory function of the latter group was caused by alterations in both glomerular function and tubular reabsorption of fluid and sodium.

CONCLUSIONS

The coexistence of systemic hypertension and glucose intolerance result in impaired renal excretory function. This defect could be important, as hypertension can develop before the onset of overt type 2 diabetes.

Rapid effects of glucose on the insulin signaling of endothelial NO generation and epithelial Na transport

Insulin resistance is associated with deficits in glucose metabolism. We tested whether the vascular and renal responses to insulin might contribute to insulin resistance. Generation of endothelial-derived vasodilator nitric oxide (NO), estimated after a 2-h period of insulin stimulation, was inhibited in the presence of high glucose. Immunoprecipitations indicated that insulin-induced endothelial signal transduction was mediated through an immediate complex formation of insulin receptor substrate (IRS) with phosphatidylinositol 3-kinase, which caused serine phosphorylation of a protein complex that was comprised of Akt kinase and endothelial NO synthase. The enzymatic complexes did not form when the endothelial insulin stimulation occurred in the presence of high glucose concentrations. By contrast, neither epithelial signal transduction nor sodium transport in renal epithelial cells was affected by high glucose. Hence, glucose does not appear to modulate either the epithelial IRS cascade or renal sodium retention. Dysfunction of the endothelial IRS cascade and NO generation, which suppresses efficient delivery of nutrients, may further exacerbate the metabolic syndrome of insulin resistance.

Altered renal sodium handling in men with abdominal adiposity: a link to hypertension

OBJECTIVES

Central adiposity, insulin resistance and hypertension are clearly interrelated but the mechanisms underlying this association have not been thoroughly elucidated. As renal sodium handling plays a central role in salt-sensitive forms of hypertension, we investigated the relation of renal tubular sodium handling to abdominal adiposity, blood pressure and insulin sensitivity.

DESIGN

Population-based study.

PARTICIPANTS

Five hundred and fifty-five untreated Olivetti male workers, aged 25-75 years.

SETTING

Olivetti factory medical centers in Pozzuoli and Marcianise (Naples, Italy)

MAIN OUTCOME MEASURES

Anthropometric indices, serum insulin, homeostatic model assessment index of insulin sensitivity, blood pressure, fractional excretions of uric acid and exogenous lithium (as markers of renal tubular sodium handling).

RESULTS

In univariate analysis, measures of central adiposity (i.e. sagittal abdominal diameter and umbilical circumference) were directly correlated with serum insulin (P < 0.001) and blood pressure levels (P < 0.001) and inversely associated with the fractional excretions of uric acid and lithium (P = 0.01-0.001). In multiple linear regression analysis, the same anthropometric indices but not the measures of peripheral adiposity (arm circumference and tricipital skinfold thickness), were significant predictors of the fractional excretion of uric acid and lithium, independently of age, blood pressure and serum insulin levels (P = 0.01-0.001).

CONCLUSIONS

Abdominal adiposity was associated with altered renal tubular sodium handling apart from insulin resistance and high blood pressure. The data indicate that men with prevalent abdominal adiposity have an enhanced rate of tubular sodium reabsorption, mainly at proximal sites. These findings provide a possible mechanistic link between central adiposity and salt-dependent hypertension.

Renal sodium handling and haemodynamics are equally affected by hyperinsulinaemia in salt-sensitive and salt-resistant hypertensives

OBJECTIVE

It is well-known that insulin induces renal sodium retention. It is not yet known whether insulin's renal effects are involved in the development of salt-sensitive hypertension. We assessed the effects of insulin on renal sodium handling and haemodynamics in 10 salt-sensitive (SS) and 10 salt-resistant (SR) essential hypertensives.

DESIGN

After a baseline period of 90 min, all subjects underwent a euglycaemic clamp with sequential infusion of a physiological and supraphysiological dose of insulin (50 and 150 mU/kg per h) during 90 min periods each. Time-control studies were performed in the same subjects. Clearances of 131I-hippuran, 125I-iothalamate, sodium and lithium were used to evaluate renal plasma flow (RPF), CNa/glomerular filtration rate (GFR) and fractional proximal and distal sodium reabsorption.

RESULTS

Plasma insulin levels and insulin-mediated glucose uptake did not differ between both groups. RPF and GFR showed similar increases during both insulin infusions in both groups. During physiological hyperinsulinaemia, fractional sodium excretion decreased 38% (P = 0.009) in the SS group and 36% (P = 0.002) in the SR group. During supraphysiological hyperinsulinaemia, fractional sodium excretion decreased 49% (P = 0.01) in the SS group and 19% (P = 0.2) in the SR group, not statistically different between both groups. Fractional proximal sodium reabsorption was unaffected and fractional distal sodium reabsorption increased to a similar magnitude in both groups.

CONCLUSION

The comparable renal effects of acute exogenous hyperinsulinaemia in SS and SR hypertensives do not support a role for insulin in the development of salt-sensitive hypertension. However, the results do not yet exclude a role for chronic hyperinsulinaemia.

Activation of renal signaling pathways in db/db mice with type 2 diabetes

BACKGROUND

Altered regulation of signaling pathways may contribute to the pathogenesis of renal disease. We examined renal cortical signaling pathways in type 2 diabetes.

METHODS

The status of renal cortical signaling pathways was examined in control and db/db mice with type 2 diabetes in the early phase of diabetic nephropathy associated with renal matrix expansion and albuminuria.

RESULTS

Tyrosine phosphorylation of renal cortical proteins was increased in diabetic mice. Renal cortical activities of phosphatidylinositol 3-kinase (PI 3-kinase) in antiphosphotyrosine immunoprecipitates, Akt (PKB), and ERK1/2-type mitogen-activated protein (MAP) kinase activities were significantly augmented sixfold (P < 0.01), twofold (P < 0.0003), and sevenfold (P < 0.001), respectively, in diabetic mice compared with controls. A part of the increased renal cortical PI 3-kinase activity was due to insulin receptor activation, as PI 3-kinase activity associated with beta chain of the insulin receptor was increased nearly fourfold (P < 0.0235). Additionally, the kinase activity of the immunoprecipitated insulin receptor beta chain was augmented in the diabetic renal cortex, and tyrosine phosphorylation of the insulin receptor was increased. In the liver, activities of PI 3-kinase in the antiphosphotyrosine immunoprecipitates and Akt also were increased threefold (P < 0.05) and twofold (P < 0.0002), respectively. However, there was no change in the hepatic insulin receptor-associated PI 3-kinase activity. Additionally, the hepatic ERK1/2-type MAP kinase activity was inhibited by nearly 50% (P < 0.01).

CONCLUSIONS

These studies demonstrate that a variety of receptor signaling pathways are activated in the renal cortex of mice with type 2 diabetes, and suggest a role for augmented insulin receptor activity in nephropathy of type 2 diabetes.

Metabolic complications of obesity. Pathophysiologic considerations

Given a specific research interest in human fatty acid metabolism, this article focuses primarily on the evidence surrounding the hypothesis that dysregulation of the fuel release function of fat cells (lipolysis) is an important contributing factor to the health hazards of obesity.

Insulin's acute effects on glomerular filtration rate correlate with insulin sensitivity whereas insulin's acute effects on proximal tubular sodium reabsorption correlation with salt sensitivity in normal subjects

BACKGROUND

Insulin induces sodium retention by increasing distal tubular sodium reabsorption. Opposite effects of insulin to offset insulin-induced sodium retention are supposedly increases in glomerular filtration rate (GFR) and decreases in proximal tubular sodium reabsorption. Defects in these opposing effects could link insulin resistance to blood-pressure elevation and salt sensitivity.

METHODS

We assessed the relationship between the effects of sequential physiological and supraphysiological insulin dosages (50 and 150 mU/kg/h) on renal sodium handling, and insulin sensitivity and salt sensitivity using the euglycaemic clamp technique and clearances of [131I]hippuran, [125I]iothalamate, sodium, and lithium in 20 normal subjects displaying a wide range of insulin sensitivity. Time-control experiments were performed in the same subjects. Salt sensitivity was determined using a diet method.

RESULTS

During the successive insulin infusions, GFR increased by 5.9% (P = 0.003) and 10.9% (P<0.001), while fractional sodium excretion decreased by 34 and 50% (both P<0.001). Distal tubular sodium reabsorption increased and proximal tubular sodium reabsorption decreased. Insulin sensitivity correlated with changes in GFR during physiological (r = 0.60, P = 0.005) and supraphysiological (r = 0.58, P = 0.007) hyperinsulinaemia, but not with changes in proximal tubular sodium reabsorption. Salt sensitivity correlated with changes in proximal tubular sodium reabsorption (r = 0.49, P = 0.028), but not in GFR, during physiological hyperinsulinaemia. Neither insulin sensitivity or salt sensitivity correlated with changes in overall fractional sodium excretion.

CONCLUSIONS

Insulin sensitivity and salt sensitivity correlate with changes in different elements of renal sodium handling, but not with overall sodium excretion, during insulin infusion. The relevance for blood pressure regulation remains to be proved.

Blood pressure, sodium intake, insulin resistance, and urinary nitrate excretion

The objective of this study was to investigate the relationships among various humoral factors thought to be involved in the regulation of blood pressure during high NaCl intake. Nineteen healthy subjects underwent sequential 5-day periods ingesting a low-sodium (25 mmol/d) or high-sodium (200 mmol/d) diet. Insulin resistance was assessed by the steady-state plasma glucose concentration at the end of a 3-hour insulin suppression test. Insulin resistance correlated inversely with natriuresis (P=0.04) and directly with increase in weight (P=0.03). The increase in mean arterial pressure associated with the high-sodium diet correlated directly with the gain in weight (P<0.05) and inversely with the increase in urinary nitrate excretion (P<0.0001). In a multiple regression model, more than 2/3 of the variance in mean arterial pressure was accounted for by the gain in weight and change in urinary nitrate excretion. The steady-state plasma glucose concentrations obtained with the 2 diets were similar, indicating that insulin resistance was unaffected by sodium intake. During high sodium intake, plasma renin activity and aldosterone decreased and plasma atrial natriuretic peptide increased; these changes did not correlate with the change in mean arterial pressure, insulin resistance, or change in urinary nitrate excretion. To the extent that urinary nitrate excretion reflects activity of the endogenous nitric oxide system, these results suggest that the salt sensitivity of mean arterial pressure may be related to blunted generation of endogenous nitric oxide. The results also demonstrate that insulin-resistant individuals have an impaired natriuretic response to high sodium intake.

Effect of ACE gene on diabetic nephropathy in NIDDM patients with insulin resistance

We investigated the influence of the angiotensin-converting enzyme (ACE) gene on the onset and/or progression of diabetic nephropathy in 62 Japanese patients with non-insulin-dependent diabetes mellitus (NIDDM; type II diabetes). Because a number of factors are believed to be involved in the onset and/or progression of diabetic nephropathy, especially in patients with NIDDM, we selected the patients with well-matched risk factors, duration of disease, glycemic control, blood pressure, and others. All patients had normal renal function and none were receiving ACE inhibitors. Patients were divided into three groups according to albumin excretion rate (AER): group A, patients with an AER less than 15 microg/min (n = 29); group B, patients with an AER between 15 and 70 microg/min (n = 19); and group C, patients with an AER greater than 70 microg/min (n = 14). The glucose disposal rate was estimated using a euglycemic hyperinsulinemic clamp. We determined the mean glucose disposal rate in 132 patients with NIDDM (6.49 mg/kg/min). Patients with a glucose disposal rate less than the mean rate were considered to have a high degree of insulin resistance (n = 36). The presence of an insertion/deletion (I/D) polymorphism of the ACE gene was determined by the polymerase chain reaction method. Among patients with a high degree of insulin resistance, diabetic nephropathy was present in 2 of 11 patients with the II genotype of the ACE gene compared with 19 of 25 patients with the ID or DD genotype (P = 0.0024). The prevalence of diabetic nephropathy was greater in patients with both significant insulin resistance and the D allele (19 of 25) than in the remaining patients (14 of 37; odds ratio, 5.20). These results suggest that the ACE gene influences the onset and/or progression of diabetic nephropathy in patients with NIDDM with significant insulin resistance.

Insulin and coronary artery disease: is syndrome X the unifying hypothesis?

OBJECTIVE

To review data supporting the hypothesis that syndrome X plays a major role in the pathogenesis of coronary artery disease (CAD), and the effects of lifestyle factors and pharmacologic interventions on insulin, other metabolic parameters, and outcomes.

DATA SOURCES

MEDLINE (January 1966-August 1997) and Current Contents database searches identified applicable English-language experimental trials, epidemiologic studies, reviews, and editorials.

STUDY SELECTION AND DATA EXTRACTION

Studies that were included addressed the role of insulin resistance and hyperinsulinemia in the pathogenesis of CAD or the effects of lifestyle factors and pharmacologic interventions on metabolic parameters and outcomes.

DATA SYNTHESIS

The main characteristics of syndrome X are hyperinsulinemia and insulin resistance. These result in secondary syndrome X features, including hyperglycemia, increased very-low-density lipoprotein concentrations, decreased high-density lipoprotein cholesterol, and hypertension. Insulin resistance is worsened by obesity, and insulin has been shown to contribute to the development of hypertension. Other studies demonstrate that smoking adversely affects glucose and insulin concentrations. Animal studies have linked hyperinsulinemia and atherogenesis. These animal data have been confirmed by several large prospective and population studies that have identified associations between hyperinsulinemia and CAD.

CONCLUSIONS

Strong evidence links insulin resistance and hyperinsulinemia to CAD. Lifestyle modifications play an important role in decreasing cardiovascular risk, and clinicians should strongly encourage such changes. Clinicians must also carefully consider the effects of antihypertensive, antihyperglycemic, and antidyslipidemic agents on patients' metabolic profiles when choosing appropriate therapeutic regimens. However, outcome data on many potentially beneficial agents, including calcium antagonists, alpha 1-adrenergic antagonists, angiotensin-converting enzyme inhibitors, metformin, acarbose, and troglitazone, are not yet available.

The kidney: an unwilling accomplice in syndrome X

The ability of insulin to stimulate glucose disposal by muscle varies widely within the population at large. Individuals with muscle insulin resistance develop type 2 diabetes if they cannot compensate for this defect by secreting large amounts of insulin. Although this philanthropic effort on the part of the pancreatic B-cell may prevent gross decompensation of glucose homeostasis, it renders such individuals at increased risk to develop a cluster of abnormalities (syndrome X) associated with coronary heart disease. Although the kidney is not considered to be an insulin sensitive tissue, two features of syndrome X, hyperuricemia and hypertension, are likely to be dependent on the retention of normal insulin action on the kidney. More specifically, there is evidence to support the hypothesis that elevated plasma insulin concentrations may enhance renal sodium retention and decrease urinary uric acid clearance. As such, it is possible that a normal kidney response to the compensatory hyperinsulinemia associated with insulin resistance in nondiabetic subjects contributes to the development of hyperuricemia and hypertension in such individuals.

Impaired autoregulation of GFR in hypertensive non-insulin dependent diabetic patients

We investigated the effect of acute lowering of blood pressure (BP) upon glomerular filtration rate (GFR) in hypertensive non-insulin-dependent diabetes mellitus (NIDDM) patients, 14 with diabetic nephropathy and 12 with normoalbuminuria. The study was performed twice with the subjects receiving an intravenous injection of either clonidine (150 to 225 micrograms) or saline (0.154 mmol/liter). We assessed GFR, albuminuria, and BP. The two groups were well matched with respect to demographic data, baseline GFR and BP. Clonidine induced similar reductions in mean arterial blood pressure 19 (SE +/- 4) and 21 (SE +/- 3) mm Hg in patients with and without nephropathy, respectively. In the nephropathy group GFR diminished in average from 90 (SE +/- 6) to 81 (SE +/- 7) ml/min/1.73 m2 (P = 0.006), fractional clearance of albumin (x 10(-6)) declined from a geometric mean of 219 (antilog SE /divided by 1.3) to 186 (antilog SE /divided by 1.3) (P = 0.04), and four patients had a complete pressure-passive vasculature, defined as delta GFR% = delta MABP%. A significant correlation between relative reductions in MABP and GFR (r = 0.78, P < 0.001) was demonstrated in albuminuric patients. None of the normoalbuminuric patients had a complete pressure-passive vasculature and there were no significant differences in GFR between the two examinations, but five had abnormal autoregulation of GFR. Mean difference between changes in GFR (95% confidence interval) between the nephropathic and normoalbuminuric group was 5.5 (divided by 2.7 to 13.7) ml/min/1.73 m2 (P = 0.18). Our study suggests that hypertensive NIDDM patients, particularly patients with nephropathy, frequently suffer from impaired or abolished autoregulation of GFR.

Renal function in the noninsulin-dependent diabetic rat: effects of unilateral nephrectomy

A new model of noninsulin-dependent diabetic (NIDD) is described which exhibits more prominent defects in renal function than does the standard neonatal NIDD model. To produce this model, 2-day-old neonatal male Wistar Kyoto (WKY) rats were injected intraperitoneally with streptozotocin (90 mg/kg; NIDD), while their corresponding nondiabetic controls were administered vehicle (citrate buffer, pH: 4.5; control). At 3 weeks of age, the animals were weaned, and 1 week later, under ether anesthesia, the animals underwent a right nephrectomy or a sham operation. Diabetes was confirmed by intraperitoneal administration of a glucose load (2g/kg), which resulted in significantly higher blood glucose concentration in the NIDD, compared to the nondiabetic rats. Surgical reduction of renal mass had no effect on the glycemic response to a glucose tolerance test in either group. Intravenous administration of an isotonic saline load resulted in a similar pattern of enhanced sodium and fluid excretion in the two-kidney sham-operated nondiabetic and NIDD rats. These responses were significantly higher than those observed in their counterparts with one remaining kidney. Yet, the natriuretic and diuretic responses to the saline load were significantly lower in the nephrectomized NIDD, compared to the nephrectomized nondiabetic rats. The glomerular filtration rate was similar in the sham-operated (two kidneys) NIDD and nondiabetic rats. In contrast, both the basal and saline-stimulated glomerular filtration rate were lower in the nephrectomized NIDD rats compared to the nephrectomized nondiabetic group. Mean arterial pressure was similar between the two nephrectomized groups, thereby ruling out a significant contribution from the pressure-diuresis-natriuresis mechanism to the reduction in sodium and fluid excretion in the nephrectomized NIDD rats. Thus, unilateral nephrectomy is an effective method of accelerating the manifestation of NIDD-related renal alterations. The mild, but progressive, nature of diabetes in this model should facilitate the investigation of temporal changes in renal function in NIDDM.

Effects of insulin and lipid emulsion on renal haemodynamics and renal sodium handling in IDDM patients

To evaluate the role of insulin and hypertriglyceridaemia in the regulation of renal haemodynamics and sodium handling in insulin-dependent diabetes mellitus (IDDM), 11 IDDM patients without microalbuminuria and 13 weight-, age-, protein intake- and sex-matched healthy control subjects were studied. Clearances of inulin (Cin), para-amino-hippuric acid (CPAH), sodium (CNa), and lithium (CLi) were measured in four 60-min clearance periods (periods I, II, III and IV) during isoinsulinaemia with lipid emulsion infusion (study 1), a hyperinsulinaemic isoglycaemic clamp with Intralipid infusion (study 2), and during time-controlled isoinsulinaemia (study 3). We found that Cin, CPAH and filtration fraction were comparable in IDDM and control subjects, whereas CNa was decreased in diabetic subjects (2.01 +/- 1.11 vs 3.03 +/- 1.32 ml/min; p < 0.05) due to elevations of proximal tubular fractional and absolute reabsorptions of sodium (p < 0.05). Insulin infusion did not affect Cin, increased CPAH (p < 0.05) and, consequently, lowered the filtration fraction (p < 0.01) in both groups. While acute hyperinsulinaemia resulted in increases in distal tubular fractional and absolute reabsorptions of sodium (p < 0.01) contributing to a fall in CNa (p < 0.01) in control subjects, in diabetic subjects the sodium-retaining effect of insulin was not significant. The lipid emulsion did not alter any of the estimated parameters. We conclude that IDDM without microalbuminuria is associated with a tendency to sodium retention which is not aggravated by insulin when compared to control subjects. Acutely induced hypertriglyceridaemia does not alter renal haemodynamics or renal sodium handling.

Acquired oligonephropathy and diabetic nephropathy

Congenital or acquired nephron number reduction and diabetes mellitus both induce hyperfiltration and intrarenal hypertension. These hemodynamic factors have been suggested to play an important role in the initiation and progression of diabetic and nondiabetic glomerulopathies. In a prevalence cohort of all 50 albuminuric (> or = 300 mg/24 hr), non-insulin-dependent diabetic mellitus (NIDDM) patients (aged < 66 years) attending a diabetic clinic during 1987, we identified four patients with acquired oligonephropathy who had developed diabetic nephropathy. Three patients only had a single functioning kidney (nephrectomy in two cases), and the remaining patient had unilateral severely reduced kidney function and elevated plasma renin concentration. These patients represent 8% of our NIDDM population with albuminuria, and are the only patients in our NIDDM population younger than 66 years (n = 363) known to have single kidneys. However, systematic renography screening was not performed. During the observation period, which ranged from 16 to 100 months, albuminuria increased from 2.3 g/24 hr (range, 1.2 to 3.4 g/24 hr) to 3.2 g/24 hr (range, 0.7 to 9.4 g/24 hr), glomerular filtration rate decreased to 6 mL/min/yr (range, 4 to 12 mL/min/yr), and blood pressure remained stable at 142/92 mm Hg. Three patients received antihypertensive medication. Two of the patients died after 16 and 26 months of observation, respectively, due to acute myocardial infarction and stroke. The results of our study of NIDDM patients support the concept that reduced nephron number predispose patients to the development of diabetic nephropathy.

Combined renal effects of overweight and hypertension

The existence of a direct relationship between body mass and arterial pressure is well recognized; however, the effect of obesity on known target organs of hypertension is not clearly understood. We undertook the present studies to assess the influence of obesity on renal function and urinary albumin excretion in 40 normotensive subjects and 80 nevertreated hypertensive patients matched for age, sex, arterial pressure level, and known duration of hypertension in whom an oral glucose tolerance test was within normal limits. Glomerular filtration rate and effective renal plasma flow (expressed as absolute values or values normalized for height) were increased in overweight compared with lean subjects whether normotensive or hypertensive. Glomerular filtration rate was positively correlated with protein intake (as assessed from urinary excretion of urea) and fasting serum insulin level. Urinary excretion of albumin but not IgG and beta 2 microglobulin was higher in hypertensive patients compared with normotensive subjects. The overweight condition clearly enhanced the influence of arterial pressure on albuminuria; in fact, a steeper regression line between albumin excretion rate and arterial pressure was found in overweight compared with lean subjects. These results indicate that the overweight condition is associated with renal hyperfiltration and hyperperfusion, irrespective of the presence of hypertension, and that obesity magnifies the effect of hypertension on albuminuria, thus raising the possibility of an increased susceptibility of obese hypertensive patients to the development of renal damage.

Serum insulin level versus blood pressure: a cross-sectional, case-controlled study in non-obese, middle-aged Japanese subjects with normal glucose tolerance

To assess the relationship between blood pressure (BP) and serum insulin level in non-obese (body mass index (BMI) < or = 27 kg m-2), middle-aged (40-64 years of age) Japanese subjects with normal glucose tolerance, a three-phase study protocol was designed. First, the responses of plasma glucose and serum insulin to an oral glucose load were compared between 40 patients with untreated essential hypertension and 40 age-, sex- and BMI-matched normotensive control subjects. Second, the glucose and insulin responses to an i.v. glucose load were evaluated in 7 non-obese hypertensive, 7 non-obese normotensive and 7 obese hypertensive subjects. Third, BP and serum lipid profile were compared between 21 hyperinsulinaemic (serum insulin level (while fasting, after glucose loading, or both) > 2 SDs higher than the mean) and 21 age-, sex- and BMI-matched normoinsulinaemic subjects (serum insulin level within 1 SD of the mean). The glucose and insulin responses to the oral glucose load were comparable between the hypertensive and normotensive groups. Similarly, the glucose and insulin responses to the i.v. glucose load were comparable between the non-obese hypertensive and normotensive groups, whereas the mean AUCinsulin in the obese hypertensive group was significantly greater (p < 0.01) than that in either of the non-obese groups. The respective mean values for systolic and diastolic BPs did not differ between the hyperinsulinaemic and normoinsulinaemic groups. The mean serum triglyceride and HDL cholesterol concentrations were significantly higher (p < 0.01) and lower (p < 0.05), respectively, in the hyperinsuslinaemic than in the normoinsulinaemic group.(ABSTRACT TRUNCATED AT 250 WORDS)

Acute reduction of renal 11 beta-hydroxysteroid dehydrogenase activity by several antinatriuretic stimuli

The 11 beta-hydroxysteroid dehydrogenase (11 beta-HSD) activity of the kidney prevents access of cortisol or corticosterone to the renal mineralocorticoid receptor. Reduction of 11 beta-HSD activity by nutritional, hormonal, or pharmacologic factors might enhance the mineralocorticoid effect of these corticosteroids, thus causing sodium retention. To test this concept, we studied the effect on 11 beta-HSD activity of several antinatriuretic factors given orally to rats or exposed in vitro to rat renal tissue. Renal 11 beta-HSD activity was higher in fasted than fed rats (P < .05). Glucose, ethanol, and Toradol (Syntex Laboratories, Palo Alto, CA) given orally to fasted rats all reduced renal 11 beta-HSD activity by 20% to 40% (P < .05-.005) to levels similar to those observed in fed animals. Incubation of renal tissue from fasted rats with physiologic concentrations of insulin, ethanol, and Toradol also reduced 11 beta-HSD activity by 20% to 40% (P < .05-.01). These findings are consistent with the hypothesis that the antinatriuretic actions of these stimuli are due in part to alteration of renal 11 beta-HSD leading to greater mineralocorticoid effects in kidney.

Effective antihypertensive treatment postpones renal insufficiency in diabetic nephropathy

The effect of long-term, aggressive, antihypertensive treatment on kidney function in diabetic nephropathy was studied prospectively in 11 insulin-dependent diabetic patients (mean age, 30 years). Renal function was assessed every 4 months by measurement of glomerular filtration rate (GFR) (single-bolus 51Cr-EDTA technique) and by albuminuria (radial immunodiffusion). During the median pretreatment period of 2.4 years (range, 1.9 to 5.5 years), the GFR decreased significantly and albuminuria and the arterial blood pressure increased significantly. During the 9.7-year (range, 2.8 to 10.4 year) period of antihypertensive treatment with metoprolol, hydralazine, and furosemide, the arterial blood pressure decreased from 143/96 mm Hg to 130/84 mm Hg and albuminuria decreased from 1,038 micrograms/min to 547 micrograms/min. The rate of decline in GFR decreased from 10.7 mL/min/yr (range, 5.3 to 17.5 mL/min/yr) before treatment to 2.5 mL/min/yr (range, 0.5 to 4.8 mL/min/yr) during treatment. The rate of decline in GFR is significantly smaller during the last 6 years compared with the first 3 years in patients who received long-term antihypertensive treatment (> or = 9 years). One patient died from acute myocardial infarction (GFR, 46 mL/min/1.73 m2). Effective antihypertensive treatment postpones renal insufficiency in diabetic nephropathy.

Role of hyperglycemia and insulin resistance in determining sodium retention in non-insulin-dependent diabetes

Sodium retention has been advocated to give rise to hypertension in humans. Increases in blood glucose and insulin concentrations ensue in the stimulation of sodium reabsorption by the kidney. Although the combined occurrence of hyperglycemia and hyperinsulinemia, frequently secondary to insulin resistance with regard to carbohydrate metabolism, is a hallmark of non-insulin dependent diabetes (NIDDM), the role of these abnormalities in determining an impaired natriuresis in NIDDM is not yet fully understood. We studied sodium homeostasis in 14 control subjects and 59 NIDDM normotensive, normoalbuminuric patients who were divided into two groups with markedly impaired (Group 2 NIDDM: 30) and less severely impaired (Group 1 NIDDM: 29) insulin sensitivity during euglycemic-hyperinsulinemic (80 to 90 microU/ml plasma insulin) clamp. A hyperglycemic (9 mmol/liter plasma glucose)--nearly euinsulinemic (20 to 40 microU/ml plasma insulin) clamp was also performed in the same 14 controls and in two cohorts of 22 Group 2 and 17 Group 1 NIDDM patients. The two groups of patients had similar overnight fasting glucose levels (Group 1 NIDDM vs. Group 2 NIDDM: 176 +/- 13 vs. 185 +/- 15 mg/dl, mean +/- SE). Conversely, overnight fasting plasma insulin was significantly higher in Group 2 NIDDM than in Group 1 NIDDM patients (Group 1 NIDDM vs. Group 2 NIDDM: 12 +/- 3 vs. 18 +/- 3 microU/ml, P < 0.05). Both NIDDM Groups had higher plasma glucose and insulin than controls (75 +/- 4 mg/dl and 6 +/- 3 microU/ml). Blood pressure levels and albumin excretion rates were slightly but significantly higher in Group 2 NIDDM, but not in Group 1 NIDDM patients, than in controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of hyperglycemia per se on glucose turnover rates in patients with insulin-dependent diabetes

The effect of hyperglycemia, per se, on glucose utilization and hepatic glucose production was reevaluated in eight C-peptide-negative insulin-dependent diabetic patients using primed-continuous noncontaminated 3-3H-glucose infusion and labeled glucose infusates. The night before study, euglycemia was maintained by a variable insulin infusion. During the studies, insulin was infused at basal replacement rates determined as the rate required to maintain euglycemia in the morning. After a 2-hour equilibration period, either plasma glucose level was increased to 12 mmol/L for 4 hours using a variable glucose infusion, or no glucose was infused (control day). On the hyperglycemic day, glucose utilization increased 16% (86 +/- 2 to 99 +/- 4 mg.m-2.min-1, P < .02) and glucose production decreased 45% (85 +/- 3 to 47 +/- 4 mg.m-2.min-1, P < .01). On the control day, both glucose utilization and glucose production decreased (84 +/- 3 to 68 +/- 3 and 84 +/- 3 to 65 +/- 3 mg.m-2.min-1, respectively; both P < .01). Therefore, comparing rates at the end of the hyperglycemic and control studies, glucose utilization was increased by 45% and glucose production was decreased by 28% in response to hyperglycemia (both P < .01). Thus hyperglycemia, at basal insulin levels enhanced glucose utilization and suppressed glucose production in insulin-dependent diabetic patients. Quantitatively, the enhancement of glucose utilization was more important than the suppressive effect on glucose production.