The effect of an aldose reductase inhibitor on cardiovascular performance in patients with diabetes mellitus

Asymptomatic Diabetic Cardiomyopathy: an Underrecognized Entity in Type 2 Diabetes

PURPOSE OF REVIEW

Type 2 diabetes (T2D) is associated with an increased risk of heart failure (HF), with diabetic cardiomyopathy (DbCM) referring to abnormal heart structure in the absence of other driving cardiac factors such as hypertension, coronary artery disease (CAD), and valvular heart disease. Stage B DbCM is commonly asymptomatic and represents a form of stage B HF; DbCM thus represents a transitional phenotype prior to onset of symptomatic HF. The pathogenesis of DbCM is not fully elucidated but involves hyperglycemia, insulin resistance, increased free fatty acids (FFA), lipotoxicity, oxidative stress, advanced glycation end product (AGE) formation, activation of the renin-angiotensin-aldosterone system (RAAS) with an increase in angiotensin II, and dyshomeostasis of calcium, which all contribute to left ventricular hypertrophy (LVH) and cardiac systolic and diastolic dysfunction. Although DbCM is an established pathogenic process, it is underrecognized clinically due to its commonly asymptomatic nature. Raising awareness to identify high-risk individuals with stage B HF due to DbCM, who may subsequently progress to overt HF (stage C/D HF), as well as identifying new pharmacological agents and approaches to prevent functional decline, may help reduce this global health problem. The aim of this review is to focus on stage B DbCM; provide data on diagnostic approaches, current therapies, and potential therapies under investigation; and highlight the need to raise awareness and interdisciplinary dialogue among clinicians and researchers.

RECENT FINDINGS

There are no currently approved therapeutic strategies to treat or prevent progression of stage B DbCM, but multiple attempts are being made to target different pathogenic mechanisms involved in the development of DbCM. Recent cardiovascular (CV) outcome trials (CVOTs) have identified newer therapeutic agents with CV benefit, such as sodium-glucose cotransporter-2 (SGLT-2) inhibitors that reduce hospitalization for HF and glucagon-like peptide-1 (GLP-1) receptor agonists that reduce major adverse CV events (MACE), though without consistent effect on HF outcomes. Recent clinical practice guidelines recommend screening patients at high risk for HF. Further definition and interdisciplinary discussion of high-yield populations to screen, appropriate subsequent evaluation and intervention are needed to advance this area. DbCM is a complex entity that results from multiple pathogenic mechanisms triggered by impairment of glucose and lipid metabolism over many years. DbCM is commonly asymptomatic and represents a form of stage B HF. It is an underrecognized process that may progress to functional decline and overt HF. Although newer medications approved for the treatment of T2D may play an important role in reducing the risk of HF complications, less focus has been placed on earlier recognition and treatment of DbCM while asymptomatic. Additional efforts should be made to further study and target this stage in order to decrease the overall burden of HF.

Development of Aldose Reductase Inhibitors for the Treatment of Inflammatory Disorders and Cancer: Current Drug Design Strategies and Future Directions

Aldose Reductase (AR) is an enzyme that converts glucose to sorbitol during the polyol pathway of glucose metabolism. AR has been shown to be involved in the development of secondary diabetic complications due to its involvement in causing osmotic as well as oxidative stress. Various AR inhibitors have been tested for their use to treat secondary diabetic complications, such as retinopathy, neuropathy, and nephropathy in clinical studies. Recent studies also suggest the potential role of AR in mediating various inflammatory complications. Therefore, the studies on the development and potential use of AR inhibitors to treat inflammatory complications and cancer besides diabetes are currently on the rise. Further, genetic mutagenesis studies, computer modeling, and molecular dynamics studies have helped design novel and potent AR inhibitors. This review discussed the potential new therapeutic use of AR inhibitors in targeting inflammatory disorders and cancer besides diabetic complications. Further, we summarized studies on how AR inhibitors have been designed and developed for therapeutic purposes in the last few decades.

Aldo-Keto Reductases: Multifunctional Proteins as Therapeutic Targets in Diabetes and Inflammatory Disease

Aldose reductase (AR) is an NADPH-dependent aldo-keto reductase that has been shown to be involved in the pathogenesis of several blinding diseases such as uveitis, diabetic retinopathy (DR) and cataract. However, possible mechanisms linking the action of AR to these diseases are not well understood. As DR and cataract are among the leading causes of blindness in the world, there is an urgent need to explore therapeutic strategies to prevent or delay their onset. Studies with AR inhibitors and gene-targeted mice have demonstrated that the action of AR is also linked to cancer onset and progression. In this review we examine possible mechanisms that relate AR to molecular signaling cascades and thus explain why AR inhibition is an effective strategy against colon cancer as well as diseases of the eye such as uveitis, cataract, and retinopathy.

Protective mechanisms of mitochondria and heart function in diabetes

SIGNIFICANCE

The heart depends on continuous mitochondrial ATP supply and maintained redox balance to properly develop force, particularly under increased workload. During diabetes, however, myocardial energetic-redox balance is perturbed, contributing to the systolic and diastolic dysfunction known as diabetic cardiomyopathy (DC).

CRITICAL ISSUES

How these energetic and redox alterations intertwine to influence the DC progression is still poorly understood. Excessive bioavailability of both glucose and fatty acids (FAs) play a central role, leading, among other effects, to mitochondrial dysfunction. However, where and how this nutrient excess affects mitochondrial and cytoplasmic energetic/redox crossroads remains to be defined in greater detail.

RECENT ADVANCES

We review how high glucose alters cellular redox balance and affects mitochondrial DNA. Next, we address how lipid excess, either stored in lipid droplets or utilized by mitochondria, affects performance in diabetic hearts by influencing cardiac energetic and redox assets. Finally, we examine how the reciprocal energetic/redox influence between mitochondrial and cytoplasmic compartments shapes myocardial mechanical activity during the course of DC, focusing especially on the glutathione and thioredoxin systems.

FUTURE DIRECTIONS

Protecting mitochondria from losing their ability to generate energy, and to control their own reactive oxygen species emission is essential to prevent the onset and/or to slow down DC progression. We highlight mechanisms enforced by the diabetic heart to counteract glucose/FAs surplus-induced damage, such as lipid storage, enhanced mitochondria-lipid droplet interaction, and upregulation of key antioxidant enzymes. Learning more on the nature and location of mechanisms sheltering mitochondrial functions would certainly help in further optimizing therapies for human DC.

The impact of Cardiovascular Autonomic Neuropathy in diabetes: Is it associated with left ventricular dysfunction?

Cardiovascular Autonomic Neuropathy (CAN) is one of the least understood of all serious complications of diabetes. Besides increasing mortality, CAN may have various clinical sequelae including exercise intolerance, arrhythmias and painless myocardial infarction. But does it also cause left ventricular dysfunction? Patients with diabetes have a greater risk of developing congestive heart failure. Coronary artery disease and hypertension have been notorious in causing left ventricular dysfunction in many of these patients. However, even in their absence, diabetes itself, through several studies, has been proposed to cause the controversial entity, Diabetic Cardiomyopathy (DCM). Various mechanisms have been suggested. CAN through alteration in myocardial blood flow and sympathetic denervation, and through changes in myocardial neurotransmitters, including catecholamines and neurotransmitters of the neuropeptidergic system, has been and is still being studied as one of the main mechanisms to cause left ventricular dysfunction. Earlier detection of CAN and instant initiation of upcoming treatments may be a way to help prevent DCM, and thus improve the morbidity and mortality this causes to patients with diabetes.

Vascular endothelial dysfunction in diabetic cardiomyopathy: pathogenesis and potential treatment targets

Cardiovascular complications account for significant morbidity and mortality in the diabetic population. Diabetic cardiomyopathy, a prominent cardiovascular complication, has been recognized as a microvascular disease that may lead to heart failure. Pathogenesis of diabetic cardiomyopathy involves vascular endothelial cell dysfunction, as well as myocyte necrosis. Clinical trials have identified hyperglycemia as the key determinant in the development of chronic diabetic complications. Sustained hyperglycemia induces several biochemical changes including increased non-enzymatic glycation, sorbitol-myoinositol-mediated changes, redox potential alterations, and protein kinase C (PKC) activation, all of which have been implicated in diabetic cardiomyopathy. Other contributing metabolic abnormalities may include defective glucose transport, increased myocyte fatty acid uptake, and dysmetabolism. These biochemical changes manifest as hemodynamic alterations and structural changes that include capillary basement membrane (BM) thickening, interstitial fibrosis, and myocyte hypertrophy and necrosis. Diabetes-mediated biochemical anomalies show cross-interaction and complex interplay culminating in the activation of several intracellular signaling molecules. Studies in both animal and human diabetes have shown alteration of several factors including vasoactive molecules that may be instrumental in mediating structural and functional deficits at both the early and the late stages of the disease. In this review, we will highlight some of the important vascular changes leading to diabetic cardiomyopathy and discuss the emerging potential therapeutic interventions.

The influence of sulindac on diabetic cardiomyopathy: A non-invasive evaluation by Doppler echocardiography in streptozotocin-induced diabetic rats

The aim of the present study was to investigate the cardioprotective activity of sulindac as an aldose reductase inhibitor in the development of cardiomyopathy by non-invasive techniques; M-mode and Doppler echocardiography. Diabetes was induced by streptozotocin (45 mg/kg, iv) in the Sprague-Dawley rats. Echocardiography, biochemical and histological studies were carried out in normal control, diabetic untreated, diabetic vehicle (sodium carboxy methyl cellulose, 1%, po) and sulindac (6 mg/kg and 20 mg/kg, po) treated animals at varying time intervals. In the diabetic untreated and vehicle treated rats at 12 weeks after induction of diabetes, there was a significant decrease in the E-wave, an increase in the A-wave and corresponding decrease in the E/A ratio was observed. Significant decrease in the Eat was found after 12 weeks (P < 0.05). Whereas systolic function variables; ejection fraction and fractional shortening were significantly decreased (P < 0.05) after 12 weeks compared to their baseline data. In the sulindac treated animals, there were no significant alterations in the systolic and diastolic parameters were found throughout the study period. Myocardial fructose levels were significantly increased in the diabetic untreated animals compared to normal control rats (P < 0.05), whereas these were significantly decreased in the sulindac (6 mg/kg and 20 mg/kg) treated animals (301.11+/-37.98, 214.11+/-25.31, vs. 914.88+/-56.01 nmol/g) compared to diabetic vehicle treated group (P < 0.05). Extensive focal ischemic myocyte degeneration was observed in the diabetic untreated and vehicle treated rats, whereas in the sulindac (6 mg/kg) treated rats, minimal necrosis was found, with no evidence of necrosis in sulindac (20 mg/kg) group. Our results show for the first time that sulindac has a cardioprotective activity as this agent prevented the development of left ventricular dysfunction in STZ-induced diabetic rats in the 12-week chronic study.

Diabetic cardiomyopathy: evidence, mechanisms, and therapeutic implications

The presence of a diabetic cardiomyopathy, independent of hypertension and coronary artery disease, is still controversial. This systematic review seeks to evaluate the evidence for the existence of this condition, to clarify the possible mechanisms responsible, and to consider possible therapeutic implications. The existence of a diabetic cardiomyopathy is supported by epidemiological findings showing the association of diabetes with heart failure; clinical studies confirming the association of diabetes with left ventricular dysfunction independent of hypertension, coronary artery disease, and other heart disease; and experimental evidence of myocardial structural and functional changes. The most important mechanisms of diabetic cardiomyopathy are metabolic disturbances (depletion of glucose transporter 4, increased free fatty acids, carnitine deficiency, changes in calcium homeostasis), myocardial fibrosis (association with increases in angiotensin II, IGF-I, and inflammatory cytokines), small vessel disease (microangiopathy, impaired coronary flow reserve, and endothelial dysfunction), cardiac autonomic neuropathy (denervation and alterations in myocardial catecholamine levels), and insulin resistance (hyperinsulinemia and reduced insulin sensitivity). This review presents evidence that diabetes is associated with a cardiomyopathy, independent of comorbid conditions, and that metabolic disturbances, myocardial fibrosis, small vessel disease, cardiac autonomic neuropathy, and insulin resistance may all contribute to the development of diabetic heart disease.

Cardiac abnormalities in diabetic patients with neuropathy: effects of aldose reductase inhibitor administration

OBJECTIVE

The goal of this study was to determine whether treatment with an aldose reductase inhibitor (ARI) has beneficial effects on asymptomatic cardiac abnormalities in diabetic patients with neuropathy.

RESEARCH DESIGN AND METHODS

Diabetic subjects with neuropathy (n = 81) with either a low diastolic peak filling rate or impaired augmentation of left ventricular (LV) ejection fraction (LVEF) during maximal bicycle exercise were identified by gated radionuclide ventriculography. Coronary artery disease, left ventricular hypertrophy, and valvular heart disease were excluded by clinical evaluation, myocardial perfusion imaging, and echocardiography. Subjects were randomized to receive blinded treatment with either the placebo or the ARI zopolrestat 500 or 1,000 mg daily for 1 year.

RESULTS

After 1 year of ARI treatment, there were increases in resting LVEF (P < 0.02), cardiac output (P < 0.03), LV stroke volume (P < 0.004), and exercise LVEF (P < 0.001). In placebo-treated subjects, there were decreases in exercise cardiac output (P < 0.03), stroke volume (P < 0.02), and end diastolic volume (P < 0.04). Exercise LVEF increased with ARI treatment independent of blood pressure, insulin use, or the presence of baseline abnormal heart rate variability. There was no change in resting diastolic filling rates in either group.

CONCLUSIONS

Diabetic patients with neuropathy have LV abnormalities that can be stabilized and partially reversed by ARI treatment.

The effect of sorbinil, an aldose reductase inhibitor, on aortic function in control and streptozotocin-induced diabetic rats

1. The present study investigates the effect of treatment of 14-day streptozotocin-diabetic rats with the aldose reductase inhibitor, sorbinil, on changes ex vivo in aortic vasoconstriction and vasodilation. 2. Maximum contractile responses and aortic sensitivity to phenylephrine were significantly enhanced in aortae from 14-day diabetic rats, in accordance with our previous data. 3. Endothelium-dependent relaxations to carbachol were, in contrast, depressed, although endothelium-independent relaxations to forskolin and sodium nitroprusside were unaltered. 4. Sorbinil treatment of diabetic animals failed to prevent any of these diabetes-induced alterations in aortic function, and indeed exacerbated some of these alterations. In addition, sorbinil treatment caused altered aortic responses in control animals, which sometimes mirrored those found in diabetic animals. 5. It can be concluded that sorbinil may have actions in addition to, and independent of, polyol pathway inhibition. Thus, sorbinil may not be an effective tool for the investigation of aldose reductase inhibition within the vascular system of the rat.

Fructose-3-phosphate production and polyol pathway metabolism in diabetic rat hearts

Previous studies have suggested that polyol-pathway and nonenzymatic glycation may be involved in the development of cardiac myopathy, a well-known manifestation of diabetes. Although the exact etiology of this complication is not fully understood, it is likely to be multifactorial. In this study, we investigated the metabolic consequences of diabetes and the effect of aldose reductase inhibitor (ARI) treatment on cardiac tissues of Sprague-Dawley rats. Perchloric acid (PCA) extracts of hearts from the animals were examined using 31P-nuclear magnetic resonance (NMR), gas chromatography/mass spectrometry (GC/MS), and high-performance liquid chromatography (HPLC). In 31P-NMR spectra of diabetic animals, a peak resonating at the chemical shift of 5.8 ppm with a coupling constant of 10 Hz was identified as fructose-3-phosphate (F3P). Undetectable in controls (< approximately 20 nmol/g), this metabolite was present at a concentration of 81.3 +/- 16.3 nmol/g wet weight (n = 4) in diabetic rat hearts. GC/MS analysis of these extracts from diabetics also identified a decomposition product of F3P, 3-deoxyglucosone (3DG), at a concentration of 9.4 +/- 3.5 nmol/g (n = 3), compared with 0.98 +/- 0.43 nmol/g (n = 3) in controls. No evidence was found for the expected detoxification products of 3-DG, 3-deoxyfructose and 2-keto 3-deoxygluconate. Concomitant with the elevation of F3P and 3DG, fructose and sorbitol levels were also elevated in diabetic animals. Surprisingly, ARI treatment was found to have no effect on the levels of these metabolites. These data suggest that either the heart may be unique in its production of fructose or it may not readily transport the ARI sorbinil. Production of the potent glycating agents F3P and 3DG in diabetics suggests that these compounds may be contributing factors in the glycation of cardiac proteins in the diabetic rat heart.

The efficacy of aldose reductase inhibitors in the management of diabetic complications. Comparison with intensive insulin treatment and pancreatic transplantation

Recently, aldose reductase inhibitors (ARIs) have been registered in several countries for the improvement of glycaemic control. However, their efficacy is still controversial. ARIs inhibit the enhanced flux of glucose through the polyol pathway. As such, they can never be more effective than normoglycaemia, and so their potential benefits and limitations should be considered relative to the effects of prolonged euglycaemia. The clinical effects of ARIs can be put into perspective by assessing the effects of improved glycaemic control attained in randomised trials of intensive insulin treatment [such as the Diabetes Control and Complications Trial (DCCT)] and after pancreatic transplantation. Although direct comparison of these 3 interventions is hampered by differences in patient populations, duration and methods of follow-up and in the potency of ARIs, the effects of these 3 metabolic interventions and their course in time appear remarkably similar. For neuropathy, all 3 interventions induce an increase in average motor nerve conduction velocity of approximately 1 m/sec during the first months of treatment. At the same time, improvement of painful symptoms may occur. These changes probably largely represent a metabolic amelioration of the condition of the nerves. Around the second year of treatment with all 3 forms of metabolic improvement, an acceleration of nerve conduction of a similar magnitude occurs, with signs of structural nerve regeneration and some sensory recuperation. Experience with ARIs in nephropathy is still limited, but similar improvements in glomerular filtration rate and, less consistently, in urinary albumin excretion were found during short term normoglycaemia produced by all 3 forms of treatment. Comparison of a small number of studies, however, shows differences between intensive insulin regimens, pancreatic transplantation and ARIs in effects on retinopathy. Retinopathy often temporarily deteriorates in the early phases of improved glycaemic control, but this is not noted with ARIs. New microaneurysm formation was slightly reduced in a single long term study with the ARI sorbinil, but the preventive effects on the overall levels of retinopathy seemed less strong than in normoglycaemia trials of similar duration. However, the pharmacodynamic effects on inhibiting the polyol pathway differ among ARIs, and the half-life of the inhibiting effect of sorbinil may have been too short for a complete reduction of polyol pathway activity. The trials of prolonged intensive insulin therapy and pancreatic transplantation have demonstrated that very strict metabolic control must be maintained continuously for many years before a significant reduction of complications can be demonstrated.(ABSTRACT TRUNCATED AT 400 WORDS)

Ascorbic acid supplementation prevents hyperlipidemia and improves myocardial performance in streptozotocin-diabetic rats

The present study investigated the effects of ascorbic acid (AA) supplementation on the cardiac performance and the plasma levels of glucose, insulin, triglycerides, cholesterol and free fatty acid in diabetic and non-diabetic rats. Diabetes was induced by intravenous injection of streptozotocin (STZ) 55 mg/kg. AA was given in drinking water in concentrations of 1 g/l or 2 g/l for 8 weeks after STZ injection. Myocardial performance was determined using the isolated perfused working heart preparations. Following AA supplementation, there were no significant changes in any of the parameters measured in non-diabetic rats; however, the occurrence of polydipsia, hyperphagia, hyperlipidemia and myocardial dysfunction in STZ-diabetic rats was significantly alleviated in a dose-dependent manner. Nevertheless, the decreased body weight gain, hypoinsulinemia and hyperglycemia in diabetic animals were not affected. The data show that AA supplementation in STZ-diabetic rats improves both hyperlipidemia and cardiac function. However, the mechanisms of these effects and the correlation between these improvements are not clear.

Effects of aldose reductase inhibition with epalrestat on diabetes-induced changes in rat isolated atria

1. Isoprenaline and cardiac responsiveness of isolated atria from 2 and 6 week streptozotocin-diabetic rats, and their age-matched controls, was examined. The effects of chronic administration of epalrestat (40 mg/kg orally, by gavage) on diabetes-induced changes were also investigated. 2. Spontaneously beating atria, bathed in either normal or high glucose (30 mmol/L) Krebs' solution, from both 2 and 6 week diabetic rats beat more slowly and with greater force than atria from control rats. These changes in basal parameters were normalized by 2 weeks of insulin (5 U/day s.c.) treatment but not by 2 or 6 weeks of chronic treatment with epalrestat. 3. Isoprenaline (0.1 nmol-0.1 mumol/L) produced concentration-dependent increases in inotropy and chronotropy in atria from both control and diabetic rats. 4. Atria from 2 week diabetic rats displayed decreased sensitivity to the positive inotropic effects of isoprenaline. This change was normalized by chronic insulin treatment but not by chronic epalrestat treatment. 5. Atria from 6 week diabetic rats displayed increased sensitivity to the positive chronotropic effects of isoprenaline which was normalized by epalrestat. 6. These results suggest that changes observed in atria from 2 week diabetic rats may be due to hyperglycaemia per se whereas in atria from 6 week diabetic rats abnormal activity of the polyol pathway may be a contributing factor.

Increase in cardiac muscle fructose content in streptozotocin-induced diabetic rats

To evaluate the activation of the sorbitol pathway in cardiac muscle in diabetic rats, we measured sorbitol, fructose, and myo-inositol content in cardiac tissue obtained from control and streptozotocin-diabetic rats, with or without an 8-week insulin treatment, using gas chromatography-mass spectrometry (GC-MS). Cardiac fructose and sorbitol content in 10-week diabetic rats increased by 60-fold and 3.9-fold of those of control rats, respectively (P less than .001). In contrast, cardiac myo-inositol content in 10-week diabetic rats decreased to 56% (P less than .025) of the control value. The abnormalities in cardiac fructose, sorbitol, and myo-inositol content were completely normalized by the 8-week insulin treatment, which was initiated 2 weeks after the induction of diabetes. There was no difference in cardiac aldose reductase activity between control and diabetic rats. However, cardiac sorbitol dehydrogenase activity in diabetic rats was 151% (P less than .005) higher than that of control rats, although hepatic sorbitol dehydrogenase activity was not different between the two groups. These results indicate that the sorbitol pathway is significantly activated in cardiac tissue obtained from streptozotocin-induced diabetic rats, which results in the marked cardiac accumulation of fructose.

Recent clinical experience with aldose reductase inhibitors

Since 1981 a number of aldose reductase inhibitors (ARIs) have been extensively investigated in clinical trials for the treatment or prevention of diabetic complications. In general, the results from these trials have varied from no effect to improvement. In part, the inconclusive results are due to differences in the study designs. Investigation of some ARIs has been discontinued because of lack of efficacy (ponalrestat) or major toxicity (sorbinil). Of the ARIs that have been evaluated, only tolrestat is currently undergoing clinical investigation. Although the most recent studies have shown promising results, the role of ARIs in the treatment or prevention of secondary complications of diabetic neuropathy will be determined in ongoing or future clinical trials.

One-year treatment with the aldose reductase inhibitor, ponalrestat, in diabetic neuropathy

A double blind placebo controlled trial was performed to evaluate the effects of the aldose reductase inhibitor, ponalrestat, on symptomatic diabetic neuropathy. After a 4-week placebo run-in phase, 60 patients were 2:1 randomized to receive either 600 mg ponalrestat or placebo once daily over 12 months. Forty-six patients, 30 of whom were treated with ponalrestat and 16 with placebo, completed the study. Motor and sensory nerve conduction, thermal and vibration sensation thresholds, heart rate variation at rest, E/I ratio, pupillary dilation velocity and pupillary reflex latency were determined at baseline and after 6 and 12 months. Neuropathic symptom scores were assessed every 3 months. Among the fifteen nerve function parameters studied, only trends in favour of ponalrestat were noted for heart rate variation and E/I ratio after 6 months (P = 0.06), but no significant differences between the groups could be demonstrated during the study. No adverse reactions were observed. It is concluded that one-year treatment with ponalrestat has no beneficial effects on symptoms or electrophysiological parameters in diabetic neuropathy.

Cardiomyopathy associated with noninsulin-dependent diabetes

Cardiovascular disease represents the major cause of morbidity and mortality in noninsulin-dependent diabetic patients. While it was once thought that atherosclerotic vascular disease was responsible for all of these adverse effects, recent studies support the notion that one of the major adverse complications of diabetes is the development of a diabetic cardiomyopathy characterized by defects in both diastolic and systolic function. Contributing to the development of the cardiomyopathy is a shift in myosin isozyme content in favor of the least active V3 form. Also defective in the noninsulin-dependent diabetic heart is regulation of calcium homeostasis. While transport of calcium by the sarcolemmal and sarcoplasmic reticular calcium pumps are minimally affected by noninsulin-dependent diabetes, significant impairment occurs in sarcolemmal Na(+)-Ca2+ exchanger activity. This defect limits the ability of of the diabetic heart to extrude calcium, contributing to an elevation in [Ca2+]i. Also promoting the accumulation of calcium by the diabetic cell is a decrease in Na+, K+ ATPase activity, which is known to increase [Ca2+]i secondary to a rise in [Na+]i. In addition, calcium influx via the calcium channel is stimulated. Although the molecular mechanisms underlying these defects are presently unknown, the possibility that they may be related to aberrations in glucose or lipid metabolism are considered. The evidence suggests that classical theories of glucose toxicity, such as excessive polyol production or glycosylation, appear to be insignificant factors in heart. Also insignificant are defects in lipid metabolism leading to accumulation of toxic lipid amphiphiles or triacylglycerol. Rather, the major defects involve membrane changes, such as phosphatidylethanolamine N-methylation and protein phosphorylation, which can be attributed to the state of insulin resistance.

Cardiovascular autonomic dysfunction: diagnosis and prognosis

The symptoms of cardiovascular autonomic dysfunction may be subtle and occur late in the course of diabetes. They include abnormal exercise-induced cardiovascular performance, postural hypotension, and cardiac denervation syndrome. Autonomic nervous system testing involves an evaluation of the responses of complex reflex pathways. Some of the most commonly used and validated cardiovascular autonomic tests are RR-variation, the Valsalva manoeuvre, and postural testing. Sinus arrhythmia during breathing is termed RR-variation. In diabetic patients with autonomic neuropathy the magnitude of the RR-variation is decreased. Abnormal exercise-induced cardiovascular performance has been observed in diabetic subjects with abnormal RR-variation due to autonomic neuropathy. The Valsalva manoeuvre consists of forced expiration against a standardized resistance for a specified period of time. The reflex bradycardia that follows the Valsalva period in normal subjects is lacking in diabetic patients with clinical evidence of autonomic neuropathy. Postural hypotension in diabetics may be due to neuropathy or to a variety of secondary causes. An algorithm is presented to facilitate assessment of diabetic patients with postural symptoms. Treatment of postural hypotension should be directed primarily to the correction of secondary causes, in the absence of which the symptoms can be controlled by mechanical measures, plasma volume expansion, and vasoconstriction. Cardiac denervation syndrome may result in denervation supersensitivity and afferent (pain) nerve dysfunction. The RR-variation is a sensitive indicator of impairment of cardiac autonomic innervation and is a simple method for identifying asymptomatic patients at risk for painless ischaemia. Formal cardiovascular stress testing may be prudent before initiating an exercise programme in such individuals.