Insulin and heart failure

Insulin resistance in chronic heart failure

Insulin resistance is an important risk factor for the development of hypertension, atherosclerotic heart disease, left ventricular hypertrophy and dysfunction, and heart failure. It reflects a disturbance of glucose metabolism and potentially worsens metabolic efficiency of both skeletal muscle and cardiac muscle. The exact mechanisms of insulin resistance are not known, but the finding of significant insulin resistance occurring as a consequence of heart failure raises interesting possibilities as to its pathogenesis. While sympathetic nervous system overactivity can acutely reduce insulin sensitivity, it is not clear to what extent, in stable optimally treated chronic heart failure (CHF), the neurohormonal overactivity of this syndrome is the major cause of insulin resistance. Other potential mechanisms include the loss of skeletal muscle bulk, impaired endothelial function and reduced skeletal muscle blood flow, and a possible direct action of proinflammatory cytokines such as tumour necrosis factor-alpha. The consequences of insulin resistance in heart failure are not known, but the severity of the abnormality appears to parallel symptomatics and exercise limitation in this condition, and, in particular, be related to the impairment of gross skeletal muscle function. While specific therapies to correct insulin resistance in CHF have not been evaluated, there are several exciting possibilities on the horizon. Several nonpharmacological therapies have been shown to increase insulin sensitivity in patients with normal left ventricular function, and if these benefits could be duplicated in CHF, they may offer symptomatic benefit. These include weight reduction in the obese, regular exercise training and the use of dietary manipulation such as low-fat, high-fibre diets. Drug treatments with positive effects on insulin sensitivity include some angiotensin converting enzyme-inhibitors as well as newer drug groups, such as the glitazones and moxonidine, a centrally active agent with effects on the recently described imidazoline I-1 receptor that inhibits central sympathetic tone. The role of these agents in reversing the insulin resistance of chronic heart failure warrants further investigation.

Reversible and irreversible damage in reoxygenated 'ischemic' ventricular myocytes in culture

The LDH release pattern from cardiomyocytes under 'ischemia-like' conditions shows two phases. In the initial slow phase, reoxygenation immediately stops further enzyme release. Accelerated LDH release, which occurs concomitantly with Iysosomal enzyme release, characterizes the second phase of 'ischemia.' Reoxygenation at this stage does not put a stop to further enzyme release. Reoxygenation during the first phase of 'ischemia' rapidly restored ATP level, while in the second phase, ATP levels remained low even after 6 h of reoxygenation. This study as well as previous data seem to suggest that irreversible cellular damage leading to cell death, occurs by synergistic action of many effectors, each of which does not necessarily cause irreversible damage.

Vectorcardiographic evaluation of diabetic cardiomyopathy and of its contributing factors

In order to investigate the prevalence of vectorcardiographic bites, expression of small areas of fibrosis, atrophy or degeneration of the myocardium, we studied, using the vectorcardiograms (VCG) of 101 diabetic patients (35 with insulin-dependent and 66 with non-insulin-dependent diabetes mellitus, aged from 25 to 60 years, without hypertension, coronary artery disease, or intraventricular conduction defects) and 228 normal control subjects, matched for age and sex. The prevalence of bites was 38.6% in diabetic patients and 10.0% in the control group (p less than 0.001). Diabetic patients were also subdivided into groups according to age, sex, metabolic control, risk factors for coronary heart disease, type of diabetes, duration of diabetes and diabetic microangiopathy. No correlation was found between any of the variables investigated nor of a combination of these, and the presence of bites. We conclude that VCG is a sensitive test for cardiac involvement in diabetic patients but that it cannot be used to identify any specific factor able to influence the onset and evolution of this involvement.

Effects of excess glucose and insulin on glycolytic metabolism during experimental myocardial ischemia

The selective metabolic effects of glucose and insulin were tested in an intact working swine heart preparation. Supplements of glucose (26.6 millimolar [mM] and insulin (0.025 units/ml) were provided to 18 hearts, 9 control hearts (coronary flow 151 ml/min) and 9 hearts rendered globally ischemic (coronary flow reduced from 167 to 85 ml/min). These hearts were compared with 14 additional hearts (6 control and 8 ischemic) given no supplements (glucose 8.6 mM, no excess insulin). In hearts without supplements, ischemic significantly decreased mechanical performance, myocardial oxygen consumption, fatty acid oxidation and tissue high energy phosphate stores. Glucose consumption was reduced from 133 micromoles (mumol)/hr per g (before ischemia) to 58 mumol/hr per g (P less than 0.05), presumably from inhibition at glyceraldehyde-3-phosphate dehydrogenase. Data for control hearts with excess glucose and insulin were similar to data in control hearts without supplements except that glucose consumption and glycolytic flux were increased. Ischemia in treated hearts, as compared with untreated ischemic hearts, effected similar significant decreases in myocardial oxygen consumption, fatty acid oxidation and high energy phosphate stores and resulted in greater reductions in mechanical performance and in 10 minutes' less average survival time. Glucose consumption was reduced from 483 (before ischemia) to 242 mumol/hr per g (P less than 0.005) and inhibition at glyceraldehyde-3-phosphate dehydrogenase was again noted. Thus, excess carbohydrate and insulin hormone, when infused directly into the ischemic myocardium, did not provide an efficacious increase in either glycolytic flux or energy production. These findings suggest that an alternative explanation for the reported efficacy of glucose-insulin-potassium infusions must be sought.

Metabolic studies following thoracotomy for lung cancer with particular reference to postoperative atrial fibrillation

Atrial fibrillation has proved to be a frequent complication after thoracotomy for lung cancer. In order to study the possibility that metabolic changes are the trigger mechanism of these arrhythmias, free fatty acids (FFA), glycerol, triglycerides, serum insulin and glucose were determined pre- and postoperatively in patients both with and without this postoperative complication. In the primary series of patients, FFA were found to be increased immediately after the start of the atrial fibrillation, partly in relation to the morning values and partly to the 24-hour curve. This is a possible expression of an increased lipolysis in a causal relationship to the arrhythmia. In another series of patients, the degree of lypolysis was demonstrated using the glycerol values. With this method, no signs of increased lipolysis were found in connection with the onset of atrial fibrillation. Thus, changes in the lipoid metabolism were not found to be the trigger mechanism of the postoperative atrial fibrillation. Reduced values of serum insulin and glucose were not demonstrated either during the uncomplicated postoperative period or at the start of atrial fibrillation.

Ineffectiveness of glucose, potassium, and insulin infusion during pacing stress in chronic ischemic heart disease

Eight patients with coronary artery disease and positive exercise electrocardiograms undergoing cardiac catheterization were subjected to periods of atrial pacing-induced tachycardia at identical rates, before and during a glucose, potassium, and insulin (GKI) infusion. Steady state elevation of glucose and insulin was obtained prior to repeating the atrial pacing test in the GKI state. With each patient serving as his own control, five developed augmented clinical signs of acute ischemia during the paced GKI state, two no change, and one subjective improvement. Left ventricular end-diastolic pressure (LVEDP) was elevated in all patients (1-8 mm Hg) at rest during GKI infusion as compared to the rest control state. LVEDP during pacing was greater during the GKI paced state as compared to the control paced state in three of the eight patients whereas after pacing LVEDP was higher in four of eight patients during GKI. ST-segment depression was less during pacing in the GKI as compared to control state in four patients. A decrease in ST-segment depression was noted in two patients after pacing in the GKI state as compared to this measure after pacing in the control state. Augmented myocardial glucose uptake was demonstrated in the paced GKI state but lactate analysis failed to demonstrate a stoichiometric relationship between enhanced glucose uptake and lactate production.

It is concluded that tolerance to ischemia is not extended and in the majority of cases may be adversely affected by GKI.