Aorta and muscle metabolism in pigs with peripheral hyperinsulinaemia

The glycolytic pathway to coronary heart disease: a hypothesis

Coronary heart disease (CHD) is pathogenetically linked to numerous metabolic disturbances. These are inextricably interrelated, constituting identifiable clusters or syndromes of cardiovascular risk. Prominent among these is the insulin resistance syndrome, whose components, including hyperuricemia, have all been linked to CHD pathogenesis. Many mechanisms have been put forward to account for the emergence of this syndrome, but none offer a satisfactory explanation for the involvement of hyperuricemia. Possible explanations relate to the observation of glycolytic disturbances in insulin-resistant and hyperuricemic states. This might be expected from the fact that uric acid production is linked to glycolysis and that glycolysis is controlled by insulin. Phosphoribosylpyrophosphate (PPRP) is an important metabolite in this respect. Its availability depends on ribose-5-phosphate (R-5-P), the production of which is governed by glycolytic flux. Diversion of glycolytic intermediates toward R-5-P, PPRP, and uric acid will follow if there is diminished activity of glyceraldehyde-3-phosphate dehydrogenase (GA3PDH), which is regulated by insulin. Serum triglyceride concentrations may also increase, as might be expected from accumulation of glycerol-3-phosphate. Thus, intrinsic defects in GA3PDH and a loss of its responsiveness to insulin, by causing accumulation of glycolytic intermediates, may explain the association between insulin resistance, hyperuricemia, and hypertriglyceridemia. This scenario raises the possibility that disturbances of a single glycolytic enzyme may be pivotal in the modulation of metabolic risk factors for CHD.

Insulin as a mitogenic factor: role in the pathogenesis of cardiovascular disease

Evidence has been accumulating that insulin has actions that may promote the development of atherosclerosis. Research has involved three broad areas: actions of insulin on cultured arterial cells, the effect of insulin on isolated artery preparations, and the development of lipid-containing lesions in the arteries of experimental animals. Insulin, in concentrations similar to those found in physiologic conditions, stimulates proliferation of cultured arterial smooth muscle cells from a number of species, including humans. Insulin also stimulates migration of smooth muscle cells. Cholesterol synthesis and low-density lipoprotein interaction with its receptor in smooth muscle cells are stimulated by insulin. Insulin's mitogenic action appears to be mediated by the insulin-like growth factor receptor. Endothelial cells cultured from large vessels are resistant to the actions of insulin, but hyperglycemia inhibits their proliferation. Insulin deficiency protects animals from experimental atherosclerosis; this protection is lost with insulin treatment. Insulin administration results in lipid-containing lesions in chickens and rats fed a normal diet, and in increased lipid synthesis in the arteries of pigs and dogs. Isolated artery preparations from insulin-deficient or insulin-treated animals undergo lipid metabolism at a rate that correlates with the insulin concentrations in the donor animals. The biological actions of insulin (and glucose) on arterial tissue suggest that hyperglycemia and hyperinsulinemia may promote the development of atherosclerosis.

Influence of portal delivery of insulin on intracellular glucose and lipid metabolism

We have investigated whether portal delivery of insulin as a result of intrahepatic islet cell autografts would prevent the development of metabolic alterations. Seven pancreatectomized dogs received islet autografts transplanted into the liver through the portal vein (PD). One year after transplantation, their intravenous glucose tolerance and insulin responses were similar to age-matched control (C) dogs (n = 5). Also, normal triglyceride content in arterial smooth muscle and striated muscle was observed in the dogs with portal insulin delivery in contrast to the substantial increases we observed in pancreatectomized dogs (n = 7) with pancreatic autografts that drained into the systemic circulation (SD). In these dogs, the tissue samples were taken at the age of 3 to 4 years. Triglyceride content (mean +/- SEM) in the aorta was 4.9 +/- 1.2 versus 2.6 +/- 0.6 versus 20.7 +/- 8.0 mumol/g (P less than .01) in C, PD, and SD models, respectively. The corresponding values for triglyceride content in striated muscles were 29.1 +/- 1.2, 25.9 +/- 1.5, and 171.4 +/- 46.6 mumol/g (P less than .01). Glucose-6-phosphate dehydrogenase (G-6-PDH) and malic enzyme, key enzymes for lipid synthesis, were also normal in the PD model, in contrast to the fivefold increased activity of these enzymes in the SD model (P less than .01). The glycolytic enzymes, hexokinase (HK) and phosphofructokinase (PFK), were normal compared with the decreased values in the SD. These data indicate that it is possible to normalize glucose and lipid metabolism in arterial walls by portal delivery of insulin, following intrahepatic islet cell transplantation.(ABSTRACT TRUNCATED AT 250 WORDS)

[Intolerance to carbohydrates: the seven questions]

The borderline between diabetes and intolerance to carbohydrates has been drawn on the basis of prospective studies which determined a glycaemic threshold marking the risk for microangiopathy. On the other hand, the borderline between intolerance to carbohydrates and normal glucose tolerance remains arbitrary: 25% for subjects who are intolerant to carbohydrates return to normal glucose tolerance within 10 years. This is due to the fact that intolerance to carbohydrates is a heterogeneous entity which should be dismembered according to the severity of insulin deficiency and to the degree of insulin resistance. Alteration of insulin secretion is perhaps the most specific marker of susceptibility to non insulin dependent diabetes, but insulin resistance is certainly the principal factor exhausting insulin secretion and leading to non insulin dependent diabetes. Insulin resistance and the hyperinsulinism it creates seem to facilitate atherogenesis, even when glucose tolerance is still normal, so that the oral glucose tolerance test is not only poorly reproducible but loses a great deal of its value in the early detection of vascular risk. Measurements of fasting and post-prandial glucose levels and of A1C haemoglobin, cholesterol, triglyceride, and HDL cholesterol levels usually make it possible to classify subjects into one of the three following categories: (1) no risk of macro- or microangiopathy; (2) diabetes with a risk of macro- or microangiopathy; (3) intolerance to glucose with risk of atherogenesis but no risk of microangiopathy. The oral glucose tolerance test probably remains useful within a small set of values that are either very slightly above normal or dissociated. Measuring blood insulin levels might be a better way of assessing the risk of atherogenesis, but the clinical use of this test requires evaluation.

Activity of enzymes of glucose and triglyceride metabolism in adipose and muscle tissue from normal pregnant women at term

In order to further investigate insulin insensitivity in pregnancy, the activities of key enzymes in glycolysis and lipid metabolism were measured in adipose and muscle tissue biopsies from 20 normal pregnant women undergoing caesarean section at term, and 23 non-pregnant women of similar age and body weight undergoing gynaecological surgery. The activity of pyruvate kinase was decreased in pregnant women in both adipose tissue (0.015 (0.009-0.024) (median and range) vs 0.020 (0.009-0.038) Ug-1 wet weight, p less than 0.05) and muscle tissue (6.7 (3.6-10.9) vs 12.0 (2.8-16.2) U g-1 wet weight, p less than 0.001). The activity of hexokinase was decreased in adipose tissue only (0.045 (0.022-0.085) vs 0.057 (0.025-0.097) U g-1 wet weight, p less than 0.05), while the activity of phosphofructokinase was decreased in muscle tissue only (1.3 (0.7-2.6) vs 2.1 (0.3-4.5) U g-1 wet weight, p less than 0.01). Glucose-6-phosphate dehydrogenase activity was increased in muscle tissue (0.30 (0.11-0.59) vs 0.17 (0.09-0.48) U g-1 wet weight, p less than 0.05), while the activity of hydroxyacyl-CoA dehydrogenase was decreased in adipose tissue (0.5 (0.3-1.1) vs 1.0 (0.5-2.3) U g-1 wet weight p less than 0.001) from the pregnant women. Similar results were found when enzyme activities were calculated per gram of protein, but with poorer reproducibility.(ABSTRACT TRUNCATED AT 250 WORDS)

Carbohydrate and lipid metabolism of skeletal muscle in type 2 diabetic patients

Peripheral hyperinsulinaemia is the cause of metabolic changes that might contribute to the high incidence of macrovascular disease in patients with diabetes mellitus. In order to test this hypothesis muscle biopsies from 12 Type 2 diabetic patients and 14 age and sex matched non-diabetic patients, undergoing minor surgery, were obtained. The diabetic patients had significantly elevated fasting serum insulin (0.29 +/- 0.05 vs 0.06 +/- 0.03 nmol-1) and glucose (8.3 +/- 1.5 vs 4.6 +/- 0.5 mmol-1) and HbA1 levels (8.4 +/- 0.4 vs 5.0 +/- 0.2 per cent). The fasting and 2-h postprandial C-peptide levels were 0.99 +/- 0.25 vs 0.39 +/- 0.12 and 3.12 +/- 0.75 vs 1.09 +/- 0.34 nmol/l, respectively. The diabetic patients showed a marked elevation of triglyceride in the striated muscle biopsies compared to the non-diabetic controls (290 +/- 52 vs 48 +/- 6 mumol/g wet weight, p less than 0.001). Moreover, the activities of glucose-6-phosphate dehydrogenase (0.25 +/- 0.03 vs 0.13 +/- 0.01 U/g wet weight) and malic enzyme (0.15 +/- 0.01 vs 0.05 +/- 0.01 U/g wet weight), necessary for lipid synthesis, were significantly increased (both p less than 0.001) in the diabetic patients while the glycolytic enzymes, hexokinase (0.65 +/- 0.09 vs 1.82 +/- 0.11 U/g wet weight), pyruvate kinase (7.3 +/- 0.9 vs 13.2 +/- 0.9 U/g wet weight), phosphofructokinase (1.3 +/- 0.2 vs 2.6 +/- 0.2 U/g wet weight), and alpha-glycerophosphate dehydrogenase (7.3 +/- 0.5 vs 12.5 +/- 0.7 U/g wet weight) were decreased (all p less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Clenbuterol-induced muscle growth: investigation of possible mediation by insulin

The role of insulin as a possible mediator of the beta-adrenergic agonist stimulation of muscle growth was investigated. To exclude possible action of the beta-agonist on the pancreatic release of insulin, diabetes was induced in rats by a streptozotocin injection (100 mg/kg). Insulin levels were almost not detectable in these rats. Feeding either normal diet or diet containing the beta-adrenergic agonist clenbuterol (10 parts/million) did not alter plasma insulin concentrations. The effects of clenbuterol on muscle and weight gain were determined in diabetic rats given daily insulin replacement (D + I) and fed either a normal diet or clenbuterol-treated diet. Clenbuterol, fed for 1 wk, increased the wet weight of the gastrocnemius, soleus, and extensor digitorum longus muscles (15-23%) in both normal and D + I rats. Although clenbuterol increased body weight gain, it did not alter feed consumption and, therefore, feed efficiency (g gain/g food) was improved. Activities of cathepsin B and N-acetyl-beta-glucosaminidase, but not cathepsin D, were elevated in the soleus muscles of clenbuterol-treated rats. The clenbuterol-induced increase in muscle growth in the insulin-replaced diabetic rats indicated that this beta-adrenergic agonist effect was not mediated by an alteration of circulating levels of insulin, secondary to beta-agonist action on pancreatic insulin release.

Sulphonylurea failure in type 2 diabetes: treatment with a basal insulin supplement

Many diabetic patients continue to have hyperglycaemia on maximal sulphonylurea therapy. Five different therapeutic options, with the prime aim of achieving normal fasting plasma glucose concentrations, have been compared in 15 asymptomatic, sulphonylurea-treated type 2 diabetic patients in a randomized crossover study of 8-week periods. In 24 h metabolic profiles the overnight mean (+/- 1SD) basal plasma glucose level on sulphonylurea therapy was 8.9 +/- 4.2 mmol/l. This was slightly improved with added metformin therapy (7.3 +/- 4.3 mmol/l, p = 0.013), but reduced to normal by added ultralente insulin (5.2 +/- 3.2 mmol/l, p less than 0.001), ultralente insulin alone (5.1 +/- 1.6 mmol/l, p = 0.005) or by ultralente and soluble insulin (4.7 +/- 1.4 mmol/l, p = 0.003). The mean glycosylated haemoglobin concentration was reduced significantly only by the treatments which included insulin. None of the patients had severe or incapacitating hypoglycaemia and only when on additional soluble insulin did patients show a significant gain in weight. Combining sulphonylurea therapy with ultralente insulin did not significantly improve overall glucose control over treatment with ultralente alone, although the insulin dose required to restore fasting normoglycaemia was significantly lower (median (interquartile range), 25 (12-41) versus 40 (27-80) U/day, p = 0.001). In type 2 diabetic patients who continue to have fasting hyperglycaemia on maximal sulphonylurea therapy, fasting normoglycaemia can be achieved easily, without minimal changes in diet or lifestyle, by means of a basal insulin supplement.

The effects of hyperinsulinemia on arterial wall and peripheral muscle metabolism in dogs

Peripheral hyperinsulinemia may be associated with metabolic consequences that could contribute to the high incidence of macrovascular disease in patients with diabetes mellitus. Arterial wall and striated muscle cells were studied in dogs to examine the effect of hyperinsulinemia on the lipid content and on lipogenic and glycolytic enzyme activity. Eight pancreatectomized dogs received segmental pancreatic autografts with venous drainage into the iliac vein. Glucose disappearance rates (K values) were normal four years after transplantation, but both fasting serum insulin levels (48.9 +/- 4.8 v 11.8 +/- 1.9 microU/mL) and the total area under the glucose-insulin response curve (1797 +/- 196 v 1110 +/- 158 microU X min/mL) were significantly greater than in control animals (P less than 0.05). The hyperinsulinemic dogs had a marked triglyceride elevation in arterial smooth muscle (20.6 +/- 8.0 v 0.5 +/- 0.4 mumol/g) and striated muscle (171.4 +/- 46.6 v 41.2 +/- 7.7 mumol/g) (P less than 0.001). Moreover, key enzymes in lipid synthesis (glucose-6-phosphate dehydrogenase, malic enzyme, and 3-hydroxyacyl-CoA DH) were significantly increased (P less than 0.01) in the hyperinsulinemic animals, while the glycolytic enzymes, (phosphofructokinase, hexokinase, pyruvate kinase, and alpha-glycerophosphate DH) were not significantly different. These data demonstrate substantial enhancement of lipid synthesis in arterial wall and striated muscle in hyperinsulinemic dogs. Altered substrate metabolism in arterial walls, in association with hyperinsulinemia, may have important implications with regard to macrovascular disease in diabetes, particularly in insulin-treated patients. In addition, these studies may serve to stimulate longer term assessments of macroangiopathy in the increasing number of patients with functioning pancreatic allografts draining into the systemic circulation.