Insulin receptors and the clinician

Evidence that the insulin resistance of pregnancy may not involve a post-receptor defect in human adipocytes

Insulin binding and the capacity of insulin to stimulate the conversion of glucose to carbon dioxide and lipid, and to activate the protein tyrosine kinase associated with the insulin receptor have been investigated in adipocytes isolated from pregnant and non-pregnant women. Insulin binding and the conversion of glucose to lipid were the same for both groups. However, conversion of glucose to CO2 was higher in the non-pregnant group due to an elevated basal activity, and the increase produced by insulin was similar in both groups. The tyrosine kinase activity of the isolated receptor preparations was higher in the pregnant group due to an increase in the basal non-insulin dependent activity, and the increase produced by insulin was similar in both groups. These findings show the in vitro insulin responsiveness of isolated adipocytes is similar for both groups, and suggests that the in vivo insulin resistance of late pregnancy, as far as adipose tissue is concerned, is not due to any inherent defect in insulin action at the receptor or post-receptor level. In vivo insulin resistance may result from an increased level of circulating insulin antagonists.

The effects of metformin on adipocyte insulin action and metabolic control in obese subjects with type 2 diabetes

To investigate the mechanisms of action of metformin, insulin receptor binding and the activity of several insulin-controlled metabolic pathways were measured in adipocytes taken from 10 obese Type 2 diabetic patients treated for 4 weeks with either metformin (0.5 g x 3 daily) or matching placebo using a double-blind crossover design. Metformin therapy was associated with a significant fall in serum fructosamine levels (3.1 +/- 0.4 vs 2.8 +/- 0.4 mmol l-1, p less than 0.02) as well as fasting (10.8 +/- 2.4 vs 9.4 +/- 2.1 mmol l-1) and daytime (11.5 +/- 2.4 vs 10.0 +/- 2.2 mmol l-1) plasma glucose concentrations (p less than 0.05). Fasting and postprandial plasma levels of C-peptide and insulin were unchanged. While fasting plasma lactate concentrations remained unaltered after metformin, a rise was noted in response to meals (from 1.4 +/- 0.1 to 1.8 +/- 0.2 mmol l-1, p less than 0.05). Adipocyte insulin receptor binding was unaffected by drug treatment. Moreover, no insulin-like effects or post-binding potentiation of insulin action could be found on adipocyte glucose transport, glucose oxidation, lipogenesis, glycolysis or antilipolysis. A complementary in vitro study using adipocytes from non-obese healthy volunteers failed to show any direct effect of metformin on adipocyte insulin binding or glucose transport and metabolism, at media drug concentrations corresponding to therapeutic plasma levels.

Glucose tolerance and insulin receptor binding to monocytes and erythrocytes in patients with cystic fibrosis

Cystic fibrosis (CF) is the most frequent life threatening hereditary disease in the Western World with an incidence of approximately 1:2000. Due to increasing survival rates the high frequency of abnormal glucose tolerance has become an important problem. We compared insulin concentrations during oral glucose tolerance test and insulin receptor binding to both monocytes and erythrocytes from 9 patients with CF, with results from 10 healthy controls of similar body weight. The insulin: glucose ratio was increased in the fasting state (p less than 0.05) in patients with CF compared to controls, indicating an increased insulin resistance in CF-patients. The total insulin secretion during oral glucose tolerance test as judged by the area beneath the insulin curve was similar in the two groups, but insulin secretion was significantly delayed in patients with CF. Insulin receptor binding to monocytes and the number of receptors were significantly increased (p less than 0.01 and 0.02, respectively) in patients with CF whereas the dissociation constant was similar in patients with CF and controls. No difference was observed in insulin receptor binding to erythrocytes between the two groups. No correlations were found between insulin receptor binding to monocytes or erythrocytes and glucose tolerance or insulin concentrations.

Insulin receptor binding to blood cells: an outdated concept for clinical studies on insulin resistance?

Clinical data on insulin binding to blood cells contribute to our present knowledge of insulin resistance. Binding data have been frequently presented to indicate alterations in insulin action. It is now evident that insulin binding data are of limited value for our understanding of insulin resistance. This is mainly due to the tissue-specific binding of insulin as well as the "pleiotypic" nature of insulin action. As insulin action is determined at (a) the "pre-" receptor level, (b) the receptor, and/or (c) different postbinding sites, it is unlikely that receptor data alone can explain defective insulin action. As knowledge of the molecular biology of insulin receptor morphology and function as well as of the action of insulin on intermediary metabolism increases, clinicians should not be further overloaded with binding data. Nevertheless, binding data on blood cells may be still of some value in investigating patients with severe insulin resistance due to genetic disorders of the insulin receptor or insulin receptor autoantibodies.

Insulin dependent diabetes in Asians

Type 1 diabetes is said to be extremely rare in children in India, where diabetes treated with insulin may be due to chronic pancreatic disease or malnutrition. To see whether typical type 1 diabetes occurred in Asian children in the United Kingdom, all known Asian children with diabetes in industrial West Yorkshire were ascertained. A total of 17 such children were studied; of these, seven were from three multiplex families and two fathers from these families had diabetes. All children were ketosis prone and developed diabetes while resident in the UK. There were significant increases in HLA-B8 and HLA-DR3 and increases in HLA-DR4 and HLA-DR3/DR4, while HLA-B15 was absent. Islet cell antibodies, either IgG or complement fixing, were present in four of 18 subjects tested, all of whom had disease of short duration. The prevalence of type 1 diabetes in Asian children aged 15 years or less in West Yorkshire was 36/100,000, assuming complete ascertainment. It is concluded that typical type 1 diabetes may occur in Asian children and this condition may be more common in families who have migrated to the UK.