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The proportion of active, dephosphorylated, pyruvate dehydrogenase complex was decreased in the mouse heart by obesity (by 56%), and this decrease in enzyme activity persisted during preparation and extraction of heart mitochondria. Phosphorylation and inactivation of pyruvate dehydrogenase may be a major factor in mediating the inhibitory effects of obesity on glucose oxidation in muscle, and this may represent an important mechanism in the development and/or expression of cellular insulin-resistance.

Lipogenic response to feeding was measured in vivo in liver, epididymal white adipose tissue (WAT) and interscapular brown adipose tissue (BAT), during the development of obesity in gold-thioglucose (GTG)-injected mice. The fatty acid synthesis after a meal was higher in all tissues of GTG-treated mice on a total-tissue basis, but the magnitude of this increase varied, depending on the tissue and the time after the initiation of obesity. Lipogenesis in BAT from GTG mice was double that of control mice for the first 2 weeks, but subsequently decreased to near control values. In WAT, lipogenesis after feeding was highest 2-4 weeks after GTG injection, and in liver, lipid synthesis in fed obese mice was greatest at 7-12 weeks after the induction of obesity. The post-prandial insulin concentration was increased after 2 weeks of obesity, and serum glucose concentration was higher in fed obese mice after 4 weeks. These results indicate that increased lipogenesis in GTG-injected mice may be due to an increase in insulin concentration after feeding and that insulin resistance (assessed by lipogenic response to insulin release) is apparent in BAT before WAT and liver.

The amount of pyruvate dehydrogenase in the active form (PDHa) was increased 1.7-fold compared with controls in heart muscle of mice 1 week after induction of obesity with a single injection of gold-thioglucose. At 4 weeks post injection, the amount of PDHa was decreased to 32% of control, a value which was observed in later stages of the obesity syndrome. In contrast, liver PDHa was increased and remained at an increased activity during the development of obesity. Despite normal post-prandial serum insulin contents, liver membrane insulin-receptor numbers were decreased 1 week after gold-thioglucose injection, and there was no change in receptor affinity. The decrease in heart PDHa in the obese animals was reversed by a single dose of 2-tetradecylglycidic acid, but this inhibitor of mitochondrial fatty acid oxidation did not affect liver PDHa in these animals. These early and diverse changes in PDHa argue for a multifactorial aetiology in the development of the whole-body insulin resistance seen in older gold-thioglucose-treated obese animals.

The proportion of pyruvate dehydrogenase complex in the active, dephosphorylated form was decreased (compared with lean controls) in heart muscle in gold thioglucose-treated obese hyperinsulinaemic mice, and the extent of enzyme inactivation was significantly linearly correlated with both body weight and body fat content. A single oral dose (25 mg/kg body wt.) of the beta-oxidation inhibitor 2-tetradecylglycidic acid to obese animals restored pyruvate dehydrogenase complex activity to that of lean controls. It is suggested that increased fatty acid oxidation may be a major factor in mediating the phosphorylation and inactivation of pyruvate dehydrogenase complex in mouse heart muscle in obesity, and this may represent an important mechanism in the development and/or expression of insulin resistance in respect of abnormalities of cellular glucose homoeostasis in these animals.

Phosphorylation of the insulin receptor of isolated rat adipocytes in response to insulin has been studied. Immunoprecipitation of adipocyte membrane protein demonstrated increased incorporation of 32P after exposure to insulin for 15 min, but this was dependent on the presence of physiological concentrations of Ca2+ and Mg2+. Autoradiography of solubilized immunoprecipitated membrane protein after sodium dodecyl sulphate/polyacrylamide-gel electrophoresis revealed that most of the 32P incorporation occurred in a band corresponding to Mr 95 000, which has been identified previously as the beta-subunit of the insulin receptor. 32P incorporation was inhibited by 2,4-dinitrophenol and trifluoperazine. It is suggested that insulin-receptor phosphorylation is an energy-requiring process that is Ca2+-dependent and may be modulated by calmodulin. Phosphorylation may proceed independently of glucose transport.