Peter Staehr

Altered basal and insulin-stimulated phosphotyrosine phosphatase (PTPase) activity in skeletal muscle from NIDDM patients compared with control subjects

SummaryTo measure possible changes in basal and insulin-stimulated phosphotyrosine phosphatase (PTPase) activity in skeletal muscle from insulin-resistant individuals, soluble and particulate muscle fractions were prepared from biopsies taken before and after a 3-h hyperinsulinaemic euglycaemic clamp in eight non-insulin-dependent diabetic (NIDDM) patients and nine control subjects. We used a sensitive sandwich-immunofluorescence assay and the human insulin receptor as the substrate. PTPase activity was expressed as percentage of dephosphorylation of phosphotyrosyl-residues in immobilized insulin receptors per 2 h incubation time per 83 µg and 19 µg muscle fraction protein (soluble and particulate fraction, respectively). In the diabetic soluble muscle fractions, the basal PTPase activity was decreased compared with that of control subjects (11.5±5.5 vs 27.5±3.3,p<0.04, mean±SEM). In the particulate muscle fractions from the control subjects, PTPase activity was increased after 3 h hyperinsulinaemia (20.0±3.2 vs 30.2±3.6,p<0.03) and in the corresponding soluble fractions PTPase activity seemed decreased (27.5±3.3 vs 19.9±5.9, NS). No effect of insulin on PTPase activity was found in NIDDM patients (25.1±4.1 vs 27.2±5.2, 11.5±5.5 vs 15.1±4.5 [particulate and soluble fractions], NS). In conclusion, we found that the basal PTPase activity in soluble muscle fractions was decreased in NIDDM patients; furthermore, insulin stimulation was unable to increase PTPase activities in the particulate fractions, as opposed to the effect of insulin in control subjects.

Effect of insulin on protein phosphatase 2A expression in muscle in type 2 diabetes.

Background Protein phosphatase 2A (PP2A) acts on a number of enzymes involved in the insulin regulation of glucose uptake and glycogen synthesis. This study was carried out to investigate the effect of insulin on PP2A expression in skeletal muscles of type 2 diabetic and control subjects.

GLUT4 expression in human muscle fibres is not correlated with intracellular triglyceride (TG) content. Is TG a maker or a marker of insulin resistance

We have recently reported a progressive decline in the expression of glucose transporter isoform 4 (GLUT4) from control subjects through obese non‐diabetics to obese type 2 diabetic subjects, indicating that the reduced GLUT4 in slow twitch fibres could be secondary to obesity. In this study we investigate the association of GLUT4 expression with the intracellular triglyceride (TG) content in the same muscle fibres and with plasma lipid parameters. We used histochemistry and stereology to study the relationship between TG content and GLUT4 expression in muscle fibres from obese, obese type 2 diabetic subjects, and young lean controls. TG density was significantly higher in slow compared to fast fibres in all studied subjects (p<0.05). We found an increased TG density in slow twitch fibres of obese diabetic subjects compared to obese (p<0.05) and lean controls (p<0.008). Intracellular TG densities in slow and fast fibres did not correlate with the corresponding GLUT4 density in the same fibres in our study groups (p>0.05). Plasma TG and FFA did not correlate with GLUT4 expression in slow or fast fibres (p>0.05). In conclusion, TG content was increased in diabetic slow fibres with a reduced GLUT4 expression. The GLUT4 expression was not associated with an increased intracellular triglyceride content or with increased plasma FFA levels. Thus, intracellular TG content and circulating FFA may not influence glucose transport directly through GLUT4 expression.

The role of the liver in type 2 diabetes.

Type 2 diabetes is a disease with a heterogeneous phenotype, which is not only characterized by hyperglycaemia, but also by dyslipidaemia i.e. elevated triglyceride (TG), reduced HDL-cholesterol and elevated plasma free fatty acid (FFA) levels. More than 90% of type 2 diabetic subjects are obese, and in this review we will focus on this phenotype. Central to the type 2 diabetic syndrome is insulin resistance in skeletal muscle and adipocytes, as well as in the liver [1]. Obesity may play a primary role for the development of insulin resistance, whereas insulin resistance itself may lead to hyperglycaemia and dyslipidaemia [2]. The liver seems to play an important role in the development of the type 2 diabetic phenotype, contributing to dyslipidaemia as well as hyperglycaemia [2,3]. The liver is centrally placed in relation to insulin secretion and nutrient intake, since both insulin and nutrients must pass through the liver. The liver affects insulin concentrations, since about 50% of insulin is extracted at first passage, and this fraction may be reduced in the insulin resistant liver, leading to hyperinsulinemia [4]. Increasing carbohydrate and fat intake may challenge the liver, thereby influencing the net hepatic glucose balance and the degree of hepatic lipoprotein accumulation i.e. steatosis (“fatty liver”)—a phenomenon linked to insulin resistance [3]. Therefore, in this review we will not only discuss the role of the liver in the development of hyperglycaemia, but also the role of hepatic steatosis for the development of insulin resistance and dyslipidaemia. Previously, it was assumed that elevated rates of hepatic glucose production (HGP) was a major cause of fasting hyperglycaemia, and therefore treatment regimes have been developed in order to specifically control HGP in the postabsorptive state. However, recent data, and between these some from our own lab, indicate that basal HGP is close to normal in type 2 diabetic subjects and that the liver may play a more important role during a meal, where a lack of suppression of HGP may contribute to postprandial hyperglycaemia [5,6] Through an increased secretion of TG, the liver may also contribute to peripheral insulin resistance and thereby further increase postprandial hyperglycaemia [1]. Finally, an accumulation of TG in beta cells may impair specifically first phase insulin secretion and thereby further reduce the insulin mediated glucose metabolism postprandially.

Does overnight normalization of plasma glucose by insulin infusion affect assessment of glucose metabolism in Type 2 diabetes

Aims  In order to perform euglycaemic clamp studies in Type 2 diabetic patients, plasma glucose must be reduced to normal levels. This can be done either (i) acutely during the clamp study using high‐dose insulin infusion, or (ii) slowly overnight preceding the clamp study using a low‐dose insulin infusion. We assessed whether the choice of either of these methods to obtain euglycaemia biases subsequent assessment of glucose metabolism and insulin action.