Endothelin-1 reduces glucose uptake in human skeletal muscle in vivo and in vitro

OBJECTIVE

Endothelin (ET)-1 is a vasoconstrictor and proinflammatory peptide that may interfere with glucose uptake. Our objective was to investigate whether exogenous ET-1 affects glucose uptake in the forearm of individuals with insulin resistance and in cultured human skeletal muscle cells.

RESEARCH DESIGN AND METHODS

Nine male subjects (aged 61 ± 3 years) with insulin resistance (M value <5.5 mg/kg/min or a homeostasis model assessment of insulin resistance index >2.5) participated in a protocol using saline infusion followed by ET-1 infusion (20 pmol/min) for 2 h into the brachial artery. Forearm blood flow (FBF), endothelium-dependent vasodilatation, and endothelium-independent vasodilatation were assessed. Molecular signaling and glucose uptake were determined in cultured skeletal muscle cells.

RESULTS

ET-1 decreased forearm glucose uptake (FGU) by 39% (P < 0.05) after the 2-h infusion. ET-1 reduced basal FBF by 36% after the 2-h infusion (P < 0.05) and impaired both endothelium-dependent vasodilatation (P < 0.01) and endothelium-independent vasodilatation (P < 0.05). ET_A and ET_B receptor expression was detected on cultured skeletal muscle cells. One-hour ET-1 incubation increased glucose uptake in cells from healthy control subjects but not from type 2 diabetic patients. Incubation with ET-1 for 24 h reduced glucose uptake in cells from healthy subjects. ET-1 decreased insulin-stimulated Akt phosphorylation and increased phosphorylation of insulin receptor substrate-1 serine 636.

CONCLUSIONS

ET-1 not only induces vascular dysfunction but also acutely impairs FGU in individuals with insulin resistance and in skeletal muscle cells from type 2 diabetic subjects. These findings suggest that ET-1 may contribute to the development of insulin resistance in skeletal muscle in humans.Endothelial dysfunction, characterized by reduced bioactivity of nitric oxide (NO) and increased activity of the vasoconstrictor and proinflammatory peptide endothelin (ET)-1, is an important factor promoting the development of atherosclerosis (1). Several observations demonstrate that endothelial dysfunction is present in insulin-resistant states, including diabetes, obesity, and the metabolic syndrome (1,2). Insulin exerts important vascular actions via stimulation of NO production in the endothelium, leading to vasodilatation and increased blood flow, which in turn stimulates glucose uptake in skeletal muscle (3). These antiatherogenic effects are mediated via activation of the phosphatidylinositol 3-kinase (PI3-kinase) pathway, resulting in phosphorylation of Ser-Thr kinases, such as Akt, as well as activation of endothelial NO synthase (4). Insulin resistance is associated with reduced activation of this pathway in vascular endothelial cells (5) and in skeletal muscle (6). Instead, the mitogenic-signaling pathway mediated by mitogen-activated protein (MAP) kinase (extracellular signal–related kinase [ERK] MAP) is stimulated. In endothelial cells, this change in intracellular signaling results in the stimulation of cell growth, proinflammatory effects, increased production of ET-1, and reduced bioavailability of NO (2,4). These observations indicate that endothelial dysfunction, including increased activity of ET-1, is of functional importance in insulin-resistant states.The vascular responses to ET-1 are mediated via the two receptor subtypes, ET_A and ET_B (7,8). Both types of receptors are located on vascular smooth muscle cells and mediate vasoconstriction. The ET_B receptor also is located on endothelial cells and mediates vasodilatation by stimulating the release of NO and prostacyclin (9). Recent studies suggest that ET-1 inhibits insulin-mediated glucose uptake via a plasma membrane–dependent mechanism. ET-1 impairs insulin-stimulated glucose transporter GLUT4 translocation in adipocytes (10,11) and decreases PI3-kinase activity via insulin receptor substrate (IRS)-2 Ser and Tyr phosphorylation in isolated vascular smooth muscle cells (12). Furthermore, ET-1 reduces peripheral glucose utilization (13) and insulin sensitivity in healthy volunteers (14). Selective ET_A receptor blockade was shown to augment insulin-mediated glucose uptake in obese but not lean subjects (15). We have demonstrated that the dual ET_A/ET_B receptor blockade acutely increases total body glucose uptake and insulin sensitivity in obese patients with insulin resistance and coronary artery disease (16). These observations suggest that endogenous ET-1 plays a role in the regulation of glucose uptake. However, it still remains unclear whether ET receptors are expressed on skeletal muscle cells and whether ET-1 affects glucose uptake in the skeletal muscle tissue of subjects with insulin resistance.The current study was therefore designed to investigate the direct effect of ET-1 on skeletal muscle glucose uptake and blood flow in insulin-resistant individuals in vivo. Furthermore, we aimed to identify ET-1 receptors as well as the effects of ET-1 on basal and insulin-stimulated glucose uptake and signaling in human skeletal muscle cells.