Molecular and Physiologic Actions of Insulin Related to Production of Nitric Oxide in Vascular Endothelium Michelle A. Vincent, PhD, Monica Montagnani, MD, PhD, and Michael J. Quon, MD, PhD* Address *Diabetes Unit, Laboratory of Clinical Investigation, NCCAM, National Institutes of Health, 10 Center Drive, Building 10, Room 6C-205, Bethesda, MD 20892-1632, USA. E-mail: quonm@nih.gov Current Diabetes Reports 2003, 3:279–288 Current Science Inc. ISSN 1534-4827 Copyright © 2003 by Current Science Inc. Introduction The vascular endothelium is the first organ that insulin encounters after it is secreted into the circulation by pan- creatic β cells. One of the endothelium’s primary func- tions is to serve as a semipermeable barrier that controls transport of macromolecules such as insulin into the interstitial space [1]. In addition to regulating delivery of insulin to target tissues, the endothelium also directly responds to insulin by increasing production of nitric oxide (NO), a potent vasodilator [2]. Recent studies in primary cultures of endothelial cells have elucidated a complete biochemical signaling pathway from the insulin receptor to activation of endothelial NO synthase (eNOS) [2,3•,4••,5•]. This pathway requires activation of the insulin receptor tyrosine kinase, subsequent phosphory- lation of insulin receptor substrate-1 (IRS-1), binding and activation of phosphatidylinositol 3 (PI3)-kinase, activa- tion of the serine kinase phosphoinositide-dependent kinase-1 (PDK-1) that in turn phosphorylates and acti- vates Akt, which then directly phosphorylates and acti- vates eNOS leading to increased production of NO in a matter of minutes. In vivo studies in humans [6] and ani- mals [7••,8,9,10•] have clearly demonstrated that insu- lin-stimulated production of NO from vascular endothelium plays an important physiologic role in increasing capillary recruitment and total limb blood flow in skeletal muscle. Capillaries are more sensitive and quicker to respond to the NO-dependent vasodilator effects of insulin than larger conduit blood vessels [7••,8]. These hemodynamic actions of insulin greatly enhance delivery of both glucose and insulin into meta- bolic targets such as skeletal muscle and fat, tissues that contribute substantially to whole-body insulin-mediated glucose disposal. Striking parallels between insulin sig- naling pathways in metabolic targets and signaling path- ways in vascular endothelium regulating production of NO suggest a mechanism that links metabolic insulin resistance with endothelial dysfunction [11]. Blunted pro- duction of NO in endothelium may lead to reduced capil- lary recruitment and decreased total limb blood flow, which contributes to the impaired glucose disposal char- acteristic of insulin-resistant states [10•]. This review focuses on recent in vitro studies investigat- ing molecular mechanisms of insulin-stimulated produc- tion of NO in vascular endothelium, as well as complementary in vivo evidence relevant to physiologic actions of insulin to enhance capillary recruitment and total limb blood flow in skeletal muscle. The role of vascu- lar actions of insulin to couple regulation of hemodynamic and metabolic homeostasis is discussed, with an emphasis on relating metabolic insulin resistance with vascular and endothelial dysfunction. Insulin has important vascular actions that regulate blood flow, in addition to its classical actions to coordinate glu- cose homeostasis. Insulin-stimulated production of nitric oxide in vascular endothelium results in capillary recruit- ment and vasodilation that diverts and increases blood flow to skeletal muscle and consequently increases glu- cose disposal. Thus, vascular actions of insulin may be essential for coupling hemodynamic and metabolic homeostasis. A complete biochemical signaling pathway linking the insulin receptor to activation of endothelial nitric oxide synthase in vascular endothelium has recently been elucidated. Moreover, the time course and dose response for capillary recruitment in response to physio- logic concentrations of insulin parallels that of insulin- mediated glucose uptake in vivo. Taken together, these observations suggest a molecular mechanism that may help to explain how insulin resistance contributes to car- diovascular components of the metabolic syndrome and vascular complications of diabetes.