We hypothesize that insulin acts on endothelial cells in 2 ways to enhance its delivery (and that of glucose) to skeletal muscle. One action is to recruit microvascular elements (capillaries) in muscle to increase the surface area available for insulin and glucose to enter skeletal muscle interstitium. A second action is to bind to its receptor on the capillary endothelium and via a receptor-mediated pathway cross the endothelium to the interstitium. We further hypothesize that insulin-resistance (e.g. type 2 diabetes and obesity) inhibits both of these vascular actions and this impairs overall insulin-mediated glucose disposal. Inasmuch as exercise also recruits muscle microvasculature, we further propose that even very modest exercise will enhance insulin and glucose delivery to muscle interstitium and improve muscle insulin sensitivity and glucose uptake. Establishment of 4 methods by our laboratory now allows critical testing of these hypotheses in humans: 1) the human forearm method;2) contrast enhanced ultrasound (CEU) to measure microvascular recruitment;3) a high sensitivity, precise immunoassay for insulin;and 4) immunohistochemical, confocal microscopic measurement of insulin, insulin receptor and insulin signaling proteins. We will study healthy as well as obese and type 2 diabetic volunteers to quantify the rate of insulin uptake by human forearm skeletal muscle. We will define whether insulin and glucose uptake are impaired by obesity or diabetes or by experimental manipulations which limit microvascular recruitment (nitric oxide synthase inhibition or intralipid infusion). We will as well ascertain whether exercise restores microvascular recruitment and insulin and glucose uptake by skeletal muscle in diabetic and obese volunteers. Together these measurements will allow a quantitative determination of the role of the endothelium in insulin delivery to muscle cells and the impact of altered insulin delivery on overall glucose homeostasis within skeletal muscle. Inasmuch as obesity and type 2 diabetes are expanding public health problems, defining the effects of altered insulin delivery produced by endothelial insulin resistance on glucose homeostasis will introduce the opportunity for novel interventions (either pharmacologic or behavioral) to mitigate a rising epidemic.
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