Original Article Reduced Access to Insulin-Sensitive Tissues in Dogs With Obesity Secondary to Increased Fat Intake Martin Ellmerer, 1,2 Marianthe Hamilton-Wessler, 1 Stella P. Kim, 1 Katrin Huecking, 1 Erlinda Kirkman, 1 Jenny Chiu, 1 Joyce Richey, 1 and Richard N. Bergman 1 Physiological hyperinsulinemia provokes hemodynamic ac- tions and augments access of macromolecules to insulin- sensitive tissues. We investigated whether induction of insulin resistance by a hypercaloric high-fat diet has an effect on the extracellular distribution of macromolecules to insulin-sensitive tissues. Male mongrel dogs were ran- domly selected into two groups: seven dogs were fed an isocaloric control diet (3,900 kcal, 35% from fat), and six dogs were fed a hypercaloric high-fat diet (5,300 kcal, 54% from fat) for a period of 12 weeks. During hyperinsu- linemic-euglycemic clamps, we determined transport pa- rameters and distribution volumes of [ 14 C]inulin by applying a three-compartment model to the plasma clear- ance data of intravenously injected [ 14 C]inulin (0.8 Ci/ kg). In another study with direct cannulation of the hindlimb skeletal muscle lymphatics, we investigated the effect of physiological hyperinsulinemia on the appearance of intravenously injected [ 14 C]inulin in skeletal muscle interstitial fluid and compared the effect of insulin be- tween control and high-fat diet groups. The hypercaloric high-fat diet resulted in significant weight gain (18%; P < 0.001) associated with marked increases of subcutaneous (140%; P < 0.001) and omental (83%; P < 0.001) fat depots, as well as peripheral insulin resistance, measured as a significant reduction of insulin-stimulated glucose uptake during clamps (35%; P < 0.05). Concomitantly, we observed a significant reduction of the peripheral distribu- tion volume of [ 14 C]inulin (26%; P < 0.05), whereas the vascular distribution volume and transport and clearance parameters did not change as a cause of the diet. The second study directly confirmed our findings, suggesting a marked reduction of insulin action to stimulate access of macromolecules to insulin-sensitive tissues (control diet 32%, P < 0.01; high-fat diet 18%, NS). The present results indicate that access of macromolecules to insulin-sensitive tissues is impaired during diet-induced insulin resistance and suggest that the ability of insulin itself to stimulate tissue access is diminished. We speculate that the observed diet-induced defects in stimulation of tissue perfusion contribute to the development of peripheral insulin resis- tance. Diabetes 55:1769 –1775, 2006 B esides its effect to stimulate glucose uptake at the cellular level, insulin can regulate blood flow (1–5) as well as alter perfusion of insulin-sensi- tive tissues (6 – 8). Rattigan et al. (9) reported that insulin recruits microvascular vessels in skeletal muscle, changing flow distribution independent of its effect to alter total blood flow. Clark et al. (10) suggested that insulin’s microvascular actions contribute to its over- all effect on nutrient and hormone delivery to skeletal muscle. Yet the importance of insulin’s hemodynamic effects to the regulation of in vivo insulin action remains unclear. In a recent study (8), we introduced an alternative approach to examine the effects of physiological insulin concentrations on perfusion of insulin-sensitive tissues. We measured the effect of systemic insulin infusion on transport and distribution kinetics of the extracellular marker [ 14 C]inulin in an animal model that allowed access to hindlimb lymph, a surrogate for interstitial fluid. Insulin, at physiological concentrations, augments the access of the labeled inulin to insulin-sensitive tissues. The latter study supported the concept that the in vivo effect of insulin is determined, at least in part, by insulin’s own effect to reach metabolically active tissues by changing local blood flow distribution patterns. The question remains whether insulin resistance can be attributed to alterations in the blood flow–altering effects of the peptide. The objective of the present study was to test whether the hypercaloric induction of fat-induced peripheral insulin resistance alters perfusion of insulin- sensitive tissues. In fact, our results suggest reduced tissue perfusion of [ 14 C]insulin after induction of diet-induced insulin resistance in dogs. These studies implicate changes in tissue perfusion in the etiology of nutritive fat-induced insulin resistance. RESEARCH DESIGN AND METHODS Experiments were performed in male mongrel dogs (n = 13, 27.5  5.4 kg body wt [means SD]). Animals were housed under controlled kennel conditions (12-h light/dark cycle) in the University of Southern California Medical School Vivarium. Animals were accepted into the study only if judged by the veterinary staff to be in good health. One week before experimentation, under general anesthesia, chronic catheters were implanted. Catheters were placed in the carotid artery for sampling of arterial blood and into the left jugular and femoral veins for infusions. On the morning of conscious experiments, an acute catheter was inserted into the saphenous vein for the variable infusion of glucose and for the injection of [ 14 C]inulin. All experi- mental protocols were approved by the University Institutional Animals Care and Use Committee. From the 1 Department of Physiology and Biophysics, University of Southern California School of Medicine, Los Angeles, California; and the 2 Medical University Graz, Department of Internal Medicine, Diabetes and Metabolism, Graz, Austria. Address correspondence and reprint requests to Richard N. Bergman, PhD, Keck Professor of Medicine, Department of Physiology and Biophysics, University of Southern California School of Medicine, 1333 San Pablo St., MMR 626, Los Angeles, CA 90033. E-mail rbergman@usc.edu. Received for publication 18 November 2005 and accepted in revised form 16 March 2006. MRI, magnetic resonance imaging. DOI: 10.2337/db05-1509 © 2006 by the American Diabetes Association. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. DIABETES, VOL. 55, JUNE 2006 1769