Metabolic basis of decreased transient outward KS current in ventricular myocytes from diabetic rats ZHI XU, KAUSHIK P. PATEL, AND GEORGE J. ROZANSKI Department of Physiology and Biophysics, University of Nebraska Medical Center, Omaha, Nebraska 68198-4575 Xu, Zhi, Kaushik P. Patel, and George J. Rozanski. Metabolic basis of decreased transient outward K+ current in ventricular myocytes from diabetic rats. Am. J. Physiol. 271 (Heart Circ. Ph ysiol. 40): H2190-H2196,1996.-The purpose of this study was to examine the mechanisms of alterations in cardiac K+ channel function in early stages of experimental diabetes mellitus induced by streptozotocin. Transient out- ward (IJ and inward rectifier (IK1) K+ currents were recorded by the whole cell voltage-clamp technique in ventricular myocytes isolated from hearts of 2- to 4-wk diabetic and age-matched control rats. Ito density in myocytes from dia- betic rats was -30% less than control (at +60 mV; P < 0.01) under basal recording conditions in the presence of 18 mM external glucose, whereas IK1 density was not different be- tween groups. When external glucose concentration was decreased to 5 mM for 4-6 h, basal Ito density was not changed in either group of myocytes. To further examine the possible metabolic basis of reduced Ito density in myocytes from diabetic rats, we separately tested three structurally different compounds that affect substrate utilization in car- diac myocytes: insulin (0.1 pM), dichloroacetate (1.5 mM), and L-carnitine (10 mM). Each compound completely normal- ized Ito density in myocytes from diabetic rats treated in vitro for 4-6 h. The same agents had no effect on Ito density in control myocytes, nor was I Kl altered in either group of cells. These data provide the first evidence to support the hypoth- esis that there is a metabolic basis for decreased Ito density in diabetic rat ventricular myocytes in early stages of this model. Furthermore, our data suggest that depressed glucose metabolism in the diabetic heart may be a key factor underly- ing changes in Ito channel function, because agents that increase glucose utilization normalize Ito density within a short time period. heart; insulin; dichloroacetate; L-carnitine CARDIOVASCULARCOMPLICATIONS ofdiabetesmellitushave a significant impact on the clinical prognosis of patients with this disorder. The pathogenic conditions of coro- nary artery disease and autonomic neuropathy are considered to be important factors accounting for the increased incidence of heart failure and sudden death in diabetics (24). However, there may also be significant defects in cardiac cell function, independent of vascular pathology, characterized by impaired contractility and aberrant electrical properties (23, 24). Nevertheless, the etiology of cardiomyocyte dysfunction in diabetes is not well understood, and several factors have been proposed to be involved. For example, it was suggested that diabetes-induced metabolic disturbances may pro- vide the initial biochemical signals leading to abnormali- ties in cellular proteins controlling ion movements across the sarcolemma and sarcoplasmic reticulum (23). In support of this hypothesis, several experimen- tal studies demonstrated abnormal ion regulatory mechanisms in myocytes from diabetic animals that can be normalized by administering insulin (12,21,22) or other pharmacological agents (10) that alter utiliza- tion of glucose or fatty acids by cardiac cells. Sarcolemmal ion channels are also markedly altered in the diabetic heart, which likely underlies the abnor- mal electrophysiology of this disease state (24). One cellular electrophysiological characteristic that has been consistently observed in experimental models of diabe- tes mellitus is a significant increase in action potential duration (APD) compared with control (&II, 14,17,25, 27,30). Experimental data suggest that APD prolonga- tion in diabetic myocytes is due partly to a net decrease in outward repolarizing current, in particular the Ca2+- independent transient outward K+ current (&,; Refs. 11, 14, 25, 29, 30), which also controls APD in human ventricular myocytes (16). Nonetheless, the mecha- nisms responsible for the decrease in Ito and prolonged APD in ventricular myocytes from diabetic rats are not known. The present investigation tested whether early, diabe- tes-induced alterations in cellular metabolism are re- sponsible for the marked decrease in Zto density that characterizes cardiac myocytes from diabetic rats. Our results suggest that hyperglycemia is not directly in- volved in downregulating this current but that de- pressed glucose utilization may be linked to abnormal Zto channel behavior. Thus compounds that are known to directly or indirectly enhance glucose oxidation completely normalize Ito density in myocytes from diabetic rats. METHODS Rat model of diabetes mellitus: Isolation of cardiac myo- cytes. Male Sprague-Dawley rats weighing 150-200 g were made diabetic by a single intraperitoneal injection of strepto- zotocin (STZ; 65 mg/kg). Normal rats of comparable age and weight to be used as controls were injected with vehicle only (100 mM citrate buffer, pH 4.5). The diabetic rats in this study exhibited a nearly fourfold increase in blood glucose concentra- H2190 0363-6135196 $5.00 Copyright o 1996 the American Physiological Society