Oxidative stress and the resulting change in cell redox state are proposed to contribute to pathogenic alterations in ion channels that underlie electrical remodeling of the diseased heart. The present study examined whether K⁺ channel remodeling is controlled by endogenous oxidoreductase systems that regulate redox-sensitive cell functions. Diabetes was induced in rats by streptozotocin and experiments conducted after 3-5 wk of hyperglycemia. Spectrophotometric assays of ventricular tissue extracts from diabetic rat hearts revealed divergent changes in two major oxidoreductase systems. The thioredoxin (Trx) system in diabetic rat heart was characterized by a 52% decrease in thioredoxin reductase activity from control (p<0.05), whereas Trx activity was 1.7 fold greater than control (p<0.05). Diabetes elicited similar changes in the glutaredoxin (Grx) system: glutathione reductase was decreased 35% from control (p<0.05) and Grx activity was 2.5 fold greater than control (p<0.05). The basal activity of glucose-6- phosphate dehydrogenase (G6PD), which generates NADPH required by the Trx and Grx systems, was not altered by diabetes. Voltage-clamp studies showed that the characteristically decreased density of the transient outward K⁺ current (I_to) in isolated diabetic rat myocytes was normalized by in vitro treatment with 0.1 µM insulin or the metabolic activator dichloroacetate (DCA; 1.5 mM). The effect of these agonists on I_to was blocked by inhibitors of G6PD. Moreover, inhibitors of thioredoxin reductase, which controls the reducing activity of Trx also blocked up-regulation of I_to by insulin and DCA. These data suggest that K⁺ channels underlying I_to are regulated in a redox-sensitive manner by the Trx system, and that the remodeling of I_to that occurs in diabetes may be due to decreased thioredoxin reductase activity. We propose that oxidoreductase systems are an important repair mechanism that protects ion channels and associated regulatory proteins from irreversible oxidative damage.