Up-regulation of K(+) channels in diabetic rat ventricular myocytes by insulin and glutathione.

Abstract

The cardiac pathogenesis of diabetes mellitus involves oxidative stress that elicits profound changes in myocardial glutathione, an endogenous regulator of cell function. This study examined the role of glutathione in regulating K(+) channel activity in isolated ventricular myocytes from diabetic rats and its relationship to insulin signaling. Colorimetric analysis of extracts of ventricular tissue from Sprague-Dawley rats showed that the basal level of reduced glutathione (GSH) was significantly less in rats with experimental diabetes compared with sham controls, consistent with oxidative stress conditions. This change in GSH status paralleled a significant decrease in the activity of gamma-glutamylcysteine synthetase, a major pathway involved in GSH homeostasis. Voltage-clamp studies confirmed that, compared with control myocytes, K(+) channels carrying the transient outward current (I(to)) are down-regulated in the diabetic state and that this electrophysiological change is reversed by in vitro treatment with insulin for 2-3 h. Incubation of diabetic rat myocytes with GSH also normalized I(to) density compared with untreated myocytes, but with a longer time course than insulin. To determine if up-regulation of I(to) by insulin was mediated by alterations in myocyte GSH, insulin-responsiveness of diabetic rat myocytes was tested in the presence of 1,3-bis-chloroethyl-nitrosourea, an inhibitor of glutathione reductase, or buthionine sulfoximine, a blocker of gamma-glutamylcysteine synthetase. Neither blocker alone altered I(to) density in diabetic rat myocytes when compared with untreated cells, but each blocked the effect of insulin to up-regulate I(to). These data suggest that oxidative stress-induced alteration in GSH redox state plays an important role in regulating I(to) channel function and that GSH homeostasis in ventricular myocytes is functionally coupled to insulin signaling.