Role of γ-glutamyl transpeptidase in redox regulation of K ⁺ channel remodeling in postmyocardial infarction rat hearts

γ-Glutamyl transpeptidase (γ-GT) is a key enzyme in GSH metabolism that regulates intracellular GSH levels in response to extracellular GSH (GSH_o). The objective of this study was to identify the role of γ-GT in reversing pathogenic K⁺ channel remodeling in the diseased heart. Chronic ventricular dysfunction was induced in rats by myocardial infarction (MI), and studies were done after 6-8 wk. Biochemical assays of tissue extracts from post-MI hearts revealed significant increases in γ-GT activity in left ventricle (47%) and septum (28%) compared with sham hearts, which paralleled increases in protein abundance and mRNA. Voltage-clamp studies of isolated left ventricular myocytes from post-MI hearts showed that downregulation of transient outward K⁺ current (I_to) was reversed after 4-5 h by 10 mmol/l GSH_o or N-acetylcysteine (NAC _o), and that the effect of GSH_o but not NAC_o was blocked by the γ-GT inhibitors, acivicin or S-hexyl-GSH. Inhibition of γ-glutamylcysteine synthetase by buthionine sulfoximine did not prevent upregulation of I_to by GSH_o, suggesting that intracellular synthesis of GSH was not directly involved. However, pretreatment of post-MI myocytes with an SOD mimetic [manganese (III) tetrapyridylporphyrin] and catalase completely blocked recovery of I_to by GSH_o. Confocal microscopy using the fluorogenic dye 2′,7′- dichlorodihydrofluorescein diacetate confirmed that GSHo increased reactive oxygen species (ROS) generation by post-MI myocytes and to a lesser extent in myocytes from sham hearts. Furthermore, GSH_o-mediated upregulation of I_to was blocked by inhibitors of tyrosine kinase (genistein, lavendustin A, and AG1024) and thioredoxin reductase (auranofin and 13-cis-retinoic acid). These data suggest that GSH_o elicits γ-GT- and ROS-dependent transactivation of tyrosine kinase signaling that upregulates K⁺ channel activity or expression via redox-mediated mechanisms. The signaling events stimulated by γ-GT catalysis of GSH _o may be a therapeutic target to reverse pathogenic electrical remodeling of the failing heart.