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

Abstract

γ-Glutamyl transpeptidase (γ-GT) is a key enzyme in GSH metabolism that regulates intracellular GSH levels in response to extracellular GSH (GSHo). 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 ( Ito) was reversed after 4–5 h by 10 mmol/l GSHo or N-acetylcysteine (NACo), and that the effect of GSHo but not NACo 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 Ito by GSHo, 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 Ito by GSHo. 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, GSHo-mediated upregulation of Ito 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 GSHo 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 GSHo may be a therapeutic target to reverse pathogenic electrical remodeling of the failing heart.