Objective: Sustained β-adrenergic receptor (β-AR) activation augments oxidative stress in the heart; whether alterations in antioxidant enzymes contribute to this effect is unknown. Methods and results: Adult male Wistar rats were implanted with osmotic minipumps to infuse either l-isoproterenol (ISO, 25 μg/kg/h) or saline (SAL). After 7-days, ISO-treated hearts exhibited significant (p < 0.005): 1) concentric hypertrophy and augmentation of systolic function, 2) reductions of end-systolic wall stress, and 3) augmentation of oxidative stress, with a ∼ 3-fold increase in 4-hydroxy-2-nonenal-and malondialdehyde-protein adducts. ISO-treated hearts also exhibited significant (p < 0.01) reductions of CuZn-superoxide dismutase (SOD) enzyme activity (30%), protein (40%), and mRNA (60%), without changes in Mn-SOD, catalase, or glutathione peroxidase. Elk-1 and YinYang1 (YY1) are transcription factors that positively and negatively regulate CuZn-SOD expression, respectively. ISO-treated hearts exhibited a 3-fold increase in YY1 and a 2-fold reduction in Elk-1 DNA binding activity, strongly favoring CuZn-SOD gene repression. In isolated cardiomyocytes, sustained (24 h) ISO stimulation significantly (p < 0.01) increased reactive oxygen species (ROS), an effect blocked by CGP20712A, a β1-AR antagonist, but not by ICI118,551, a β2-AR antagonist. CuZn-SOD downregulation paralleled the increase in ROS, and were similarly blocked by β1- but not β2-AR blockade. There were no changes in CuZn-SOD mRNA stability or myocyte size with ISO treatment. However, nuclear run-on revealed a 40% reduction in CuZn-SOD mRNA expression (p < 0.01), consistent with transcriptional repression. ISO also depressed total cellular antioxidant capacity, reduced glutathione (GSH) levels, and the GSH:GSSG ratio. Moreover, CuZn-SOD siRNA transfection of H9c2 cardiomyocytes to suppress CuZn-SOD protein by ∼ 40-50% (analogous to the in vivo changes) induced cellular apoptosis. Conclusions: Sustained β-AR stimulation transcriptionally downregulates CuZn-SOD in myocardium via the β1-AR, thereby contributing to β-AR-mediated oxidative stress. © 2007 European Society of Cardiology.