Decreasing Cellular Hydrogen Peroxide With Catalase Mimics the Effects of Hypoxia on the Sensitivity of the L-Type Ca 2+ Channel to β-Adrenergic Receptor Stimulation in Cardiac Myocytes

Abstract

In cardiac myocytes, hypoxia inhibits the basal L-type Ca 2+ current ( I Ca-L ) and increases the sensitivity of I Ca-L to β-adrenergic receptor stimulation. We investigated whether hydrogen peroxide (H 2 O 2 ) is involved in the hypoxic response. Guinea pig ventricular myocytes were dialyzed with catalase, which specifically catalyzes the conversion of H 2 O 2 to H 2 O and oxygen, and then I Ca-L was recorded during exposure to isoproterenol (Iso). Catalase decreased the K 0.5 for activation of I Ca-L by Iso from 2.7±0.3 nmol/L (in cells dialyzed with heat-inactivated catalase) to 0.4±0.1 nmol/L. The increase in sensitivity to Iso by catalase may be attenuated when cells are preexposed to H 2 O 2 . A significant increase in sensitivity of I Ca-L to Iso was recorded when mitochondrial function was inhibited with myxothiazol or FCCP, suggesting that the source of H 2 O 2 was from the mitochondria. Prior exposure of cells to H 2 O 2 attenuated the inhibition of basal I Ca-L during hypoxia and the increase in sensitivity of I Ca-L to Iso during hypoxia. Additionally, extracellularly applied catalase mimicked the effect of hypoxia on basal I Ca-L . Measurement of the rate of production of hydrogen peroxide using 5- (and 6-)chloromethyl-2′, 7′-dichlorodihydrofluorescein diacetate acetyl ester indicated that hypoxia was associated with a significant decrease in the production of hydrogen peroxide in the cells. These data suggest that hypoxia mediates changes in channel activity through a lowering in H 2 O 2 levels and that H 2 O 2 is a key intermediate in modifying basal channel activity and the β-adrenergic responsiveness of the channel during hypoxia.