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 2O 2) is involved in the hypoxic response. Guinea pig ventricular myocytes were dialyzed with catalase, which specifically catalyzes the conversion of H 2O 2 to H 2O 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 2O 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 2O 2 was from the mitochondria. Prior exposure of cells to H 2O 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 2O 2 levels and that H 2O 2 is a key intermediate in modifying basal channel activity and the β-adrenergic responsiveness of the channel during hypoxia.