We sought to understand the effect of a transient exposure of cardiac myocytes to H(2)O(2) at a concentration that did not induce apoptosis. Myocytes were exposed to 30 micromol/L H(2)O(2) for 5 minutes followed by 10 U/mL catalase for 5 minutes to degrade the H(2)O(2). Cellular superoxide was measured using dihydroethidium. Transient exposure to H(2)O(2) caused a 66.4% increase in dihydroethidium signal compared with controls exposed to only catalase, without activation of caspase 3 or evidence of necrosis. The increase in dihydroethidium signal was attenuated by the mitochondrial inhibitors myxothiazol or carbonyl cyanide p-(trifluoromethoxy)phenyl-hydrazone and when calcium uptake by the mitochondria was inhibited with Ru360. We investigated the L-type Ca(2+) channel (I(Ca-L)) as a source of calcium influx. Nisoldipine, an inhibitor of I(Ca-L), attenuated the increase in superoxide. Basal channel activity increased from 5.4 to 8.9 pA/pF. Diastolic calcium was significantly increased in quiescent and contracting myocytes after H(2)O(2). The response of I(Ca-L) to beta-adrenergic receptor stimulation was used as a functional reporter because decreasing intracellular H(2)O(2) alters the sensitivity of I(Ca-L) to isoproterenol. H(2)O(2) increased the K(0.5) required for activation of I(Ca-L) by isoproterenol from 5.8 to 27.8 nmol/L. This effect and the increase in basal current density persisted for several hours after H(2)O(2). We propose that extracellular H(2)O(2) is associated with an increase in superoxide from the mitochondria caused by an increase in Ca(2+) influx from I(Ca-L). The effect persists because a positive feedback exists among increased basal channel activity, elevated intracellular calcium, and superoxide production by the mitochondria.