Protein kinase C and Ca2+–calmodulin‐dependent protein kinase II mediate the enlarged reverse INCX induced by ouabain‐increased late sodium current in rabbit ventricular myocytes

New Findings What is the central question of this study? What are the effects of protein kinase C (PKC) and Ca 2+ –calmodulin‐dependent protein kinase II (CaMKII) on late sodium current ( I NaL ), reverse Na + –Ca 2+ exchange current (reverse I NCX ) or intracellular Ca 2+ levels changed by ouabain? What is the main finding and its importance? Ouabain, even at low concentrations (0.5–8.0 μ m ), can increase I NaL and reverse I NCX , and these effects may contribute to the effect of the glycoside to increase Ca 2+ transients and contractility. Both PKC and CaMKII activities may mediate or modulate these processes. It has been reported that the cardiac glycoside ouabain can increase the late sodium current ( I NaL ), as well as the diastolic intracellular calcium concentration and contractile shortening. Whether an increase of I NaL participates in a pathway that can mediate the positive inotropic response to ouabain is unknown. We therefore determined the effects of ouabain on I NaL , reverse Na + –Ca 2+ exchange current (reverse I NCX ), intracellular Ca 2+ ([Ca 2+ ] i ) levels and contractile shortening in rabbit isolated ventricular myocytes. Ouabain (0.1–8 μ m ) markedly increased I NaL and reverse I NCX in a concentration‐dependent manner, with significant effects at concentrations as low as 0.5 and 1 μ m . These effects of ouabain were suppressed by the I NaL inhibitors TTX and ranolazine, the protein kinase C inhibitor bisindolylmaleimide and the Ca 2+ –calmodulin‐dependent protein kinase II inhibitor KN‐93. The enhancement by 0.5 μ m ouabain of ventricular myocyte contractility and intracellular Ca 2+ transients was suppressed by 2.0 μ m TTX. We conclude that ouabain, even at low concentrations (0.5–8.0 μ m ), can increase I NaL and reverse I NCX , and these effects may contribute to the effect of the glycoside to increase Ca 2+ transients and contractility. Both protein kinase C and Ca 2+ –calmodulin‐dependent protein kinase II activities may mediate or modulate these processes.