Background: 

The cardiac Na/K ATPase (NKA) is an important regulator of contractility by controlling the intracellular sodium concentration ([Na]_i). A change in [Na]_i affects the activity of the Na/Ca exchanger (NCX), and thus Ca handling. We recently reported that blockade of the NKA a₂-isoform increased Ca influx via the NCX, possibly through accumulation of Na in subsacolemmal microdomains.

Methods and Results: 

Here, we tested whether blockade of the NKA a₂ isoform reduced the rate of Ca extrusion through the NCX by changing [Na] _i in the microdomain and independently of secondary effects due to alterations in Ca load of the sarcoplasmic reticulum (SR). In isolated voltage clamped cardiomyocytes from mice, we determined that 5 mM of ouabain blocked ~91% of the a₂ isoforms, but less than 7% of the a₁ isoforms. Mouse cardiomyocytes pretreated with 0.1 mM thapsigargin and 10 mM ryanodine to exclude SR function were loaded with caged Ca (4 mM DM- nitrophen) through a patch pipette. At a holding potential of 50 mV, Ca was photoreleased by a UV flash to activate NCX and the associated currents were measured. Changes in [Ca] _i were simultaneously recorded with a confocal microscope. Preliminary data suggest that Ca removal from the cytosol, measured as the rate of decay in flash-induced Ca transient, was slower in cardiomyocytes treated with 5 mM ouabain than in control. This was accompanied by a slower decay of the NCX current.

Conclusion: 

These data indicate that a reduction in NKA a₂ isoform activity, leading to local Na accumulation, reduces the rate of Ca extrusion through NCX.