Na/K pump alpha subunit expression in rabbit ventricle and regional variations of intracellular sodium regulation

We examined the isoform distribution and expression of the alpha subunit of the Na/K-ATPase in the left ventricular muscle of rabbit heart in order to determine whether previously reported regional differences in intracellular sodium regulation derive from differences in pump expression. Immunohistochemical techniques show that only the alpha1 isoform is present in rabbit ventricle; therefore, regional variation in isoform distribution is not possible. Western blots of samples taken from subendocardial and sub-epicardial regions confirm the absence of alpha2 and alpha3 isoforms but also show that levels of the alpha1 isoform do not differ significantly in the two regions. The ratio of densitometric readings from blot bands was 1.18+/-0.17 (epicardial:endocardial; mean +/-SEM). Measurements of fully activated pump current in voltage-clamped cells were achieved by dialysing the cell via the patch pipette with 50 mM Na and applying 1 mM dihydroouabain. The current measured was 0.16+/-0.02 pA/pF in epicardial cells and 0.17+/-0.02 pA/pF in endocardial cells. These results indicate that the capacity of cells from the two regions to generate sodium efflux is identical. Regional differences in intracellular sodium regulation, therefore, are more likely to arise from differences in influx of sodium.