Na/K pump a subunit expression in rabbit ventricle and regional variationsof 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 intracellula r sodium regulation derive from differences in pump expression. Immunohisto chemical techniques show that only the alpha1 isoform is present in rabbit ventricle; therefore, regional variation in isoform distribution is not pos sible, Western blots of samples taken from subendocardial and sub-epicardia l regions confirm the absence of alpha2 and alpha3 isoforms but also show t hat levels of the al isoform do not differ significantly in the two regions . The ratio of densitometric readings from blot bands was 1.18+/-0.17 (epic ardial:endocardial; mean +/-SEM). Measurements of fully activated pump curr ent in voltage-clamped cells were achieved by dialysing the cell via the pa tch pipette with 50 mM Na and applying mM dihydroouabain. The current measu red was 0.16+/-0.02 pA/pF in epicardial cells and 0.17+/-0.02 pA/pF in endo cardial cells. These results indicate that the capacity of cells from the t wo regions to generate sodium efflux is identical. Regional differences in intracellular sodium regulation, therefore, are more likely to arise from d ifferences ill influx of sodium.