Na + pump low and high ouabain affinity α subunit isoforms are differently distributed in cells

Abstract

Three isoforms (alpha1, alpha2, and alpha3) of the catalytic (alpha) subunit of the plasma membrane (PM) Na+ pump have been identified in the tissues of birds and mammals. These isoforms differ in their affinities for ions and for the Na+ pump inhibitor, ouabain. In the rat, alpha1 has an unusually low affinity for ouabain. The PM of most rat cells contains both low (alpha1) and high (alpha2 or alpha3) ouabain affinity isoforms, but precise localization of specific isoforms, and their functional significance, are unknown. We employed high resolution immunocytochemical techniques to localize alpha subunit isoforms in primary cultured rat astrocytes, neurons, and arterial myocytes. Isoform alpha1 was ubiquitously distributed over the surfaces of these cells. In contrast, high ouabain affinity isoforms (alpha2 in astrocytes, alpha3 in neurons and myocytes) were confined to a reticular distribution within the PM that paralleled underlying endoplasmic or sarcoplasmic reticulum. This distribution is identical to that of the PM Na/Ca exchanger. This raises the possibility that alpha1 may regulate bulk cytosolic Na+, whereas alpha2 and alpha3 may regulate Na+ and, indirectly, Ca2+ in a restricted cytosolic space between the PM and reticulum. The high ouabain affinity Na+ pumps may thereby modulate reticulum Ca2+ content and Ca2+ signaling.