Na+ Pump Low and High Ouabain Affinity α Subunit Isoforms are Differently Distributed in Cells

Three isoforms (α 1, α 2, and α 3) of the catalytic (α ) 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, α 1 has an unusually low affinity for ouabain. The PM of most rat cells contains both low (α 1) and high (α 2 or α 3) ouabain affinity isoforms, but precise localization of specific isoforms, and their functional significance, are unknown. We employed high resolution immunocytochemical techniques to localize α subunit isoforms in primary cultured rat astrocytes, neurons, and arterial myocytes. Isoform α 1 was ubiquitously distributed over the surfaces of these cells. In contrast, high ouabain affinity isoforms (α 2 in astrocytes, α 3 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 α 1 may regulate bulk cytosolic Na+, whereas α 2 and α 3 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.