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, Ca$^2+$ in
a restricted cytosolic space between the PM and reticulum. The high
ouabain affinity Na$^+$ pumps may thereby modulate reticulum
Ca$^2+$ content and Ca$^2+$ signaling.