Na+ pump α2-subunit expression modulates Ca2+ signaling

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Abstract
The role of the Na+ pump α2-subunit in Ca2+ signaling was examined in primary cultured astrocytes from wild-type (α2 +/+ = WT) mouse fetuses and those with a null mutation in one [α2 +/− = heterozygote (Het)] or both [α2 −/− = knockout (KO)] α2 genes. Na+ pump catalytic (α) subunit expression was measured by immunoblot; cytosol [Na+] ([Na+]cyt) and [Ca2+] ([Ca2+]cyt) were measured with sodium-binding benzofuran isophthalate and fura 2 by using digital imaging. Astrocytes express Na+ pumps with both α1- (≈80% of total α) and α2- (≈20% of total α) subunits. Het astrocytes express ≈50% of normal α2; those from KO express none. Expression of α1 is normal in both Het and KO cells. Resting [Na+]cyt = 6.5 mM in WT, 6.8 mM in Het ( P > 0.05 vs. WT), and 8.0 mM in KO cells ( P < 0.001); 500 nM ouabain (inhibits only α2) equalized [Na+]cyt at 8 mM in all three cell types. Resting [Ca2+]cyt = 132 nM in WT, 162 nM in Het, and 196 nM in KO cells (both P < 0.001 vs. WT). Cyclopiazonic acid (CPA), which inhibits endoplasmic reticulum (ER) Ca2+ pumps and unloads the ER, induces transient (in Ca2+-free media) or sustained (in Ca2+-replete media) elevation of [Ca2+]cyt. These Ca2+ responses to 10 μM CPA were augmented in Het as well as KO cells. When CPA was applied in Ca2+-free media, the reintroduction of Ca2+ induced significantly larger transient rises in [Ca2+]cyt (due to Ca2+ entry through store-operated channels) in Het and KO cells than in WT cells. These results correlate with published evidence that α2 Na+ pumps and Na+/Ca2+ exchangers are confined to plasma membrane microdomains that overlie the ER. The data suggest that selective reduction of α2 Na+ pump activity can elevate local [Na+] and, via Na+/Ca2+ exchange, [Ca2+] in the tiny volume of cytosol between the plasma membrane and ER. This, in turn, augments adjacent ER Ca2+ stores and thereby amplifies Ca2+ signaling without elevating bulk [Na+]cyt.