The alpha-2 isomer of the sodium pump is inhibited by calcium at physiological levels

Coupling of calcium homeostasis to axonal sodium in axons of mouse optic nerve

Axonal populations in neonatal and mature optic nerves were selectively stained with calcium dyes for analysis of calcium homeostasis and its possible coupling to axonal Na. Repetitive nerve stimulation causes a rise in axonal [Ca(2+)](i) the posttetanus recovery of which is impeded by increasing the number of action potentials in the tetanus. This effect is augmented in 4-aminopyridine (4-AP; 1 mM), which dramatically increases the calcium and presumably sodium load during the tetanus. Increasing axonal [Na](i) with the Na-ionophore monensin (4-50 microM) and ouabain (30 microM) retards posttetanus calcium decline, suggesting that efficient calcium clearance depends on a low level of axonal [Na](i). Posttetanus calcium clearance is not affected by K-mediated depolarization. To further examine coupling between axonal [Na](i) and [Ca(2+)](i), the resting axonal [Ca(2+)](i) was monitored as axonal [Na(+)](i) was elevated with ouabain, veratridine, and monensin. In all cases, elevation of axonal [Na(+)](i) evokes a calcium influx into axons. This influx is unrelated to activation of calcium channels but is consistent with calcium influx via reversal of the Na/Ca exchanger expected as a consequence of axonal [Na(+)](i) elevation. In conclusion, this study demonstrates that calcium homeostasis in the axons of the optic nerve is strongly coupled to axonal [Na(+)](i) in a manner consistent with the Na/Ca exchanger playing a major role in extruding calcium following nerve activity.

Repolarization of the plasma membrane shapes NMDA-induced cytosolic [Ca2+ ] transients

After inactivation of NMDA receptors, restoration of basal cytosolic [Ca2+] ([Ca2+]c) is delayed. This may be caused by Ca2+ influx via reverse Na/Ca exchange or voltage-gated Ca2+ channels, and/or by Ca2+ efflux from internal stores. Monitoring of [Na+]c, [Ca2+]c, and plasma membrane potential in cultured cerebellar granule cells showed that repolarization of the plasma membrane and inactivation of voltage-gated Ca channels plays the most critical role in restoration of low [Ca2+]c following NMDA receptor inactivation. During NMDA receptor activation, however, an Na-dependent mechanism enhanced NMDA-induced elevation in [Ca2+]c. This mechanism did not involve Na,K-ATPase activation by Na+, because it operated even when Na,K-ATPase was inhibited.

Focal cerebral ischaemia induces a decrease in activity and a shift in ouabain affinity of Na+, K+-ATPase isoforms without modifications in mRNA and protein expression

In a mouse model of focal cerebral ischaemia, we observed after 1 h of ischaemia, that the total Na+, K+-ATPase activity was decreased by 39.4%, and then did not vary significantly up to 6 h post-occlusion. In the sham group, the dose-response curves for ouabain disclosed three inhibitory sites of low (LA), high (HA) and very high (VHA) affinity. In ischaemic animals, we detected the presence of only two inhibitory sites for ouabain. After 1 h of permanent occlusion, the first site exhibited a low affinity while the second site presented an affinity intermediate between those of HA and VHA sites, which evolved after 3 h and 6 h of occlusion towards that of the VHA site. The presence of only two ouabain sites for Na+, K+-ATPase after ischaemia could result from a change in ouabain affinity of both HA and VHA sites (alpha2 and alpha3 isoforms, respectively) to form a unique component. Irrespective of the duration of ischaemia, the smaller activity of this second site accounted entirely for the loss in total activity. Surprisingly, no modifications in protein and mRNA expression of any alpha or beta isoforms of the enzyme were observed, thus suggesting that ischaemia could induce intrinsic modifications of the Na+, K+-ATPase.

Isozymes of the Na-K-ATPase: heterogeneity in structure, diversity in function

The Na-K-ATPase is characterized by a complex molecular heterogeneity that results from the expression and differential association of multiple isoforms of both its alpha- and beta-subunits. At present, as many as four different alpha-polypeptides (alpha1, alpha2, alpha3, and alpha4) and three distinct beta-isoforms (beta1, beta2, and beta3) have been identified in mammalian cells. The stringent constraints on the structure of the Na pump isozymes during evolution and their tissue-specific and developmental pattern of expression suggests that the different Na-K-ATPases have evolved distinct properties to respond to cellular requirements. This review focuses on the functional properties, regulation, and possible physiological relevance of the Na pump isozymes. The coexistence of multiple alpha- and beta-isoforms in most cells has hindered the understanding of the roles of the individual polypeptides. The use of heterologous expression systems has helped circumvent this problem. The kinetic characteristics of different Na-K-ATPase isozymes to the activating cations (Na+ and K+), the substrate ATP, and the inhibitors Ca2+ and ouabain demonstrate that each isoform has distinct properties. In addition, intracellular messengers differentially regulate the activity of the individual Na-K-ATPase isozymes. Thus the regulation of specific Na pump isozymes gives cells the ability to precisely coordinate Na-K-ATPase activity to their physiological requirements.

A calnaktin-like inhibitor of Na,K-ATPase in rat brain: regulation of alpha 1 and alpha 2 isozymes

This study was designed to determine if a Ca(2+)-dependent, calnaktin-like inhibitor of Na,K-ATPase existed in rat brain and to compare the inhibition of different Na,K-ATPase isozymes in brain, heart and kidney. Based on the size and characteristics of human red blood cell calnaktin, a soluble protein fraction was obtained from rat brain and subjected to ultrafiltration and gel filtration to restrict the proteins to an appropriate molecular range of 6-50 kDa (6/50 fraction) for a crude calnaktin preparation. The 6/50 fraction was reconstituted with semipurified rat brain Na,K-ATPase and resulted in Ca(2+)-dependent inhibition of Na,K-ATPase activity. A 6/50 fraction was also prepared from rat heart ventricles, and, in its presence, Ca(2+)-dependent inhibition of cardiac Na,K-ATPase activity was observed. With brain preparations, the threshold for inhibition was approximately 100 nM free Ca2+, and inhibition was half maximal at 3-10 microM free Ca2+. Different isozymes of Na,K-ATPase were examined using differential sensitivity to ouabain and differential tissue distribution in brain, heart and kidney. The alpha 1 activity was inhibited in all three tissues. The alpha 2 activity of heart and the alpha 2 and/or alpha 3 activity of brain were also inhibited by the brain 6/50 fraction. In synaptosomal preparations from rat forebrain, resting intracellular (intrasynaptosomal) free Ca2+ was close to the threshold for calnaktin-like inhibition. The results are consistent with the presence of a calnaktin-like inhibitor of Na,K-ATPase in rat brain and indicate that calnaktin could be a widespread regulator of the alpha 1 isozyme. In addition, this study provides the first evidence that calnaktin also inhibits the alpha 2 activity of heart and the alpha 2 and/or alpha 3 isozymes of brain.

Extracellular Mg2+ regulates the intracellular Na+ concentration in rat sublingual acini

The intracellular free Na+ concentration ([Na+]i) increases during muscarinic stimulation in salivary acinar cells. The present study examined in rat sublingual acini the role of extracellular Mg2+ in the regulation of the stimulated [Na+]i increase using the fluorescent sodium indicator benzofuran isophthalate (SBFI). The muscarinic induced rise in [Na+]i was approximately 4-fold greater in the absence of extracellular Mg2+. When Na+ efflux was blocked by the Na+,K+-ATPase inhibitor ouabain, the stimulated [Na+]i increase was comparable to that seen in an Mg2+-free medium. Moreover, ouabain did not add further to the stimulated [Na+]i increase in an Mg2+-free medium suggesting that removal of extracellular Mg2+ may inhibit the Na+ pump. In agreement with this assumption, ouabain-sensitive Na+ efflux and rubidium uptake were reduced by extracellular Mg2+ depletion. Our results suggest that extracellular Mg2+ may regulate [Na+]i in sublingual salivary acinar cells by modulating Na+ pump activity.

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

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.

Spatiotemporal gradients of intra-axonal [Na+] after transection and resealing in lizard peripheral myelinated axons

1. Post-transection changes in intracellular Na+ ([Na+]i) were measured in lizard peripheral axons ionophoretically injected with the Na(+)-sensitive ratiometric dye, sodium-binding benzofuran isophthalate (SBFI). 2. Following axonal transection in physiological saline [Na+]i increased to more than 100 mM in a region that quickly extended hundreds of micrometers from the transection site. This post-transection increase in [Na+]i was similar when the bath contained 5 microM tetrodotoxin, but was absent in Na(+)-free solution. Depolarization of uncut axons in 50 mM K+ produced little or no elevation of [Na+]i until veratridine was added. These results suggest that the post-transection increase in [Na+]i was due mainly to Na+ entry via the cut end, rather than via depolarization-activated Na+ channels. 3. The spatiotemporal profile of the post-transection increase in [Na+]i could be accounted for by movement of Na+ from the cut end with an apparent diffusion coefficient of 1.3 x 10(-5) cm2 s-1. 4. [Na+]i began to decline toward resting levels by 20 +/- 15 min (mean +/- S.D.) post-transection, except in regions of the axon within 160 +/- 85 microns of the transection site, where [Na+]i remained high. The boundary between axonal regions in which [Na+]i did or did not recover probably defines a locus of resealing of the axonal membrane. 5. [Na+]i returned to resting values within about 1 h after resealing, even in axonal regions where the normal transmembrane [Na+] gradient had completely dissipated. The recovery of [Na+]i was faster and reached lower levels than expected by diffusional redistribution of Na+ along the axon. Partial recovery occurred even in an isolated internode, indicating that the internodal axolemma can actively extrude Na+.

Stimulation of synaptosomal Na+,K(+)-ATPase by ethanol: possible involvement of an isozyme-specific inhibitor of Na+,K(+)-ATPase

In synaptosomal preparations from rat cerebral cortex, ouabain-sensitive Rb+ uptake was stimulated by ethanol (20-80 mM). Based on differential sensitivity to ouabain, 80% of this Na+,K(+)-ATPase activity represented activity of the alpha 1 isozyme while 20% was due to the alpha 2 and/or alpha 3 isozymes (alpha 2/ alpha 3). Stimulation of Na+,K(+)-ATPase was selective for the activity of alpha 2/alpha 3 which was increased by 167% in the presence of 80 mM ethanol. In this concentration range, ethanol had no effect on alpha 1 activity. Exposure of synaptosomal preparations to EGTA increased basal (no ethanol) alpha 2/alpha 3 activity with no effect on alpha 1 activity. Further, ethanol no longer stimulated alpha 2/alpha 3 activity after EGTA treatment. An EGTA extract was concentrated and desalted to yield a fraction that selectively inhibited alpha 2/alpha 3 activity when reconstituted with EGTA-treated synaptosomal preparations. This inhibition was trypsin-sensitive, suggesting protein involvement, and was prevented by 80 mM ethanol. In the presence of the inhibitory protein fraction, ethanol stimulated Na+, K(+)-ATPase activity in EGTA-treated membranes with a dose-response like that observed with the crude (no EGTA) synaptosomes. We propose that the alpha 2/alpha 3 activity of Na+,K(+)-ATPase is subject to inhibitory regulation and that ethanol stimulates this activity by releasing it from inhibition, an effect that may mimic in vivo deregulation of the enzyme by ethanol.

Vasopressin stimulation of vanadate-sensitive Na+ transport by liver plasma membrane vesicles. Evidence for regulation via phospholipase C and protein kinase C activities

The rate of vanadate-sensitive 22Na+ uptake by isolated liver membrane vesicles, reflecting transport by Na+/K(+)-ATPase, was measured to study the role played by phospholipase C and protein kinase C in the regulation of this process by vasopressin. Na+ uptake was enhanced 2-3-fold by 100 nM [Arg8]vasopressin and the hormone effect was mimicked by 0.1 microM inositol 1,4,5-trisphosphate as well as by 1.0 microM myo-inositol. The stimulation by vasopressin was potentiated by phosphatidylinositol-specific phospholipase C from Bacillus thuringiensis (5-10 mU/ml). No effect of the bacterial enzyme was observed in the absence of the hormone. Phorbol myristate acetate (0.5-1 microM) suppressed the stimulation by vasopressin but had no effect in the absence of the hormone. High concentrations of bacterial phosphatidylinositol-specific phospholipase C (50-100 mU/ml) also antagonized the hormone stimulation. Staurosporine (50-100 nM) prevented the antagonistic effect of bacterial phospholipase C (50 mU/ml) and EGTA (1 mM) partially protected the hormonal stimulation in the presence of phorbol myristate acetate. Our results suggest that the stimulatory effect of vasopressin on Na+ transport is mediated by phospholipase C and products derived from the inositol moiety of membrane phospholipids. Membrane-associated protein kinase C appears to be at least partially responsible for the desensitization to stimulation by vasopressin.

Glucose and oxygen deprivation induces a Ca(2+)-mediated decrease in (Na(+)+K+)-ATPase activity in rat brain slices

Exposure of rat brain cortical slices to a medium lacking in glucose, oxygen or both glucose and oxygen, resulted in a decrease of the tissue ATP content and a reduction of (Na(+)+K+)-ATPase activity in membranes prepared from the slices. These treatments also inhibited partial reactions of (Na(+)+K+)-ATPase such as Na(+)-dependent phosphorylation and K(+)-stimulated phosphatase, as well as specific binding of [3H]ouabain in membranes prepared from the slices. Glucose deprivation and hypoxia decreased (Na(+)+K+)-ATPase activity in the absence of extracellular Ca2+, but the effects were blocked by 1,2-bis(2-amino-phenoxy)ethane-N,N,N',N'-tetraacetic acid tetra-acetomethyl ester (BAPTA-AM), a chelator of intracellular Ca2+. Metabolic inhibitors mimicked the effects of glucose deprivation and hypoxia. The effect of glucose-free hypoxia was dependent on extracellular Ca2+. It was blocked by Mg2+ at high concentration, bepridil or amiloride, but not by voltage-sensitive Ca2+ channel antagonists and glutamate receptor antagonists. None of the drugs tested here, except for dithiothreitol, affected the inhibitory effect of glucose-free hypoxia on the enzyme activity. In contrast to brain (Na(+)+K+)-ATPase, the kidney enzyme was insensitive to glucose and oxygen deprivation and metabolic inhibitors which depleted the tissue ATP.

Postnatal change in a Ca(2+)-mediated decrease in (Na+ + K+)-ATPase activity in rat brain slices

The treatment of brain slices from immature rats with veratrine and monensin did not cause any change in (Na+ + K+)-adenosine triphosphatase (ATPase) activity or [3H]ouabain binding in membranes prepared from the slices, though these reagents remarkably stimulated Ca2+ uptake in the slices. Exposure of the slices from adult rats to a glucose-free, hypoxic or both glucose-free and hypoxic medium resulted in a decrease in the enzyme activity, but the enzyme from immature rats was resistant to the conditions.