Regulation of endogenous and expressed Na+/K+ pumps in Xenopus oocytes by membrane potential and stimulation of protein kinases

Citrus Genetic Engineering for Disease Resistance: Past, Present and Future

Worldwide, citrus is one of the most important fruit crops and is grown in more than 130 countries, predominantly in tropical and subtropical areas. The healthy progress of the citrus industry has been seriously affected by biotic and abiotic stresses. Several diseases, such as canker and huanglongbing, etc., rigorously affect citrus plant growth, fruit quality, and yield. Genetic engineering technologies, such as genetic transformation and genome editing, represent successful and attractive approaches for developing disease-resistant crops. These genetic engineering technologies have been widely used to develop citrus disease-resistant varieties against canker, huanglongbing, and many other fungal and viral diseases. Recently, clustered regularly interspaced short palindromic repeats (CRISPR)-based systems have made genome editing an indispensable genetic manipulation tool that has been applied to many crops, including citrus. The improved CRISPR systems, such as CRISPR/CRISPR-associated protein (Cas)9 and CRISPR/Cpf1 systems, can provide a promising new corridor for generating citrus varieties that are resistant to different pathogens. The advances in biotechnological tools and the complete genome sequence of several citrus species will undoubtedly improve the breeding for citrus disease resistance with a much greater degree of precision. Here, we attempt to summarize the recent successful progress that has been achieved in the effective application of genetic engineering and genome editing technologies to obtain citrus disease-resistant (bacterial, fungal, and virus) crops. Furthermore, we also discuss the opportunities and challenges of genetic engineering and genome editing technologies for citrus disease resistance.

Suppression of Na+/K+-ATPase activity during estivation in the land snail Otala lactea

Entry into the hypometabolic state of estivation requires a coordinated suppression of the rate of cellular ATP turnover, including both ATP-generating and ATP-consuming reactions. As one of the largest consumers of cellular ATP, the plasma membrane Na+/K+-ATPase is a potentially key target for regulation during estivation. Na+/K+-ATPase was investigated in foot muscle and hepatopancreas of the land snail Otala lactea, comparing active and estivating states. In both tissues enzyme properties changed significantly during estivation: maximal activity was reduced by about one-third, affinity for Mg.ATP was reduced (Km was 40% higher), and activation energy (derived from Arrhenius plots) was increased by approximately 45%. Foot muscle Na+/K+-ATPase from estivated snails also showed an 80% increase in Km Na+ and a 60% increase in Ka Mg2+ as compared with active snails, whereas hepatopancreas Na+/K+-ATPase showed a 70% increase in I50 K+ during estivation. Western blotting with antibodies recognizing the alpha subunit of Na+/K+-ATPase showed no change in the amount of enzyme protein during estivation. Instead, the estivation-responsive change in Na+/K+-ATPase activity was linked to posttranslational modification. In vitro incubations manipulating endogenous kinase and phosphatase activities indicated that Na+/K+-ATPase from estivating snails was a high phosphate, low activity form, whereas dephosphorylation returned the enzyme to a high activity state characteristic of active snails. Treatment with protein kinases A, C or G could all mediate changes in enzyme properties in vitro that mimicked the effect of estivation, whereas treatments with protein phosphatase 1 or 2A had the opposite effect. Reversible phosphorylation control of Na+/K+-ATPase can provide the means of coordinating ATP use by this ion pump with the rates of ATP generation by catabolic pathways in estivating snails.

Functional role of the N-terminus of Na(+),K(+)-ATPase alpha-subunit as an inactivation gate of palytoxin-induced pump channel

The N-terminus of the Na(+),K(+)-ATPase alpha-subunit shows some homology to that of Shaker-B K(+) channels; the latter has been shown to mediate the N-type channel inactivation in a ball-and-chain mechanism. When the Torpedo Na(+),K(+)-ATPase is expressed in Xenopus oocytes and the pump is transformed into an ion channel with palytoxin (PTX), the channel exhibits a time-dependent inactivation gating at positive potentials. The inactivation gating is eliminated when the N-terminus is truncated by deleting the first 35 amino acids after the initial methionine. The inactivation gating is restored when a synthetic N-terminal peptide is applied to the truncated pumps at the intracellular surface. Truncated pumps generate no electrogenic current and exhibit an altered stoichiometry for active transport. Thus, the N-terminus of the alpha-subunit appears to act like an inactivation gate and performs a critical step in the Na(+),K(+)-ATPase pumping function.

Electrophysiology of the sodium-potassium-ATPase in cardiac cells

Like several other ion transporters, the Na(+)-K(+) pump of animal cells is electrogenic. The pump generates the pump current I(p). Under physiological conditions, I(p) is an outward current. It can be measured by electrophysiological methods. These methods permit the study of characteristics of the Na(+)-K(+) pump in its physiological environment, i.e., in the cell membrane. The cell membrane, across which a potential gradient exists, separates the cytosol and extracellular medium, which have distinctly different ionic compositions. The introduction of the patch-clamp techniques and the enzymatic isolation of cells have facilitated the investigation of I(p) in single cardiac myocytes. This review summarizes and discusses the results obtained from I(p) measurements in isolated cardiac cells. These results offer new exciting insights into the voltage and ionic dependence of the Na(+)-K(+) pump activity, its effect on membrane potential, and its modulation by hormones, transmitters, and drugs. They are fundamental for our current understanding of Na(+)-K(+) pumping in electrically excitable cells.

Extracellular pH modulates kinetics of the Na(+),K(+)-ATPase

To investigate effects of pH on the Na(+),K(+)-ATPase, we used the Xenopus oocytes to measure transient charge movements in the absence of extracellular K(+), and steady-state currents mediated by the pump as well as ATPase activity. The activity of purified Na(+), K(+)-ATPase strongly depends on pH, which has been attributed to protonation of intracellular sites. The steady-state current reflects pump activity, the transient charge movement voltage-dependent interaction of external Na(+) ions with the pump molecule and/or conformational changes during Na(+)/Na(+) exchange. The steady-state current exhibits a characteristic voltage dependence with maximum at about 0 mV at low external K(+) (< or =2 mM) and with 50 Na(+). This dependency is not significantly affected by changes in external pH in the range from pH 9 to pH 6. Only below pH 6, the voltage dependence of pump current becomes less steep, and may be attributed to a pH-dependent inhibition of the forward pump cycle by external Na(+). External stimulation of the pump by K(+) in the absence of Na(+) can be described by a voltage-dependent K(m) value with an apparent valency z(K). At higher external pH the z(K) value is reduced. The transient current signal in the absence of external K(+) can be described by the sum of three exponentials with voltage-dependent time constants of about 50 ms, 700 micros and less than 100 micros during pulses to 0 mV. The charge distribution was calculated by integration of the transient current signals. The slowest component and the associated charge distributions do not significantly depend on external pH changes. The intermediate component of the transients is represented by a voltage-dependent rate constant which shows a minimum at about -120 mV and increases with decreasing pH. Nevertheless, the contribution to the charge movement is not altered by pH changes due to a simultaneous increase of the amplitude of this component. We conclude that reduction of external pH counteracts external K(+) and Na(+) binding.

Identification of a phospholemman-like protein from shark rectal glands. Evidence for indirect regulation of Na,K-ATPase by protein kinase c via a novel member of the FXYDY family

The Na,K-ATPase provides the driving force for many ion transport processes through control of Na(+) and K(+) concentration gradients across the plasma membranes of animal cells. It is composed of two subunits, alpha and beta. In many tissues, predominantly in kidney, it is associated with a small ancillary component, the gamma-subunit that plays a modulatory role. A novel 15-kDa protein, sharing considerable homology to the gamma-subunit and to phospholemman (PLM) was identified in purified Na,K-ATPase preparations from rectal glands of the shark Squalus acanthias, but was absent in pig kidney preparations. This PLM-like protein from shark (PLMS) was found to be a substrate for both PKA and PKC. Antibodies to the Na, K-ATPase alpha-subunit coimmunoprecipitated PLMS. Purified PLMS also coimmunoprecipitated with the alpha-subunit of pig kidney Na, K-ATPase, indicating specific association with different alpha-isoforms. Finally, PLMS and the alpha-subunit were expressed in stoichiometric amounts in rectal gland membrane preparations. Incubation of membrane bound Na,K-ATPase with non-solubilizing concentrations of C(12)E(8) resulted in functional dissociation of PLMS from Na,K-ATPase and increased the hydrolytic activity. The same effects were observed after PKC phosphorylation of Na,K-ATPase membrane preparations. Thus, PLMS may function as a modulator of shark Na,K-ATPase in a way resembling the phospholamban regulation of the Ca-ATPase.

Mutations of Ser-23 of the alpha1 subunit of the rat Na+/K+-ATPase to negatively charged amino acid residues mimic the functional effect of PKC-mediated phosphorylation

The Na+/K+-ATPase is a target protein for protein kinase C (PKC). The PKC-mediated phosphorylation of the rat alpha1 subunit at Ser-23 results in the inhibition of its transport function. To understand the molecular basis of the inhibition by PKC, the Ser-23 in the rat alpha1 subunit has been replaced by negatively (Asp, Glu) or positively (Lys) charged, or uncharged (Gln, Ala) residues, and the mutants were expressed in Xenopus oocytes. Ouabain-specific 86Rb uptake and pump-generated current as well as sensitivity to ouabain and to external K+ have been investigated. When Ser-23 was replaced by the negatively charged residues, transport function was inhibited, and simultaneously synthesis of the alpha subunits was enhanced. In addition, if Ser-23 was substituted by Glu, the K(I) value for inhibition of transport by ouabain was drastically increased from 46.5 microM to 1.05 mM. The data suggest that insertion of a negative charge within the N-terminus of alpha subunit of the Na+/K+-ATPase due to phosphorylation of Ser-23 plays an important role in the PKC-mediated inhibition of transport function.

[Ca2+]i determines the effects of protein kinases A and C on activity of rat renal Na+,K+-ATPase

1. It is well established that the activity of Na+,K+-ATPase (NKA) is regulated by protein kinases A (PKA) and C (PKC), but results on their effects have been conflicting. The aim of this study was to examine if this is ascribed to the intracellular concentration of Ca2+ ([Ca2+]i). 2. Rat renal NKA was stably expressed in COS cells (green monkey kidney cells). Increases in [Ca2+]i were achieved with the Ca2+ ionophore A23187 and verified by direct measurements of [Ca2+]i using fura-2 AM as an indicator. The activity of NKA was measured as ouabain-sensitive 86Rb+ uptake and the state of phosphorylation of NKA was monitored with two site-directed phosphorylation state-specific antibodies. 3. Activation of PKA with forskolin decreased NKA activity by 45.5 +/- 8.9 % at low [Ca2+]i (120 nM) and increased it by 40.5 +/- 6.4 % at high [Ca2+]i (420 nM). The change in NKA activity by forskolin correlated with the level of increase in [Ca2+]i. 4. The effect of 1-oleoyl-2-acetoyl-sn-glycerol (OAG), a specific PKC activator, on the activity of NKA was also Ca2+ dependent, being inhibitory when [Ca2+]i was low (29.3 +/- 3.6 % decrease at 120 nM Ca2+) and stimulatory when [Ca2+]i was high (36.6 +/- 10.1 % increase at 420 nM Ca2+). 5. The alpha subunit of NKA was phosphorylated under both low and high [Ca2+]i conditions upon PKA or PKC activation. PKA phosphorylates Ser943. PKC phosphorylates Ser23. 6. To see if the observed effects on NKA activity are secondary to changes in Na+ entry, we measured NKA hydrolytic activity using permeabilized membranes isolated from cells under controlled Na+ conditions. A decreased activity at low [Ca2+]i and no change in activity at high [Ca2+]i were observed following forskolin or OAG treatment. 7. Purified NKA from rat renal cortex was phosphorylated and inhibited by PKC. This phosphorylation-associated inhibition of NKA was neither affected by Ca2+ nor by calmodulin, tested alone or together. 8. We conclude that effect of PKA/PKC on NKA activity is dependent on [Ca2+]i. This Ca2+ dependence may provide an explanation for the diversity of responses of NKA to activation of either PKA or PKC.

Phosphorylation of the alpha-subunits of the Na+/K+-ATPase from mammalian kidneys and Xenopus oocytes by cGMP-dependent protein kinase results in stimulation of ATPase activity

Phosphorylation of Na+/K+-ATPase by cGMP-dependent protein kinase (PKG) has been studied in enzymes purified from pig, dog, sheep and rat kidneys, and in Xenopus oocytes. PKG phosphorylates the alpha-subunits of all animal species investigated. Phosphorylation of the beta-subunit was not observed. The stoichiometry of phosphorylation estimated for pig, sheep and dog renal Na+/K+-ATPase is 3.5, 2.2 and 2.1 mol Pi per mol alpha-subunit, respectively. Proteolytic fingerprinting of the pig alpha1-subunits phosphorylated by PKG using specific antibodies raised against N-terminus or C-terminus reveals that phosphorylation sites are located within the intracellular loop of the alpha-subunit between the 35 kDa N-terminal and 27 kDa C-terminal fragments. Phosphorylation sites within the alpha1-subunit of the purified Na+/K+-ATPase do not appear to be easily accessible for PKG since incorporation of Pi requires 0.2% of Triton X-100. Administration of cGMP and PKG in the presence of 5 mm ATP, which prevents inactivation of the Na+/K+-ATPase by detergent, leads to stimulation of hydrolytic activity by 61%. Administration of 50 microm of cGMP or dbcGMP in yolk-free homogenates of Xenopus oocytes leads to stimulation of ouabain-dependent ATPase activity by 130-198% and to incorporation of 33P into the alpha-subunit without the detergent. Hence, PKG plays regulatory role in active transmembraneous transport of Na+ and K+ via phosphorylation of the catalytic subunit of the Na+/K+-ATPase.

Voltage-dependent inhibition of the Na+,K+ pump by tetraethylammonium

Tetraethylammonium (TEA+) is an effective inhibitor of a variety of K+ channels, and has been widely used to reduce K+-sensitive background conductances in electrophysiological investigations of the Na+,K+-ATPase. Here we demonstrate by combination of two-electrode voltage clamp (TEVC) and giant patch clamp of Xenopus oocytes, and measurements of the activity of purified ATPase of pig kidney that TEA+ directly inhibits the Na+,K+-ATPase from the outside. The KI value in TEVC experiments at 0 mV is about 10 mM increasing with more negative potentials. A similar voltage-dependent inhibition by TEA+ was observed in the excised membrane patches except that the apparent KI value at 0 mV is about 100 mM, a value nearly identical to that found for inhibition of purified kidney ATPase. The voltage-dependent inhibition can be described by an effective valency of 0.39 and is attributed to an interference with the voltage-dependent binding of K+ at an external access channel. The apparent dielectric length of the access channel for K+ is not affected by TEA+.

alpha-Adrenergic effects on Na+-K+ pump current in guinea-pig ventricular myocytes

1. The whole-cell patch clamp was employed to study Na+-K+ pump current (Ip) in acutely isolated myocytes. alpha-Adrenergic receptors were activated with noradrenaline (NA) after blocking beta-adrenergic receptors with propranolol. Ip was measured as the current blocked by strophanthidin (Str). 2. Activation of alpha-receptors by NA increased Ip in a concentration-dependent manner. The K0.5 depended on intracellular calcium ([Ca2+]i), however maximal stimulation did not. At 15 nM [Ca2+]i the K0.5 was 219 nM NA whereas at 1.4 microM [Ca2+]i it was 3 nM. 3. The voltage dependence of Ip was not shifted by NA at either high or low [Ca2+]i. At each voltage, maximal stimulation of Ip was 14-15 %. 4. Staurosporine (St), an inhibitor of protein kinase C (PKC), eliminated the alpha-receptor-mediated stimulation of Ip at either high or low[Ca2+]i. 5. The stimulation of Ip was independent of changes in intracellular sodium or external potassium concentrations, and did not reflect a change in affinity for Str. 6. Phenylephrine, methoxamine and metaraminol, three selective alpha1-adrenergic agonists, stimulate Ip in a similar manner to NA. Stimulation of Ip by NA was eliminated by prazosin, an alpha1-antagonist, but was unaffected by yohimbine, an alpha2-antagonist. 7. We conclude noradrenaline activates ventricular alpha1-receptors, which are specifically coupled via PKC to increase Na+-K+ pump current. The sensitivity of the coupling is [Ca2+]i dependent, however the maximal increase in pump current is [Ca2+]i and voltage independent.

Significance of the glutamic acid residues Glu334, Glu959, and Glu960 of the alpha subunits of Torpedo Na+, K+ pumps for transport activity and ouabain binding

Glutamic acid residues in transmembrane segments of the alpha subunit of the Na+,K+-ATPase have been discussed as possible candidates for the binding sites of the transported cations. Here we report on effects of mutations of Glu334, Glu959, and Glu960 to alanine in ouabain-sensitive (OS) as well as ouabain-resistant (OR) ATPases of Torpedo electroplax expressed in Xenopus oocytes. All mutants are incorporated to about the same extend as the wild-type ATPases into the plasma membrane. None of the mutations produces complete inhibition of transport activity as judged from measurements of 86Rb+ uptake, membrane current, and ATPase activity. After conversion of OS to OR by mutation of the bordering residues of the first extracellular loop Gln118 to Arg and Asp129 to Asn, the Km value for inhibition by ouabain increases to 59 microM. Substitution of Glu334 to Ala in the OR pump variant restores ouabain sensitivity with a Km value of 0.12 microM, which is similar to that of the endogenous Xenopus pump. After substitution of Glu960 by Ala in the OR pump, ouabain sensitivity is partially restored. The Km values for pump stimulation by external K+ appear to be reduced in the OR compared to the OS pump. Mutation of Glu959 and Glu960 to Ala has no pronounced effects on the potential-dependent Km values at external pH 7.8; only in the Glu959-mutated OR pump, the apparent Km at 0 mV is raised. We conclude that none of the mutated glutamic acid residues is essential for cation coordination, but that GIu334, and in part also Glu960, seems to be involved in preserving the ouabain-resistant conformation of the enzyme.

Assembly of the chimeric Na+/K+-ATPase and H+/K+-ATPase beta-subunit with the Na+/K+-ATPase alpha-subunit

Two sets of chimeric beta-subunits were constructed from subunits of Torpedo californica Na+/K+-ATPase and pig gastric H+/K+-ATPase. Five unique restriction sites (SnaBI, EcoRV, MunI, SphI and EcoT22I) were created at equivalent positions of the respective cDNAs and were used as joining points for the construction. One set of chimeras (HxN series) was made by exchanging the 5' portion of the Na+/K+-ATPase beta-subunit cDNA with the corresponding portion of the H+/K+-ATPase beta-subunit cDNA at the respective joining point. Complementary constructs were also prepared (NxH series). In the HxN series, the chimera joined at the SnaBI site formed a stable trypsin resistant complex with the Na+/K+-ATPase alpha-subunit, which was functional with respect to ATP hydrolysis and pump current generation, although the activities were less than those of the complex with the Na+/K+-ATPase beta-subunit. Trypsin resistance decreased for the complex of the chimera joined at the EcoRV site. In the NxH series, the chimeras joined at the SnaBI site and the EcoRV site formed rather trypsin-resistant complexes, but the expressions of the alpha-subunits were below 50% of the control. The chimeras joined at the MunI, SphI and EcoT22I site formed complexes susceptible to tryptic digestion. None of the chimeras in the NxH series were functional. These results suggest that at least two regions of the Na+/K+-ATPase beta-subunit [SnaBI site(Tyr40) to EcoRV site(Ile89) and EcoT22I site(Cys176) to C-terminus)] are involved in stable assembly with the Na+/K+-ATPase alpha-subunit and that the cytoplasmic domain [N-terminus to SnaBI site(Tyr40)] is functionally replaceable with the corresponding domain of the H+/K+-ATPase beta-subunit.

Role of serine/threonine protein phosphatases in insulin regulation of Na+/K+-ATPase activity in cultured rat skeletal muscle cells

In this study, we examined the potential role of serine/threonine protein phosphatase-1 (PP-1) and PP-2A in the mechanism of Na+/K+-ATPase activation by insulin in the rat skeletal muscle cell line L6. Incubation of L6 cells with insulin caused a time- and dose-dependent stimulation of ouabain-sensitive plasma membrane Na+/K+-ATPase activity. Pretreatment with okadaic acid (OA; 0.1-1 microM) or calyculin A (1 microM) blocked insulin's effect on Na+/K+-ATPase activation. Low concentrations of OA that specifically inhibit PP-2A were ineffective. Immunoprecipitation of the enzyme from 32P-labeled cells with an antibody directed against the alpha-1 subunit of the enzyme revealed a 60% decrease in 110-kDa protein phosphorylation in insulin-treated cells. The presence of calyculin A blocked insulin-mediated dephosphorylation of Na+/K+-ATPase, whereas low concentrations of OA were ineffective. To further confirm the role of PP-1, we used L6 cell lines that overexpress the glycogen/SR-associated regulatory subunit of PP-1, PP-1G. Overexpression of PP-1G resulted in a 3-fold increase in insulin-stimulated PP-1 catalytic activity. This was accompanied by a 30% increase in basal Na+/K+-ATPase activity and a >2-fold increase in insulin's effect on pump activity. Inhibition of phosphatidylinositol-3 kinase with wortmannin blocked insulin-stimulated PP-1 activation as well as the dephosphorylation and activation of Na+/K+-ATPase. We conclude that insulin regulates the activity of Na+/K+-ATPase by promoting dephosphorylation of the alpha subunit via an insulin-stimulated PP-1 and that phosphatidylinositol-3 kinase-generated signals may mediate insulin activation of PP-1 and Na+/K+-ATPase.

The Ca2+/calmodulin binding domain of the Ca2+-ATPase linked to the Na+,K+-ATPase alters transport stoichiometry

Using Xenopus oocytes as an expression system, we have investigated ion-transport and ouabain-binding properties of a chimeric ATPase (alpha1-CBD; Ishii and Takeyasu (1995) EMBO J. 14, 58-67) formed by the alpha1-subunit of chicken Na+,K(+)-ATPase (alpha1) and the calmodulin binding domain (CBD) of the rat plasma membrane Ca2(+)-ATPase. alpha1-CBD can be expressed and transported to the oocyte plasma membrane without the beta-subunit, and shows ouabain binding. In contrast to ouabain binding, this chimera requires the beta-subunit for its cation (Na+ and K+) transport activity. alpha1-CBD exhibits an altered stoichiometry of Na(+)-K+ exchange. A detailed analysis of 22Na+ efflux, 86Rb+ uptake, pump current and ouabain binding suggests that the chimeric molecule can operate in an electrically silent 2Na(+)-2K+ exchange mode and, with much lower probability, in its normal 3Na(+)-2K+ exchange mode.

20-Hydroxyeicosa-tetraenoic acid (20 HETE) activates protein kinase C. Role in regulation of rat renal Na+,K+-ATPase

It is well documented that the activity of Na+,K+-ATPase can be inhibited by the arachidonic acid metabolite, 20-hydroxyeicosa-tetraenoic acid (20 HETE). Evidence is presented here that this effect is mediated by protein kinase C (PKC). PKC inhibitors abolished 20 HETE inhibition of rat Na+,K+-ATPase in renal tubular cells. 20 HETE caused translocation of PKC alpha from cytoplasm to membrane in COS cells. It also inhibited Na+,K+-ATPase activity in COS cells transfected with rat wild-type renal Na+,K+-ATPase alpha1 subunit, but not in cells transfected with Na+,K+-ATPase alpha1, where the PKC phosphorylation site, serine 23, had been mutated to alanine. PKC-induced phosphorylation of rat renal Na+,K+-ATPase, as well as of histone was strongly enhanced by 20 HETE at the physiologic calcium concentration of 1.3 microM, but not at the calcium concentration of 200 microM. The results indicate that phospholipase A2-arachidonic acid-20 HETE pathway can exert important biological effects via activation of PKC and that this effect may occur in the absence of a rise in intracellular calcium.

Activation of the Na(+)-K+ pump in frog erythrocytes by catecholamines and phosphodiesterase blockers

K+ and Na+ influx into frog erythrocytes incubated in standard saline was studied using 86Rb and 22Na as tracers. 10 microM isoproterenol (ISP) produced a significant increase in K+ influx for the first 15 min, which was sustained during the entire 60 min of cell incubation. Treatment of red cells with the phosphodiesterase (PDE) blockers theophylline (THEO, 1 and 5 mM) or 3-isobutyl-1-methylxanthine (IBMX, 0.5 mM) was also accompanied by an enhancement in K+ influx. A distinct additive effect on K+ influx into red cells was found when ISP and THEO or IBMX were added together. The increase in K+ transport induced by ISP plus IBMX was totally abolished by pretreatment of red cells with 0.1 mM ouabain. The ouabain-sensitive K+ influx in frog erythrocytes was elevated in the presence of ISP plus IBMX to 2.05 +/- 0.45, as compared with the control level of 0.39 +/- 0.11 mmol/L cells/hr (P < 0.001). Similar effects of ISP and IBMX on K+ influx were observed after chloride was replaced by nitrate. A dose-related increase in K+ influx into frog erythrocytes was observed at ISP concentrations of 10(-8)-10(-6) M, with a half-maximal stimulatory concentration of approximately 0.02 microM. The effects of ISP (10(-8)-10(-5) M) on K+ transport were completely abolished with 10 microM of the beta-adrenergic blocker propranolol, but alpha-adrenergic antagonists (phentolamine, prazosin, and yohimbine) did not alter the ISP-induced increase in K+ influx. The drugs tested had no effect on 22Na influx in frog red cells, but ISP produced a small decline (13%) in intracellular Na+ concentration. Thus, our study indicates that catecholamines and PDE blockers enhance K+ (86Rb) transport in frog erythrocytes mediated by Na(+)-K+ pump activity. The frog erythrocyte membrane may serve as a convenient model to investigate the hormonal modulation of the Na(+)-K+.

The effects of beta-stimulation on the Na(+)-K+ pump current-voltage relationship in guinea-pig ventricular myocytes

1. The whole cell patch clamp technique was used to study effects of the beta agonist isoprenaline (Iso) on the current-voltage (I-V) relationship of the Na(+)-K+ pump current (Ip) in acutely isolated guinea-pig ventricular myocytes. 2. The effect of Iso on Ip at high [Ca2+]i (1.4 microM) was voltage dependent. The I-V relationship of Ip in Iso shifted by approximately 30 mV in the negative direction on the voltage axis, increasing Ip at negative voltages but leaving Ip unchanged at positive voltages. 3. Intracellular application of the calmodulin antagonist, calmodulin-dependent protein kinase II fragment 290-309, did not eliminate or reduce the Iso-induced voltage shift, suggesting calmodulin-dependent protein kinase II was not involved. 4. The Iso inhibition of Ip at low [Ca2+]i (15 nM) was not voltage dependent. Ip was reduced by 20 to 30% in the presence of Iso at each holding potential. 5. When the voltage dependence of Ip was largely reduced by substitution of N-methyl-D-glucamine+ for external Na+, the magnitude of the low [Ca2+]i, Iso-induced inhibition of Ip was progressively eliminated by increasing the [Ca2+]i. At a [Ca2+]i of 1.4 microM, this inhibition disappeared. 6. At intermediate values of [Ca2+]i, the I-V curves in Na(+)-containing solution in the presence and the absence of Iso crossed over. The higher the [Ca2+]i, the more positive the voltage at which the two I-V curves intersected. 7. During beta-adrenergic activation our results suggest intracellular Ca2+ has two effects: (a) It prevents protein kinase A (PKA) phosphorylation-induced inhibition of Ip. (b) It causes a PKA phosphorylation-induced shift of the pump I-V relationship in the negative direction on the voltage axis. These effects may have important physiological significance in the regulation of heart rate and cardiac contractility.

Activation of Na+/K(+)-ATPase by fatty acids, acylglycerols, and related amphiphiles: structure-activity relationship

A number of fatty acids and derivatives have been shown to activate Na+/K(+)-ATPase when ATP is suboptimal. To explore the relation of the structures of these amphiphiles to enzyme activation, the effects of varying amphiphile concentrations on the activity of the highly purified kidney Na+/K(+)-ATPase at 50 microM ATP were determined. Among fatty acids, efficacy (maximal level of activation) and potency were found to be dependent, in different ways, on chain length and unsaturation. Compared to fatty acids, the corresponding alcohols had lower efficacies. Methyl esters of fatty acids inhibited, but CoA esters and monoacyl esters of glycerol activated the enzyme. Relation between chain length and potency among CoA esters and monoacylglycerols was the same as that observed with acids. Diacylglycerols did not activate, but they antagonized the effects of the activator amphiphiles. The substantial specificities of the amphiphile effects support the hypothesis that these ligands bind to a distinct amphipathic peptide segment of the intracellular central loop of the alpha-subunit to regulate ATP binding to the enzyme. The findings also suggest that direct effects of the changing intracellular levels of fatty acids and derivatives on Na+/K(+)-ATPase should be considered as a possible mechanism for the regulation of its function in the intact cell.

Regulation of renal Na+,K(+)-ATPase in rat thick ascending limb during K+ depletion: evidence for modulation of Na+ affinity

1. NaCl reabsorption along the loop of Henle is reduced in K(+)-depleted rats. Because Na+,K(+)-ATPase energizes this transport and because K+ depletion is known to induce an upregulation of Na+,K(+)-ATPase in most tissues, the regulation of this enzyme was investigated at the level of single thick ascending limbs of the loop of Henle freshly microdissected from rats fed either a normal (control rats) or a low-K+ diet (LK rats). 2. Within 2 weeks of K+ depletion, Na+,K(+)-ATPase activity and [3H]ouabain binding were increased by 30-50% in the medullary portion of the thick ascending limb (MTAL). 3. Despite this increase in the number of Na+,K(+)-ATPase units, the transport capacity of the Na+,K+ pump, determined by ouabain-sensitive Rb+ uptake in the presence of an extracellular concentration of Rb+ mimicking the kalaemia determined in control (4.0 mM Rb+) and LK rats (2.3 mM Rb+), was reduced in MTAL from LK rats. 4. Inhibition of the Na+,K+ pump was not accounted for by changes in either extracellular K+ or intracellular Na+ concentrations, but by a decrease in the pump affinity for Na+. 5. Because this change in the apparent affinity of the Na+,K+ pump for Na+ was detectable in intact but not in permeabilized MTAL cells, it is probably induced by a rapidly reversible cytosolic factor.

Adrenergic, dopaminergic, and muscarinic receptor stimulation leads to PKA phosphorylation of Na-K-ATPase

Na-K-adenosinetriphosphatase (Na-K-ATPase) is a potential target for phosphorylation by protein kinase A (PKA) and C (PKC). We have investigated whether the Na-K-ATPase alpha-subunit becomes phosphorylated at its PKA or PKC phosphorylation sites upon stimulation of G protein-coupled receptors primarily linked either to the PKA or the PKC pathway. COS-7 cells, transiently or stably expressing Bufo marinus Na-K-ATPase wild-type alpha- or mutant alpha-subunits affected in its PKA or PKC phosphorylation site, were transfected with recombinant DNA encoding beta 2- or alpha 1-adrenergic (AR), dopaminergic (D1A-R), or muscarinic cholinergic (M1-AChR) receptor subspecies. Agonist stimulation of beta 2-AR or D1A-R led to phosphorylation of the wild-type alpha-subunit, as well as the PKC mutant, but not of the PKA mutant, indicating that these receptors can phosphorylate the Na-K-ATPase via PKA activation. Surprisingly, stimulation of the alpha 1B-AR, alpha 1C-AR, and M1-AChR also increased the phosphorylation of the wild-type alpha-subunit and its PKC mutant but not of its PKA mutant. Thus the phosphorylation induced by these primarily phospholipase C-linked receptors seems mainly mediated by PKA activation. These data indicate that the Na-K-ATPase alpha-subunit can act as an ultimate target for PKA phosphorylation in a cascade starting with agonist-receptor interaction and leading finally to a phosphorylation-mediated regulation of the enzyme.

Hydrophobic ion interaction on Na+ activation and dephosphorylation of reconstituted Na+,K(+)-ATPase

In liposomes with reconstituted shark Na+,K(+)-ATPase an uncoupled Na(+)-efflux and a Na+/Na+ exchange can be induced on inside-out oriented pumps by the addition of external (cytoplasmic) Na+ and MgATP to liposomes that either do not contain Na+ (and other alkali cations), or include 130 mM Na+ internally (extracellular). Both modes of exchange are electrogenic and accompanied by a net hydrolysis of ATP. The coupling ratio of positive net charges translocated per ATP split is found to be close to 3:1 and 1:1, respectively, for the two modes of exchange reactions at pH 7.0. By addition of the hydrophobic anion tetraphenylboron (TPB-), which imposes a negative electrostatic membrane potential inside the lipid bilayer, the ATP hydrolysis accompanying uncoupled Na+ efflux is increased with increasing TPB- concentrations. Cholesterol which increases the inner positive dipole potential of the bilayer counteracted this activation by TPB- of uncoupled Na+ efflux. Using the structural analog tetraphenylphosphonium (TPP+), which elicits an inside positive membrane potential, ATP hydrolysis accompanying uncoupled Na(+)-efflux is decreased. The rate of dephosphorylation in the absence of extracellular alkali cations was affected in a similar manner, whereas the dephosphorylation in the presence of extracellular Na+ inducing Na+/Na+ exchange was unaffected by the hydrophobic ions. In both modes of exchange the phosphorylation reaction was independent of the presence of hydrophobic ions. The hydrophobic ions affected the apparent affinity for cytoplasmic Na+, indicating that binding of cytoplasmic Na+ may involve the migration of cations to binding sites through a shallow cytoplasmic access channel. The results are in accordance with the simple electrostatic model for charge translocation in which two negative charges in the cytoplasmic binding domain of the Na+,K(+)-ATPase co-migrate during cation transport.

The effect of cardiac glycosides on the Na+ pump current-voltage relationship of isolated rat and guinea-pig heart cells

1. Whole-cell recording from isolated rat and guinea-pig ventricular myocytes revealed a change of the cardiac Na+ pump current (Ip)-voltage (V) relationship by cardiac glycosides, specific inhibitors of the Na(+)-K+ pump. 2. Dihydro-ouabain (DHO) diminished Ip in rat ventricular cells at 0 mV in a concentration-dependent manner. 3. The concentration-response curve of Ip inhibition caused by DHO was shifted to higher [DHO] at higher extracellular K+ concentrations ([K+]o) or at more negative membrane potentials. 4. In rat myocytes, DHO immediately flattened the normalized cardiac Ip-V curve and evoked or enhanced a region of negative slope. 5. Ouabain, at concentrations which caused a comparable inhibition of Ip, exerted DHO-like effects on the Ip-V relationship of rat ventricular myocytes. However, the effects developed more slowly. 6. A slowly developing alteration of the Ip-V curve was also observed upon application of DHO to guinea-pig ventricular cells. The range of [DHO] used was about 100-fold lower than that applied to rat ventricular cells, but was equally effective for Ip inhibition. 7. Increasing the K+ concentration of DHO-containing media affected the existing equilibrium of DHO binding to the cardiac Na(+)-K+ pump. A new equilibrium was reached within about 3 s in rat ventricular myocytes, but only within about 50 s in guinea-pig ventricular cells under the experimental conditions chosen. 8. It is concluded that the changes of the cardiac Ip-V curve induced by cardiac glycosides are mediated by voltage-dependent variations of the local [K+]o at the K+ binding sites of the Na(+)-K+ pump in an 'access channel'. The variations were estimated by means of the Boltzmann equation. The estimations agreed with those derived from the measured DHO binding to the Na(+)-K+ pump at various [K+]o. A new equilibrium of glycoside binding to the pump is established at the altered [K+]o. The time necessary to reach the new binding equilibrium varies with the cardioactive steroid, its concentration and the glycoside sensitivity of the cardiac cells.

Regulation of the beta-stimulation of the Na(+)-K+ pump current in guinea-pig ventricular myocytes by a cAMP-dependent PKA pathway

1. The whole-cell patch-clamp technique was employed with the free intracellular [Ca2+] fixed at 1.4 microM in order to study the isoprenaline (Iso)-induced increase in the Na(+)-K+ pump current (Ip) in acutely isolated guinea-pig ventricular myocytes. 2. The non-specific protein kinase inhibitor, H-7, eliminated the stimulatory effect of Iso, suggesting a phosphorylation step is involved in the beta-agonist stimulation of Ip. 3. H-7 or the phosphatase inhibitor calyculin A individually had no effect on basal Ip; however, when Ip was first increased by Iso, H-7 inhibited and calyculin A further increased Ip. This suggests phosphorylation is not important to the basal regulation of Ip, but does have an effect during beta-stimulation. 4. The Iso-induced increase in Ip could be mimicked by adding the membrane-permanent cAMP analogue chlorophenylthio-cAMP, blocking cAMP degradation with IBMX or stimulating cAMP production with forskolin. Alternatively the protein kinase A inhibitor PKI blocked the stimulatory effect of Iso. This suggests the Iso-induced phosphorylation responsible for increasing Ip is mediated by cAMP, which then activates protein kinase A (PKA). 5. We conclude that the beta-agonist-induced increase in Ip in the presence of high intracellular [Ca2+] is mediated by a phosphorylation step via the cAMP-dependent PKA pathway. During beta-stimulation, this increase in active Na(+)-K+ transport can serve to offset the effects of increases in passive membrane conductances.

Isoproterenol stimulates rapid extrusion of sodium from isolated smooth muscle cells

beta-Agonists cause an inhibition of contractility and a transient stimulation of Na+/K+ pumping in smooth muscle cells of the stomach from the toad Bufo marinus. To determine if the stimulation of Na+/K+ pumping causes changes in intracellular [Na+] ([Na+]i) that might link Na+ pump stimulation to decrease Ca2+ availability for contraction, [Na+]i was measured in these cells with SBFI, a Na(+)-sensitive fluorescent indicator. Basal [Na+]i was 12.8 +/- 4.2 mM (n = 32) and was uniform throughout the cell. In response to isoproterenol, [Na+]i decreased an average of 7.1 +/- 1.1 mM in 3 sec. Since this decrease in [Na+]i could be completely blocked by inhibition of the Na+ pump, or by blockade of the beta-receptor, [Na+]i reduction is the result of occupation of the beta-receptor by isoproterenol and subsequent stimulation of the Na+ pump. 8-Bromoadenosine 3',5'-cyclic monophosphate and forskolin mimicked the effect of isoproterenol, indicating that the sequence of events linking beta-receptor occupation to Na+ pump stimulation most likely includes activation of adenylate cyclase, production of cAMP, and stimulation of cAMP-dependent protein kinase. The decrease in [Na+]i is sufficiently large and fast that it is expected to stimulate turnover of the Na+/Ca2+ exchanger in the Ca2+ extrusion mode, thereby accounting for the observed linkage between stimulation of the Na+/K+ pump and inhibition of contractility in response to beta-adrenergic agonists.

Electrogenic Na+/K(+)-transport in human endothelial cells

Na+/K+ pump currents were measured in endothelial cells from human umbilical cord vein using the whole-cell or nystatin-perforated-patch-clamp technique combined with intracellular calcium concentration ([Ca2+]i) measurements with Fura-2/AM. Loading endothelial cells through the patch pipette with 40 mmol/l [Na+] did not induce significant changes of [Ca2+]i. Superfusing the cells with K(+)-free solutions also did not significantly affect [Ca2+]i. Reapplication of K+ after superfusion of the cells with K(+)-free solution induced an outward current at a holding potential of 0 mV. This current was nearly completely blocked by 100 mumol/l dihydroouabain (DHO) and was therefore identified as a Na+/K+ pump current. During block and reactivation of the Na+/K+ pump no changes in [Ca2+]i could be observed. Pump currents were blocked concentration dependently by DHO. The concentration for half-maximal inhibition was 21 mumol/l. This value is larger than that reported for other tissues and the block was practically irreversible. Insulin (10-1000 U/l) did not affect the pump currents. An increase of the intracellular Na+ concentration ([Na+]i) enhanced the amplitude of the pump current. Half-maximal activation of the pump current by [Na+]i occurred at about 60 mmol/l. The concentration for half-maximal activation by extracellular K+ was 2.4 +/- 1.2 mmol/l, and 0.4 +/- 0.1 and 8.7 +/- 0.7 mmol/l for Tl+ and NH4+ respectively. The voltage dependence of the DHO-sensitive current was obtained by applying linear voltage ramps. Its reversal potential was more negative than -150 mV. Pump currents measured with the conventional whole-cell technique were about four times smaller than pump currents recorded with the nystatin-perforated-patch method.(ABSTRACT TRUNCATED AT 250 WORDS)

Na+ pump current-voltage relationships of rabbit cardiac Purkinje cells in Na(+)-free solution

1. The Na+ pump current (Ip) of isolated, single rabbit cardiac Purkinje cells in Na(+)-free solution was measured at 32-34 degrees C by means of whole-cell recording. 2. The Ip amplitude was studied as a function of clamp potential (Vc) and external concentration of various monovalent cations known to activate the Na(+)-K+ pump. 3. Under conditions which strongly activated Ip the Ip-Vc curve of the cells displayed a positive slope at membrane potentials negative to -20 mV and little variation at more positive potentials. 4. The Ip-Vc relationship showed an extended region of negative slope at positive and negative potentials in solutions containing low concentrations of activator cations which caused little Ip activation. A positive slope of the Ip-Vc curve was occasionally observed at clamp potentials negative to -60 mV under these conditions. 5. The shape of the Ip-Vc relation was independent of the cation species used as external Ip activator. 6. At zero membrane potential half-maximum Ip activation (K0.5(Vc = 0 mV) occurred at 0.05 mM Tl+, 0.08 mM K+, 0.4 mM NH4+ and 1.5 mM Cs+. The Hill coefficient derived amounted to 0.9 for Tl+, 1.2 for K+, 1.04 for NH4+ and 1.5 for Cs+. 7. The concentrations of external activator cations required for half-maximum Ip activation increased with depolarization. The voltage dependence of the K0.5 values could be described by a single exponential function for clamp potentials positive to -40 mV. 8. The steepness of the function is determined by a factor alpha, indicating the apparent fraction of an elementary charge which moves in the electrical field across the sarcolemma when external monovalent cations bind to the Na(+)-K+ pump. 9. The alpha values were calculated to be 0.32 for Tl+, 0.24 for K+, 0.29 for NH4+ and 0.18 for Cs+. Possible interpretations of the alpha values are considered. 10. It is suggested that binding of external monovalent activator cations to the Na(+)-K+ pump (or a process related to the binding) is voltage dependent. This potential-dependent process determines mainly the shape of the Ip-Vc curve in cardiac Purkinje cells superfused with Na(+)-free media containing low concentrations (< K0.5(Vc = 0 mV)) of K+ or its congeners.

Annual review prize lecture. 'All hands to the sodium pump'

Images

Phosphorylation of the Na,K-ATPase by Ca,phospholipid-dependent and cAMP-dependent protein kinases. Mapping of the region phosphorylated by Ca,phospholipid-dependent protein kinase

Ca,phospholipid-dependent (PKC) and cAMP-dependent (PKA) protein kinases phosphorylate the alpha-subunit of the Na,K-ATPase from duck salt gland with the incorporation of 0.3 and 0.5 mol 32P/mol of alpha-subunit, respectively. PKA (in contrast to PKC) phosphorylates the alpha-subunit only in the presence of detergents. Limited tryptic digestion of the Na,K-ATPase phosphorylated by PKC demonstrates that 32P is incorporated into the N-terminal 41-kDa fragment of the alpha-subunit. Selective chymotrypsin cleavage of phosphorylated enzyme yields a 35-kDa radioactive fragment derived from the central region of the alpha-subunit molecule. These findings suggest that PKC phosphorylates the alpha-subunit of the Na,K-ATPase within the region restricted by C3 and T1 cleavage sites.

Voltage-dependent inhibition of the sodium pump by external sodium: species differences and possible role of the N-terminus of the alpha-subunit

Currents generated by the Na+/K+ ATPase were measured under voltage clamp in oocytes of Xenopus laevis. The dependence of pump current on external [Na+] was investigated for the endogenous Xenopus pump as well as for wild-type and mutated pumps of electroplax of Torpedo californica expressed in the oocytes. The mutants had alpha-subunits truncated before position Lys28 (alpha delta K28) or Thr29 (alpha delta T29) of the N-terminus. The currents generated by all variants of pump molecules in the presence of 5 mM K+ show voltage-dependent inhibition by external [Na+]. The apparent KI values increase with membrane depolarisation, and the potential dependence can be described by the movement of effective charges in the electrical potential gradient across the membrane. Taking into account Na(+)-K+ competition for external binding to the E2P form, apparent KI values and effective charges for the interaction of the Na+ ions with the E2P form can be estimated. For the Xenopus pump the effective charge amounts to 1.1 of an elementary charge and the KI value at 0 mV to 44 mM. For the wild-type Torpedo pump, the analysis yields values of 0.73 of an elementary charge and 133 mM, respectively. Truncation at the N-terminus removing a lysine-rich cluster of the alpha-subunit of the Torpedo pump leads to an increase of the effective charge and decrease of the KI value. For alpha delta K28, values of 0.83 of an elementary charge and 117 mM are obtained, respectively. If Lys28 is included in the truncation (alpha delta T29), the effective charge increases to 1.5 of an elementary charge and the apparent KI value is reduced to 107 mM. The KI values for pump inhibition by external Na+, calculated by taking into account Na(+)-K+ competition, are smaller than the K1/2 values determined in the presence of 5 mM [K+]. The difference is more pronounced for those pump variants that have higher Km values. The variations of the parameters describing inhibition by external [Na+] are qualitatively similar to those described for the stimulation of the pumps by external [K+] in the absence of extracellular [Na+]. The observations may be explained by an access channel within the membrane dielectric that has to be passed by the external Na+ and K+ ions to reach or leave their binding sites. The potential-dependent access and/or the interaction with the binding sites shows species differences and is affected by cytoplasmic lysine residues in the N-terminus.

Voltage-dependent stimulation of Na+/K(+)-pump current by external cations: selectivity of different K+ congeners

Currents generated by the endogenous Na+/K+ pump in the oocytes of Xenopus laevis were determined under voltage-clamp as currents activated by different K+ congeners. The voltage dependence of the pump current reflects voltage-dependent steps in the reaction cycle. The decrease of K(+)-activated pump current at positive potentials has been attributed to voltage-dependent stimulation by the external K+ (Rakowski, Vasilets, LaTona and Schwarz (1991) J. Membr. Biol. 121, 177-187). In Na(+)-free solution, activation of the pump by external cations seems to be the dominating voltage-dependent and rate-determining step in the reaction cycle. Under these conditions, the voltage dependence of apparent Km values for pump activation can be analyzed. The dependence suggests voltage-dependent binding of extracellular cations assuming that an effective charge of about 0.4 of an elementary charge is moved in the electrical field during a step associated with the cation binding. The apparent Km values at 0 mV differ for various cations that stimulate pump activity. The values are in mM: 0.10 for Tl+, 0.63 for K+, 0.71 for Rb+, 9.3 for NH4+, and 12.9 for Cs+. The corresponding apparent affinities follow the same sequence as the cation permeability of the K(+)-selective delayed rectifier channel of nerve cells. The results are compatible with the interpretation that the cations have to pass an ion-selective access channel to reach their binding sites in the pump molecule.