[26] Isolation of (Na+ + K+)-ATPase

Partial Reactions of the Na,K-ATPase: Determination of Activation Energies and an Approach to Mechanism

Abstract

Kinetic experiments were performed with preparations of kidney Na,K-ATPase in isolated membrane fragments or reconstituted in vesicles to obtain information of the activation energies under turnover conditions and for selected partial reactions of the Post-Albers pump cycle. The ion transport activities were detected with potential or conformation sensitive fluorescent dyes in steady-state or time-resolved experiments. The activation energies were derived from Arrhenius plots of measurements in the temperature range between 5 °C and 37 °C. The results were used to elaborate indications of the respective underlying rate-limiting reaction steps and allowed conclusions to be drawn about possible molecular reaction mechanisms. The observed consequent alteration between ligand-induced reaction and conformational relaxation steps when the Na,K-ATPase performs the pump cycle, together with constraints set by thermodynamic principles, provided restrictions which have to be met when mechanistic models are proposed. A model meeting such requirements is presented for discussion.

Graphic Abstract

Overview on solubilization and lipid reconstitution of Na,K-ATPase: enzyme kinetic and biophysical characterization

Na,K-ATPase is a membrane protein which plays a vital role. It pumps Na⁺ and K⁺ ions across the cellular membranes using energy from ATP hydrolysis, and is responsible for maintaining the osmotic equilibrium and generating the membrane potential. Moreover, Na,K-ATPase has also been involved in cell signaling, interacting with partner proteins. Cardiotonic steroids bind specifically to Na,K-ATPase triggering a number of signaling pathways. Because of its importance, many efforts have been employed to study the structure and function of this protein. Difficulties associated with its removal from natural membranes and the concomitant search for appropriate replacement conditions to keep the protein in solution have presented a challenge that had to be overcome prior to carrying out biophysical and biochemical studies in vitro. In this review, we summarized all of the methods and techniques applied by our group in order to obtain information about Na,K-ATPase in respect to solubilization, reconstitution into mimetic system, influence of lipid composition, stability, oligomerization, and aggregation.

Attenuation of diabetic nephropathy by dietary fenugreek (Trigonella foenum-graecum) seeds and onion (Allium cepa) via suppression of glucose transporters and renin-angiotensin system

OBJECTIVES

The aim of this study was to determine the effects of dietary fenugreek (Trigonella foenum-graecum) seeds and onion on the hyperglycemia-stimulated glucose transporters and activation of renin-angiotensin system-mediated cascade of events leading to renal lesions in diabetic animals.

METHODS

The mechanistic aspects of nephroprotective influence of dietary fenugreek seeds (10%) and onion (3%) on diabetic renal lesions was investigated in streptozotocin diabetic rats. Renal damage was assessed by measuring proteinuria, enzymuria, expression of glucose transporters, renin-angiotensin system, and activities of polyol pathway enzymes.

RESULTS

Diabetes resulted in an upregulation of glucose transporters in kidney tissue, which was countered by these dietary interventions. The upregulation of renal angiotensin-converting enzyme and its receptor was also countered by these dietary interventions. Dietary fenugreek and onion significantly reduced metabolites of polyol pathway, nitric oxide, and N-acetyl-β-d-glucosaminidase activity. Markers of podocyte damage in kidney (nephrin, podocin, and podocalyxin) and their urinary excretion were normalized along with downregulation of the expression of kidney injury molecule-1 by these dietary interventions. Dietary fenugreek and onion effectively countered the diabetes-induced structural abnormalities of renal tissue.

CONCLUSION

Feeding fiber-rich fenugreek seeds and sulfur compounds-rich onion produced a blockade in glucose translocation and renin-angiotensin system in the early stage of diabetic nephropathy. This involved a downregulation of the expression of polyol pathway enzymes, partial restoration of the podocyte damage, revival of renal architecture and functional abnormality. The present study also suggested that these two dietary interventions offer a higher renoprotective influence when consumed together.

Ameliorative Influence of Dietary Fenugreek (Trigonella foenum-graecum) Seeds and Onion (Allium cepa) on Eye Lens Abnormalities via Modulation of Crystallin Proteins and Polyol Pathway in Experimental Diabetes

PURPOSE AND METHODS

Hyperglycemia-induced osmotic and oxidative stress is thought to be involved in the pathogenesis of diabetes-related secondary complications including cataract. In continuation of our previous observation of the ameliorative influence of these spices on hyperglycemia, attendant metabolic abnormalities, and oxidative stress in tissues of diabetic rats, the beneficial influence of dietary (10%) fenugreek seeds, (3%) onion, or their combination was investigated on diabetes-induced alteration in the eye lens of streptozotocin-induced diabetic rats.

RESULTS

Animals maintained on these spices showed significantly countered oxidative stress markers (reactive oxygen species, lipid peroxidation and protein carbonyl), advanced glycation end products, and expression of their receptor in the eye lens. Increased activity of polyol pathway enzymes, their protein, and mRNA expression was significantly countered in the cataractogenic lens as a result of these dietary interventions. Altered crystallin (αA and αB) distribution profile, their expression, activity of carbohydrate metabolizing enzymes, and antioxidant status were significantly annulled by these dietary treatments. Physical and visual observation of the photomicrographs of the lenses of treated rats indicated that these dietary interventions delayed cataractogenesis in diabetic rats.

CONCLUSIONS

Overall, the present investigation evidenced a beneficial modulation of the progression of cataractogenesis by dietary fenugreek seeds and onion, implicating their potential in ameliorating cataract in diabetics.

Haemato-protective influence of dietary fenugreek (Trigonella foenum-graecum L.) seeds is potentiated by onion (Allium cepa L.) in streptozotocin-induced diabetic rats

We have recently reported the beneficial modulation of metabolic abnormalities and oxidative stress in diabetic rats by dietary fenugreek seeds and onion. This investigation evaluated the protective influence of dietary fenugreek seeds (100 g kg^-1) and onion (30 g kg^-1) on erythrocytes of streptozotocin-induced diabetic rats, through modulation of reduced haematological indices and antisickling potency. This study also evaluated the altered erythrocyte membrane lipid profile and beneficial countering of increased lipid peroxidation, osmotic fragility, along with reduced membrane fluidity and deformability, nitric oxide production and echinocyte formation. Dietary fenugreek seeds and onion appeared to counter the deformity and fragility of erythrocytes partially in diabetic rats by their antioxidant potential and hypocholesterolemic property. The antisickling potency of these spices was accomplished by a substantial decrease in echinocyte population and AGEs in diabetic rats. Further insight into the factors that might have reduced the fluidity of erythrocytes in diabetic rats revealed changes in the cholesterol: phospholipid ratio, fatty acid profile, and activities of membrane-bound enzymes. Dietary fenugreek seeds and onion offered a beneficial protective effect to the red blood cells, the effect being higher with fenugreek + onion. This is the first report on the hemato-protective influence of a nutraceutical food component in diabetic situation.

Water Deprivation Increases (Pro)renin Receptor Levels in the Kidney and Decreases Plasma Concentrations of Soluble (Pro)renin Receptor

Water deprivation activates the renin-angiotensin system. We have hypothesized that the renal expression of (pro)renin receptor ((P)RR), a specific receptor for renin and prorenin, could be changed under dehydration. Moreover, plasma levels of soluble (P)RR (s(P)RR) comprising of the extracellular domain of (P)RR may reflect the renal (P)RR expression. In the present study, we therefore aimed to clarify changes of plasma s(P)RR concentrations and kidney tissue (P)RR levels using rats with dehydration. Male Wister-Kyoto rats were divided into two groups; dehydrated (DH) rats deprived of water for 72 hours with free access to food, and control rats. Plasma s(P)RR concentrations measured by enzyme-linked immunosorbent assay were significantly lower in DH rats (6.94 ± 2.08 ng/mL, mean ± SD, n = 5) than in control (12.54 ± 2.00 ng/mL, n = 5) (p < 0.05). Western blot analysis confirmed lower expression levels of s(P)RR in plasma in DH rats than in control. By contrast, western blot analysis showed higher levels of full-length (P)RR and lower levels of furin (an enzyme responsible for generation of s(P)RR from full-length (P)RR) in the kidney tissues obtained from DH rats compared to control. There was no significant difference in the renal (P)RR mRNA levels between DH rats and control. These findings suggest that water deprivation may elevate the renal full-length (P)RR levels via reducing the expression of furin. Increased full-length (P)RR may contribute to the up-regulation of the renal renin-angiotensin system and the production of concentrated urine under dehydration.

Modulation of the Na,K-ATPase by Magnesium Ions

Since the beginning of investigations of the Na,K-ATPase, it has been well-known that Mg²⁺ is an essential cofactor for activation of enzymatic ATP hydrolysis without being transported through the cell membrane. Moreover, experimental evidence has been collected through the years that shows that Mg²⁺ ions have a regulatory effect on ion transport by interacting with the cytoplasmic side of the ion pump. Our experiments allowed us to reveal the underlying mechanism. Mg²⁺ is able to bind to a site outside the membrane domain of the protein's α subunit, close to the entrance of the access channel to the ion-binding sites, thus modifying the local concentration of the ions in the electrolyte, of which Na⁺, K⁺, and H⁺ are of physiological interest. The decrease in the concentration of these cations can be explained by electrostatic interaction and estimated by the Debye-Hückel theory. This effect provokes the observed apparent reduction of the binding affinity of the binding sites of the Na,K-ATPase in the presence of various Mg²⁺ concentrations. The presence of the bound Mg²⁺, however, does not affect the reaction kinetics of the transport function of the ion pump. Therefore, stopped-flow experiments could be performed to gain the first insight into the Na⁺ binding kinetics on the cytoplasmic side by Mg²⁺ concentration jump experiments.

Assaying P-Type ATPases Reconstituted in Liposomes

Reconstitution of P-type ATPases in unilamellar liposomes is a useful technique to study functional properties of these active ion transporters. Experiments with such liposomes provide an easy access to substrate-binding affinities of the ion pumps as well as to the lipid and temperature dependence of the pump current. Here, we describe two reconstitution methods by dialysis and the use of potential-sensitive fluorescence dyes to study transport properties of two P-type ATPases, the Na,K-ATPase from rabbit kidney and the K(+)-transporting KdpFABC complex from E. coli. Several techniques are introduced how the measured fluorescence signals may be analyzed to gain information on properties of the ion pumps.

Probing the extracellular access channel of the Na,K-ATPase

When the Na,K-ATPase pumps at each turnover two K(+) ions into the cytoplasm, this translocation consists of several reaction steps. First, the ions diffuse consecutively from the extracellular phase through an access pathway to the binding sites where they are coordinated. In the next step, the enzyme is dephosphorylated and the ions are occluded inside the membrane domain. The subsequent transition to the E1 conformation produces a deocclusion of the binding sites to the cytoplasmic side of the membrane and allows in the last steps ion dissociation and diffusion to the aqueous phase. The interaction and competition of K(+) with various quaternary organic ammonium ions have been used to gain insight into the molecular mechanism of the ion binding process from the extracellular side in the P-E2 conformation of the enzyme. Using the electrochromic styryl dye RH421, evidence has been obtained that the access pathway consists of a wide and water-filled funnel-like part that is accessible also for bulky cations such as the benzyltriethylammonium ion, and a narrow part that permits passage only of small cations such as K(+) and NH4(+) in a distinct electrogenic way. Benzyltriethylammonium ions inhibit K(+) binding in a competitive manner that can be explained by a stopper-like function at the interface between the wide and narrow parts of the access pathway. In contrast to other quaternary organic ammonium ions, benzyltriethylammonium ions show a specific binding to the ion pump in a position inside the access pathway where it blocks effectively the access to the binding sites.

Surface charges of the membrane crucially affect regulation of Na,K-ATPase by phospholemman (FXYD1)

The human α1/His10-β1 isoform of Na,K-ATPase has been reconstituted as a complex with and without FXYD1 into proteoliposomes of various lipid compositions in order to study the effect of the regulatory subunit on the half-saturating Na⁺ concentration (K(½)) of Na⁺ ions for activation of the ion pump. It has been shown that the fraction of negatively charged lipid in the bilayer crucially affects the regulatory properties. At low concentrations of the negatively charged lipid DOPS (<10 %), FXYD1 increases K(½) of Na⁺ ions for activation of the ion pump. Phosphorylation of FXYD1 by protein kinase A at Ser68 abrogates this effect. Conversely, for proteoliposomes made with high concentrations of DOPS (>10 %), little or no effect of FXYD1 on the K(½) of Na⁺ ions is observed. Depending on ionic strength and lipid composition of the proteoliposomes, FXYD1 can alter the K(½) of Na⁺ ions by up to twofold. We propose possible molecular mechanisms to explain the regulatory effects of FXYD1 and the influence of charged lipid and protein phosphorylation. In particular, the positively charged C-terminal helix of FXYD1 appears to be highly mobile and may interact with the cytoplasmic N domain of the α-subunit, the interaction being strongly affected by phosphorylation at Ser68 and the surface charge of the membrane.

A method to measure hydrolytic activity of adenosinetriphosphatases (ATPases)

The detection of small amounts (nanomoles) of inorganic phosphate has a great interest in biochemistry. In particular, phosphate detection is useful to evaluate the rate of hydrolysis of phosphatases, that are enzymes able to remove phosphate from their substrate by hydrolytic cleavage. The hydrolysis rate is correlated to enzyme activity, an extremely important functional parameter. Among phosphatases there are the cation transporting adenosinetriphosphatases (ATPases), that produce inorganic phosphate by cleavage of the γ-phosphate of ATP. These membrane transporters have many fundamental physiological roles and are emerging as potential drug targets. ATPase hydrolytic activity is measured to test enzyme functionality, but it also provides useful information on possible inhibitory effects of molecules that interfere with the hydrolytic process. We have optimized a molybdenum-based protocol that makes use of potassium antimony (III) oxide tartrate (originally employed for phosphate detection in environmental analysis) to allow its use with phosphatase enzymes. In particular, the method was successfully applied to native and recombinant ATPases to demonstrate its reliability, validity, sensitivity and versatility. Our method introduces significant improvements to well-established experimental assays, which are currently employed for ATPase activity measurements. Therefore, it may be valuable in biochemical and biomedical investigations of ATPase enzymes, in combination with more specific tests, as well as in high throughput drug screening.

Kinetic comparisons of heart and kidney Na+,K(+)-ATPases

Most kinetic measurements of the partial reactions of Na(+),K(+)-ATPase have been conducted on enzyme from mammalian kidney. Here we present a kinetic model that is based on the available equilibrium and kinetic parameters of purified kidney enzyme, and allows predictions of its steady-state turnover and pump current in intact cells as a function of ion and ATP concentrations and the membrane voltage. Using this model, we calculated the expected dependence of the pump current on voltage and extracellular Na(+) concentration. The simulations indicate a lower voltage dependence at negative potentials of the kidney enzyme in comparison with heart muscle Na(+),K(+)-ATPase, in agreement with experimental results. The voltage dependence is enhanced at high extracellular Na(+) concentrations. This effect can be explained by a voltage-dependent depopulation of extracellular K(+) ion binding sites on the E2P state and an increase in the proportion of enzyme in the E1P(Na(+))(3) state in the steady state. This causes a decrease in the effective rate constant for occlusion of K(+) by the E2P state and hence a drop in turnover. Around a membrane potential of zero, negligible voltage dependence is observed because the voltage-independent E2(K(+))(2) → E1 + 2K(+) transition is the major rate-determining step.

Loss of caveolin-1 impairs retinal function due to disturbance of subretinal microenvironment

Caveolin-1 (Cav-1), an integral component of caveolar membrane domains, is expressed in several retinal cell types, including photoreceptors, retinal vascular endothelial cells, Müller glia, and retinal pigment epithelium (RPE) cells. Recent evidence links Cav-1 to ocular diseases, including autoimmune uveitis, diabetic retinopathy, and primary open angle glaucoma, but its role in normal vision is largely undetermined. In this report, we show that ablation of Cav-1 results in reduced inner and outer retinal function as measured, in vivo, by electroretinography and manganese-enhanced MRI. Somewhat surprisingly, dark current and light sensitivity were normal in individual rods (recorded with suction electrode methods) from Cav-1 knock-out (KO) mice. Although photoreceptor function was largely normal, in vitro, the apparent K(+) affinity of the RPE-expressed α1-Na(+)/K(+)-ATPase was decreased in Cav-1 KO mice. Cav-1 KO retinas also displayed unusually tight adhesion with the RPE, which could be resolved by brief treatment with hyperosmotic medium, suggesting alterations in outer retinal fluid homeostasis. Collectively, these findings demonstrate that reduced retinal function resulting from Cav-1 ablation is not photoreceptor-intrinsic but rather involves impaired subretinal and/or RPE ion/fluid homeostasis.

Post-transcriptional control of Na,K-ATPase activity and cell growth by a splice variant of FXYD2 protein with modified mRNA

In kidney, FXYD proteins regulate Na,K-ATPase in a nephron segment-specific way. FXYD2 is the most abundant renal FXYD but is not expressed in most renal cell lines unless induced by hypertonicity. Expression by transfection of FXYD2a or FXYD2b splice variants in NRK-52E cells reduces the apparent Na(+) affinity of the Na,K-ATPase and slows the cell proliferation rate. Based on RT-PCR, mRNAs for both splice variants were expressed in wild type NRK-52E cells as low abundance species. DNA sequencing of the PCR products revealed a base alteration from C to T in FXYD2b but not FXYD2a from both untreated and hypertonicity-treated NRK-52E cells. The 172C→T sequence change exposed a cryptic KKXX endoplasmic reticulum retrieval signal via a premature stop codon. The truncation affected trafficking of FXYD2b and its association with Na,K-ATPase and blocked its effect on enzyme kinetics and cell growth. The data may be explained by altered splicing or selective RNA editing of FXYD2b, a supplementary process that would ensure that it was inactive even if transcribed and translated, in these cells that normally express only FXYD2a. 172C→T mutation was also identified after mutagenesis of FXYD2b by error-prone PCR coupled with a selection for cell proliferation. Furthermore, the error-prone PCR alone introduced the mutation with high frequency, implying a structural peculiarity. The data confirm truncation of FXYD2b as a potential mechanism to regulate the amount of FXYD2 at the cell surface to control activity of Na,K-ATPase and cell growth.

Seasonal changes in microsomal fraction enriched with Na,K-ATPase from kidneys of the ground squirrel Spermophilus undulatus

The Na,K-ATPase activity in microsomal fraction isolated from kidneys of winter hibernating ground squirrels was found to be 1.8-2.0-fold lower than that in active animals in summer. This is partially connected with a decrease in Na,K-ATPase protein content in these preparations (by 25%). Using antibodies to different isoforms of Na,K-ATPase α-subunit and analysis of enzyme inhibition by ouabain, it was found that the decrease in Na,K-ATPase activity during hibernation is not connected with change in isoenzyme composition. Seasonal changes of Na,K-ATPase α-subunit phosphorylation level by endogenous protein kinases were not found. Proteins which could be potential regulators of Na,K-ATPase activity were not found among phosphorylated proteins of the microsomes. Analysis of the composition and properties of the lipid phase of microsomes showed that the total level of unsaturation of fatty acids and the lipid/protein ratio are not changed significantly during hibernation, whereas the cholesterol content in preparations from kidneys of hibernating ground squirrels is approximately twice higher than that in preparations from kidneys of active animals. However, using spin and fluorescent probes it was shown that this difference in cholesterol content does not affect the integral membrane microviscosity of microsomes. Using the cross-linking agent cupric phenanthroline, it was shown that Na,K-ATPase in membranes of microsomes from kidneys of hibernating ground squirrels is present in more aggregated state in comparison with membranes of microsomes from kidneys of active animals. We suggest that the decrease in Na,K-ATPase activity in kidneys of ground squirrels during hibernation is mainly connected with the aggregation of proteins in plasma membrane.

Confining the sodium pump in a phosphoenzyme form: the effect of lead(II) ions

The effect of Pb(2+) ions on the Na(+),K(+)-ATPase was investigated in detail by means of steady-state fluorescence spectroscopy. Experiments were performed by using the electrochromic styryl dye RH421. It is shown that Pb(2+) ions can bind reversibly to the protein and do not affect the Na(+) and K(+) binding affinities in the E(1) and P-E(2) conformations of the enzyme. The pH titrations indicate that lead(II) favors binding of one H(+) to the P-E(2) conformation in the absence of K(+). A model scheme is proposed that accounts for the experimental results obtained for backdoor phosphorylation of the enzyme in the presence of Pb(2+) ions. Taken together, our results clearly indicate that Pb(2+) bound to the enzyme stabilizes an E(2)-type conformation. In particular, under conditions that promote enzyme phosphorylation, Pb(2+) ions are able to confine the Na(+),K(+)-ATPase into a phosphorylated E(2) state.

Proton diet for the sodium pump

In the absence of Na(+) and K(+) ions the Na,K-ATPase shows a pH-dependent ATP hydrolysis that can be inhibited by ouabain. At pH 7.2 this activity is 5% of the maximal under physiological conditions. It could be inferred that this activity is associated with H(+) transport in both directions across the membrane and facilitates an H-only mode of the sodium pump under such unphysiological conditions. By the analysis of experiments with reconstituted proteoliposomes an overall electroneutral transport mode has been proven. The stoichiometry was determined to be 2 H(+)/2 H(+)/1 ATP and is comparable to what is known from the closely related H,K-ATPase. By time-resolved ATP-concentration jump experiments it was found that at no time was the third, Na(+)-specific binding site of the pump occupied by protons. A modified Post-Albers pump cycle is proposed, with H(+) ions as congeners for Na(+) and K(+), by which all experiments performed can be explained.

Interaction of ATP with the phosphoenzyme of the Na+,K+-ATPase

The interaction of ATP with the phosphoenzyme of Na(+),K(+)-ATPase from pig kidney, rabbit kidney, and shark rectal gland was investigated using the voltage-sensitive fluorescent probe RH421. In each case, ATP concentrations >or=100 microM caused a drop in fluorescence intensity, which, because RH421 is sensitive to the formation of enzyme in the E2P state, can be attributed to ATP binding to the E2P phosphoenzyme. Simulations of the experimental behavior using kinetic models based on either a monomeric or a dimeric enzyme mechanism yielded a K(d) for ATP binding in the range 140-500 muM. Steady-state activity measurements and independent measurements of the phosphoenzyme level via a radioactive assay indicated that ATP binding to E2P causes a deceleration in its dephosphorylation when acting in the Na(+)-ATPase mode, i.e., in the absence of K(+) ions. Both the ATP-induced drop in RH421 fluorescence and the effect on the dephosphorylation reaction could be attributed to an inhibition of dissociation from the E2P(Na(+))(3) state of the one Na(+) ion necessary to allow dephosphorylation. Stopped-flow studies on the shark enzyme indicated that the ATP-induced inhibition of dephosphorylation is abolished in the presence of 1 mM KCl. A possible physiological role of allosteric binding of ATP to the phosphoenzyme could be to stabilize the E2P state and stop the enzyme running backward, which would cause dissipation of the Na(+) electrochemical potential gradient and the resynthesis of ATP from ADP. ATP binding to E2P could also fix ATP within the enzyme ready to phosphorylate it in the subsequent turnover.

Ion channels versus ion pumps: the principal difference, in principle

The incessant traffic of ions across cell membranes is controlled by two kinds of border guards: ion channels and ion pumps. Open channels let selected ions diffuse rapidly down electrical and concentration gradients, whereas ion pumps labour tirelessly to maintain the gradients by consuming energy to slowly move ions thermodynamically uphill. Because of the diametrically opposed tasks and the divergent speeds of channels and pumps, they have traditionally been viewed as completely different entities, as alike as chalk and cheese. But new structural and mechanistic information about both of these classes of molecular machines challenges this comfortable separation and forces its re-evaluation.

Two gears of pumping by the sodium pump

The kinetics of the phosphorylation and subsequent conformational change of Na(+),K(+)-ATPase was investigated via the stopped-flow technique using the fluorescent label RH421 (pH 7.4, 24 degrees C). The enzyme was preequilibrated in buffer containing 130 mM NaCl to stabilize the E1(Na(+))(3) state. On mixing with ATP in the presence of Mg(2+), a fluorescence increase occurred, due to enzyme conversion into the E2P state. The fluorescence change accelerated with increasing ATP concentration until a saturating limit in the hundreds of micromolar range. The amplitude of the fluorescence change (DeltaF/F(0)) increased to 0.98 at 50 microM ATP. DeltaF/F(0) then decreased to 0.82 at 500 microM. The decrease was attributed to an ATP-induced allosteric acceleration of the dephosphorylation reaction. The ATP concentration dependence of the time course and the amplitude of the fluorescence change could not be explained by either a one-site monomeric enzyme model or by a two-pool model. All of the data could be explained by an (alphabeta)(2) dimeric model, in which the enzyme cycles at a low rate with ATP hydrolysis by one alpha-subunit or at a high rate with ATP hydrolysis by both alpha-subunits. Thus, we propose a two-gear bicyclic model to replace the classical monomeric Albers-Post model for kidney Na(+),K(+)-ATPase.

Palytoxin-induced effects on partial reactions of the Na,K-ATPase

The interaction of palytoxin with the Na,K-ATPase was studied by the electrochromic styryl dye RH421, which monitors the amount of ions in the membrane domain of the pump. The toxin affected the pump function in the state P-E2, independently of the type of phosphorylation (ATP or inorganic phosphate). The palytoxin-induced modification of the protein consisted of two steps: toxin binding and a subsequent conformational change into a transmembrane ion channel. At 20 degrees C, the rate-limiting reaction had a forward rate constant of 10(5) M(-1)s(-1) and a backward rate constant of about 10(-3) s(-1). In the palytoxin-modified state, the binding affinity for Na+ and H+ was increased and reached values between those obtained in the E1 and P-E2 conformation under physiological conditions. Even under saturating palytoxin concentrations, the ATPase activity was not completely inhibited. In the Na/K mode, approximately 50% of the enzyme remained active in the average, and in the Na-only mode 25%. The experimental findings indicate that an additional exit from the inhibited state exists. An obvious reaction pathway is a slow dephosphorylation of the palytoxin-inhibited state with a time constant of approximately 100 s. Analysis of the effect of blockers of the extracellular and cytoplasmic access channels, TPA+ and Br2-Titu3+, respectively, showed that both access channels are part of the ion pathway in the palytoxin-modified protein. All experiments can be explained by an extension of the Post-Albers cycle, in which three additional states were added that branch off in the P-E2 state and lead to states in which the open-channel conformation is introduced and returns into the pump cycle in the occluded E2 state. The previously suggested molecular model for the channel state of the Na,K-ATPase as a conformation in which both gates between binding sites and aqueous phases are simultaneously in their open state is supported by this study.

New evidence for ATP binding induced catalytic subunit interactions in pig kidney Na/K-ATPase

Pig kidney Na/K-ATPase preparations showed a positive cooperative effect for pNPP in Na-pNPPase activity. Measurements of the Na-pNPPase activity, Na-ATPase activity and the accumulation of phosphoenzyme (EP) under conditions of pNPP saturation showed several different ATP affinities. The presence of pNPP reduced both the maximum amount of EP and Na-ATPase activity to half showing a value of 4 and a 3,700-fold reduced ATP affinity for EP formation, and a 7 and 1,300-fold reduced affinity for Na-ATPase activity. The presence of low concentrations of ATP in the phosphorylation induced a 2-fold enhancement in Na-pNPPase activity despite a reduction in available pNPP sites. However, higher concentrations of ATP inhibited the Na-pNPPase activity and a much higher concentration of ATP increased both the phosphorylation and Na-ATPase activity to the maximum levels. The maximum Na-pNPPase activity was 1.7 and 3.4-fold higher without and with ATP, respectively, than the maximum Na-ATPase activity. These data and the pNPP dependent reduction in both Na-ATPase activity and the amount of enzyme bound ATP provide new evidence to show that ATP, pNPP and ATP with pNPP, respectively, induce different subunit interactions resulting a difference in the maximum Na(+)-dependent catalytic activity in tetraprotomeric Na/K-ATPase.

Photochemical behavior and Na+,K+-ATPase sensitivity of voltage-sensitive styrylpyridinium fluorescent membrane probes

RH421 is a widely used voltage-sensitive fluorescent membrane probe. Its exposure to continuous illumination with 577 nm light from an Hg lamp leads, however, to an increase in its steady-state fluorescence level when bound to lipid membranes. The increase occurs on the second time scale at typical light intensities and was found to be due to a single-photon excited-state isomerization. Modifications to the dye structure are, therefore, necessary to increase photochemical stability and allow wider application of such dyes in kinetic studies of ion-transporting membrane proteins. The related probe ANNINE 5, which has a rigid polycyclic structure, shows no observable photochemical reaction when bound to DMPC vesicles on irradiation with 436 nm light. The voltage sensitivity of ANNINE 5 was tested with the use of Na+,K+-ATPase membrane fragments. As long as ANNINE 5 is excited on the far red edge of its visible absorption band, it shows a similar sensitivity to RH421 in detecting charge-translocating reactions triggered by ATP phosphorylation. Unfortunately the wavelengths necessary for ANNINE 5 excitation are in a region where the Hg lamps routinely used in stopped-flow apparatus have no significant lines available for excitation.

Altered expression of rat renal cortical OAT1 and OAT3 in response to bilateral ureteral obstruction

BACKGROUND

Bilateral ureteral obstruction (BUO) is characterized by the development of hemodynamic and tubular lesions. However, little is known about the expression of organic anion renal transporters. The objective of this work was to study the renal excretion of p-aminohippurate (PAH) and the cortical renal expression of the organic anion transporter 1 (OAT1) and organic anion transporter 3 (OAT3) in BUO rats.

METHODS

Male Wistar rats underwent bilateral obstruction of the proximal ureters for 24 hours (BUO) or sham operation. After 24 hours of ureteral releasing, the following studies were performed: PAH renal excretion employing conventional clearance techniques and OAT1 and OAT3 abundance (homogenates, intracellular and basolateral plasma membrane fractions from renal cortex) using immunoblotting and immunocytochemical techniques (light microscopic and confocal immunofluorescence microscopic analysis).

RESULTS

BUO rats showed a lower renal excretion of PAH. In obstructed kidneys, immunoblotting revealed a significant decrease in the abundance of both OAT1 and OAT3 in basolateral plasma membranes from renal cortex. An increase of OAT1 expression was observed in homogenates and in intracellular membrane fractions in kidneys from BUO rats compared with sham-operated ones, indicating an internalization of this carrier. Immunocytochemical techniques confirmed these results. On the contrary, OAT3 expression was reduced both in homogenates and in intracellular membrane fractions in obstructed kidneys.

CONCLUSION

BUO was associated with down-regulation of OAT1 and OAT3 in basolateral plasma membranes from proximal tubule cells, thus these carriers may play important roles in the impaired organic anion excretion displayed in the obstructed kidney.

Effect of chaotropic anions on the sodium transport by the Na,K-ATPase

The effect of choline iodide, bromide and chloride on the kinetics of the electrogenic sodium transport by the Na,K-ATPase was investigated in a model system of ATPase-containing membrane fragments adsorbed on the lipid bilayer membrane. The kinetic parameters of Na(+) transport were determined from short circuit currents after fast release of ATP from its caged precursor. The falling phase of the current transients could be fitted by a single exponential with the time constant, tau (2). Its temperature dependence allowed an estimation of the activation energy of the rate-limiting reaction step, the conformation transition E(1)/E(2). Choline iodide and bromide caused a decrease of the activation energy as well as the overall rate of the process expressed as the pre-exponential factor A of the Arrhenius equation. If choline iodide or bromide were present on the cytoplasmic and extracellular sides of the protein, the temperature dependent changes were more pronounced than when present on the cytoplasmic side only. These results can be explained by an effect of the anions on water structure on the extracellular surface of the protein, where a deep access channel connects the ion-binding sites with the solution. Chloride ions also caused a deceleration of the electrogenic transport, however, in contrast to iodide or bromide, they did not affect the activation energy, and were more effective when added on the cytoplasmic side. This effect can be explained by asymmetric screening of the negative surface charges which leads to a transmembrane electric potential that modifies the ion transfer.

Mechanism of the Na,K-ATPase Inhibition by MCS Derivatives

The previously reported class of potent inorganic inhibitors of Na,K-ATPase, named MCS factors, was shown to inhibit not only Na,K-ATPase but several P-type ATPases with high potency in the sub-micromolar range. These MCS factors were found to bind to the intracellular side of the Na, K-ATPase. The inhibition is not competitive with ouabain binding, thus excluding its role as cardiac-steroid-like inhibitor of the Na,K-ATPase. The mechanism of inhibition of Na,K-ATPase was investigated with the fluorescent styryl dye RH421, a dye known to report changes of local electric fields in the membrane dielectric. MCS factors interact with the Na,K-ATPase in the E(1) conformation of the ion pump and induce a conformational rearrangement that causes a change of the equilibrium dissociation constant for one of the first two intracellular cation binding sites. The MCS-inhibited state was found to have bound one cation (H(+), Na(+) or K(+)) in one of the two unspecific binding sites, and at high Na(+) concentrations another Na(+) ion was bound to the highly Na(+)-selective ion-binding site.

Photodynamic inactivation of the Na,K-ATPase occurs via different pathways

The photodynamic, i.e., the light-induced, inactivation of the Na,K-ATPase in the presence of the sensitizer rose bengal was studied under different conditions. The shape of inactivation curves of the enzyme activity was analyzed as well as partial reactions of the pump cycle. Both experimental approaches showed the existence of two different time constants of inactivation of the ion pump, which reflect two pathways of a photodynamic modification. This is supported by the following observations: (1) The amplitude of the initial fast decay of enzyme activity was enhanced in the presence of D2O and reduced in the presence of the singlet oxygen scavenger imidazole. (Similar results were found for the SR Ca-ATPase.) (2) Contrary to the fast enzyme inactivation the slow process shows an inverse dose-rate behavior. (3) Inactivation of the partial reactions of Na+ -binding and of K+-binding to the membrane domain of the Na,K-ATPase showed only a single time constant, which corresponded to the slower time constant of enzyme inactivation. In the presence of high concentrations of singlet oxygen the fast time constant dominated the inactivation of the ATP-induced partial reaction for which the cytoplasmic domains of the enzyme play an important role. The data support the conclusion that fast inactivation is due to modification of the cytoplasmic domains and slow inactivation due to modifications of the membrane domain of the ion pumps.

Structure-function relationship in P-type ATPases--a biophysical approach

P-type ATPases are a large family of membrane proteins that perform active ion transport across biological membranes. In these proteins the energy-providing ATP hydrolysis is coupled to ion-transport that builds up or maintains the electrochemical potential gradients of one or two ion species across the membrane. P-type ATPases are found in virtually all eukaryotic cells and also in bacteria, and they are transporters of a broad variety of ions. So far, a crystal structure with atomic resolution is available only for one species, the SR Ca-ATPase. However, biochemical and biophysical studies provide an abundance of details on the function of this class of ion pumps. The aim of this review is to summarize the results of preferentially biophysical investigations of the three best-studied ion pumps, the Na,K-ATPase, the gastric H,K-ATPase, and the SR Ca-ATPase, and to compare functional properties to recent structural insights with the aim of contributing to the understanding of their structure-function relationship.

Morphology, fine structure, biochemistry, and function of the spermatic ducts in marine fish

The spermatic ducts and the testicular efferent ducts were investigated in different marine teleost fish species (Diplodus sargus, Mullus barbatus, Thalassoma pavo, Trachinus draco, Uranuscopus scaber, Sparisoma cretense, Synodon saurus). From the morphological, histological, fine structural and biochemical investigations it appeared that the testicular main ducts and spermatic ducts of the investigated marine fish have the following functions: storage of spermatozoa, monosacharide synthesis for nutrition of spermatozoa, synthesis of steroid glucuronides, synthesis of seminal plasma proteins, formation of a ionic gradient in the seminal fluid and phagocytotic activity. Species-specific differences were only found in the morphology of the gonads and in the histology of the spermatic duct epithelium.

Specific expression of an A-kinase anchoring protein subtype, AKAP-150, and specific regulatory mechanism for Na(+),K(+)-ATPase via protein kinase A in the parotid gland among the three major salivary glands of the rat

We have examined the expression of A-kinase anchoring protein (AKAP) in the three major salivary glands, i.e. the parotid gland (PG), submandibular gland (SMG), and sublingual gland (SLG), of the rat to elucidate the functional relevance between saliva secretion and Na(+),K(+)-ATPase regulation by protein kinase A (PKA)-dependent phosphorylation, since an AKAP subtype, AKAP-150, is known to be involved in the regulation of the ATPase in PG. Although AKAP-150 and its mRNA were clearly detected in the PG, they were hardly detectable in either the SMG or SLG. The membrane-bound form of the RII regulatory subunit of PKA, an index for the total amount of AKAP subtypes and therefore of the anchored PKA holoenzyme, was also undetectable in membranes from the SMG and SLG but was found in the PG; though a substantial and comparable amount of Na(+),K(+)-ATPase was present in all of these membrane preparations. Incubation with [gamma-32P]ATP revealed that Na(+),K(+)-ATPase in the PG membranes was quickly phosphorylated upon the addition of cAMP, whereas the ATPases in the membranes from SMG and SLG were not; though they were readily and equally phosphorylated by the exogenously added PKA catalytic subunit. AKAP-150 in the basolateral membranes of PG acinar cells was co-immunoprecipitated with RII by an anti-RII antiserum; and AKAP-150 and Na(+),K(+)-ATPase were immunohistochemically co-localized predominantly on the basolateral membranes, suggesting a possibility that the ATPase might directly interact with the AKAP to form an ATPase/AKAP/PKA complex or associate with the AKAP, such association being mediated via some scaffolding molecule. Expression of AKAP-150 and quick down-regulation of Na(+),K(+)-ATPase by AKAP-anchored PKA in response to cAMP elevation are characteristics specific to PG among the three major salivary glands, suggesting the presence of PG-specific regulatory mechanisms for saliva production/secretion.

Characterization of the macrocyclic carbon suboxide factors as potent Na,K-ATPase and SR Ca-ATPase inhibitors

Recently discovered macrocyclic carbon suboxide (MCS) factors with the general formula (C(3)O(2))(n) were found to strongly inhibit rabbit and rat Na,K-ATPase as well as SR Ca-ATPase. Highly active MCS factors were obtained by a base/acid treatment of their lipophilic precursor isolated from plants. In the ESI-MS spectra, the dominant molar mass ion of 431 Da corresponds to a 1:1 complex of the carbon suboxide hexamer (n=6; M(r)=408 Da) with a Na(+) ion. Additional mass ions identified in positive and negative ion mode were assigned as complexes of the MCS hexamer (n=6) and octamer (n=8) with Na(+) or with TFA(-) in various ratios. The dominant mass ion values of these active MCS factors from plants are also found in mass spectra of previously described endogenous digitalis-like factors (EDLF) from animals. This would suggest that ubiquitously distributed MCS factors may function as putative endogenous regulatory substances of Na,K-ATPase and possibly of other ATPases. With the symmetric display of several equivalent carbonyl or hydroxy groups, the structure of MCS factors is particularly suited for interactions with proteins and other bio-molecules. This could explain the high biological activity and the unusual properties of the MCS factors.

Do H+ ions obscure electrogenic Na+ and K+ binding in the E1 state of the Na,K-ATPase?

In contrast to other P-type ATPases, the Na,K-ATPase binding and release of ions on the cytoplasmic side, to the state called E1, is not electrogenic with the exception of the third Na+. Since the high-resolution structure of the closely related SR Ca-ATPase in state E1 revealed the ion-binding sites deep inside the transmembrane part of the protein, the missing electrogenicity in state E1 can be explained by an obscuring counter-movement of H+ ions. Evidence for such a mechanism is presented by analysis of pH effects on Na+ and K+ binding and by electrogenic H+ movements in the E1 conformation of the Na,K-ATPase.

Conventional estrogen receptors are found in the plasma membrane of vaginal epithelial cells of the rat

Estrogens induce rapid (non-genomic) and delayed (genomic) effects on the target cells. The early effects include induction of signal transduction pathway within seconds, whereas the delayed responses require hours and involve transcription and translation. The rapid effects of estradiol (E) on the vaginal epithelial cells (VEC) involved calcium uptake within seconds via the induction of phosphoinositol lipid metabolism as reported in our earlier studies. In this study, we demonstrate the presence of classical estrogen receptors (ER) on the plasma membrane of VEC of the rats. Immunoreactive bands of 67, 56 and 35 kDa are detectable in the membrane fractions (mf) using antibodies recognizing different epitopes of ER alpha. We have also been able to purify a protein having a mass of 67 kDa from the detergent-soluble fraction of the plasma membrane of VEC, which shows properties identical to the classical receptor purified from the cytosolic fraction of the cells. The membrane receptors get dissociated upon binding to the ligand. Besides a role in signal transduction events induced by estradiol, the membrane estrogen receptors may have an important role to play in translocation of the steroid to the cytosolic compartment.

Intracellular sodium modulates the state of protein kinase C phosphorylation of rat proximal tubule Na+,K+-ATPase

The natriuretic hormone dopamine and the antinatriuretic hormone noradrenaline, acting on alpha-adrenergic receptors, have been shown to bidirectionally modulate the activity of renal tubular Na+,K+-adenosine triphosphate (ATPase). Here we have examined whether intracellular sodium concentration influences the effects of these bidirectional forces on the state of phosphorylation of Na+,K+-ATPase. Proximal tubules dissected from rat kidney were incubated with dopamine or the alpha-adrenergic agonist, oxymetazoline, and transiently permeabilized in a medium where sodium concentration ranged between 5 and 70 mM. The variations of sodium concentration in the medium had a proportional effect on intracellular sodium. Dopamine and protein kinase C (PKC) phosphorylate the catalytic subunit of rat Na+,K+-ATPase on the Ser23 residue. The level of PKC induced Na+,K+-ATPase phosphorylation was determined using an antibody that only recognizes Na+,K+-ATPase, which is not phosphorylated on its PKC site. Under basal conditions Na+,K+-ATPase was predominantly in its phosphorylated state. When intracellular sodium was increased, Na+,K+-ATPase was predominantly in its dephosphorylated state. Phosphorylation of Na+,K+-ATPase by dopamine was most pronounced when intracellular sodium was high, and dephosphorylation by oxymetazoline was most pronounced when intracellular sodium was low. The oxymetazoline effect was mimicked by the calcium ionophore A23187. An inhibitor of the calcium-dependent protein phosphatase, calcineurin, increased the state of Na+,K+-ATPase phosphorylation. The results imply that phosphorylation of renal Na+,K+-ATPase activity is modulated by the level of intracellular sodium and that this effect involves PKC and calcium signalling pathways. The findings may have implication for the regulation of salt excretion and sodium homeostasis.

Differential regulation of renal Na,K-ATPase by splice variants of the gamma subunit

Sodium and potassium-exchanging adenosine triphosphatase (Na,K-ATPase) in the kidney is associated with the gamma subunit (gamma, FXYD2), a single-span membrane protein that modulates ATPase properties. Rat and human gamma occur in two splice variants, gamma(a) and gamma(b), with different N termini. Here we investigated their structural heterogeneity and functional effects on Na,K-ATPase properties. Both forms were post-translationally modified during in vitro translation with microsomes, indicating that there are four possible forms of gamma. Site-directed mutagenesis revealed Thr(2) and Ser(5) as potential sites for post-translational modification. Similar modification can occur in cells, with consequences for Na,K-ATPase properties. We showed previously that stable transfection of gamma(a) into NRK-52E cells resulted in reduction of apparent affinities for Na(+) and K(+). Individual clones differed in gamma post-translational modification, however, and the effect on Na(+) affinity was absent in clones with full modification. Here, transfection of gamma(b) also resulted in clones with or without post-translational modification. Both groups showed a reduction in Na(+) affinity, but modification was required for the effect on K(+) affinity. There were minor increases in ATP affinity. The physiological importance of the reduction in Na(+) affinity was shown by the slower growth of gamma(a), gamma(b), and gamma(b') transfectants in culture. The differential influence of the four structural variants of gamma on affinities of the Na,K-ATPase for Na(+) and K(+), together with our previous finding of different distributions of gamma(a) and gamma(b) along the rat nephron, suggests a highly specific mode of regulation of sodium pump properties in kidney.

Distribution and oligomeric association of splice forms of Na(+)-K(+)-ATPase regulatory gamma-subunit in rat kidney

Renal Na(+)-K(+)-ATPase is associated with the gamma-subunit (FXYD2), a single-span membrane protein that modifies ATPase properties. There are two splice variants with different amino termini, gamma(a) and gamma(b). Both were found in the inner stripe of the outer medulla in the thick ascending limb. Coimmunoprecipitation with each other and the alpha-subunit indicated that they were associated in macromolecular complexes. Association was controlled by ligands that affect Na(+)-K(+)-ATPase conformation. In the cortex, the proportion of the gamma(b)-subunit was markedly lower, and the gamma(a)-subunit predominated in isolated proximal tubule cells. By immunofluorescence, the gamma(b)-subunit was detected in the superficial cortex only in the distal convoluted tubule and connecting tubule, which are rich in Na(+)-K(+)-ATPase but comprise a minor fraction of cortex mass. In the outer stripe of the outer medulla and for a short distance in the deep cortex, the thick ascending limb predominantly expressed the gamma(b)-subunit. Because different mechanisms maintain and regulate Na(+) homeostasis in different nephron segments, the splice forms of the gamma-subunit may have evolved to control the renal Na(+) pump through pump properties, gene expression, or both.

Molecular activity of Na+/K+-ATPase from different sources is related to the packing of membrane lipids

The activity of the ubiquitous Na+/K+-ATPase represents a substantial portion of the resting metabolic activity of cells, and the molecular activity of this enzyme from tissues of different vertebrates can vary several-fold. Microsomes were prepared from the kidney and brain of the rat (Rattus norvegicus) and the cane toad (Bufo marinus), and Na+/K+-ATPase molecular activity was determined. The membrane lipids surrounding this enzyme were isolated and phospholipids prepared. ‘Surface pressure/area’ isotherms were measured in monolayers for both membrane lipids and phospholipids using classic Langmuir trough techniques. Microsomal lipid composition was also measured. Whilst significant correlations were observed between membrane composition and Na+/K+-ATPase molecular activity, the strongest correlations were found between the molecular activity and parameters describing the packing of the surrounding membrane lipids and phospholipids. The influence of membrane lipid composition, especially membrane acyl composition, on the activity of a membrane protein mediated by physical properties of the lipids may represent a fundamental principle applicable to other membrane proteins.

Rate limitation of the Na(+),K(+)-ATPase pump cycle

The kinetics of Na(+)-dependent phosphorylation of the Na(+),K(+)-ATPase by ATP were investigated via the stopped-flow technique using the fluorescent label RH421 (saturating [ATP], [Na(+)], and [Mg(2+)], pH 7.4, and 24 degrees C). The well-established effect of buffer composition on the E(2)-E(1) equilibrium was used as a tool to investigate the effect of the initial enzyme conformation on the rate of phosphorylation of the enzyme. Preincubation of pig kidney enzyme in 25 mM histidine and 0.1 mM EDTA solution (conditions favoring E(2)) yielded a 1/tau value of 59 s(-1). Addition of MgCl(2) (5 mM), NaCl (2 mM), or ATP (2 mM) to the preincubation solution resulted in increases in 1/tau to values of 129, 167, and 143 s(-1), respectively. The increases can be attributed to a shift in the enzyme conformational equilibrium before phosphorylation from the E(2) state to an E(1) or E(1)-like state. The results thus demonstrate conclusively that the E(2) --> E(1) transition does in fact limit the rate of subsequent reactions of the pump cycle. Based on the experimental results, the rate constant of the E(2) --> E(1) transition under physiological conditions could be estimated to be approximately 65 s(-1) for pig kidney enzyme and 90 s(-1) for enzyme from rabbit kidney. Taking into account the rates of other partial reactions, computer simulations show these values to be consistent with the turnover number of the enzyme cycle (approximately 48 s(-1) and approximately 43 s(-1) for pig and rabbit, respectively) calculated from steady-state measurements. For enzyme of the alpha(1) isoform the E(2) --> E(1) conformational change is thus shown to be the major rate-determining step of the entire enzyme cycle.

Phosphorylation of the Catalytic Subunit of Rat Renal Na+,K+-ATPase by Classical PKC Isoforms

In this study we have evaluated the specificity of different PKC isozymes for the phosphorylation of the catalytic alpha1 subunit of rat renal Na+,K+-ATPase (alpha1 Na+,K+-ATPase). Using in vitro phosphotransferase assays we found that classical PKCs (cPKCs) alpha, betaI, and gamma efficiently phosphorylate alpha1 Na+,K+-ATPase. However, alpha1 Na+,K+-ATPase was a poor substrate for the novel PKCs (nPKCs) delta and epsilon. Two-dimensional phosphopeptide mapping revealed a similar pattern of phosphorylation by all cPKCs. The functional significance of this finding was evaluated by measuring Na+,K+-ATPase activity (assessed by 86Rb+ uptake) in COS-7 cells expressing the rat alpha1 Na+,K+-ATPase. 1-oleoyl-2-acetoyl-sn-glycerol (OAG), a nonselective PKC activator, inhibited Na+,K+-ATPase activity in this system. On the other hand, 12-deoxyphorbol-13-phenylacetate (DPP), which preferentially activates nPKCepsilon, did not affect 86Rb+ uptake. These results indicate a differential pattern of phosphorylation and regulation of rat renal Na+,K+-ATPase activity by PKC isoforms and suggest an important role for cPKCs in the physiological regulation of the pump.

Carbachol inhibits Na(+)-K(+)-ATPase activity in choroid plexus via stimulation of the NO/cGMP pathway

Secretion of cerebrospinal fluid by the choroid plexus can be inhibited by its cholinergic innervation. We demonstrated that carbachol inhibits the Na(+)-K(+)-ATPase in bovine choroid tissue slices and investigated the mechanism. Many of the actions of cholinergic agents are mediated by nitric oxide (NO), which plays important roles in fluid homeostasis. The inhibition of Na(+)-K(+)-ATPase was blocked by the NO synthase inhibitor [N(omega)-nitro-L-arginine methyl ester] and was quantitatively mimicked by the NO agonists sodium nitroprusside (SNP) and diethylenetriamine NO. Inhibition by SNP correlated with an increase in tissue cGMP and was abolished by 1H-[1,2,4]oxadiazolo[4, 3-a]quinoxalin-1-one, an inhibitor of soluble guanylate cyclase. Inhibition was mimicked by the protein kinase G activator 8-bromo-cGMP and by okadaic acid, an inhibitor of protein phosphatases 1 and 2A. cGMP-dependent protein kinase inhibitors Rp-8-pCPT-cGMP (0.5-5 microM) and KT-5823 (2.0 microM) did not block the effects of SNP, but higher concentrations of the more selective inhibitor (Rp-8-pCPT-cGMP) had a pharmacological inhibitory effect on Na(+)-K(+)-ATPase. The data suggest that cholinergic regulation of the Na(+)-K(+)-ATPase is mediated by NO and involves activation of guanylate cyclase and elevation of cGMP.

Regulation of Na(+)-K(+)-ATPase by cAMP-dependent protein kinase anchored on membrane via its anchoring protein

Na(+)-K(+)- ATPase alpha-subunits in basolateral membrane vesicles (BLMVs) purified from rat parotid glands were (32)P-labeled within 5 s by incubation with [gamma-(32)P]ATP at 37 degrees C in the presence of cAMP, but no labeling occurred without cAMP. Phosphorylation of Na(+)-K(+)-ATPase was associated with a decrease in its activity. This alpha-subunit phosphorylation disappeared when BLMVs were briefly incubated with cAMP and subsequent washing before the incubation with [gamma-(32)P]ATP, indicating that catalytic subunit of protein kinase A (PKA) associated to BLMVs via binding with its RII regulatory subunit anchored on the membrane. In the absence of cAMP, a PKA catalytic subunit readily reassociated with the membrane-bound RII subunit. HT-31 peptide inhibited the Na(+)-K(+)-ATPase phosphorylation by membrane-bound endogenous PKA, indicating an involvement of A-kinase anchoring protein (AKAP). AKAP-150 protein in BLMVs was shown by immunoblotting and an RII overlay assay and was coimmunoprecipitated by anti-RII antibody. These results show that Na(+)-K(+)-ATPase of rat parotid gland acinar cells is regulated in vivo by membrane-anchored PKA via AKAP rather than by free cytosolic PKA.

Effect of anti-inflammatory bowel disease drug, E3040, on urate transport in rat renal brush border membrane vesicles

To confirm the assumption that 6-hydroxy-5, 7-dimethyl-2-methylamino-4-(3-pyridylmethyl)benzothiazole (E3040) acts on urate reabsorption by inhibiting urate-anion exchange at the luminal membrane of renal tubules, we investigated the inhibitory effect of E3040 and its two conjugated metabolites on hydroxyl ion (OH(-)) gradient-dependent urate uptake into brush border membrane vesicles from rat renal cortex and compared it with other uricosuric agents. The order of potency was AA193 (5-chloro-7, 8-dihydro-3-phenylfuro[2,3-g]-1,2-benzisoxazole-7-carboxylic acid)>benzbromarone>E3040>probenecid>E3040 sulfate>E3040 glucuronide. Furthermore, kinetic analysis revealed that E3040 may be a competitive inhibitor of the OH(-) gradient-dependent uptake of urate into brush border membrane vesicles.

P(3)-[2-(4-hydroxyphenyl)-2-oxo]ethyl ATP for the rapid activation of the Na(+),K(+)-ATPase

P(3)-[2-(4-hydroxyphenyl)-2-oxo]ethyl ATP (pHP-caged ATP) has been investigated for its application as a phototrigger for the rapid activation of electrogenic ion pumps. The yield of ATP after irradiation with a XeCl excimer laser (lambda = 308 nm) was determined at pH 6.0-7.5. For comparison, the photolytic yields of P(3)-[1-(2-nitrophenyl)]ethyl ATP (NPE-caged ATP) and P(3)-[1, 2-diphenyl-2-oxo]ethyl ATP (desyl-caged ATP) were also measured. It was shown that at lambda = 308 nm pHP-caged ATP is superior to the other caged ATP derivatives investigated in terms of yield of ATP after irradiation. Using time-resolved single-wavelength IR spectroscopy, we determined a lower limit of 10(6) s(-1) for the rate constant of release of ATP from pHP-caged ATP at pH 7.0. Like NPE-caged ATP, pHP-caged ATP and desyl-caged ATP bind to the Na(+), K(+)-ATPase and act as competitive inhibitors of ATPase function. Using pHP-caged ATP, we investigated the charge translocation kinetics of the Na(+),K(+)-ATPase at pH 6.2-7.4. The kinetic parameters obtained from the electrical measurements are compared to those obtained with a technique that does not require caged ATP, namely parallel stopped-flow experiments using the voltage-sensitive dye RH421. It is shown that the two techniques yield identical results, provided the inhibitory properties of the caged compound are taken into account. Our results demonstrate that under physiological (pH 7.0) and slightly basic (pH 7.5) or acidic (pH 6. 0) conditions, pHP-caged ATP is a rapid, effective, and biocompatible phototrigger for ATP-driven biological systems.