Ouabain-insensitive active sodium transport in rat jejunum: Evidence from ATPase activities,Na uptake by basolateral membrane vesicles and in vitro transintestinal transport

The basolateral membrane of the jejunal enterocyte of the rat was separated by self-orienting Percoll-gradient centrifugation and further purified from brush border contamination. Pellets were analysed for Mg-, Na- and (Na, K)-ATPase activities. The uptake of 0·02 M NaCl was also followed by the rapid micro-filtration technique. Transintestinal transport of fluid and electrolytes, and cell water, Na and K were determined in the in vitro everted and incubated jejunum. There is ouabain-insensitive Na-ATPase in addition to the well-known (Na, K)-ATPase in the basolateral membrane. These are differently inhibited by furosemide and ethacrynate. Na uptake by osmotically active basolateral membrane vesicles is enhanced by ATP and a further enhancement is obtained if there is intravesicular K. The ATP effect is inhibited differently by strophanthidin, furosemide and ethacrynate. In the everted sac preparation, transintestinal transport of Na and fluid still occurs when the Na/K pump is totally inhibited by ouabain. These experimental results suggest that there is also a ouabain-insensitive Na pump, different from the Na/K pump, in the basolateral membrane.     

Ouabain-insensitive Na(+)-ATPase activity in Trypanosoma cruzi epimastigotes

In the present paper, the presence of a ouabain-insensitive Na(+)-stimulated, Mg(2+)-dependent ATPase activity in T. cruzi epimastigotes CL14 clone and Y strain was investigated. The increase in Na+ concentration (from 5 to 170 mM), in the presence of 2 mM ouabain, increases the ATPase activity in a saturable manner along a rectangular hyperbola. The Vmax was 18.0 +/- 1.0 and 21.1 +/- 1.1 nmoles Pi x mg-1 x min-1 and the half-activation value (K50) for Na+ was 34.3 +/- 5.8 mM and 37.7 +/- 5.3 in CL14 clone and in Y strain, respectively. The Na(+)-stimulated ATPase activity was inhibited by 5-[aminosulfonyl]-4-chloro-2-[(2-furanylmethyl)-amino] benzoic acid (furosemide) in a dose-dependent manner. The half-inhibition value (I50) was 0.22 +/- 0.03 and 0.24 +/- 0.07 mM, and the Hill number (n) was 0.99 +/- 0.2 and 2.16 +/- 0.29 for CL14 clone and Y strain, respectively. These data indicate that both cell types express the ouabain-insensitive Na(+)-ATPase activity, which might be considered the biochemical expression of the second Na+ pump.     

Backdoor phosphorylation of basolateral plasma membranes of small intestinal epithelial cells: characterization of a furosemide-induced phosphoprotein related to the second sodium pump

Enterocyte has two different Na+-stimulated ATPases, the ouabain-sensitive Na+/K+ ATPase and a furosemide-inhibitable Na+ ATPase. To identify the polypeptide associated with the Na+-ATPase, 32Pi phosphorylation into basolateral membranes of enterocyte was investigated. Both, ouabain and furosemide induced Mg2+-dependent, vanadate-sensitive 32Pi incorporation into a 100kDa polypeptide. K(m) for Pi was 17.7+/-1.82 microM and 16.8+/-0.69 microM for ouabain-induced and furosemide-induced phosphorylation, respectively. K(m) for furosemide was 1.3+/-0.21 mM. Furosemide-induced 32Pi incorporation was sensitive to alkaline pH and hydroxylamine suggesting an acyl-phosphate bond. Na+ and K+ inhibited 32Pi incorporation induced by ouabain. In contrast, Na+ stimulated furosemide-induced phosphorylation with a K(m) of 16.5+/-5.59 mM while K+ had no effect. Purified Na+/K+ ATPase only presented ouabain-induced phosphoprotein, indicating that furosemide-induced phosphorylation is not related to this enzyme and appears to correspond to a new member of P-type ATPases associated with the second Na+ pump.     

Cell volume-sensitive Na+-ATPase activity in rat kidney cortex cell membranes

A ouabain-insensitive, K+-independent, sodium pump, has been demonstrated in guinea-pig and rat kidney proximal tubular cells. This pump is thought to be distinct from the ouabain-sensitive Na+/K+ pump. We present evidence here indicating the modulation of the biochemical expression of the Na+ pump, i.e. the ouabain-insensitive Na+-ATPase, by the cell volume in rat kidney proximal tubular cells. Thus, basolateral plasma membranes from swollen cells show a ouabain-insensitive Na+-ATPase activity 10-times higher than that in membranes from control cells. If the swollen cells recover their volume, the activity decreases ten times to control values. The ouabain-sensitive Na+/K+-ATPase is not affected by changes in the cell volume.     

Leishmania amazonensis: characterization of an ouabain-insensitive Na+-ATPase activity

We characterized ouabain-insensitive Na+-ATPase activity present in the plasma membrane of Leishmania amazonensis and investigated its possible role in the growth of the parasite. An increase in Na+ concentration in the presence of 1mM ouabain, increased the ATPase activity with a V(max) of 154.1+/-13.5nmol Pi x h(-1) x mg(-1) and a K0.5 of 28.9+/-7.7mM. Furosemide and sodium orthovanadate inhibited the Na+-stimulated ATPase activity with an IC(50) of 270microM and 0.10microM, respectively. Furosemide inhibited the growth of L. amazonensis after 48h incubation, with maximal effect after 96h. The IC50 for furosemide was 840. On the other hand, ouabain (1mM) did not change the growth of the parasite. Taken together, these results show that L. amazonensis expresses a P-type, ouabain-insensitive Na+-ATPase that could be involved with the growth of the parasite.     

Characterization of (Na+, K+)-ATPase in gill microsomes of the freshwater shrimp Macrobrachium olfersii

To better understand the adaptive strategies that led to freshwater invasion by hyper-regulating Crustacea, we prepared a microsomal (Na+, K+)-ATPase by differential centrifugation of a gill homogenate from the freshwater shrimp Macrobrachium olfersii. Sucrose gradient centrifugation revealed a light fraction containing most of the (Na+, K+)-ATPase activity, contaminated with other ATPases, and a heavy fraction containing negligible (Na+, K+)-ATPase activity. Western blotting showed that M. olfersii gill contains a single alpha-subunit isoform of about 110 kDa. The (Na+, K+)-ATPase hydrolyzed ATP with Michaelis Menten kinetics with K5, = 165+/-5 microM and Vmax = 686.1+/-24.7 U mg(-1). Stimulation by potassium (K0.5 = 2.4+/-0.1 mM) and magnesium ions (K0.5 = 0.76+/-0.03 mM) also obeyed Michaelis-Menten kinetics, while that by sodium ions (K0.5 = 6.0+/-0.2 mM) exhibited site site interactions (n = 1.6). Ouabain (K0.5 = 61.6+/-2.8 microM) and vanadate (K0.5 = 3.2+/-0.1 microM) inhibited up to 70% of the total ATPase activity, while thapsigargin and ethacrynic acid did not affect activity. The remaining 30% activity was inhibited by oligomycin, sodium azide and bafilomycin A. These data suggest that the (Na+, K+)-ATPase corresponds to about 70% of the total ATPase activity; the remaining 30%, i.e. the ouabain-insensitive ATPase activity, apparently correspond to F0F1- and V-ATPases, but not Ca-stimulated and Na- or K-stimulated ATPases. The data confirm the recent invasion of the freshwater biotope by M. olfersii and suggest that (Na+, K+)-ATPase activity may be regulated by the Na+ concentration of the external medium.     

S-adenosyl-L-methionine modulates Na+ + K+-ATPase activity in rat colonic basolateral membranes.

Rat colonic basolateral membranes were incubated with S-adenosyl-L-[methyl-3H]methionine (0.3 mM) at 37 degrees C for 2 h at pH 9.0. This resulted in an increase in the specific activity of Na+ + K+-ATPase by 60%. Kinetic parameter analysis revealed a 2-fold increase in the Vmax. of this enzymatic activity, whereas the Km for ATP was unchanged. The methylation inhibitor S-adenosyl-L-homocysteine (2 mM) significantly reduced these S-adenosyl-L-methionine-stimulated increases in specific activity and the Vmax. of Na+ + K+-ATPase. S-Adenosyl-L-methionine treatment of basolateral membranes was also found to significantly increase the fluidity of these preparations, as assessed by steady-state fluorescence polarization techniques using the fluorophore 1,6-diphenyl-1,3,5-hexatriene; S-adenosyl-L-homocysteine (2 mM) again markedly reduced this S-adenosyl-L-methionine-induced increase in fluidity. While transmethylation reactions involving phospholipids, non-polar lipids and proteins were all found to exist in rat colonic basolateral membranes, based on a number of observations, the results of the present studies suggest that transmethylation of membrane phospholipids, but not membrane non-polar lipids or proteins, influenced the fluidity of basolateral membranes which, in turn, modified Na+ + K+-ATPase activity in these membranes.     

Na+-dependent transport of alpha-aminoisobutyrate in isolated basolateral membrane vesicles from rat parotid glands

Basolateral plasma membranes were prepared from rat parotid gland after centrifugation in a self-orienting Percoll gradient. K+-dependent phosphatase [Na+ + K+)-ATPase), a marker enzyme for basolateral membranes, was enriched 10-fold from tissue homogenates. Using this preparation, the transport of alpha-aminoisobutyrate was studied. The uptake of alpha-aminoisobutyrate was Na+-dependent, osmotically sensitive, and temperature-dependent. In the presence of a Na+ gradient between the extra- and intravesicular solutions, vesicles showed an 'overshoot' accumulation of alpha-aminoisobutyrate. Sodium-dependent alpha-aminoisobutyrate uptake was saturable, exhibiting an apparent Km of 1.28 +/- 0.35 mM and Vmax of 780 +/- 170 pmol/min per mg protein. alpha-Aminoisobutyrate transport was inhibited considerably by monensin, but incubating with ouabain was without effect. These results suggest that basolateral membrane vesicles, which possess an active amino acid transport system (system A), can be prepared from the rat parotid gland.     

Kinetic studies on Na+/K+-ATPase and inhibition of Na+/K+-ATPase by ATP

Na+/K+-ATPase (EC 3.6.1.3) is an important membrane-bound enzyme. In this paper, kinetic studies on Na+/K+-ATPase were carried out under mimetic physiological conditions. By using microcalorimeter, a thermokinetic method was employed for the first time. Compared with other methods, it provided accurate measurements of not only thermodynamic data (deltarHm) but also the kinetic data (Km and Vmax). At 310.15K and pH 7.4, the molar reaction enthalpy (deltarHm) was measured as -40.514 +/- 0.9kJmol(-1). The Michaelis constant (Km) was determined to be 0.479 +/- 0.020 mM and consistent with literature data. The reliability of the thermokinetic method was further confirmed by colorimetric studies. Furthermore, a simple and reliable kinetic procedure was presented for ascertaining the true substrate for Na+/K+-ATPase and determining the effect of free ATP. Results showed that the MgATP complex was the real substrate with a Km value of about 0.5mM and free ATP was a competitive inhibitor with a Ki value of 0.253 mM.     

Intestinal calcium transport in spontaneously hypertensive rats (SHR) and their genetically matched WKY rats

Calcium transport across the basolateral membranes of the enterocyte represents the active step in calcium translocation. This step occurs by two mechanisms, an ATP-dependent pump and a Ca2+/Na+ exchange process. These studies were designed to investigate these two processes in jejunal basolateral membrane vesicles (BLMV) of the spontaneously hypertensive rats (SHR) and their genetically matched controls, Wistar-Kyoto (WKY) rats. The ATP-dependent calcium uptake was stimulated several-fold compared with no ATP condition in both SHR and WKY, but no differences were noted between rate of calcium uptake in SHR and WKY. Kinetics of ATP-dependent calcium uptake at concentrations between 0.01 and 1.0 microM revealed a Vmax of 0.67 +/- 0.03 nmol/mg protein/20 sec and a Km of 0.2 +/- 0.03 microM in SHR and Vmax of 0.69 +/- 0.12 and a Km of 0.32 +/- 0.14 microM in WKY rats. Ca2+/Na+ exchange in jejunal BLMV of SHR and WKY was investigated in two ways. First, sodium was added to the incubation medium (cis-Na+). Second, Ca2+ efflux from BLMV was studied in the presence of extravesicular Na+ (trans-Na+). Both studies suggest a decreased exchange of calcium and Na+. Kinetic parameters of Na(+)-dependent Ca2+ uptake at concentrations between 0.01 and 1.0 microM exhibited Vmax of 0.05 +/- 0.01 nanmol/mg protein/5 sec and a Km of 0.21 +/- 0.13 microM in SHR and Vmax of 0.11 +/- 0.02 nanmol/mg protein/5 sec and a Km of 0.09 +/- 0.05 in WKY, respectively. These results confirm that the intestinal BLMV of SHR and WKY rats have two mechanisms for calcium extrusion, an ATP-dependent Ca2+ transport process and a Na+/Ca2+ exchange process. The ATP-dependent process appears to be functional in SHR; however, the Ca2+/Na+ exchange mechanism appears to have a marked decrease in its maximal capacity. These findings suggest that calcium extrusion via Ca2+/Na+ is impaired in the SHR, which may lead to an increase in intracellular calcium concentration. These findings may have relevance to the development of hypertension.     

Na+/HCO3-co-transport in basolateral membrane vesicles isolated from rabbit renal cortex

Recent studies suggest that the major pathway for exit of HCO3- across the basolateral membrane of the proximal tubule cell is electrogenic Na+/HCO3- co-transport. We therefore evaluated the possible presence of Na+/HCO3- co-transport in basolateral membrane vesicles isolated from the rabbit renal cortex. Imposing an inward HCO3- gradient induced the transient uphill accumulation of Na+, and imposing an outward Na+ gradient caused HCO3- -dependent generation of an inside-acid pH gradient as monitored by quenching of acridine orange fluorescence, findings consistent with the presence of Na+/HCO3- co-transport. In the absence of other driving forces, generating an inside-positive membrane potential by imposing an inward K+ gradient in the presence of valinomycin caused net Na+ uptake via a HCO3- -dependent pathway, indicating that Na+/HCO3- co-transport is electrogenic and associated with a flow of negative charge. Imposing transmembrane Cl- gradients did not appreciably affect HCO3- gradient-stimulated Na+ influx, suggesting that Na+/HCO3- co-transport is not Cl- -dependent. The rate of HCO3- gradient-stimulated Na+ influx was a simple, saturable function of the Na+ concentration (Km = 9.7 mM, Vmax = 160 nmol/min/mg of protein), was inhibited by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (I50 = 100 microM), but was inhibited less than 10% by up to 1 mM amiloride. We could not demonstrate a HCO3- -dependent or 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid-sensitive component of Na+ influx in microvillus membrane vesicles. This study thus indicates the presence of a transport system mediating electrogenic Na+/HCO3- co-transport in basolateral, but not luminal, membrane vesicles isolated from the rabbit renal cortex. Analogous to the use of renal microvillus membrane vesicles to study Na+/H+ exchange, renal basolateral membrane vesicles may be a useful model system for examining the kinetics and possible regulation of Na+/HCO3- co-transport.     

Characterization of basolateral membrane Na/H antiport in rat jejunum

Na uptake studies were performed in order to examine the activity of a Na/H exchanger in basolateral membrane vesicles isolated from rat jejunum. Experiments were carried out under voltage-clamped conditions in order to avoid electrodiffusional ionic movements. 1 mM Na uptake was found to be enhanced by an outward proton gradient and its initial rate was further increased by the presence of monensin or nigericin. The pH gradient-driven Na uptake was inhibited by 2 mM amiloride and unaffected by 0.1 mM 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid. The initial rate of the proton gradient-induced Na uptake was saturable with respect to external Na, with a Km of 13.6 +/- 1.4 mM and a Vmax of 35.4 +/- 2.2 nmol/mg protein per min. Li competed with Na for the exchange process, whereas K, Rb, Cs, tetramethylammonium had no effect. We conclude that rat jejunal basolateral membrane contains a Na/H exchanger whose properties are similar to those of the antiporter identified in the brush-border membrane.     

Glutamine transport by rat basolateral membrane vesicles

Glutamine, a neutral amino acid, is unlike most amino acids, has two amine moieties which underlies its importance as a nitrogen transporter and a carrier of ammonia from the periphery to visceral organs. The gastrointestinal tract utilizes glutamine as a respiratory substrate. The intestinal tract receives glutamine from the luminal side and from the arterial side through the basolateral membranes of the enterocyte. This study characterizes the transport of glutamine by basolateral membrane vesicles of the rat. Basolateral membranes were prepared by a well validated technique of separation on a percoll density gradient. Membrane preparations were enriched with Na+/K+-ATPase and showed no 'overshoot' phenomena with glucose under sodium-gradient conditions. Glutamine uptake represented transport into the intravesicular space as evident by an osmolality study. Glutamine uptake was temperature sensitive and driven by an inwardly directed sodium gradient as evident by transient accumulation of glutamine above the equilibrium values. Kinetics of glutamine uptake under both sodium and potassium gradients at glutamine concentrations between 0.01 and 0.6 mM showed saturable processes with Vmax of 0.39 +/- 0.008 and 0.34 +/- 0.05 nmol/mg protein per 15 s for both sodium-dependent and sodium-independent processes, respectively. Km values were 0.2 +/- 0.01 and 0.55 +/- 0.01 mM, respectively. pH optimum for glutamine uptake was 7.5. Imposition of negative membrane potential by valinomycin and anion substitution studies enhanced the sodium-dependent uptake of glutamine suggesting an electrogenic process, whereas the sodium-independent uptake was not enhanced suggesting an electroneutral process. Other neutral amino acids inhibited the initial uptake of glutamine under both sodium-dependent and sodium-independent conditions. We conclude that glutamine uptake by basolateral membranes occurs by carrier-mediated sodium-dependent and sodium-independent processes. Both processes exhibit saturation kinetics and are inhibited by neutral amino acids. The sodium-dependent pathway is electrogenic whereas the sodium-independent pathway is electroneutral.     

Increase in membrane fluidity modulates sodium-coupled uptakes and cyclic AMP synthesis by renal proximal tubular cells in primary culture.

In order to evaluate the influence of membrane fluidization on three apical transport systems and on a basolateral enzyme, and to analyse the mechanisms involved, we studied, in cultured rabbit proximal tubular cells, the effect of increasing concentrations of the local anesthetic drug benzyl alcohol on Na(+)-dependent uptakes of phosphate (Pi), methyl alpha-D-glucopyranoside (MGP), and L-alanine, as well as on basal and stimulated cyclic AMP content. At 10 mM, benzyl alcohol increased the Vmax of Pi uptake by 31%, decreased that of MGP uptake by 24%, and did not affect alanine uptake. Km values were not affected. Benzyl alcohol, up to 40 mM, increased in a concentration-dependent manner basal, PTH-stimulated, and cholera toxin-stimulated, but not forskolin-stimulated cyclic AMP accumulation. In the presence of 40 mM benzyl alcohol, the magnitude of PTH-induced inhibition of Pi uptake was enhanced from 11% to 24%. It is concluded that: (i) fluidization of apical membranes affected differently Na+/Pi, Na+/MGP, and Na+/alanine cotransports, reflecting differences in the lipidic environments of these transport system; (ii) fluidization of basolateral membranes enhanced PTH-stimulated cyclic AMP generation through improved coupling between the receptor-GS complex and the catalytic subunit of adenylate cyclase; (iii) these variations may result in physiological and pathophysiological modulation of the renal handling of solutes and of the phosphaturic effect of PTH.     

Characterization of gill (Na+ + K+)-ATPase in the sea bass (Dicentrarchus labrax L.)

Bass gill microsomal preparations contain both a Na+, K+ and Mg2+-dependent ATPase, which is completely inhibited by 10(-3)M ouabain and 10(-2)M Ca2+, and also a ouabain insensitive ATP-ase activity in the presence of both Mg2+ and Na+. Under the optimal conditions of pH 6.5, 100 mM Na+, 20 mM K+, 5 mM ATP and 5 mM Mg2+, (Na+ + K+)-ATPase activity at 30 degrees C is 15.6 mumole Pi hr/mg protein. Bass gill (Na+ + K+)-ATPase is similar to other (Na+ + K+)-ATPases with respect to the sensitivity to ionic strength, Ca2+ and ouabain and to both Na+/K+ and Mg2+/ATP optimal ratios, while pH optimum is lower than poikilotherm data. The enzyme requires Na+, whereas K+ can be replaced efficiently by NH+4 and poorly by Li+. Both Km and Vm values decrease in the series NH+4 greater than K+ greater than Li+. The break of Arrhenius plot at 17.7 degrees C is close to the adaptation temperature. Activation energies are scarcely different from each other and both lower than those generally reported. The Km for Na+ poorly decreases as the assay temperature lowers. The comparison with literature data aims at distinguishing between distinctive and common features of bass gill (Na+ + K+)-ATPase.     

Sodium-phosphate cotransport in human red blood cells. Kinetics and role in membrane metabolism

Orthophosphate (Pi) uptake was examined in human red blood cells at 37 degrees C in media containing physiological concentrations of Pi (1.0- 1.5 mM). Cells were shown to transport Pi by a 4,4'-dinitro stilbene- 2,2'-disulfonate (DNDS) -sensitive pathway (75%), a newly discovered sodium-phosphate (Na/Pi) cotransport pathway (20%), and a pathway linearly dependent on an extracellular phosphate concentration of up to 2.0 mM (5%). Kinetic evaluation of the Na/Pi cotransport pathway determined the K1/2 for activation by extracellular Pi ([Na]o = 140 mM) and extracellular Na [( Pi]o = 1.0 mM) to be 304 +/- 24 microM and 139 +/- 8 mM, respectively. The phosphate influx via the cotransport pathway exhibited a Vmax of 0.63 +/- 0.05 mmol Pi (kg Hb)-1(h)-1 at 140 mM Nao. Activation of Pi uptake by Nao gave Hill coefficients that came close to a value of 1.0. The Vmax of the Na/Pi cotransport varied threefold over the examined pH range (6.90-7.75); however, the Na/Pi stoichiometry of 1.73 +/- 0.15 was constant. The membrane transport inhibitors ouabain, bumetanide, and arsenate had no effect on the magnitude of the Na/Pi cotransport pathway. No difference was found between the rate of incorporation of extracellular Pi into cytosolic orthophosphate and the rate of incorporation into cytosolic nucleotide phosphates, but the rate of incorporation into other cytosolic organic phosphates was significantly slower. Depletion of intracellular total phosphorus inhibited the incorporation of extracellular Pi into the cytosolic nucleotide compartment; and this inhibition was not reversed by repletion of phosphorus to 75% of control levels. Extracellular 32Pi labeled the membrane-associated compounds that migrate on thin-layer chromatography (TLC) with the Rf values of ATP and ADP, but not those of 2,3-bisphosphoglycerate (2,3-DPG), AMP, or Pi. DNDS had no effect on the level of extracellular phosphate incorporation or on the TLC distribution of Pi in the membrane; however, substitution of extracellular sodium with N-methyl-D-glucamine inhibited phosphorylation of the membranes by 90% and markedly altered the chromatographic pattern of the membrane-associated phosphate.(ABSTRACT TRUNCATED AT 400 WORDS)     

Characterization of the mitochondrial Na+-H+ exchange. The effect of amiloride analogues

The kinetic properties and inhibitor sensitivity of the Na+-H+ exchange activity present in the inner membrane of rat heart and liver mitochondria were studied. (1) Na+-induced H+ efflux from mitochondria followed Michaelis-Menten kinetics. In heart mitochondria, the Km for Na+ was 24 +/- 4 mM and the Vmax was 4.5 +/- 1.4 nmol H+/mg protein per s (n = 6). Basically similar values were obtained in liver mitochondria (Km = 31 +/- 2 mM, Vmax = 5.3 +/- 0.2 nmol H+/mg protein per s, n = 4). (2) Li+ proved to be a substrate (Km = 5.9 mM, Vmax = 2.3 nmol H+/mg protein per s) and a potent competitive inhibitor with respect to Na+ (Ki approximately 0.7 mM). (3) External H+ inhibited the mitochondrial Na+-H+ exchange competitively. (4) Two benzamil derivatives of amiloride, 5-(N-4-chlorobenzyl)-N-(2',4'-dimethyl)benzamil and 3',5'-bis(trifluoromethyl)benzamil were effective inhibitors of the mitochondrial Na+-H+ exchange (50% inhibition was attained by approx. 60 microM in the presence of 15 mM Na+). (5) Three 5-amino analogues of amiloride, which are very strong Na+-H+ exchange blockers on the plasma membrane, exerted only weak inhibitory activity on the mitochondrial Na+-H+ exchange. (6) The results indicate that the mitochondrial and the plasma membrane antiporters represent distinct molecular entities.     

Distinction between the two basic mechanisms of cation transport in the cardiac Na(+)-Ca2+ exchange system

In order to distinguish between the Ping-Pong and sequential mechanisms of cation transport in the cardiac Na(+)-Ca2+ exchange system, the initial rates of the Nai-dependent 45Ca uptake (t = 1 s) were measured in reconstituted proteoliposomes, loaded with a Ca chelator. Under "zero-trans" conditions ([Na]o = [Ca]i = 0) at a fixed [Na]i = 10-160 mM with varying [45Ca]o = 2.5-122 microM for each [Na]i, the Km and Vmax values increased from 7.7 to 33.5 microM and from 2.3 to 9.0 nmol.mg-1.s-1, respectively. The Vmax/Km values show a +/- 2-10% deviation from the average value of 0.274 nmol.mg-1.s-1.microM-1 over the whole range of [Na]i. These deviations are within the standard error of Vmax (+/- 3-7%), Km (+/- 11-17%), and Vmax/Km (+/- 11-19%). This suggests that, under conditions in which Vmax and Km are [Na]i dependent and vary 4-5-fold, the Vmax/Km values are constant within the experimental error. In the presence of K(+)-valinomycin the Vmax/Km values are 0.85 +/- 0.17 and 1.08 +/- 0.18 nmol.mg-1.s-1.microM-1 at [Na]i = 20 and 160 mM, respectively, suggesting that under conditions of "short circuit" of the membrane potential the Vmax/Km values still exhibit the [Na]i independence. At a very low fixed [45Ca]o = 1.1 microM with varying [Na]i = 10-160 mM, the initial rates were found to be [Na]i independent. At a high fixed [45Ca]o = 92 microM the initial rates show a sigmoidal dependence on the [Na]i with Vmax = 13.8 nmol.mg-1.s-1, KmNa = 21 mM, and Hill coefficient nH = 1.5. The presented data support a Ping-Pong (consecutive) mechanism of cation transport in the Na(+)-Ca2+ exchanger.     

Stoichiometry and voltage dependence of the sodium pump in voltage- clamped, internally dialyzed squid giant axon

The stoichiometry and voltage dependence of the Na/K pump were studied in internally dialyzed, voltage-clamped squid giant axons by simultaneously measuring, at various membrane potentials, the changes in Na efflux (delta phi Na) and holding current (delta I) induced by dihydrodigitoxigenin (H2DTG). H2DTG stops the Na/K pump without directly affecting other current pathways: (a) it causes no delta I when the pump lacks Na, K, Mg, or ATP, and (b) ouabain causes no delta I or delta phi Na in the presence of saturating H2DTG. External K (Ko) activates Na efflux with Michaelis-Menten kinetics (Km = 0.45 +/- 0.06 mM [SEM]) in Na-free seawater (SW), but with sigmoid kinetics in approximately 400 mM Na SW (Hill coefficient = 1.53 +/- 0.08, K1/2 = 3.92 +/- 0.29 mM). H2DTG inhibits less strongly (Ki = 6.1 +/- 0.3 microM) in 1 or 10 mM K Na-free SW than in 10 mM K, 390 mM Na SW (1.8 +/- 0.2 microM). Dialysis with 5 mM each ATP, phosphoenolpyruvate, and phosphoarginine reduced Na/Na exchange to at most 2% of the H2DTG-sensitive Na efflux. H2DTG sensitive but nonpump current caused by periaxonal K accumulation upon stopping the pump, was minimized by the K channel blockers 3,4-diaminopyridine (1 mM), tetraethylammonium (approximately 200 mM), and phenylpropyltriethylammonium (20-25 mM) whose adequacy was tested by varying [K]o (0-10 mM) with H2DTG present. Two ancillary clamp circuits suppressed stray current from the axon ends. Current and flux measured from the center pool derive from the same membrane area since, over the voltage range -60 to +20 mV, tetrodotoxin-sensitive current and Na efflux into Na-free SW, under K-free conditions, were equal. The stoichiometry and voltage dependence of pump Na/K exchange were examined at near-saturating [ATP], [K]o and [Na]i in both Na-free and 390 mM Na SW. The H2DTG-sensitive F delta phi Na/delta I ratio (F is Faraday's constant) of paired measurements corrected for membrane area match, was 2.86 +/- 0.09 (n = 8) at 0 mV and 3.05 +/- 0.13 (n = 6) at -60 to -90 mV in Na-free SW, and 2.72 +/- 0.09 (n = 7) at 0 mV and 2.91 +/- 0.21 (n = 4) at -60 mV in 390 mM Na SW. Its overall mean value was 2.87 +/- 0.07 (n = 25), which was not significantly different from the 3.0 expected of a 3 Na/2 K pump.(ABSTRACT TRUNCATED AT 400 WORDS)