Kinetic Properties Of Na Research Articles

Understanding rhombohedral iron hexacyanoferrate with three different sodium positions for high power and long stability sodium-ion battery

Sodium iron hexacyanoferrate (NaFeHCF) has been considered as a potential cathode for sodium-ion batteries owing to its low-cost and easily prepared procedure. However, it is still challenging to achieve long cyclic stability and superior rate capability, and the sodium storage mechanism of sodium-rich NaFeHCF is still elusive. Herein, a sodium-rich NaFeHCF with rhombohedral structure is presented with excellent electrochemical performances within 2.0–4.2 ​V. The specific capacity of ~115 ​mA ​h ​g−1 is obtained by utilizing two plateaus around 2.9 and 4.06 ​V, respectively. Remarkable rate performance from 10 to 4000 ​mA ​g−1 and 1000 cycles with high capacity retention is achieved as well. Synchrotron powder X-ray diffraction (PXRD) and structural refinement reveals that sodium-ions occupy three different sites (interstitial, face and edge) in rhombohedral unit cell, which contribute different capacities on different plateaus during Na+ extractions. Moreover, the rhombohedral structure is well-maintained after long-term Na+ extractions/insertions and reversible phase transitions with small volume variation are observed through in-situ synchrotron PXRD. The kinetic properties of Na+ in rhombohedral unit cell are identified by ab-initio molecular dynamics method and density functional theory calculations, which indicate that Na+ transport on three-dimensional diffusion paths, thus enabling the outstanding rate performance of NaFeHCF.

Unveiling the thermodynamic and kinetic properties of NaxFe(SO4)2 (x = 0–2): toward a high-capacity and low-cost cathode material

Employing the density functional theory framework, we have investigated the phase stability, electrochemical properties and ionic diffusion of an eldfellite cathode material.

Functional roles of Na(+)/K(+)-ATPase in active ammonia excretion and seawater acclimation in the giant mudskipper, Periophthalmodon schlosseri.

The giant mudskipper, Periophthalmodon schlosseri, is an amphibious fish that builds burrows in the mudflats. It can actively excrete ammonia through its gills, and tolerate high environmental ammonia. This study aimed to examine the effects of seawater (salinity 30; SW) acclimation and/or environmental ammonia exposure on the kinetic properties of Na+/K+-ATPase (Nka) from, and mRNA expression and protein abundance of nka/Nka α–subunit isoforms in, the gills of P. schlosseri pre-acclimated to slightly brackish water (salinity 3; SBW). Our results revealed that the Nka from the gills of P. schlosseri pre-acclimated to SBW for 2 weeks had substantially higher affinity to (or lower Km for) K+ than NH+4, and its affinity to NH+4 decreased significantly after 6-days exposure to 75 mmol l−1 NH4Cl in SBW. Hence, Nka transported K+ selectively to maintain intracellular K+ homeostasis, instead of transporting NH+4 from the blood into ionocytes during active NH+4 excretion as previously suggested. Two nkaα isoforms, nkaα1 and nkaα3, were cloned and sequenced from the gills of P. schlosseri. Their deduced amino acid sequences had K+ binding sites identical to that of Nkaα1c from Anabas testudineus, indicating that they could effectively differentiate K+ from NH+4. Six days of exposure to 75 mmol l−1 NH4Cl in SBW, or to SW with or without 50 mmol l−1 NH4Cl led to significant increases in Nka activities in the gills of P. schlosseri. However, a significant increase in the comprehensive Nkaα protein abundance was observed only in the gills of fish exposed to 50 mmol l−1 NH4Cl in SW. Hence, post-translational modification could be an important activity modulator of branchial Nka in P. schlosseri. The fast modulation of Nka activity and concurrent expressions of two branchial nkaα isoforms could in part contribute to the ability of P. schlosseri to survive abrupt transfer between SBW and SW or abrupt exposure to ammonia.

Effect of saliva from horse fly Hybomitra bimaculata on kinetic properties of Na, K-ATPase: possible role in regulation of relaxation

The possible involvement of salivary gland extract (SGE) from horse flies in modifying hyperpolarization and relaxation via alterations in functional properties of sarcolemmal Na,K-ATPase in the host tissue was tested in vitro by application of various amounts of SGE from Hybomitra bimaculata.SGE in the amount of 3 µg proteins representing approximately the equivalent of one salivary gland of Hybomitra bimaculata induced a stimulatory effect on Na,K-ATPase at all ATP concentrations applied. This effect resulted from the improved ATP-binding site affinity in the Na,K-ATPase molecule, as implicated by the reduction in KM. Increasing the amount of SGE to 6.5 µg resulted in inhibition of the enzyme, which was characterized by reduction in Vmax and also KM. This suggests that in the presence of relatively high Hybomitra bimaculata SGE concentration some SGE components affect Na,K-ATPase, when ATP is already bound to the enzyme.Our results indicate that SGE from the horse fly Hybomitra bimaculata contain at least two different biologically active compounds modifying the acute recovery and maintenance of excitability during contractile activity in the host tissue by affecting Na,K-ATPase with opposite effects, depending on the ratio of SGE-proteins to proteins of the host tissue.

Thermodynamic and kinetic properties of Na+-motive NADH: Quinone oxidoreductase

Thermodynamic and kinetic properties of Na+-motive NADH: Quinone oxidoreductase

Effect of treatment with cadmium on kinetic properties of Na +, K +-ATPase and glucose-6-phosphatase activity in rat liver microsomes : A correlative study on influence of lipid/phospholipid make-up

Studies on Cd hepatotoxicity have focused mainly on induction of cytochrome P 450 system and related enzymes. In the present study young adult male rats given a single intra-peritoneal injection of Cd (0.84 mg Cd/kg body weight) and effects on kinetic parameters rat liver microsomal Na +, K +-ATPase and G6Pase were evaluated at the end of 1 month and 1 week. The substrate and temperature kinetics parameters were examined and attempts were made to seek correlation with changes in lipid/phospholipid profiles. The Na +, K + ATPase activity decreased only in 1 week Cd-treated group but recovered at the end of 1 month. The activity resolved in two distinct kinetic components in control as well as the experimental groups. In 1 week Cd-treated group the K m value of both the components was unchanged, whereas V max value decreased. In 1-month Cd-treated group V max value only of component I increased. The catalytic efficiency of both the components was not affected in the experimental groups. In 1-week Cd-treated group the energy of activations at high-temperature range ( E H) and low-temperature range ( E L) decreased, whereas for 1-month Cd-treated group the energies of activations did not change. The G6Pase activity measured at 37 °C was high only in 1-month Cd-treated group. The activity resolved in two kinetically distinguishable components in control as well as in the experimental groups. K m value of component I decreased in both the Cd-treated groups. In 1-month Cd-treated group the V max value of component II increased. The catalytic efficiency of G6Pase was unchanged despite changes in K m and V max. In 1-week Cd-treated group the E H and E L decreased, whereas only E L showed decrease in 1-month Cd-treated group. Cholesterol (CHL) content increased in both the Cd-treated groups. Content of lysophospholipid (Lyso), spinghomyelin (SPM) and phosphatidic acid (PA) increased, whereas phosphatidylcholine (PC) and phosphatidylserine (PS) decreased in 1-week Cd-treated group. In 1-month Cd group the Lyso, SPM, and PC increased while PC, phosphatidylethanolamine (PE) and PA decreased. In conclusion, Cd has short-term effects on microsomal Na +, K +-ATPase which are reversed by the end of 1 month and that G6Pase does not seem to be a target of Cd insult.

Thyroid Hormone-Induced Alterations in Membrane Structure-Function Relationships: Studies on Kinetic Properties of Rat Kidney Microsomal Na+,K+-ATPase and Lipid/Phospholipid Profiles

The effects of thyroidectomy (Tx) and subsequent treatment with 3,5,3'-triiodothyronine (T(3)) or combined replacement therapy (T(R)) with T(3 )and thyroxine (T(4)) on the substrate and temperature kinetics properties of Na+,K+-ATPase and lipid/phospholipid makeup of rat kidney microsomes were examined. Enzyme activity was somewhat high in the hypothyroid (Tx) animals and increased significantly following T(3) treatment, while T(R) treatment caused a decrease. In the Tx and T(3) groups enzyme activity resolved in two kinetic components, while in the T(R) group the enzyme showed allosteric behavior up to 0.5 mM: ATP concentration. The K(m) and V(max) values of both the components decreased in Tx animals without affecting the catalytic efficiency. T(3) treatment caused a significant increase in the V(max) of both the components, with a significant increase in the catalytic efficiency, while the K(m) values were not upregulated. The T(R) regimen lowered the K(m) and V(max) of component II but improved the catalytic efficiency. Thyroid status-dependent changes were also noted in the temperature kinetics of the enzyme. Regression analysis revealed that changes in the substrate and temperature kinetics parameters correlated with specific phospholipid components.

Effect of Neonatal Hypothyroidism on the Kinetic Properties of Na+, K+‐ATPase from Rat Brain Microsomes

The effects of neonatal hypothyroidism on the kinetic properties of Na+, K+ -ATPase from rat brain microsomes were examined. Neonatal hypothyroidism resulted in decreased Na+, K+ -ATPase activity compared to control samples (7.4 +/- 1.48 and 29.8 +/- 2.30 micromol Pi/h/mg protein, respectively, P < 0.001). Substrate kinetics studies with ATP, Na+ and K+ revealed that there were generalised decreases in Vmax. For ATP, Na+ and K+, activities resolved into two kinetic components in the control group. In hypothyroid animals, the low-affinity component for ATP was absent. The opposite pattern (i.e. an absence of the high-affinity component) was noted for Na+. For K+, although both kinetic components were discernible in neonatal hypothyroid brain microsomes, the Km of the high-affinity component was significantly higher (P < 0.001) compared to control samples. In the control group, the enzyme displayed allosteric behaviour at high concentrations of Mg2+; in hypothyroid animals, the pattern was completely allosteric. The Na+, K+ -ATPase enzyme from the hypothyroid brain microsomes bound two molecules of ATP rather than one, unlike in the control animals. Our results thus indicate that neonatal hypothyroidism results in an impairment of microsomal Na+, K+ -ATPase activity in the rat brain, together with subtle alterations in the kinetic properties of the enzyme.

Development of ionic currents of zebrafish slow and fast skeletal muscle fibers

Voltage-gated Na+ and K+ channels play key roles in the excitability of skeletal muscle fibers. In this study we investigated the steady-state and kinetic properties of voltage-gated Na+ and K+ currents of slow and fast skeletal muscle fibers in zebrafish ranging in age from 1 day postfertilization (dpf) to 4-6 dpf. The inner white (fast) fibers possess an A-type inactivating K+ current that increases in peak current density and accelerates its rise and decay times during development. As the muscle matured, the V50s of activation and inactivation of the A-type current became more depolarized, and then hyperpolarized again in older animals. The activation kinetics of the delayed outward K+ current in red (slow) fibers accelerated within the first week of development. The tail currents of the outward K+ currents were too small to allow an accurate determination of the V50s of activation. Red fibers did not show any evidence of inward Na+ currents; however, white fibers expressed Na+ currents that increased their peak current density, accelerated their inactivation kinetics, and hyperpolarized their V50 of inactivation during development. The action potentials of white fibers exhibited significant changes in the threshold voltage and the half width. These findings indicate that there are significant differences in the ionic current profiles between the red and white fibers and that a number of changes occur in the steady-state and kinetic properties of Na+ and K+ currents of developing zebrafish skeletal muscle fibers, with the most dramatic changes occurring around the end of the first day following egg fertilization.

Kinetic properties of voltage-gated Na + currents in rat muscular sympathetic neurons with and without adenosine triphosphate and guanosine triphosphate in intracellular solution

Voltage-gated Na + currents were recorded from anatomically identified postganglionic muscular sympathetic neurons without and with ATP and GTP in the intracellular solution. The main findings of the study were that cells without ATP and GTP in the intracellular solution express a higher amplitude and greater density of voltage-gated Na + current, and their Na + current activates faster and also inactivates faster time dependently. The current is also steady-state inactivated to a lesser degree and recovers from inactivation more slowly in cells without added ATP and GTP. These findings suggest that the presence of ATP and GTP, substrates for channel phosphorylation, changes the kinetic properties of Na + currents.

Expression and kinetic properties of Na + currents in rat cardiac dorsal root ganglion neurons

The expression and properties of voltage-gated Na + currents in cardiac dorsal root ganglion (DRG) neurons were assessed in this study. Cardiac DRG neurons were labelled by injecting the Fast Blue fluorescent tracer into the pericardium. Recordings were performed from 138 cells. Voltage-dependent Na + currents were found in 115 neurons. There were 109 neurons in which both tetrodotoxin-sensitive (TTX-S, blocked by 1 μM of TTX) and tetrodotoxin-resistant (TTX-R, insensitive to 1 μM of TTX) Na + currents were present. Five cells expressed TTX-R current only and one cell only the TTX-S current. The kinetic properties of Na + currents and action potential waveform parameters were measured in neurons with cell membrane capacitance ranging from 15 to 75 pF. The densities of TTX-R (110.0 pA/pF) and TTX-S (126.1 pA/pF) currents were not significantly different. Current threshold was significantly higher for TTX-R (−34 mV) than for TTX-S (−40.4 mV) currents. V 1/2 of activation for TTX-S current (−19.6 mV) was significantly more negative than for TTX-R current (−9.2 mV), but k factors did not differ significantly. V 1/2 and the k constant for inactivation for TTX-S currents were −35.6 and −5.7 mV, respectively. These values were significantly lower than those recorded for TTX-R current for which V 1/2 and k were −62.3 and −7.7 mV, respectively. The action potential threshold was lower, the 10–90% rise time and potential width were shorter before than after the application of TTX. Based on this we drew the conclusion that action potential recorded before adding tetrodotoxin was mainly TTX-S current dependent, while the action potential recorded after the application of toxin was TTX-R current dependent. We also found 23 cells with mean membrane capacitance ranging from 12 to 35 pF (the smallest labelled DRG cells found in this study) that did not express the Na + current. The function of these cells is unclear. We conclude that the overwhelming majority of cardiac dorsal root ganglion neurons in which voltage-dependent Na + currents were present, exhibited both TTX-S and TTX-R Na + currents with remarkably similar expression and kinetic properties.

Kinetic properties of erythrocyte Na+-Li+ and Na+-H+ exchange in hypertensive patients

To ascertain the relationships between the kinetic properties of erythrocyte Na+-H+ exchange [maximum kinetic energy (Vmax), Michaelis constant (Km) for internal H+ and Hill's coefficient], Na+-Li+ (Vmax and Km for external Na+), and metabolic parameters in normotensive controls and hypertensive subjects. Na+-H+ exchange was measured as the Na+ influx driven by intracellular H+, and Na+-Li+ exchange as the Li+ efflux driven by extracellular Na+, in erythrocytes from normotensive (n = 59) and hypertensive (n = 93) subjects. In comparison with normotensives, the hypertensives had a higher Vmax for Na+-Li+ and Na+-H+ exchange, a higher Km for external Na+ for Na+-Li+ exchange and a lower reduced Hill's number for Na+-H+ exchange. Vmax values for Na+-Li+ and Na+-H+ exchange were significantly correlated, as were Km values for internal H+ for Na+-H+ exchange and Km for external Na+ for Na+-Li+ exchange. Insulin resistance and beta-cell function indices were higher in the hypertensives than the normotensives. Upon stepwise multiple regression analysis, Vmax for Na+-Li+ exchange was correlated significantly and independently with Km for external Na+ and with the insulin resistance index, while Km for external Na+ was correlated with Km for internal H+, Vmax for Na+-H+ exchange and mean blood pressure. Vmax and Hill's coefficient for Na+-H+ exchange were correlated only with mean blood pressure. The demonstration of functional correlations between the kinetic properties of Na+-H+ and Na+-Li+ exchange provides further evidence that erythrocyte Na+-Li+ exchange is a functioning mode of Na+-H+ exchange, which is affected by insulin resistance.

Kinetics of Na+/H+ exchange in vascular smooth muscle cells from WKY and SHR: effects of phorbol ester.

The kinetic properties of Na+/H+ exchange were investigated in vascular smooth muscle cells (VSMC) in culture from normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR). Antiport activity was measured in 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein-loaded cells after nigericin-induced cytosolic acidification. Studies were performed without (control) and with pretreatment of the cells with phorbol 12-myristate 13-acetate (PMA; 200 nM). Na+/H+ exchange markedly differed between the two strains with lower Hill coefficients [1.56 +/- 0.17 (SE) vs. 2.62 +/- 0.36] and higher maximal activity (Vmax) values (55.85 +/- 5.24 vs. 31.11 +/- 2.38 mmol H+.l-1.min-1) in SHR compared with WKY cell lines. PMA markedly altered the antiport kinetics in WKY VSMC with a decrease in the Hill coefficient (1.75 +/- 0.14) without affecting Vmax (31.88 +/- 1.55 mmol H+.l-1.min-1). In VSMC from SHR, PMA had no effect on the kinetic variables investigated. Thus two kinetic abnormalities are present with respect to Na+/H+ antiport activity in VSMC from SHR compared with WKY, i.e., increased Vmax and decreased Hill coefficient. The observation that PMA does not affect the kinetics of the Na+/H+ antiport in VSMC from SHR suggests a marked degree of antiporter prestimulation in this animal model of genetic hypertension.

Effect of fish oil diet on fatty acid composition of phospholipids of brain membranes and on kinetic properties of Na+,K(+)-ATPase isoenzymes of weaned and adult rats.

The influence of dietary (n-3) fatty acids (such as eicosapentaenoic and docosahexaenoic acids) as found in fish oil on Na+ sensitivity and ouabain affinity of Na+,K(+)-ATPase isoenzymes (alpha 1, alpha 2, alpha 3) was studied in whole brain membranes from weaned and adult rats fed diets for two generations. The long chain (n-3) fatty acids supplied by fish oil decreased the fatty acids of the (n-6) series compared with the standard diet, resulting in a decrease in the (n-6)/(n-3) molar ratio in both 21- and 60-day-old rats. On the basis of ouabain titration, three inhibitory processes with markedly different affinities were associated with isoenzymes, i.e., low affinity (alpha 1), high affinity (alpha 2), and very high affinity (alpha 3). It appears that the fish oil diet, in part via the modification of membrane fatty acid composition, altered the proportion and ouabain affinity of isoenzymes. Na+ sensitivity is the best criterion of physiologic change induced by fish oil diet. We calculated the Na+ activation for each isoenzyme and found one Na+ sensitivity and two Na+ sensitivities per isoenzyme in weanling and adult rats fed different diets, respectively. In contrast to alpha 2 and alpha 3, alpha 1 appears insensitive to membrane change induced by fish oil diet. Fish oil diet, which is known to confer cardioprotection, induced significant modulation of Na+,K(+)-ATPase isoenzymes at the brain level.

Abnormal Erythrocyte and Renal Frusemide-Sensitive Sodium Transport in Idiopathic Calcium Nephrolithiasis

1. Anomalous transmembrane anion transport has been observed in erythrocytes of patients with idiopathic calcium nephrolithiasis. 2. To verify whether cation transport is also abnormal, we investigated the frusemide-sensitive Na+ efflux from Na(+)-loaded erythrocytes and the natriuretic response to acute intravenous frusemide administration in calcium oxalate renal stone formers. 3. Frusemide administration induced a statistically significant smaller increase in the fractional excretion of Na+ in patients than in control subjects. Abnormal kinetic properties of erythrocyte Na(+)-K(+)-2Cl- cotransport were observed in approximately 60% of stone formers. The Km for Na+ of Na(+)-K(+)-2Cl- co-transport correlated with urinary Ca2+ excretion. 4. The abnormal kinetic properties of Na(+)-K(+)-2Cl- co-transport may be relevant for stone formation, hampering renal Ca2+ reabsorption in the distal nephron and determining critical physicochemical conditions for calcium/oxalate crystallization.

Regulation of Na&lt;sup&gt;+&lt;/sup&gt; /H&lt;sup&gt;+&lt;/sup&gt; Exchange in Mesenteric Arteries From Spontaneously Hypertensive Rats

The kinetic properties of Na+/H+ exchange are significantly different in segments of mesenteric arteries from spontaneously hypertensive rats (SHR) as compared to normotensive Wistar-Kyoto (WKY) rats. Both the maximal rate of Na+/H+ exchange and the sensitivity of Na+/H+ exchange to intracellular [H+] are greater in vessels from hypertensive animals. Prehypertensive SHR do not exhibit these features, thus the present studies examined the role of blood pressure in mediating the response. Both SHR and WKY rats were treated with captopril for 4 weeks, after which the characteristics of the Na+/H+ exchanger were examined. The Na+/H+ exchange was monitored in tissue segments by measuring Nao-dependent, dimethyl amiloride-sensitive recovery of intracellular pH after acid loading (pHi determined using the pH-sensitive fluorescent dye 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein). In SHR, captopril reduced mean arterial pressure from 153 to 118 mm Hg and altered Na+/H+ exchange. Maximal transport rate decreased from 0.164 to 0.056 mmol/L H+/sec and the sensitivity of the Na+/H+ exchange to internal H+ decreased (ie, the Hill coefficient decreased from 6.6 to 2.2). In WKY rats captopril reduced blood pressure from 108 to 71 mm Hg, but had no effect on the Na+/H+ exchange. Since reduced pressure affected Na+/H+ exchange in SHR, but not in WKY, it is likely that the Na+/H+ exchanger, or the cellular processes regulating its properties, differ in the two rat strains. Supporting this notion was the finding that ouabain treatment reduced the Hill coefficient of SHR vessels but had no effect in WKY vessels.

Cardiovascular remodelling and red blood cell Li+/Na+ countertransport in patients with type 1 diabetes and essential hypertension

Patients with type 1 (insulin-dependent) diabetes may develop a specific cardiac disease characterized by functional and structural abnormalities. The pathogenesis of the cardiac disease is poorly understood but cardiac and renal complications may coexist. Patients with overt diabetic nephropathy have increased red cell Li+/Na+ countertransport (CT), which reflects abnormal kinetic properties of the red cell membrane Na+/H+ exchange. Since the activation of Na+/H+ exchange has a key role in cell proliferation and cell growth, as well as in the regulation of cell sodium and cell pH and in the renal reabsorption of Na+ and bicarbonate, we have looked for relationships between red cell Li+/Na+ CT, Na+/H+ exchange and cardiovascular remodeling in patients with type 1 diabetes, essential hypertension and idiopathic familiar cardiomyopathy. In type 1 diabetes the maximal velocity of Li+/Na+ CT is positively correlated with interventricular septum thickness and the left ventricular wall to lumen ratio. Similar results were obtained in patients with essential hypertension. In these patients an increased Li+/Na+ CT is also associated with severe and drug-resistant hypertension and with significant vascular remodelling, estimated by the minimal post-ischaemic vascular resistance in the calf. Finally, Li+/Na+ CT is negatively correlated with diastolic relaxation of the left ventricle in familiar hypertrophic cardiomyopathy. From these data it appears that widespread abnormal kinetic properties of Na+/H+ exchange, estimated by increased red cell Li+/Na+ CT, may have epistatic effects on the pathogenesis of cardiac complications of type 1 diabetes and essential hypertension.

Kinetic properties of Na+/H+ exchange in cultured bovine pigmented ciliary epithelial cells.

Uptake studies with 22Na were performed in cultured bovine pigmented ciliary epithelial cells, in order to characterize mechanisms of Na+ transport. A large part of Na+ uptake was sensitive to amiloride, quinidine and harmaline. Na+ uptake was stimulated by intracellular acidification (using the NH+4 prepulse technique), and was inhibited with increasing extracellular proton concentration. Decreasing extracellular pH from 7.5 to 7.0 increased the apparent KM for Na+ from 38 to 86 mM without considerable changes in Vmax. In the presence of 5 mM Na+ half maximal inhibition of amiloride sensitive Na+ uptake by extracellular protons was observed at a hydrogen concentration of 50 nM. In the presence of 50 mM Na+ the proton concentration necessary for 50% inhibition was 139 nM. Thus, the mode of inhibition of extracellular H+ seemed to be competitive with a Ki of 20-40 nM. 10 microM amiloride increased the apparent KM for Na+ from 33 mM to 107 mM, while Vmax remained nearly unchanged. IC50 for amiloride was 6 microM at 5 mM Na+ and 36 microM in the presence of 150 mM Na+. Thus, amiloride behaves as a competitive inhibitor with a Ki of about 5 microM. The affinities of Na+ to the transport site (KM approximately 16 mM), to the inhibitory site for protons (KM approximately 21 mM), and to the inhibitory site for amiloride (KM approximately 26 mM) were in the same order of magnitude. In summary, we have presented evidence for the presence of a Na+/H+ exchanger in cultured bovine pigmented ciliary epithelial cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Kinetic properties of Na +/Ca 2+ exchange in basolateral plasma membranes of rat small intestine

The presence of an Na +/Ca 2+ exchange system in basolateral plasma membranes from rat small intestinal epithelium has been demonstrated by studying Na + gradient-dependent Ca 2+ uptake and the inhibition of ATP-dependent Ca 2+ accumulation by Na +. The presence of 75 mM Na + in the uptake solution reduces ATP-dependent Ca 2+ transport by 45%, despite the fact that Na + does not affect Ca 2+-ATPase activity. Preincubation of the membrane vesicles with ouabain or monensin reduces the Na + inhibition of ATP-dependent Ca 2+ uptake to 20%, apparently by preventing accumulation of Na + in the vesicles realized by the Na +-pump. It was concluded that high intravesicular Na + competes with Ca 2+ for intravesicular Ca 2+ binding sites. In the presence of ouabain, the inhibition of ATP-dependent Ca 2+ transport shows a sigmoidal dependence on the Na + concentration, suggesting cooperative interaction between counter transport of at least two sodium ions for one calcium ion. The apparent affinity for Na + is between 15 and 20 mM. Uptake of Ca 2+ in the absence of ATP can be enhanced by an Na + gradient (Na + inside > Na + outside). This Na + gradient-dependent Ca 2+ uptake is further stimulated by an inside positive membrane potential but abolished by monensin. The apparent affinity for Ca 2+ of this system is below 1 μM. In contrast to the ATP-dependent Ca 2+ transport, there is no significant difference in Na + gradient-dependent Ca 2+ uptake between basolateral vesicles from duodenum, midjejunum and terminal ileum. In duodenum the activity of ATP-driven Ca 2+ uptake is 5-times greater than the Na +/Ca 2+ exchange capacity but in the ileum both systems are of equal potency. Furthermore, the Na +/Ca 2+ exchange mechanism is not subject to regulation by 1α,25-dihydroxy vitamin D-3, since repletion of vitamin D-deficient rats with this seco-steroid hormone does not influence the Na +/Ca 2+ exchange system while it doubles the ATP-driven Ca 2+ pump activity.