Decreased intracellular Na+ concentration is an early event in murine erythroleukemic cell differentiation

A decrease in Na+/K+-pump activity is an early event of Friend murine erythroleukemic (MEL) cell differentiation along the erythroid pathway. This decreased Na+/K+-pump activity has been proposed to be an essential step in differentiation which would cause a rise in intracellular Na+ concentration and then, by means of Na+/Ca2+ exchange, an increase in intracellular Ca2+. An increase in intracellular Ca2+ has been proposed to be essential for induction of differentiation. A critical prediction of this Na+-Ca2+ hypothesis is the rise in intracellular Na+. To test this prediction we have measured intracellular Na+ using a novel triple isotope method involving 3H2O, [14C]sucrose, and 22Na to measure total water, extracellular fluid, and Na+, respectively. 22Na equilibration occurred in less than 10 min. In uninduced cells, intracellular Na+ was 15.2 +/- 2.2 mM (S.D., n = 22); after induction for 14-16 h with dimethyl sulfoxide, intracellular Na+ decreased significantly (p less than 0.0001) to 8.4 +/- 1.4 mM (n = 21). The time course of the decline in intracellular Na+ paralleled that of the decrease in the Na+/K+-pump activity. These results are in direct contradiction to the Na+-Ca2+ hypothesis and suggest that observed changes in Na+/K+-pump activity can be explained solely on the basis of changes in intracellular Na+. The drop in intracellular Na+ is due to a decrease in Na+ influx. We suggest, however, that the decrease in the Na+ influx is not itself an essential event of differentiation, but may be induced by a change in the flux of another ion coupled to Na+.     

Effects of intracellular and extracellular concentrations of Ca2+, K+, and Cl- on the Na+-dependent Mg2+ efflux in rat ventricular myocytes

Intracellular Mg2+ concentration ([Mg2+]i) was measured in rat ventricular myocytes with the fluorescent indicator furaptra (25 degrees C). After the myocytes were loaded with Mg2+, the initial rate of decrease in [Mg2+]i (initial Delta[Mg2+]i/Deltat) was estimated upon introduction of extracellular Na+, as an index of the rate of Na+-dependent Mg2+ efflux. The initial Delta[Mg2+]i/Deltat values with 140 mM [Na+]o were essentially unchanged by the addition of extracellular Ca2+ up to 1 mM (107.3+/-8.7% of the control value measured at 0 mM [Ca2+]o in the presence of 0.1 mM EGTA, n=5). Intracellular loading of a Ca2+ chelator, either BAPTA or dimethyl BAPTA, by incubation with its acetoxymethyl ester form (5 microM for 3.5 h) did not significantly change the initial Delta[Mg2+]i/Deltat: 115.2+/-7.5% (seven BAPTA-loaded cells) and 109.5+/-10.9% (four dimethyl BAPTA loaded cells) of the control values measured in the absence of an intracellular chelator. Extracellular and/or intracellular concentrations of K+ and Cl- were modified under constant [Na+]o (70 mM), [Ca2+]o (0 mM with 0.1 mM EGTA), and membrane potential (-13 mV with the amphotericin-B-perforated patch-clamp technique). None of the following conditions significantly changed the initial Delta[Mg2+]i/Deltat: 1), changes in [K+]o between 0 mM and 75 mM (65.6+/-5.0% (n=11) and 79.0+/-6.0% (n=8), respectively, of the control values measured at 140 mM [Na+]o without any modification of extracellular and intracellular K+ and Cl-); 2), intracellular perfusion with K+-free (Cs+-substituted) solution from the patch pipette in combination with removal of extracellular K+ (77.7+/-8.2%, n=8); and 3), extracellular and intracellular perfusion with K+-free and Cl--free solutions (71.6+/-5.1%, n=5). These results suggest that Mg2+ is transported in exchange with Na+, but not with Ca2+, K+, or Cl-, in cardiac myocytes.     

Electrogenic sodium-sodium exchange carried out by Na,K-ATPase containing the amino acid substitution Glu779Ala

Na,K-ATPase containing the amino acid substitution glutamate to alanine at position 779 of the alpha subunit (Glu779Ala) supports a high level of Na-ATPase and electrogenic Na+-Na+ exchange activity in the absence of K+. In microsomal preparations of Glu779Ala enzyme, the Na+ concentration for half maximal activation of Na-ATPase activity was 161 +/- 14 mM (n = 3). Furthermore, enzyme activity with 800 mM Na+ was found to be similar in the presence and absence of 20 mM K+. These results showed that Na+, with low affinity, could stimulate enzyme turnover as effectively as K+. To gain further insight into the mechanism of this enzyme activity, HeLa cells expressing Glu779Ala enzyme were voltage clamped with patch electrodes containing 115 mM Na+ during superfusion in K+-free solutions. Electrogenic Na+-Na+ exchange was observed as an ouabain-inhibitable outward current whose amplitude was proportional to extracellular Na+ (Na+(o)) concentration. At all Na+(o) concentrations tested (3-148 mM), exchange current was maximal at negative membrane potentials (V(M)), but decreased as V(M) became more positive. Analyzing this current at each V(M) with a Hill equation showed that Na+-Na+ exchange had a high-affinity, low-capacity component with an apparent Na+(o) affinity at 0 mV (K0(0.5)) of 13.4 +/- 0.6 mM and a low-affinity, high-capacity component with a K0(0.5) of 120 +/- 13 mM (n = 17). Both high- and low-affinity exchange components were V(M) dependent, dissipating 30 +/- 3% and 82 +/- 6% (n = 17) of the membrane dielectric, respectively. The low-affinity, but not the high-affinity exchange component was inhibited with 2 mM free ADP in the patch electrode solution. These results suggest that the high-affinity component of electrogenic Na+-Na+ exchange could be explained by Na+(o) acting as a low-affinity K+ congener; however, the low-affinity component of electrogenic exchange appeared to be due to forward enzyme cycling activated by Na+(o) binding at a Na+-specific site deep in the membrane dielectric. A pseudo six-state model for the Na,K-ATPase was developed to simulate these data and the results of the accompanying paper (Peluffo, R.D., J.M. Argüello, and J.R. Berlin. 2000. J. Gen. Physiol. 116:47-59). This model showed that alterations in the kinetics of extracellular ion-dependent reactions alone could explain the effects of Glu779Ala substitution on the Na,K-ATPase.     

Activation of specific membrane mechanisms in the myocardium cells on early stages of ischemia

Electron probe microanalysis was employed to determine the elemental concentration (K,Na,Cl) in a myocyte on cryosections of the papillary muscle of the isolated rat (Wistar) heart. Protocols of global ischemia and ischemic conditions under glucose-free anoxic perfusion were applied. It was shown that global ischemia induces potassium deficiency (94 +/- 2 mM) in the myocyte and an increase in the level of sodium (72 +/- 4 mM) and chlorine (42 +/- 1 mM) in the cytoplasm compared with intact cell (122 +/- 2; 36 +/- 1; 24 +/- 1 mM). Glucose-free anoxic perfusion leads to a smooth fall of potassium concentration in the cell up to 54 +/- 2 mM with the retention of intracellular sodium (40 +/- 1 mM) and chlorine (26 +/- 1 mM) level. The present finding suggest that, in early ischemia, specific membrane mechanisms of ion transport are activated. Among these are KNa channel, Hi(+)-Nao+ exchange, KATP channel, lactate transport from the cell, associated either with potassium efflux to the extracellular space or chlorine influx into the myocyte. It is assumed that Na/K-ATPase is also activated under ischemic conditions.     

Postexercise rehydration: effect of Na(+) and volume on restoration of fluid spaces and cardiovascular function.

Our purpose was to study the interaction between Na(+) content and fluid volume on rehydration (RH) and restoration of fluid spaces and cardiovascular (CV) function. Ten men completed four trials in which they exercised in a 35 degrees C environment until dehydrated by 2. 9% body mass, were rehydrated for 180 min, and exercised for an additional 20 min. Four RH regimens were tested: low volume (100% fluid replacement)-low (25 mM) Na(+) (LL), low volume-high (50 mM) Na(+) (LH), high volume (150% fluid replacement)-low Na(+) (HL), and high volume-high Na(+) (HH). Blood and urine samples were collected and body mass was measured before and after exercise and every hour during RH. Before and after the dehydration exercise and during the 20 min of exercise after RH, cardiac output was measured. Fluid compartment (intracellular and extracellular) restoration and percent change in plasma volume were calculated using the Cl(-) and hematocrit/Hb methods, respectively. RH was greater (P < 0.05) in HL and HH (102.0 +/- 15.2 and 103.7 +/- 14.7%, respectively) than in LL and LH (70.7 +/- 10.5 and 75.9 +/- 6.3%, respectively). Intracellular RH was greater in HL (1.12 +/- 0.4 liters) than in all other conditions (0.83 +/- 0.3, 0.69 +/- 0.2, and 0.73 +/- 0.3 liter for LL, LH, and HH, respectively), whereas extracellular RH (including plasma volume) was greater in HL and HH (1.35 +/- 0.8 and 1.63 +/- 0.4 liters, respectively) than in LL and LH (0.83 +/- 0.3 and 1.05 +/- 0.4 liters, respectively). CV function (based on stroke volume, heart rate, and cardiac output) was restored equally in all conditions. These data indicate that greater RH can be achieved through larger volumes of fluid and is not affected by Na(+) content within the range tested. Higher Na(+) content favors extracellular fluid filling, whereas intracellular fluid benefits from higher volumes of fluid with lower Na(+). Alterations in Na(+) and/or volume within the range tested do not affect the degree of restoration of CV function.     

Evaluation of ouabain-insensitive red blood cell cation transport in uremic patients

The authors present the results of a simultaneous assay of: intracellular Na+ and K+ concentrations, Na+ and K+ outward bumetanide-sensitive effluxes (Na+, K+ cotransport), Na+ efflux stimulated by extracellular Li+ (Na+, Li+ countertransport), and ouabain- and bumetanide-resistant Na+ and K+ effluxes (passive membrane permeability) performed in red blood cells from 15 uremic patients an regular hemodialysis and from 12 normal subjects, with an established flux assay. Na+ and K+ effluxes by the Na+, K+ cotransport system were significantly (p less than 0.01) lower in uremic patients then in normals (219 +/- 37 vs 82 +/- 17 mumol/l RBC/h and 251 +/- 29 vs 139 +/- 14 mumol/l RBC/h respectively). In normal subjects the bumetanide sensitive Na+ and K+ effluxes were strongly (r = 0.89; p less than 0.01) intercorrelated; and the intracellular Na+ concentration was related to the outward Na+ cotransport flux (r = 0.53; p approximately 0.05). Among uremic patients these correlations were not found. Na+ and K+ intracellular concentrations, passive Na+ and K+ permeability, and Na+, Li+ countertransport activity were not different among uremic patients and normal controls. In conclusion, in uremic dialyzed patients, red blood cell Na+, K+ cotransport activity is quite uniformly suppressed. The possible pathogenesis of this disfunction is still speculative and deserves further studies.     

Two functionally different Na/K pumps in cardiac ventricular myocytes

The whole-cell patch-clamp technique was used to voltage clamp acutely isolated myocytes at -60 mV and study effects of ionic environment on Na/K pump activity. In quiescent guinea pig myocytes, normal intracellular Na+ is approximately 6 mM, which gives a total pump current of 0.25 +/- 0.09 pA/pF, and an inward background sodium current of 0.75 +/- 0.26 pA/pF. The average capacitance of a cell is 189 +/- 61 pF. Our main conclusion is the total Na/K pump current comprises currents from two different types of pumps, whose functional responses to the extracellular environment are different. Pump current was reversibly blocked with two affinities by extracellular dihydro-ouabain (DHO). We determined dissociation constants of 72 microM for low affinity (type-1) pumps and 0.75 microM for high affinity (type-h) pumps. These dissociation constants did not detectably change with two intracellular Na+ concentrations, one saturating and one near half- saturating, and with two extracellular K+ concentrations of 4.6 and 1.0 mM. Ion effects on type-h pumps were therefore measured using 5 microM DHO and on total pump current using 1 mM DHO. Extracellular K+ half- maximally activated the type-h pumps at 0.4 mM and the type-1 at 3.7 mM. Extracellular H+ blocked the type-1 pumps with half-maximal blockade at a pH of 7.71 whereas the type-h pumps were insensitive to extracellular pH. Both types of pumps responded similarly to changes in intracellular-Na+, with 9.6 mM causing half-maximal activation. Neither changes in intracellular pH between 6.0 and 7.2, nor concentrations of intracellular K+ of 140 mM or below, had any effect on either type of pump. The lack of any effect of intracellular K+ suggests the dissociation constants are in the molar range so this step in the pump cycle is not rate limiting under normal physiological conditions. Changes in intracellular-Na+ did not affect the half-maximal activation by extracellular K+, and vice versa. We found DHO-blockade of Na/K pump current in canine ventricular myocytes also occurred with two affinities, which are very similar to those from guinea pig myocytes or rat ventricular myocytes. In contrast, isolated canine Purkinje myocytes have predominantly the type-h pumps, insofar as DHO-blockade and extracellular K+ activation are much closer to our type-h results than type-1. These observations suggest for mammalian ventricular myocytes: (a) the presence of two types of Na/K pumps may be a general property. (b) Normal physiological variations in extracellular pH and K+ are important determinants of Na/K pump current. (c) Normal physiological variations in the intracellular environment affect Na/K pump current primarily via the Na+ concentration. Lastly, Na/K pump current appears to be specifically tailored for a tissue by expression of a mix of functionally different types of pumps.     

The Resting Potential of Mouse Leydig Cells: Role of an Electrogenic Na+/K+ Pump

Resting potentials (Vm) were measured in mouse Leydig cells, using the whole-cell patch-clamp technique. In contrast to conventional microelectrode measurements, where a biphasic potential was observed, we recorded a stable Vm around -32.2 +/- 1.2 mV (mean +/- SEM, n = 159), at 25 degrees C, and an input resistance larger than 2.7 x 109 W. Although Vm is sensitive to changes in the extracellular concentrations of potassium and chloride, the relationship between Vm and these ions' concentrations cannot be described by either the Goldman-Hodgkin-Katz or the Nernst equation. Perifusing cells with potassium-free solution or 10?3 M ouabain induced a marked depolarization averaging 20.1 +/- 3.2 mV (n = 9) and 23.1 +/- 2.8 mV, (n = 7), respectively. Removal of potassium or addition of ouabain with the cell voltage-clamped at its Vm, resulted in an inwardly directed current, due to inhibition of the Na+K+ATPase. The pump current increased with temperature with a Q10 coefficient of 2.3 and had an average value of -6.5 +/- 0.4 pA (n = 21) at 25 degrees C. Vm also varied strongly with temperature, reaching values as low as -9.2 +/- 1.2 mV (n = 22) at 15 degrees C. Taking the pump current at 25 degrees C and a minimum estimate for the membrane input resistance, we can see that the Na+K+ATPase could directly contribute with 17.7 mV to the Vm of Leydig cells, which is a major fraction of the ?32.2 +/- 1.2 mV (n = 159) observed.     

Permeation of Na+ through a delayed rectifier K+ channel in chick dorsal root ganglion neurons

In whole-cell patch clamp recordings from chick dorsal root ganglion neurons, removal of intracellular K+ resulted in the appearance of a large, voltage-dependent inward tail current (Icat). Icat was not Ca2+ dependent and was not blocked by Cd2+, but was blocked by Ba2+. The reversal potential for Icat shifted with the Nernst potential for [Na+]. The channel responsible for Icat had a cation permeability sequence of Na+ >> Li+ >> TMA+ > NMG+ (PX/PNa = 1:0.33:0.1:0) and was impermeable to Cl-. Addition of high intracellular concentrations of K+, Cs+, or Rb+ prevented the occurrence of Icat. Inhibition of Icat by intracellular K+ was voltage dependent, with an IC50 that ranged from 3.0-8.9 mM at membrane potentials between -50 and -110 mV. This voltage- dependent shift in IC50 (e-fold per 52 mV) is consistent with a single cation binding site approximately 50% of the distance into the membrane field. Icat displayed anomolous mole fraction behavior with respect to Na+ and K+; Icat was inhibited by 5 mM extracellular K+ in the presence of 160 mM Na+ and potentiated by equimolar substitution of 80 mM K+ for Na+. The percent inhibition produced by both extracellular and intracellular K+ at 5 mM was identical. Reversal potential measurements revealed that K+ was 65-105 times more permeant than Na+ through the Icat channel. Icat exhibited the same voltage and time dependence of inactivation, the same voltage dependence of activation, and the same macroscopic conductance as the delayed rectifier K+ current in these neurons. We conclude that Icat is a Na+ current that passes through a delayed rectifier K+ channel when intracellular K+ is reduced to below 30 mM. At intracellular K+ concentrations between 1 and 30 mM, PK/PNa remained constant while the conductance at -50 mV varied from 80 to 0% of maximum. These data suggest that the high selectivity of these channels for K+ over Na+ is due to the inability of Na+ to compete with K+ for an intracellular binding site, rather than a barrier that excludes Na+ from entry into the channel or a barrier such as a selectivity filter that prevents Na+ ions from passing through the channel.     

Active ion transport in the renal proximal tubule. II. Ionic dependence of the Na pump

The dependence of Na pump activity on intracellular and extracellular Na+ and K+ was investigated using a suspension of rabbit cortical tubules that contained mostly (86%) proximal tubules. The ouabain- sensitive rate of respiration (QO2) was used to measure the Na pump activity of intact tubules, and the Na,K-ATPase hydrolytic activity was measured using lysed proximal tubule membranes. The dependence (K0.5) of the Na pump on intracellular Na+ was affected by the relative intracellular concentration of K+, ranging from approximately 10 to 15 mM at low K+ and increasing to approximately 30 mM as the intracellular K+ was increased. The Na pump had a K0.5 for extracellular K+ of 1.3 mM in the presence of saturating concentrations of intracellular Na+. Measurements of the Na,K-ATPase activity under comparable conditions rendered similar values for the K0.5 of Na+ and K+. The Na pump activity in the intact tubules saturated as a function of extracellular Na at approximately 80 mM Na, with a K0.5 of 30 mM. Since Na pump activity under these conditions could be further stimulated by increasing Na+ entry with the cationophore nystatin, these values pertain to the Na+ entry step and not to the Na+ dependence of the intracellular Na+ site. When tubules were exposed to different extracellular K+ concentrations and the intracellular Na+ concentration was subsaturating, the Na pump had an apparent K0.5 of 0.4 mM for extracellular K. Under normal physiological conditions, the Na pump is unsaturated with respect to intracellular Na+, and indirect analysis suggests that the proximal cell may have an intracellular Na+ concentration of approximately 35 mM.     

Membrane permeability changes induce hyperpolarization in transformed lymphoid cells under high-density culture conditions

BACKGROUND: Membrane potential changes in cells from the human lymphoid B cell line, JY, evoked by increasing cell density in culture were investigated, as data published on other cell types are controversial. An attempt was also made to clear the underlying mechanism. METHODS: Nonadherent JY cells were isolated from high-density plateau-phase cultures (type A cells), medium-density log-phase cultures (type B cells), and low-density lag-phase cultures (type C cells). They were analyzed for transmembrane potential, intracellular free concentration of potassium and sodium, membrane permeability for monovalent cations, cell cycle distribution by measuring DNA content, and glucose uptake. RESULTS: C type cells proved to be relatively depolarized (-41 +/- 3 mV) and cells obtained from the highest density cultures hyperpolarized (-60 +/- 3 mV). Intracellular concentrations ([K](i) = 92-97 mM and [Na](i) = 34-35 mM) were almost identical for each type of cell. The sodium/potassium permeability constant ratio in the A and C type of cells was 0.047 and 0.094, respectively. High-density culture conditions resulted in a pronounced G(1)-phase arrest. CONCLUSIONS: Differences in the membrane potential values induced by high-density culture conditions were maintained by changes in the membrane permeability for the monovalent cations.     

Cooperative phenomena in the membrane potential of parathyroid cells induced by divalent cations

Membrane potentials of mouse parathyroid cells were measured by means of the intracellular microelectrode method. The membrane potential in external Krebs solution containing 2.5 mM of Ca++ was -23.6 +/- 0.4 mV (mean +/- standard error of mean). The low concentration of Ca++ (1.0 mM) caused hyperpolarization of the membrane potential to -61.7 +/- 0.8 mV. The membrane potential was proportional to the logarithm of the concentration of K ion in the solution of low Ca ion. The concentration of external Na+, C1- and HPO4-- had no effect on the membrane potential. The sigmoidal transition of membrane potentials was induced by the change of Ca ion concentration in the range from 2.5 to 1.0 mM. The change of the membrane potentials in low Ca ion is originated from increase in potassium permeability of the cell membrane. The similar sigmoidal changes of the membrane potentials were observed in the solution containing 4 to 3 mM of Sr ion. The Mg and Ba ion showed smaller effect on the membrane potential. The Goldman equation was extended to divalent ions. Appling the extended membrane potential equation, ratios of the permeability coefficients were obtained as follows: PK/PCa = 0.067 for 2.5 mM Ca++, 0.33 for 1.0 mM Ca++; PK/PSr = 0.08 for 4 mM Sr++ and 0.4 for 3 mM Sr++; PK/PMg = 0.5; PK/PBa = 0.67 for all range of concentration. The Hill constants of Sr ion and Ca ion were 20; the relationship between Sr ion and Ca ion was competitive. The Hill constants of Mg and Ba ion were 1 each. The Hill constant of Ca ion was depend of the temperature; nmax = 20 at 36 degrees C, n = 9 at 27 degrees C, n = 2 at 22 degrees C. The enthalpy of Ca-binding reaction was obtained from the Van't Hoff plot as 0.58 kcal. The activation energies of the K+ permeability increase were obtained from the Arrhenius plots as 3.3 kcal and 4 kcal. The difference, 0.7 kcal, corresponds to the enthalpy change of this reaction, of which value is close to that of the Ca-binding reaction.     

Insulin affects the sodium affinity of the rat adipocyte (Na+,K+)-ATPase

The K0.5 for intracellular sodium of the two forms of (Na+,K+)-ATPase which exist in rat adipocytes (Lytton, J., Lin, J. C., and Guidotti, G. (1985) J. Biol. Chem. 260, 1177-1184) has been determined by incubating the cells in the absence of potassium in buffers of varying sodium concentration; these conditions shut off the Na+ pump and allow sodium to equilibrate into the cell. The activity of Na+,K+)-ATPase was then monitored with 86Rb+/K+ pumping which was initiated by adding isotope and KCl to 5 mM, followed by a 3-min uptake period. Atomic absorption and 22Na+ tracer equilibration were used to determine the actual intracellular [Na+] under the different conditions. The K0.5 values thus obtained were 17 mM for alpha and 52 mM for alpha(+). Insulin treatment of rat adipocytes had no effect on the intracellular [Na+] nor on the Vmax of 86Rb+/K+ pumping, but did produce a shift in the sodium ion K0.5 values to 14 mM for alpha (p less than 0.025 versus control) and 33 mM for alpha(+) (p less than 0.005 versus control). This change in affinity can explain the selective stimulation of alpha(+) by insulin under normal incubation conditions. Measurement of the K0.5 for sodium ion of (Na+,K+)-ATPase in membranes isolated from adipocytes revealed only a single component of activation with a low K0.5 of 3.5 or 12 mM in the presence of 10 or 100 mM KCl, respectively. Insulin treatment of the isolated membranes or of the cells prior to membrane separation had no effect on these values.     

Intracellular activities of chloride, potassium and sodium ions in rabbit corneal epithelium

The mechanism of ion transport in the epithelium of rabbit cornea was studied by determining the intracellular ion activity of Cl-, Na+ and K+ under various conditions. Ionic activities were measured by means of microelectrodes containing liquid ion-exchangers selective for Cl-, Na+ or K+. The Cl- activity in basal cells of the epithelium in Na+ containing bathing solutions amounts to 28 +/- 2 mM (n = 11). This value is 1.9-times greater than expected on the basis of passive distribution across the tear side membrane. This finding suggests the existence of a Cl- accumulating process. Replacement of Na+ in the aqueous bathing solution by choline or tetraethylammonium results in a reversible decrease in Cl- activity to 22 +/- 1 mM (n = 11, P less than 0.025). The ratio of observed and predicted Cl- activity decreased significantly from 1.9 to 1.4 (P less than 0.05). The decrease in Cl- activity due to Na+ replacement was rather slow. In contrast, after readmittance of Na+ to the aqueous bathing solution, Cl- activity rose to a stable level within 30 min. These results indicate involvement of Na+ in Cl- accumulation into the basal cells of the epithelium. The K+ and Na+ activities of the basal cells of rabbit corneal epithelium in control bathing solutions were 75 +/- 4 mM (n = 13) and 24 +/- 3 mM (n = 12), respectively. The results can be summarized in the following model for Cl- transport across corneal epithelium. Cl- is accumulated in the basal cells across the aqueous side membrane, energized by a favourable Na+ gradient. Cl- will subsequently leak out across the tear side membranes. Na+ is extruded again across the aqueous side membrane of the epithelium by the (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)     

Permeability properties and intracellular ion concentrations of epithelial cells in rat duodenum.

Effects of the K+ concentration in the bathing fluid ([K+]l) on the intracellular K+, Na+ and Cl- concentrations ([K+]i [Na+]i and [Cl-]i) as well as on the electrical potential were studied in rat duodenum. Changes in the mucosal K+ concentration ([K+]m), bringing the sum of Na+ and K+ concentrations to 147.2 mM constant, had little effect on the transmural potential difference (PDt), but did induce marked changes in the mucosal membrane potential (Vm). As [K+]m increased, Vm was depolarized gradually and obeyed the Nernst equation for a potassium electrode in the range of [K+]m greater than approx. 60 mM. Experiments of ion analyses were carried out on strips of duodenum to determine the effect of changing the external K+ concentrations on [K+] i, [Na+]i and [Cl-]i. An increase in [K+]o resulted in increases in [K+]i and [Cl-]i and a decrease in [Na+]i, [K+]i approaching its maximum at [K+]o greater than 70 mM. Such changes in [K+]i and [Na+]i seem to correlate quantitatively with the changes in [K+]o and [Na+]o. The values of the ratio of permeability coefficients, Pna+/PK+ were estimated using the Vm values and intracellular ion concentrations measured in these experiments. The results suggested that there appeared a rather abrupt increase in the PNa+/PK+ ratio from 0 to approx. 0.1, as [K+]m decreased.     

Na,K pump stimulation by intracellular Na in isolated, intact sheep cardiac Purkinje fibers

Regulation of the Na,K pump in intact cells is strongly associated with the level of intracellular Na+. Experiments were carried out on intact, isolated sheep Purkinje strands at 37 degrees C. Membrane potential (Vm) was measured by an open-tipped glass electrode and intracellular Na+ activity (aNai) was calculated from the voltage difference between an Na+-selective microelectrode (ETH 227) and Vm. In some experiments, intracellular potassium (aiK) or chloride (aCli) was measured by a third separate microelectrode. Strands were loaded by Na,K pump inhibition produced by K+ removal and by increasing Na+ leak by removing Mg++ and lowering free Ca++ to 10(-8) M. Equilibrium with outside levels of Na+ was reached within 30-60 min. During sequential addition of 6 mM Mg++ and reduction of Na+ to 2.4 mM, the cells maintained a stable aNai ranging between 25 and 90 mM and Vm was -30.8 +/- 2.2 mV. The Na,K pump was reactivated with 30 mM Rb+ or K+. Vm increased over 50-60 s to -77.4 +/- 5.9 mV with Rb+ activation and to -66.0 +/- 7.7 mV with K+ activation. aiNa decreased in both cases to 0.5 +/- 0.2 mM in 5-15 min. The maximum rate of aiNa decline (maximum delta aNai/delta t) was the same with K+ and Rb+ at concentrations greater than 20 mM. The response was abolished by 10(-5) M acetylstrophantidin. Maximum delta aNai/delta t was independent of outside Na+, while aKi was negatively correlated with aNai (aKi = 88.4 - 0.86.aNai). aCli decreased by at most 3 mM during reactivation, which indicates that volume changes did not seriously affect aNai. This model provided a functional isolation of the Na,K pump, so that the relation between the pump rate (delta aNai/delta t) and aiNa could be examined. A Hill plot allowed calculation of Vmax ranging from 5.5 to 27 mM/min, which on average is equal to 25 pmol.cm-2.s-1.K 0.5 was 10.5 +/- 0.6 mM (the aNai that gives delta aNai/delta t = Vmax/2) and n equaled 1.94 +/- 0.13 (the Hill coefficient). These values were not different with K+ or Rb+ as an external activator. The number of ouabain-binding sites equaled 400 pmol.g-1, giving a maximum Na+ turnover of 300 s-1. The Na,K pump in intact Purkinje strands exhibited typical sigmoidal saturation kinetics with regard to aNai as described by the equation upsilon/Vmax = aNai(1.94)/(95.2 + aNai(1.94)). The maximum sensitivity of the Na,K pump to aiNa occurred at approximately 6 mM.     

Apical membrane potentials and intracellular potassium and sodium activities in the gallbladder of the freshwater turtle Mauremys caspica

Open-tip and liquid ion-exchanger microelectrodes were used to measure transapical membrane potential (Va), fractional voltage ratio (fa) and intracellular sodium and potassium activities (aiNa, aiK) in Mauremys caspica gallbladder under open circuit conditions. The average values (+/- SEM) for Va and fa were -32 +/- 3 and 0.20 +/- 0.03 mV respectively. aiNa and aiK were, respectively, 17 +/- 4 and 82 +/- 7 mM. These results suggest that the mechanisms of Na+ and K+ transport in this tissue are essentially similar to those observed in leaky epithelia in general.     

The effects of salt depletion on blood and tissue ion concentrations in the freshwater mussel,Ligumia subrostrata (Say)

Summary The extracellular and intracellular fluid volumes of pondwater acclimatedLigumia subrostrata are equal (3.9 ml/g dry tissue). Total blood solute is 47 mOsm and is composed primarily of Na (19.1 mM), Cl (10.6 mM), HCO₃ (12.7 mM), Ca (4.3 mM), and K (0.5 mM). Major intracellular solutes are K (14.0 mM), Na (7.0 mM) and Cl (2.4 mM).L. subrostrata continuously exposed to deionized water at 20°C exhibit a maximum decrease of 23% in extracellular fluid total solute within 30 days. The maximum [Na] and [Cl] losses are 40% and 76% respectively, while [Ca] and [HCO₃] increase by 44% and 37% respectively. No apparent change in extracellular [K] occurs. Intracellular [Na] decreases 53% and [Cl] decreases 79%, but [K] declines only 15%. Intracellular fluid volume, extracellular fluid volume, and total body water decrease 17%, 31%, and 22% respectively. Inulin clearance is 0.41 ml/g dry tissue·h for pondwater acclimated mussels and declines to 0.24 ml/g dry tissue·h during salt depletion. When salt depleted mussels are returned to solutions containing Na or Cl, they experience a net uptake of salt. The accumulated ions are about equally distributed in the extra- and intracellular compartments.     

Energetics of coupled Na+ and Cl- entry into epithelial cells of bullfrog small intestine.

Na+, K+ and Cl- concentrations (cij) and activities (aij), and mucosal membrane potentials (Em) were measured in epithelial cells of isolated bullfrog (Rana catesbeiana) small intestine. Segments of intestine were stripped of their external muscle layers, and bathed (at 25 degrees C and pH 7.2) in oxygenated Ringer solutions containing 105 mM Na+ and Cl- and 5.4 mM K+. Na+ and K+ concentrations were determined by atomic absorption spectrometry and Cl- concentrations by conductometric titration following extraction of the dried tissue with 0.1 M HNO3. 14C-labelled inulin was used to determine extracellular volume. Em was measured with conventional open tip microelectrodes, aiCl with solid-state Cl-selective silver microelectrodes and aiNa and aiK with Na+ and K+-selective liquid ion-exchanger microelectrodes. The average Em recorded was -34mV. ciNa, ciK and ciCl were 51, 105 and 52 mM. The corresponding values for aiNa, aiK and aiCl were 18, 80 and 33 mM. These results suggest that a large fraction of the cytoplasmic Na+ is 'bound' or sequestered in an osmotically inactive form, that all, or virtually all the cytoplasmic K+ behaves as if in free solution, and that there is probably some binding of cytoplasmic Cl-. aiCl significantly exceeds the level corresponding to electrochemical equilibrium across the mucosal and baso-lateral cell membranes. Earlier studies showed that coupled mucosal entry of Na+ and Cl- is implicated in intracellular Cl- accumulation in this tissue. This study permitted estimation of the steady-state transapical Na+ and Cl- electrochemical potential differences (deltamuNa and deltamuCl). deltamuNa (-7000 J . mol-1; cell minus mucosal medium) was energetically more than sufficient to account for deltamuCl (1000--2000 J . mol-1).