Electrolyte- and fluid-spaces of rat brain in situ after infusion with dinitrophenol

Chemical distribution measurements of radioactive sodium-thiosulfate (³⁵S) and of the brain water indicate that infusion of 2.4-dinitrophenol into a carotid artery of rats caused a water uptake and fluid shifts from the extra- into the intracellular compartments in the central nervous system. The extracellular marker compound was administered to the brain via ventriculo-cisternal perfusion and intravenous injection yielding almost equal concentrations in plasma- water and perfusate. In order to prevent an active efflux of the label from the tissue, high concentrations were utilized in the perfusate to saturate potential outward transport mechanisms. The indicator space (based on total brain water) was 16% in controls and 12% in experimental animals when marker equilibrium had been attained, which is equivalent in reduction of the extracellular space of about 1/4. Intracellular water and Na⁺ rose after DNP, while K⁺ remained all but unchanged. The fluid shift into the intracellular compartment was found to relate closely with a cellular uptake of Na⁺. The Na⁺ concentration both in plasma and in the perfusion fluid leaving the ventricular system was consistently reduced in experimental animals. The K⁺ concentration was significantly elevated in the plasma of experimental animals but virtually unchanged in the cisternal effluate.     

Effect of colchicine on estrogen action. I. Inhibition of 17 beta-estradiol-induced water and potassium uptake in the immature rat uterus

The effects of colchicine on 17 beta-estradiol-induced water and electrolyte uptake in the uterus of the immature rat have been examined 6 h after treatment with this estrogen. Estradiol stimulates an increase in total uterine Na+, K+ and water while intracellular Na+ and K+ concentrations remain relatively unchanged. Assuming the sodium space is equivalent to the extracellular space, the extracellular fluid compartment increases about 84% in response to estradiol. Similarly, the intracellular compartment increases by about 62%. The uptake of water into the cellular compartment may be a direct response to a stimulation of K+ accumulation by uterine cells. Colchicine inhibits both estradiol-induced rise in intracellular potassium and both intra- and extracellular water.     

New evidence for active sodium transport from fluid-filled rat lungs

The hypothesis that fluid reabsorption from the air spaces is mediated at least in part by active transport of Na+ was investigated in six sets of experiments conducted in isolated fluid-filled rat lungs. Fluid reabsorption was monitored by following the changes in the air space concentration of labeled albumin. We found that incorporation of bicarbonate rather than a nonvolatile buffer (N-2-hydroxy-ethylpiperazine-N'-2-ethanesulfonic acid) in the air space solution more than doubled the rate of fluid reabsorption. Addition of 10(-4) M amiloride to the air space solution reduced the rate of fluid reabsorption over a 2-h experiment from 1.2 +/- 0.1 to 0.7 +/- 0.1 ml and decreased reabsorption of both labeled and unlabeled Na+ from the air spaces. To show that Na+ could be reabsorbed from the air spaces even if the concentrations of Na+ in the perfusate increased above those in the air space, mannitol (150 mM) was added to the perfusate and air space solutions and the concentrations of Na+ and Cl- were reduced to 90 and 60 mM, respectively. Mannitol diffuses across the pulmonary epithelium very slowly, and it osmotically restrained the movement of water out of the air spaces. Na+ concentrations in the perfusate increased by 10 +/- 2 mM, but concentrations in the air space remained unchanged. Despite an increasingly unfavorable concentration gradient for Na+, 0.2 mmol Na+ and 0.6 ml water were reabsorbed from the air spaces in 2 h. Ouabain (10(-4) M) did not appear to slow fluid reabsorption in the presence of mannitol, but it reduced K+ secretion into the air spaces and increased K+ appearance in the perfusate in a manner consistent with inhibition of Na+-K+-adenosinetriphosphatase at the basolateral surface of the epithelial cells. Fluid reabsorption was not altered when the lungs were exposed to a hypotonic solution (185 mM), but secretion of K+ into the air spaces was accelerated and K+ was lost from the perfusate. These experiments are consistent with active Na+ transport from the air spaces.     

Relationship of hepatic cholate transport to regulation of intracellular pH and potassium.

Modulation of hepatic cholate transport by transmembrane pH-gradients and during interferences with the homeostatic regulation of intracellular pH and K+ was studied in the isolated perfused rat liver. Within the concentration range studied uptake into the liver was saturable and appeared to be associated with release of OH- and uptake of K+. Perfusate acidification ineffectually stimulated uptake. Application of NH4Cl caused intracellular alkalinization, release of K+ and stimulation of cholate uptake, withdrawal of NH4Cl resulted in intracellular acidification, regain of K+ and inhibition of cholate uptake. Inhibition of Na+/H(+)-exchange with amiloride reduced basal release of acid equivalents into the perfusate, initiated K(+)-release, and inhibited both, control cholate uptake and its recovery following intracellular acidification. K(+)-free perfusion caused K(+)-release and inhibited cholate uptake. K(+)-readmission resulted in brisk K(+)-uptake and recovery of cholate transport. Both effects were inhibited by amiloride. Interference with cholate transport through modulation of pH homeostasis by diisothiocyanostilbenedisulfonate (DIDS) could not be demonstrated because DIDS affected bile acid transport directly. Biliary bile acid secretion was stimulated by intracellular alkalinization and by activation of K(+)-transport. Uncoupling of the mutual interference between pH-dependent cholate uptake and K(+)-transport by amiloride indicates tertiary active transport of cholate. In this, Na+/K(+)-ATPase provides the transmembrane Na(+)-gradient to sustain Na+/H(+)-exchange which maintains the transmembrane pH-gradient and thus supports cholate uptake. Effects of canalicular bile acid secretion are consistent with a saturable, electrogenic transport.     

The volume and ionic composition of cells in incubated slices of rat renal cortex, medulla and papilla

The apparent extracellular space in incubated slices of rat renal cortex, medulla and papilla has been measured using three differently sized marker molecules, mannitol, sucrose and inulin. Cellular volumes have been estimated by following the efflux of 3-O-methyl-D-glucose from equilibrated slices. Sucrose appears to be the most accurate extracellular marker in each of the regions examined, in that the sum of its volume of distribution plus cellular volume approximates most closely to the total slice fluid volume. Inulin has the same volume of distribution as sucrose in cortical slices, but under-penetrates medullary and papillary tissue. Mannitol overestimates the extracellular space in all three regions, although its larger volume of distribution, relative to that of sucrose, was not statistically significant in papillary slices. When cell volume and composition are estimated (a) using sucrose as extracellular marker and (b) making appropriate allowance for the presence of bound tissue electrolytes, it is found that cells in each region have low Na+ and high K+ concentrations and contents. When papillary slices are incubated in medium of very high osmolality (NaCl plus urea, 2000 mosmol/kg H2O) there is a moderate (approx. 23%) decrease in cell volume and an increase in cell fluid Na+ and Cl- concentrations equal to approx. 50% of the increase in the extracellular concentrations. Cell K+ concentrations remain unchanged. The results show that cells in renal slices are able to maintain high K+-to-Na+ ratios when incubated in isosmotic (cortex) or moderately hyperosmotic media (medulla and papilla), and suggest that regulation of papillary cell volume following hyperosmotic shock can only partly be ascribed to uptake of extracellular electrolytes.     

Movement of ions and small solutes across endothelium and epithelium of perfused rabbit lungs

In situ and isolated fluid-filled rabbit lungs were used to study the transport of indicators between the air space and vascular compartments. These indicators were placed in either the perfusate or air spaces and samples were collected from the perfusate at intervals during a 1-h perfusion period. At the end of the hour, fluid was pumped out of the air space compartment into serial tubes and indicator concentrations were determined in both the air space and perfusion fluids. One hour after introducing the indicators into the air space, the relative decreases in solute concentration were (arranged from the greatest to the least decline): [14C]urea greater than 36Cl- = 125I- greater than 22Na+ greater than [3H]mannitol. The relative rates at which the indicators appeared in the perfusate were similar. When the indicators were placed in the perfusate, a similar relationship was observed in the increase in air space concentrations, but the loss of 22Na+ from the perfusate was similar to those of 36Cl- and 125I-. Losses of all indicators from the perfusate were two or more times those from the air spaces, and although the loss of [3H]mannitol from the perfusate was similar to that of 22Na+ for about 30 min, subsequent loss was much slower. Very little 125I-albumin traversed the tissue barrier, and the small changes in the concentrations of 125I-albumin in the air spaces suggested that little fluid movement had occurred. These studies suggest that the epithelium is less permeable to solutes than the endothelium and permits passage of anions at a faster rate than 22Na+.(ABSTRACT TRUNCATED AT 250 WORDS)     

Are large amounts of sodium stored in an osmotically inactive form during sodium retention? Balance studies in freely moving dogs

Alterations in total body sodium (TBSodium) that covered the range from moderate deficit to large surplus were induced by 10 experimental protocols in 66 dogs to study whether large amounts of Na+ are stored in an osmotically inactive form during Na+ retention. Changes in TBSodium, total body potassium (TBPotassium), and total body water (TBWater) were determined by 4-day balance studies. A rather close correlation was found between individual changes in TBSodium and those in TBWater (r2 = 0.83). Changes in TBSodium were often accompanied by changes in TBPotassium. Taking changes of both TBSodium and TBPotassium into account, the correlation with TBWater changes became very close (r2 = 0.93). The sum of changes in TBSodium and TBPotassium was accompanied by osmotically adequate TBWater changes, and plasma osmolality remained unchanged. Calculations reveal that even moderate TBSodium changes often included substantial Na+/K+ exchanges between extracellular and cellular space. The results support the theory that osmocontrol effectively adjusts TBWater to the body's present content of the major cations, Na+ and K+, and do not support the notion that, during Na+ retention, large portions of Na+ are stored in an osmotically inactive form. Furthermore, the finding that TBSodium changes are often accompanied by TBPotassium changes and also include Na+/K+ redistributions between fluid compartments suggests that cells may serve as readily available Na+ store. This Na+ storage, however, is osmotically active, since osmotical equilibration is achieved by opposite redistribution of K+.     

Effect of saline acclimation on body water and sodium compartmentalization in Pekin ducks (Anas platyrhynchos)

The compartmentalization of body fluids was measured in individual Pekin ducks ( Anas platyrhynchos) drinking freshwater and after sequential acclimation to 300 mM NaCl and 400 mM NaCl. Total body water, extracellular fluid volume, plasma volume and exchangeable sodium pool were measured using (3)H(2)O, [(14)C]-polyethylene glycol, Evans Blue dye, and (22)Na dilution, respectively. Following acclimation to 300 mM NaCl, body mass decreased, but total body water and total exchangeable sodium pool were unaltered. Na and water were redistributed from the extracellular fluid (interstitial fluid) compartment into the intracellular fluid compartment. Following further acclimation to 400 mM NaCl, body mass, total body water and intracellular fluid volume decreased, but exchangeable sodium pool and extracellular fluid volume were unchanged. Our results suggested that, when Pekin ducks drink high but tolerable salinities, they maintain total body water, but redistribute Na(+) and water from interstitial fluid to the intracellular fluid compartment. When stressed beyond their ability to maintain total body water, they lose water from the intracellular fluid.     

Na+ effects on intracellular pH of isolated, perfused rabbit gastric mucosal surface cells.

The effects of extracellular Na+ on intracellular pH were studied by perfusing BCECF loaded gastric mucosal surface cells adherent to glass coverslips held in a spectrophotofluorometer. Removal of Na+ from a NaCl Ringer perfusate (pH 7.4) resulted in progressive intracellular acidification, which was partially blocked by amiloride. An H+ conductance did not appear to be present. Acidification induced either by Na+ removal or by a NH4 prepulse was reversed by extracellular Na+, but this effect was not completely prevented by amiloride. Amiloride significantly, but not completely, inhibited Na22 uptake by gastric mucosal surface cells. The data suggest that extracellular Na+ maintains intracellular pH of gastric mucosal surface cells through amiloride-sensitive and -insensitive pathways. In the absence of extracellular Na+, cellular acidification seemed to be partially due to Na+/H+ exchange.     

Effect of extracellular volume expansion on erythrocyte sodium transport in rats.

The effects of extracellular volume expansion (EVE) on the major sodium transport systems and sodium and potassium contents in rat erythrocytes have been examined in the present study. Study has been performed in anesthetized Wistar rat weighing about 300 g. Acute extracellular volume expansion (EVE) was induced by a constant intravenous saline infusion (3% body wt, 3 hours). Rats anaesthetized and catheterized but not expanded were used as controls. Arterial blood samples from control and expanded rats were obtained at the same time, and assayed immediately. Intracellular sodium and potassium concentration and ouabain sensitive (Na(+)-K(+)-pump) and bumetanide sensitive (Na(+)-K(+)-cotransport system) outward Na+ fluxes in erythrocytes were measured. The effect of plasma on erythrocyte transport was also analyzed by measuring 86Rb uptake. Neither of two plasma cations (Na+ and K+) were modified by the EVE. Also intracellular Na+ and K+ levels remained unvariable. Total Na+ efflux was not modified by EVE, but pump-mediated Na+ efflux was smaller after than before EVE. The ouabain-inhibible Na+ efflux rate constant decreased after EVE (from 687 +/- 81 to 525 +/- 29 h-1 x 10(-3); P less than 0.05). Both Na(+)-K(+)cotransport-mediated Na+ efflux and passive permeability increased significantly after EVE. The incubation with plasma from saline-infused animals induced a significant decrease in Rb uptake rate constant, that was not observed after incubation with plasma from non-expanded rats.(ABSTRACT TRUNCATED AT 250 WORDS)     

Training increases the concentration of [3H]ouabain-binding sites in rat skeletal muscle

Exercise is associated with a net loss of K+ from the working muscles and an increased plasma K+ concentration, indicating that the capacity for intracellular reaccumulation of K+ is exceeded. Training reduces the exercise-induced rise in plasma K+, and an increased plasma [K+] may interfere with physical performance. Since the clearing of K+ from the extracellular space depends on the capacity for active K+ uptake in skeletal muscle, the effects of training and inactivity on the total concentration of (Na+ + K+)-ATPase was determined. Following 6 weeks of swim training, the concentration of [3H]ouabain-binding sites in rat hindlimb muscles was up to 46% (P less than 0.001) higher than in those obtained from age-matched controls. Whereas muscle Na+, K+ contents remained unchanged, the concentration of citrate synthase increased by up to 76% (P less than 0.001). Training induced no change in the [3H]ouabain-binding-site concentration in the diaphragm, but in the heart ventricles, the K+-dependent 3-O-methylfluorescein phosphatase activity increased by 20% (P less than 0.001). Muscle inactivity induced by denervation, plaster immobilisation or tenotomy reduced the [3H]ouabain-binding-site concentration by 20-30% (P less than 0.02-0.001) within 1 week. In conclusion, training leads to a significant and reversible rise in the concentration of (Na+ + K+)-ATPase in muscle cells. This may be of importance for the beneficial effects on physical performance by improving the maximum capacity for K+ clearance.     

Secretion of saliva by the rabbit mandibular gland in vitro: the role of anions

Salivary glands form their secretions by first elaborating an isotonic plasma-like primary fluid in the endpieces and then modifying the composition of this secretion during its passage along the gland duct system. We have studied the role of extracellular anions in both primary secretion and ductal modification with a recently developed technique for isolation and perfusion of the rabbit mandibular gland. Neither of the major extracellular anions (Cl- or HCO-3) is essential for primary fluid secretion. HCO-3 can be removed altogether and replaced with Cl- without diminution in secretory rate, provided that extracellular pH is maintained at 7.4, and its replacement with acetate actually enhances secretion. Complete replacement of Cl- with Br- also enhances secretion and replacement with I-, NO-3, CH3SO-4 or isethionate supports secretion but at progressively diminishing rates. Our data do not yet allow us to distinguish between an electroneutral Na+-Cl- cotransport model or a double countertransport (Na+-H+ plus Cl--HCO-3) model as the basis of primary salivary secretion, or to propose any more suitable alternative model. With respect to ductal modification of the primary saliva, HCO-3 omission inhibits ductal Na+ absorption (i.e. salivary Na+ concentration rises). This inhibition is probably related to an effect of pH on the postulated Na+-H+ exchanges mechanism in the luminal duct membrane since it can also be induced by lowering perfusate pH, and reversed by substitution of perfusate HCO-3 with acetate (which enters saliva) but not HEPES (which does not enter the saliva). Substitution of perfusate Cl- with other anions seems not to inhibit ductal Na+ and K+ transport markedly.     

Transmembrane potential and ionic content of rat alveolar macrophages

The cell volume, cell water, intracellular ionic concentrations, and transmembrane potential of rat alveolar macrophages were determined. The measurements were made on cells which had been separated from the medium by centrifugation through dibutyl phthalate in order to greatly reduce the trapped extracellular space. The mean cell volume of the alveolar macrophages is 1,525 cubic microns and 72% of this volume is water. The intracellular fluid is high in Na+ (97 mM) and lower in K+ (50 mM) and the intracellular Cl- concentration in 64 mM. The transmembrane potential, as measured from the equilibrium distribution of tritiated triphenylmethyl phosphonium and by using the fluorescent probe, Di-S-C3(5), is approximately -37 millivolts. Neither Na+, K+, nor Cl- is distributed at equilibrium. However, the K+ permeability of alveolar macrophage membranes appears to be greater than Na+ permeability.     

Interactions between glutamine metabolism and cell-volume regulation in perfused rat liver

1. In the presence of near-physiological glutamine concentrations, exposure of perfused rat liver to hypotonic perfusion media switched glutamine balance across the liver from net release to net uptake. This was due to both stimulation of flux through glutaminase and inhibition of flux through glutamine synthetase. Conversely, during exposure to hypertonic media, net glutamine release from the liver increased due to inhibition of glutaminase flux and slight stimulation of flux through glutamine synthetase. The effect of perfusate osmolarity on glutaminase flux was observed at an NH4Cl concentration (0.5 mM) sufficient for near-maximal ammonia stimulation of glutaminase. This indicates the involvement of different mechanisms of glutaminase flux control by extracellular osmolarity changes and ammonia. The effects of anisotonicity on flux through glutamine-metabolizing enzymes were fully reversible. Glutamine (0.6 mM) stimulated urea synthesis from NH4Cl (0.5 mM) during hypotonic and normotonic conditions. 2. Exposure to hypotonic and hypertonic media led, after initial liver-cell swelling and shrinkage, respectively to volume-regulatory K+ fluxes which largely restored the initial liver-cell volume despite the continuing osmotic challenge. Even after completion of cell-volume regulatory K+ fluxes, the effects of perfusate osmolarity on hepatic glutamine metabolism persisted. This indicates that in anisotonicity the liver cell is left in an altered metabolic state, even after completion of volume-regulatory responses. 3. During perfusion with isotonic media, addition of glutamine (3 mM) led to an increase of liver mass by about 4% within 2 min, which was accompanied by a net K+ uptake by the liver. Thereafter, the new steady state of increased liver mass was maintained throughout glutamine infusion. When the liver mass had reached this new steady state, a net release of K+ from the liver of about 3 mumol/g liver was observed during the following 10 min. Withdrawal of glutamine was followed by a slow reuptake of K+ and the liver mass returned to its initial value. Following exposure to glutamine (3 mM), the intracellular glutamine concentration (as calculated from glutamine tissue levels, taking into account the extracellular space determined with the [3H]inulin technique) rose from about 1 mM to 30-35 mM within about 12 min, indicating a 10-12-fold concentrative uptake of glutamine into the liver cells and an osmotic challenge for the hepatocyte. When intracellular glutamine had reached its steady-state concentration, net K+ efflux from the liver was also terminated.(ABSTRACT TRUNCATED AT 400 WORDS)     

Water and electrolyte shifts with partial fluid replacement during exercise

In this study, we examined whether athletes, who typically replace only approximately 50% of their fluid losses during moderate-duration endurance exercise, should attempt to replace their Na+ losses to maintain extracellular fluid volume. Six male cyclists performed three 90-min rides at 65% of peak O2 uptake in a 32 degrees C environment and ingested either no fluid (NF), 1.21 of water (W), or saline (S) containing 100 mmol of NaCl x l(-1) to replace their electrolyte losses. Both W and S conditions decreased final heart rates by approximately 10 betas min(-1) (P<0.005) and reduced falls in plasma volume (PV) by approximately 4% (P<0.05). Maintenance of PV after 10 min in the W trial prevented further rises in plasma concentrations of Na+ [Na+], Cl- and protein but in the S and NF trials, plasma [Na+] continued to increase by approximately 4 mEq x l(-1). Differences in plasma [Na+] had little effect on the approximately 2.4 l fluid, approximately 120 mEq Na+ and approximately 50 mEq K+ losses in sweat and urine in the three trials. The main effects of W and S were on body fluid shifts. During the NF trial, PV and interstitial fluid (ISF) and intracellular fluid (ICF) volumes decreased by approximately 0.1, 1.2 and 1.0 l, respectively. In the W trial, the approximately 1.2 l fluid and approximately 120 mEq Na+ losses contracted the ISF volume, and in the S trial, ISF volume was maintained by the movement of water from the ICF. Since the W and S trials were equally effective in maintaining PV, Na+ ingestion may not be of much advantage to athletes who typically replace only approximately 50% of their fluid losses during competitive endurance exercise.     

23Na NMR studies of rat outer medullary kidney tubules

Two reservations have previously made interpretation of biological 23Na NMR measurements difficult: the "size" of the extracellular space penetrated by the shift reagent and the possibility of a 60% reduction in the intensity of the NMR-visible 23Na signal due to quadrupolar interactions (Berendsen, H. J. C., and Edzes, H. T. (1973) Ann. N. Y. Acad. Sci. 204, 459-485; Civan, M. M., Degani, H., Margalit, Y., and Shporer, M. (1983) Am. J. Physiol. 245, C213-C219; Gupta, R. K., and Gupta, P. (1982) J. Magn. Reson. 47, 344-350). We have addressed both these issues using a suspension of rat outer medullary kidney tubules, nephron segments responsible for the fine control of total body volume and electrolyte balance. First, the extracellular space penetrated by the shift reagent dysprosium tripolyphosphate, as defined by the extracellular 23Na resonance, revealed a space similar to that which contained extracellular 35Cl- ions. Measurement of an extracellular 35Cl- space using 35Cl NMR was possible because the intracellular 35Cl- resonance was broadened beyond detection in the cells studied. Second, to characterize the reduction of the 23Na signal by quadrupolar interactions, the intracellular 23Na level was raised artificially by simultaneously inhibiting Na+ efflux and increasing the ion permeability of the plasma membrane. Under these conditions, NMR-observable intracellular Na+ reached a level which was approximately 81% of that in the medium, a level determined using chemical techniques. This observation would suggest that the resonance of the intracellular 23Na pool was not subject to a 60% reduction in signal intensity, as a result of nuclear quadrupolar interaction. The intracellular 23Na level measured, under basal conditions, was 23 +/- 2 mumol/ml of cell water (37 degrees C) (n = 3, S.D.) and was demonstrated to be responsive to a number of physiological stimuli. The level was temperature-sensitive. It was reduced by inhibitors of apical Na+ transport, furosemide and amiloride, and it was raised with (Na+ + K+)-ATPase inhibition. The furosemide and amiloride actions described would suggest that the Na+-transporting mechanisms sensitive to these agents (e.g. Na+/K+/Cl- cotransport system, Na+:H+ exchange system) contribute to the regulation of the intracellular Na+ level in the kidney tubular preparation studied.     

Electrophysiological characterization of ionic transport by the retinal exchanger expressed in human embryonic kidney cells.

The retinal Na+:Ca2+, K+exchanger cDNA was transiently expressed in human embryonic kidney (HEK 293) cells by transfection with plasmid DNA. The correct targeting of the expressed protein to the plasma membrane was confirmed by immunocytochemistry. The reverse exchange offrent (Ca2+ imported per Na+ extruded) was measured in whole-cell voltage-clamp experiments after intracellular perfusion with Na+ (Na+i, 128 mM) and extracellular perfusion with Ca2+ (Ca2o+, 1 mM) and Ko+ (20 mM). As expected, the exchange current was suppressed by removing Ca2o+. Surprisingly, however, it was also abolished by increasing Na+o to almost abolish the Na+ gradient, and it was almost unaffected by the removal of Ko+. Apparently, then, at variance with the exchanger in the rod outer segment, the retinal exchanger expressed in 293 cells acts essentially as a Na+:Ca2+ exchanger and does not require K+ for its electrogenic activity.     

Kinetic analysis of Na,K-activated adenosine triphosphatase induced by low external K+ in a rat liver cell line

Exposure of ARL 15 cells to medium containing reduced concentrations of K+ (0.65 mM) elicited a 50-100% increase in Na,K-ATPase activity. The inhibition by ouabain of both the basal and the induced enzyme conformed to a single-site model (KI = 1 x 10(-4) M). The low K+-induced increment in Na,K-ATPase activity was accompanied by an equivalent increase in the abundance of Na,K-pump sites estimated by ouabain-stabilized ("back-door") phosphorylation, such that the calculated catalytic turnover number of approximately 8000/min was minimally changed. Comparison of the dependence of ouabain-inhibitable K+ uptake on intracellular Na+ and on extracellular K+ concentrations in control and low K+-treated cells revealed no change in the respective half-maximal stimulatory concentrations for these cations, whereas the maximal rate of active K+ uptake in cells exposed to low external K+ increased by nearly 100%. The derived Hill coefficients for active K+ transport rate were also unchanged by the low K+ treatment (i.e. approximately 1.4 for extracellular K+ and 2.6 for intracellular Na+). Na,K-ATPase activity of basal and low K+-induced cells calculated from the measured maximal Na,K transport rate closely approximated the Na,K-ATPase activity measured enzymatically in unfractionated cell lysates under Vmax conditions, suggesting that all or most of the Na,K-ATPase enzymatic units present in both basal and stimulated states are functionally active. Northern blot analysis of RNA isolated from control cells indicated the presence of the Na,K-ATPase alpha-I isoform of the enzyme which increased by nearly 200% following incubation of the cells in low-K+ medium. By contrast, the alpha-II and alpha-III mRNAs were undetectable in either the basal or low K+-stimulated state. These results indicate that the Na,K-ATPase induced by incubation of ARL 15 cells in low-K+ medium is kinetically and functionally indistinguishable from the basal enzyme, and that only the alpha-I isoform is expressed under control and low-K+ conditions.     

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.     

Hyponatremia-induced brain edema in guinea pigs is reduced by treatment with the novel anion transport inhibitor L-644,711

Cerebral edema in various disease states may result from astroglial swelling due to increased NaCl uptake mediated by enhanced Cl-HC03 exchange. We evaluated this mechanism in the pathogenesis of cerebral edema in acute hyponatremia by administering L-644,711, a fluorenyloxyacetate derivative that functions as an anion exchange inhibitor, to guinea pigs with severe reductions in serum Na+ concentration. Acute hyponatremia was induced for 54 hr by daily injections of arginine vasopressin (10 U/day) and 5% dextrose in water (7.5% body wt/day). Experimental animals received L-644,711, 20 mg/kg/day, while controls were given an equal volume of the diluent. This regimen lowered the serum Na from normal levels to 108 +/- 3 and 109 +/- 4 mM in experimental and control animals, respectively. Drug treatment resulted in less cerebral edema characterized by a reduction in brain total tissue water 432 +/- 4 vs 466 +/- 8 ml/100 g dry wt experimental vs control, P less than 0.005. This difference was composed mainly of less expansion of the intracellular water space, 287 +/- 11 vs 323 +/- 9 ml/100 g dry wt experimental vs control, p less than 0.005. The cerebral cortical Na+ +Cl content was reduced from 55.5 +/- 1.3 (control) to 39.5 +/- 1.1 mEq/100 g dry wt (experimental), p less than 0.01. These results indicate that treatment of guinea pigs with L-644,711 decreases brain NaCl content and attenuates cerebral edema during severe acute hyponatremia without normalizing the serum Na+ concentration.