Magnesium sulfate affords protection against oxidative damage during severe preeclampsia

INTRODUCTION

MgSO4 is the drug of choice to prevent seizures in preeclamptic pregnant women, but its mechanism of action at the molecular level remains an enigma. In previous works, we found that treating preeclamptic women with MgSO4 reduces the lipid peroxidation of their red blood cell membranes to normal levels and leads to a significant reduction in the osmotic fragility of the red blood cells that is increased during preeclampsia. In addition, the increase in lipid peroxidation of red cell membranes induced by the Fenton reaction does not occur when MgSO4 is present.

METHODS

The antioxidant protection of MgSO4 was evaluated in UV-C-treated red blood cell ghosts and syncytiotrophoblast plasma membranes by measuring their level of lipid peroxidation. The interaction of MgSO4 with free radicals was assessed for its association with the galvinoxyl radical, the quenching of H2O2-induced chemiluminescence and its effect on sensitized peroxidation of linoleic acid.

RESULTS

a) MgSO4 protected red blood cell ghosts and the syncytiotrophoblast plasma membranes of normotensive pregnant women against lipid peroxidation induced by UV-C irradiation. b) MgSO4 does not seem to scavenge the galvinoxyl free radical. c) The quenching of the H2O2-enhanced luminol chemiluminescence is increased by the presence of MgSO4. d) The peroxidation of linoleic acid is significantly blocked by MgSO4.

DISCUSSION

MgSO4 may provide protection against oxidative damage of plasma membranes through interactions with alkyl radicals.

Phosphorylated intermediate of the ouabain-insensitive, Na(+)-stimulated ATPase in rat kidney cortex and rainbow trout gills

Several tissues from different animals, including the rat kidney and the freshwater rainbow trout gills, show an ouabain-insensitive, furosemide-sensitive, Na(+)-stimulated ATPase activity, which has been associated with the active control of the cell volume. This Na-ATPase is Mg(2+) dependent and it is inhibited by vanadate, which can be taken as an indication that this enzyme is a P-type ATPase. The P-type ATPases are known to form a phosphorylated intermediate during their catalytic cycle, where the phosphate binds an aspartyl residue at the enzyme's substrate site. In the current study, we partially characterized the phosphorylated intermediate of the ouabain-insensitive Na-ATPase of rat kidney cortex homogenates and that of gill microsomes from freshwater rainbow trout. While the kidney cortex homogenates, under our assay conditions, show both Na- and Na,K-ATPase activities, the gill microsomes, when assayed at pH 5.2, only show Na-ATPase activity. Both preparations showed a Mg(2+)-dependent, Na(+)-stimulated phosphorylated intermediate, which is enhanced by furosemide. Incubation of the phosphorylated enzyme with 0.6 N hydroxylamine (NH(2)OH) showed that it is acid-stable and sensitive to hydroxylamine, either when phosphorylated in the presence or absence of furosemide. Addition of ADP to the incubation medium drives the reaction cycle of the enzyme backward, diminishing its phosphorylation. Na(+) seems to stimulate both the phosphorylation and the dephosphorylation of the enzyme, at least for the Na-ATPase from gill microsomes. In a E1-E2 reaction cycle of the Na-ATPase, furosemide seems to be blocking the transition step from Na.E1 approximately P to Na.E2-P.

Ouabain-insensitive, Na(+)-stimulated ATPase of several rat tissues: activity during a 24 h period

Rhythmic daily changes in the Na,K-ATPase activity have been previously described for rat kidney cortex, showing two peaks: at 0900 h and 2100 h, and two valleys: at 1500 h and 0100 h -0300 h. The oscillations in Na,K-ATPase activity are produced by an inhibitor, which binds the enzyme and is present in the rat blood plasma at valley times and absent or at very low concentrations at peak times. Since it has been demonstrated that active Na(+) extrusion from the cells of several tissues depends not only on the Na,K-ATPase but also on the ouabain-insensitive Na-ATPase, we studied the activity of this latter enzyme of several rat tissues, i.e., kidney cortex, small intestine, liver, heart and red blood cells along the day. None of these tissues showed any variation of their Na-ATPase activity along the day. Preincubation of kidney cortex homogenates obtained at 0900 h, with blood plasma drawn at 0900 h and 1500 h, did not modify the Na-ATPase activity. Our results indicate that the Na-ATPase activity does not oscillate along the day. These results are in agreement with the idea that the Na-ATPase could partially compensate the Na(+) transport affected by oscillations of the Na,K-ATPase activity.

Chlamydia trachomatis and sperm lipid peroxidation in infertile men

AIM

To relate the presence of anti-Chlamydial trachomatis IgA in semen with sperm lipid membrane peroxidation and changes in seminal parameters.

METHODS

Semen samples of the male partners of 52 couples assessed for undiagnosed infertility were examined for the presence of IgA antibody against C. trachomatis. The level of sperm membrane lipid peroxidation was estimated by determining the malondialdehyde (MDA) formation.

RESULTS

Sperm membrane of infertile males with positive IgA antibodies against C. trachomatis showed a higher level of lipid peroxidation than that of infertile males with negative IgA antibody (P<0.05). There was a positive correlation (P<0.01) between the level of C. trachomatis antibody and the magnitude of sperm membrane lipid peroxidation. All the other tested semen parameters were found to be similar in the two groups.

CONCLUSION

The activation of immune system by C. trachomatis may promote lipid peroxidation of the sperm membrane. This could be the way by which C. trachomatis affects fertility.

Bioactivity of serum hCG in preeclampsia

OBJECTIVE

To compare hCG levels, obtained by biologic and immunologic means, in women with normal pregnancies and women with preeclampsia.

METHODS

Peripheral blood samples from women in the third trimester with preeclampsia (n = 30) or normal pregnancies (n = 30) were assayed for immunoactive and bioactive hCG (mouse Leydig cell testosterone production assay).

RESULTS

Serum bioactive hCG levels tended to be lower than normal, and immunoactive hCG levels tended to be higher in women with preeclampsia, but the differences were not statistically significant. However, the ratio of bioactive to immunoactive hCG was significantly lower than normal for preeclamptic women (0.70 +/- 0.28 vs. 1.15 +/- 0.35 for normotensive pregnant women [mean +/- standard deviation], P <.001).

CONCLUSION

The ratio of bioactive to immunoreactive serum hCG is lower among preeclamptic than among normotensive pregnant women.

Interleukin-6 (IL-6) in seminal plasma of infertile men, and lipid peroxidation of their sperm

Concentrations of tumor necrosis factor-alpha (TNF-alpha) and interleukin-6 (IL-6) in seminal fluid, as well as levels of sperm lipid membrane peroxidation, were investigated in fertile and infertile men. Semen samples, obtained by masturbation from 37 infertile and 14 fertile men, were examined for the presence of TNF-alpha and IL-6. The level of lipid peroxidation of the sperm membrane was measured by determining malondialdehyde (MDA) formation. The correlation between the IL-6 and the TNF-alpha concentrations in seminal plasma with the levels of lipid peroxidation of the sperm membranes was statistically evaluated. The IL-6 concentration in seminal plasma of infertile men was significantly higher than that of fertile men (p < .05). Similarly, the level of membrane lipid peroxidation was higher for the semen of infertile men than that of fertile men (p < .001). A significant positive correlation was found between IL-6 levels in seminal plasma and membrane sperm lipid peroxidation (p < .002), but not between this parameter and TNF-alpha levels in seminal plasma. These findings suggest a possible association between IL-6 seminal plasma levels and lipid peroxidation of sperm membrane. Stimulation of reactive species production by human sperm and leucocytes, induced by the high levels of IL-6, could explain these results.

Comparative study of the calcium adenosine triphosphatase of basal membranes of human placental trophoblasts from normotensive and preeclamptic pregnant women

The Ca-ATPase activity of plasma membranes from human trophoblast is diminished in about 50% in preeclamptic women, as compared to normotensive pregnant women. This diminution is not due to changes in the behavior of the enzyme towards the free calcium concentration, the pH or the temperature of the incubation medium. Neither does it appear to be due to the presence of some condensing factor in the membranes, since the apparent energies of activation for the two tested ranges of temperature (10-20 and 20-37 degrees C) are similar for both normotensive and preeclamptic patients. The possibility of a diminution in the turnover rate of the Ca-ATPase in the plasma membranes from the preeclamptic patients is considered.

Ca-ATPase of human myometrium plasma membranes

We determined and characterized the Mg2+-dependent, Ca2+-stimulated ATPase (Ca-ATPase) activity in cell plasma membranes from the myometrium of pregnant women, and compared these characteristics to those of the active Ca2+-transport already demonstrated in this tissue. Similarly to the Ca2+-transport system, the Ca2+-ATPase is Mg2+-dependent, stimulated by calmodulin, and inhibited by vanadate. The Km for Ca2+ activation is 0.40 microM, very similar to that found for active calcium transport, i.e. 0.25 microM. Consequently, this Ca2+-ATPase can be responsible for the active calcium transport across the plasma membranes of smooth muscle cells.

Preeclampsia and Na,K-ATPase activity of red blood cell ghosts from neonatal and maternal blood

We determined Na,K-ATPase activity and cholesterol/phospholipid ratio of maternal and cord red blood cell ghosts from either normotensive or preeclamptic pregnant women. The Na,K-ATPase activity of the red cell ghosts from neonatal blood is significantly lower (25-32%) as compared with the ATPase activity of the maternal red cell ghosts, regardless of the presence or not of preeclampsia. This diminution in Na,K-ATPase activity of the neonatal red blood cell ghosts could be due to an increase in the cholesterol/phospholipid molar ratio of the membrane. The Na,K-ATPase activity of the red blood cell ghosts from pregnant women was unaffected by preeclampsia; however, fetal red blood cell ghosts from infants of preeclamptic mothers showed a significantly lower ATPase activity (20%) than fetal red blood cell ghosts from infants of normotensive mothers. A low Na,K-ATPase activity in the neonatal red blood cells from mothers with preeclampsia could be an indication of an important modification of the physiological role of this enzyme.

Ca-ATPase of human syncytiotrophoblast basal plasma membranes

In the present work, a Mg(2+)-dependent, Ca(2+)-stimulated ATPase activity was determined and characterized in purified preparations of syncytiotrophoblast basal (fetal facing) plasma membranes, and its characteristics were compared to those of the active Ca(2+)-transport already demonstrated in this tissue. Similar to the active Ca(2+)transport, the Ca-ATPase is Mg(2+)-dependent, is stimulated by calmodulin, and is inhibited by vanadate. The K(m) for Ca(2+)activation is 0.25+/- 0.02microM, a value near to that described for calcium active transport in this tissue. Consequently, the Ca-ATPase activity of human syncytiotrophoblast basal plasma membrane described in this paper could be responsible for the active extrusion of calcium from the syncytiotrophoblast toward the fetal circulation.

Ouabain-insensitive Na(+)-ATPase activity of Malpighian tubules from Rhodnius prolixus

In the present paper, we show the existence of a furosemide-sensitive Na(+)-stimulated, Mg(2+)-dependent ATPase activity in cell lysates of Malpighian tubular cells from Rhodnius prolixus, which could be the biochemical expression of the Na(+)-pump. The main characteristics of this activity are: (1) K0.5 for Na+ = 1.49 +/- 0.18 mM, (2) Vmax = 2.8 +/- 0.1 nmol inorganic orthophosphate (Pi).mg prot-1.min-1, (3) it is fully abolished by 2 mM furosemide, (4)it is insensitive to ouabain concentrations up to 10(-2) M, (5) it is sensitive to the presence of vanadate in the incubation medium indicating it to be a P-type ATPase, and (6) it is stimulated by nanomolar concentrations of Ca2+ in the incubation medium.

Preeclampsia, lipid peroxidation, and calcium adenosine triphosphatase activity of red blood cell ghosts

OBJECTIVE

We evaluated the effect of lipid peroxidation on the calcium adenosine triphosphatase activity of red blood cell ghosts from normotensive pregnant women and compared it with the adenosine triphosphatase activity and lipid peroxidation in preeclampsia.

STUDY DESIGN

Ten nulliparous normotensive and 10 nulliparous preeclamptic pregnant women (38 to 39 weeks of gestation) were used as blood donors. Preeclampsia was diagnosed on the basis of blood pressure (>140/90 mm Hg) and proteinuria (>0.5 gm of urinary protein per day). Red blood cell ghosts were prepared for both groups and used for calcium adenosine triphosphatase activity and lipid peroxidation determinations. Control ghosts (normotensive) were irradiated with ultraviolet light for different lengths of time.

RESULTS

Calcium adenosine triphosphatase activity of red blood cell ghosts from normotensive women is sensitive to lipid peroxidation. The lipid peroxidation of red blood cell ghosts from preeclamptic women is higher than that from normotensive women.

CONCLUSION

The diminution of the calcium adenosine triphosphatase activity with preeclampsia could be explained by the sensitivity of this adenosine triphosphatase to lipid peroxidation.

Inhibitory effect of ethanol on the Na(+)-ATPase activity of rat kidney proximal tubular cell plasma membranes

The inhibitory effect of 2% ethanol (400 mM) in the incubation medium on several characteristics of the Na(+)-ATPase of basolateral plasma membranes from rat kidney proximal tubular cells was investigated. Ethanol did not change the Km of the enzyme for Mg2+, ATP or Na+; it did not change either the optimal pH or temperature values of the incubation medium for the enzyme to act and finally, it did not affect the apparent energy of activation of the enzyme. It was also found that 2% ethanol produced stronger inhibition of the ATPase when it is in an activated or stimulated state, than when it is working at its lower basal level. The presented results can be explained by assuming that 2% ethanol in the incubation medium inhibits Na(+)-ATPase activity by affecting the enzyme structure as well as its activating mechanism.

Na,K-ATPase activity in red blood cells from patients with Chediak-Higashi syndrome

Na,K-ATPase activity of red blood cells from Chediak-Higashi syndrome (CHS) patients and relatives (gene heterozygous) was determined and compared to that of control, healthy, individuals. The enzyme activity was found to be strongly diminished in the CHS patients and slightly lower in their relatives. This reduced activity was due to a lower turnover number of the Na, K-ATPase as well as a decreased number of pumps. The reduced enzyme activity observed in these patients could be the result of an abnormal cell membrane fluidity, and the lowered number of Na, K-pumps could be explained as a consequence of an altered or deficient cell machinery caused by the CHS gene.

Ouabain-insensitive, Na(+)-stimulated ATPase activity in rabbit cardiac sarcolemma

The rabbit cardiac sarcolemma shows an ouabain, Na,K-stimulated ATPase activity and an ouabain-insensitive, Na-stimulated ATPase activity. The Na-ATPase has the following characteristics: (i) It is also stimulated by other monovalent cations. (ii) It is inhibited by 2 mM Furosemide and by 2 mM ethacrynic acid. (iii) It reaches maximal values (Vmax) at around 20 mM Na+. (iv) The apparent Km is around 5 mM. Except for the monovalent cation stimulation, the main characteristics of this ATPase are very similar to those of the ouabain-insensitive, Na-stimulated ATPase of mammalian kidneys.

Preeclampsia and calcium adenosine triphosphatase activity of red blood cell ghosts

OBJECTIVE

The current work was undertaken to study the calcium adenosine triphosphatase activity of red blood cell membranes from pregnant women with preeclampsia.

STUDY DESIGN

Six normotensive and six preeclamptic pregnant women at 38 to 39 weeks of gestation were studied. The diagnosis of preeclampsia was made on the basis of blood pressure (> 140/90 mm Hg), proteinuria (> 0.5 gm of urinary protein per day), or edema. Hemoglobin-free red blood cell ghosts were prepared from the heparinized blood samples and were used to determine the calcium adenosine triphosphatase activity.

RESULTS

It was found that the calcium adenosine triphosphatase activity of preeclamptic women is diminished by about 50% compared with that of normotensive pregnant women.

CONCLUSION

A diminution of the calcium adenosine triphosphatase activity of erythrocytes in preeclampsia might be an indication that the in vivo activity of the calcium pump of these cells is diminished, which could, in turn, drive the cells to increase their cytoplasmic free calcium concentration.

Studies on the effect of ethanol on the Na+, and the Na+, K(+)-ATPase activities of plasma membranes of rat kidney proximal tubular cells

In the present work it was investigated the effect of 2% ethanol on the Na+ and on the Na+, K(+)-ATPase activities. The differential effect of the alcohol on the two ATPases (approximately 40% inhibition of the Na(+)-ATPase and approximately 10% inhibition of the Na+, K(+)-ATPase), is not due to a higher degree of denaturalization of the enzyme, nor to a faster effect of ethanol on the Na(+)-than on the Na+, K(+)-ATPase. Our results show that ethanol affects the selectivity of the Na+, K(+)-ATPase for Na+ and/or for K+, enhancing the Na+ affinity for the K+ sites, and/or reducing the K+ affinity for its own sites. This effect was not seen for the Na(+)-ATPase, indicating that 2% ethanol inhibits the two ATPases in a totally different way.

Effect of ethanol on the Na(+)- and the Na+,K(+)-ATPase activities of basolateral plasma membranes of kidney proximal tubular cells

The Na(+)- and the Na+,K(+)-ATPase activities of basolateral plasma membranes from rat kidney proximal tubular cells were affected differentially by ethanol. Moreover, at concentrations of ethanol that can be reached in vivo in the blood plasma (50 mM) there was a significant effect on the Na(+)-ATPase activity and practically no effect on the Na+,K(+)-ATPase activity.

Ouabain-insensitive Na-ATPase activity in homogenates from different animal tissues

1. Two Na(+)-stimulated ATPase activities were determined in gill homogenates from squid, shrimp and teleost fish; in kidney slice homogenates from teleost fish, bullfrog, toad, iguana, chicken, duck, rat, pig and cow, as well as in homogenates from rat small intestinal cells, brain cortex and liver slices. The two Na(+)-stimulated ATPase activities, the Na- and the Na,K-ATPase, showed a different behavior toward K+ and ouabain. 2. The ouabain-insensitive, K(+)-independent, Na-ATPase activity for all the studied homogenates was completely inhibited by 2 mM furosemide. 3. An increase in cell volume of the kidney, brain cortex and liver slice preparations, as well as of the rat small intestinal cells, produced a concomitant increase of the ouabain-insensitive Na-ATPase.

Effect of a high Na+ diet on cell volume and Na(+)-stimulated ATPase activities of rat kidney membranes

Proximal tubular cells from kidneys of male rats chronically fed with an isotonic NaCl solution, show a volume increase which is dependent on the length of the treatment with NaCl, when compared with control rats. Parallel to the cell volume increase, there is an increase of the ouabain-insensitive Na-ATPase activity, whereas the ouabain-sensitive Na,K-ATPase activity remains unchanged. These results establish a clear relationship between a chronic Na-diet, kidney cell volume and Na-ATPase activity.

Ouabain-insensitive, Na-ATPase activity in pure suspensions of rat kidney proximal tubules

The present work was undertaken to evaluate the distribution of the Na-ATPase activity in the different components of the rat kidney cortex. Suspensions of glomeruli, proximal and distal tubules were prepared following a collagenase digestion of outermost kidney cortex slices and a separation on a Percoll gradient. It was found that the Na-ATPase activity is higher in the fraction enriched in proximal tubules. The fraction enriched in glomeruli and in distal tubules show also a Na-ATPase activity, but it is lower.

Na-pump activity in rat kidney cortex cells and its relationship with the cell volume

The present work was undertaken to evaluate whether changes in cell water content of rat kidney cortex cells can modulate the transport activity of the ouabain-insensitive Na pump as they modulate the ouabain-insensitive Na+-ATPase. It was found that there is a close relationship between the cell volume and activity of the Na pump, whereas Na,K-pump activity is not affected by variations in cell volume. When the cell water content is low, Na-pump activity (Na+ transport and Na+-ATPase activity) is minimal. Increases in cell water content produce a concomitant increase in Na-pump activity.

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

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

High sodium diet and Na+-stimulated ATPase activities in basolateral plasma membranes from rat kidney proximal tubular cells

The ouabain-insensitive, Na+-stimulated ATPase activity of kidney proximal tubular cells from rats fed a high Na+ diet for 4 months was increased approximately 70% when compared with control (normal diet) rats. The higher ATPase activity was not due to a change in the affinity of the system toward ATP, Mg2+ or Na+. This increase in Na+-ATPase activity may be due to either a higher number of pumps or to a higher turnover rate of the enzyme or both. The ouabain-sensitive, Na+, K+-stimulated ATPase activity, on the other hand, did not change with the high sodium diet. These results can be taken as evidence that the Na+,K+-ATPase and the Na+-ATPase of basolateral plasma membranes of proximal tubular cells from rat kidney are two different entities.

Effect of a high NaCl diet on the active mechanisms of Na+ extrusion in rat kidney

Healthy male rats were exposed to a chronic ingesta of an isotonic NaCl solution during a period of four months. Two different preparations were utilized to study the effect of this treatment on the active mechanisms of Na+ reabsorption of kidney proximal tubular cells: outermost kidney cortex slices (rich in proximal tubules) and inside-out basolateral plasma membrane vesicles prepared from the same tissue. It was found that the activity of the ouabain-insensitive, Na-pump of basolateral plasma membranes of kidney proximal tubular cells was increased in about 70%, whereas the ouabain-sensitive, Na,K-pump activity did not change with the experimental treatment. These results represent a strong support to the two Na-pump's hypothesis since the treatment affected the Na-pump differentially without affecting the Na,K-pump.

Na+-ATPase is a different entity from the (Na+ + K+)-ATPase in rat kidney basolateral plasma membranes

In this work, we present evidence in agreement with the hypothesis that there exist two Na+-stimulated ATPase activities in basolateral plasma membranes from rat kidney proximal tubular cells: (1) (Na+ + K+)-ATPase activity, which is inhibited by ouabain and by treating the membranes with trypsin, is insensitive to furosemide and reaches maximal activity upon treatment with SDS at an SDS/protein ratio of 1.6; (2) the Na+-ATPase activity, which is insensitive to ouabain and to trypsin treatment, is inhibited by furosemide and reaches maximal activity upon treatment with SDS at an SDS/protein ratio of 0.4.

Inside-out basolateral plasma membrane vesicles from rat kidney proximal tubular cells

A method for preparation of highly purified basolateral plasma membranes from rat kidney proximal tubular cells is reported. These membranes were assayed for the presence of vesicles as well as for their orientation. (Na+ + K+)-ATPase activity and [3H]ouabain binding studies with membranes treated with or without SDS revealed that the preparation consisted of almost 100% vesicles. The percentage of inside-out vesicles was found to be approx. 70%. This percentage was determined measuring the (Na+ + K+)-ATPase activity in K+-loaded vesicles and in membranes treated with or without trypsin and SDS. These membranes represent a very efficient tool to assay the correlation between active transport and ATPase activities in basolateral plasma membranes from rat kidney proximal tubular cells.

Characterization of the Na+, K+-ATPase activity of basolateral plasma membranes of kidney proximal tubular cells from young and old rats

Several characteristics of the Na+, K+-ATPase activity of basolateral plasma membranes of kidney proximal tubular cells from young (3 months) and old (24 months) rats were studied. In both cases, the ATPase activity reached optimum values under the following conditions: Mg2+:ATP concentrations (mM) 5:5 (apparent Km 0.5 mM); Na+ concentration 50 mM (apparent Km 18 mM); K+ concentration 20 mM (apparent Km 2.5 mM); pH 7.2; temperature 52 degrees. The values of the apparent energy of activation of the system were similar for young and old rats in the temperature range 20-52 degrees but were 55% higher for the old rats in the temperature range 10-20 degrees.

Effect of Ca2+ on the ouabain-insensitive, active Na+ uptake in inside-out basolateral plasma membrane vesicles from rat kidney proximal tubular cells

The ouabain-insensitive, active Na+ uptake of inside-out vesicles prepared with basolateral plasma membranes from rat kidney proximal tubular cells can be increased by the presence of micromolar concentrations of Ca2+ in the assay medium. The concomitant ATP hydrolysis associated with the Na+ uptake is also increased by the presence of Ca2+. The Na+ uptake and the concomitant ATP hydrolysis are inhibited by 2 mM furosemide. The effect of Ca2+ is not due to the activity of an Na+-Ca2+ exchanger. The present results are in accordance with our previous model (Proverbio, F., Proverbio, T. and Marín, R. (1982) Biochim. Biophys. Acta 688, 757-763) in which we proposed that Ca2+ seems to modulate the activity of the ouabain-insensitive Na+ pump, in two different ways: (1) in a strong association with the membranes in which Ca2+ (stable component) is essential for the pump activity and (2) in a weak association with the membranes in which Ca2+ (labile component) can be quickly and easily removed by reducing the free Ca2+ concentration of the assay medium to values lower than 1 microM. The Ka for Ca2+ (for the labile component) is around 5 microM. The Ca2+ modulation of the ouabain-insensitive Na+ pump is an indication that Ca2+ could regulate the magnitude of the Na+ extrusion accompanied by Cl- and water present in rat kidney proximal tubular cells.

ATPase activities in kidney basolateral plasma membranes of young and old rats

The present work studied the turnover rate of (Na+ + K+)-ATPase as well as Mg2+- and Na+- ATPase activities in basolateral plasma membranes from kidney cortex cells of young and old rats. It was found that, as for the homogenates, the turnover rate of the (Na+ + K+)-ATPase was diminished by aging in about 40%. The Mg2+-ATPase activities on the other hand, were similar for the rat kidneys of young and old, in both the homogenates as well as the basolateral plasma membrane fractions.

Ion transport and oxygen consumption in kidney cortex slices from young and old rats

The effects of aging on active Na+ extrusion and oxygen consumption associated with it were studied in rat kidney cortex cells. It was found that (a) the active extrusion of Na+ undergoing Na/K exchange and the active extrusion of Na+ with Cl- and water were diminished in old rats (24 months) as compared with young rats (3 months); (b) the oxygen consumption associated with each of the two active mechanisms of Na+ extrusion was also diminished in the old rats; (c) the calculated turnover rate of the Na/K pump was significantly lower for the old rats.

Partial characterization of the ouabain-insensitive, Na+-stimulated ATPase activity of kidney basal-lateral plasma membranes

The present paper characterizes the Na+-stimulated ATPase activity present in basal-lateral plasma membranes from guinea-pig kidney proximal tubular cells. These characteristics are compared with those of the (Na+ + K+)-stimulated ATPase activity, and they are: (A) Na+-ATPase activity: (1) requires Mg2+; (2) may be activated by mu molar quantities of Ca2+; (3) optimal ratio Mg:ATP = 5:1-2 and Ka for Mg:ATP = 3:0.60 mM; (4) Ka for Na+:8 mM; (5) does not require K+; (6) is only stimulated by Na+ and Li+ (in a lower extent); (7) is similarly stimulated by the Na+ salt of different anions; (8) hydrolyzes only ATP; (9) optimal temperature: 47 degrees C; (10) optimal pH: 6.9; (11) is ouabain insensitive; (12) is totally inhibited by 1.5 mM ethacrynic acid, 2 mM furosemide and 0.75 mM triflocin. (B) (Na+ + K+)-ATPase activity: (1) also requires Mg2+; (2) is inhibited by Ca2+; (3) optimal ratio Mg:ATP = 1.25:1 and Ka for Mg:ATP = 0.50: 0.40 mM; (4) Ka for Na+: 14 mM (data not shown); (5) needs K+ together with Na+; (6) K+ may be substituted by: Rb+ greater than NH+4 greater than Cs+; (7) is anion insensitive; (8) hydrolyzes mostly ATP and to a lesser extent GTP, ITP, UTP, ADP, CTP; (9) optimal temperature: 52 degrees C; (10) optimal pH: 7.2; (11) 100% inhibited by 1 mM ouabain; (12) 63% inhibited by 1.5 mM ethacrynic acid, 10% inhibited by 2 mM furosemide and insensitive to 0.75 mM triflocin.

Ouabain-insensitive Na+-stimulated ATPase activity of basolateral plasma membranes from guinea-pig kidney cortex cells. II. Effect of Ca2+

The ouabain-insensitive, Mg2+-dependent, Na+-stimulated ATPase activity present in fresh basolateral plasma membranes from guinea-pig kidney cortex cells (prepared at pH 7.2) can be increased by the addition of micromolar concentrations of Ca2+ to the assay medium. The Ca2+ involved in this effect seems to be associated with the membranes in two different ways: as a labile component, which can be quickly and easily 'deactivated' by reducing the free Ca2+ concentration of the assay medium to values lower than 1 microM; and as a stable component, which can be 'deactivated' by preincubating the membranes for periods of 3-4 h with 2 mM EDTA or EGTA. Both components are easily activated by micromolar concentrations of Ca2+. The Ka of the system for Na+ is the same, 8 mM, whether only the stable component or both components, stable and labile, are working. In other words, the activating effect of Ca2+ on the Na+-stimulated ATPase is on the Vmax, and not on the Ka of the system for Na+. The activating effect of Ca2+ may be related to some conformational change produced by the interaction of this ion with the membranes, since it can also be obtained by resuspending the membranes at pH 7.8 or by ageing the preparations. Changes in the Ca2+ concentration may modulate the ouabain-insensitive, Na+-stimulated ATPase activity. This modulation could regulate the magnitude of the extrusion of Na+ accompanied by Cl- and water that these cells show, and to which the Na+-ATPase has been associated as being responsible for the energy supply of this mode of Na+ extrusion.

Na+-stimulated ATPase activities in kidney basal-lateral plasma membranes

In the present work we demonstrate the existence of a Na+-dependent ATPase activity, which is insensitive to ouabain, and which is 100% inhibited by 1.5 mM ethacrynic acid in freshly prepared, basal-lateral enriched, plasma-membrane fractions obtained from guinea-pig kidney cortex slices (which are mainly made up by proximal tubules). This ATPase activity has characteristics similar to those described previously in a microsomal fraction of the guinea-pig kidney cortex by a procedure which required ageing of the preparation for more than two weeks (Proverbio, F., Condrescu-Guidi, M. and Whittembury, G. (1975) Biochim. Biophys. Acta 394, 281-292), it does not seem to be due to some partially modified activity of the Mg2+-ATPase, the (Na+ + K+)- or the Ca2+-ATPase activities present in this tissue. It seems to be due to the activity of another system, which is located on the basal-lateral membrane of the kidney tubular cells.

Effect of calcium on the H+/K+ ATPase of hog gastric microsomes

The K+-stimulated, ouabain-insensitive ATPase activity present in vesicles of microsomal fractions from hog gastric mucosa can be demonstrated in fresh preparations by adding Ca2+ (micron range) to the incubation medium. Ca2+ effect is similar but not additive to the effect of gramicidin or freezing. High Ca2+ concentrations (1 mM) produce an inhibitory effect on the K+-stimulated ATPase activity. This effect is not seen in the presence of gramicidin. Calcium increases the magnitude of ATP-driven H+ uptake in vesicles exposed to K+ for periods of time up to 60 min. At longer times of exposure (120 min) the response does not differ from controls. It is concluded that Ca2+ at low concentrations (micron range) enhances the K+ permeability of the vesicular membrane. At higher concentrations (mM range), Ca2+ becomes inhibitory to the K+ permeability. A role for Ca2+ as a second messenger in stimulus-secretion coupling in the parietal cell is discussed.

Membrane compartmentalized ATP and its preferential use by the Na,K-ATPase of human red cell ghosts

This paper describes work which begins to define the molecular organization in the region of the membrane that comprises the functional domain of the Na:K pump. The membrane-bound phosphoglycerate kinase (PGK) and Na, K-ATPase appear to be directly linked via a compartmentalized form of ATP. Evidence for the membrane pool of ATP is based on the labeling characteristics of the phosphoproteins by [gamma-(32)P]ATP of ghosts incubated under various conditions. Preincubation of ghosts in the presence of ATP at 37 degrees C, but not at 0 degrees C, completely obscures the formation of the Na-phosphoprotein in ghosts washed and subsequently incubated in the presence of [gamma-(32)P]ATP. In contrast to the Na component, the Mg component of phosphorylation is only slightly altered by preincubation with ATP. ATPase activity measured as (32)P(i) liberated during the subsequent incubation at 0 degrees C, reflects completely the differential effects of preincubation with ATP on (32)P incorporation into phosphoprotein. ATP placed within the pool by preincubation can be removed by operating the Na, K-ATPase or the PGK reaction in the reverse direction by use of exogenous substrates. Alternatively, the membrane pool of ATP can be formed also from exogenous substrates by running the PGK reaction in the forward direction. These results, while providing direct support for a membrane compartment of ATP, also indicate the location of this compartment in relation to the PGK and the Na, K-ATPase. In addition, these results also imply that the Mg and Na components are different enzymatic entities since substrate ATP can be derived from separate sources.

Cell electrical potentials during enhanced sodium extrusion in guinea-pig kidney cortex slices

1. Experiments were performed on outermost slices of the guinea-pig kidney which are mainly made up of proximal tubular cells. 2. Kidney cells loaded with Na+ by chilling at 0.6 degrees C for 2.5 hr, when subsequently rewarmed to 25 degrees C in a medium containing 16 mM-K+ extrude Na+ at enhanced speed for about 10 min. This Na+ movement is accompanied by efflux of Cl and influx of K+. 3. Measurements of cell potential during enhanced Na+ extrusion show that cells hyperpolarize to values about 30 mV more negative than the K+ equilibrium potential. 4. This hyperpolarization is only partly inhibited by 1 mM ouabain or by 2 mM ethacrynic acid but both agents added together suppress it completely. 5. With 16 mM-Rb instead of 16 mM-K the hyperpolarization is smaller. 6. A diminished extracellular K+ concentration outside of the cells, within the slice, can account for only a small part of the hyperpolarization. 7. The hyperpolarization is proportional to the rate of Na+ pumping. 8. Cl- seems to shunt the hyperpolarization to a greater extent than K+. 9. It is concluded that Na+ extrusion is capable of transferring electric charge across the membrane.

Ouabain-insensitive Na+ stimulation of an Mg-2+ -dependent ATPase in kidney tissue

1. Freshly prepared microsomal fractions of the outermost cortex of guinea pig kidney show an Mg-2+-dependent ATPase activity which is partially inhibited by 100 mM NaCl, LiCl, KCl, RbCl, CsCl, NH4Cl or choline chloride. 2. If the microsomal preparation is aged by storage at 4 degrees C for 10-15 days, the Mg-2+-dependent activity shows stimulation by Na-+ and Li-+ but not by K-+, Rb-+, Cs-+, NH4-+ or choline. 3. Stimulation is similar with sodium salts of Cl-minus, HCO3-minus, CH3COO-minus, BR-minus, SO4-2-minus or methylsulphonate. 4. Stimulation is insensitive to 1 mM and 10 mM ouabain. 5. Stimulation is unaltered by the presence of 0.5 mM ethyleneglycol-bis-(beta-aminoethyl ether)N,N'-tetracetic acid. 6. Stimulation is 100% inhibited by 2 mM ethacrynic acid, a concentration which inhibits only 30% of the Mg-2+-dependent ATPase and 50% of the (Na-++K-+)-stimulated ATPase. 7. Some of these characteristics coincide with those of an ouabain-resistant, K-+-independent, ethacrynic acid-sensitive mode of Na-+ extrusion out of guinea pig kidney cortex cells.

Electrophoretic separation of different phophosproteins associated with Ca-ATPase and Na, K-ATPase in human red cell ghosts

Ca has been found to increase the quantity of (32)P incorporated into red cell ghosts from [gamma-(32)P]ATP over the levels obtained by incubation with Mg alone or with Mg + Na, in correlation with the effect of Ca on the associated ATPase activities. When the (32)P-labeled ghosts were solubilized in sodium dodecyl sulfate (SDS) and electrophoresed on acrylamide gels only two bands could be detected either by autoradiography or by counting the sliced gels. The faster moving band (P-2) had the same mobility and the same molecular weight (103,000) as the phosphoprotein found either with Mg alone or with Mg + Na. The slower moving band (P-1) was not found in extensively washed ghosts labeled in the absence of Ca. The molecular weight of P-1 is approximately 150,000. P-1 like P-2 was not affected by pretreatment of intact cells with Pronase before labeling indicating that neither the phosphorylating mechanism nor the phosphoprotein are accessible to externally applied Pronase. The demonstration that a Ca-phosphoprotein is separable from the Na-stimulated phosphoprotein suggests that the Ca-ATPase is distinct from and independent of the Na,K-ATPase. The fact that Ca blocks the dephosphorylation by K of the Na-phosphoprotein indicates that caution is required in interpreting results when the activities of the different phosphoproteins have not been separately determined.

Chemical characterization and pronase susceptibility of the Na:K pump-associated phosphoprotein of human red blood cells

The phosphoproteins formed by incubation of red cell ghosts with [gamma-(32)P]ATP in the presence of Mg and Na + Mg have been characterized by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. The (32)P-labeled phosphoprotein was seen as a single peak confined to the region of the diffuse 90,000 dalton polypeptide band; labeling with Na + Mg considerably increased the quantity of (32)P-phosphoprotein contained in this band relative to labeling with Mg alone. Treatment of intact cells with Pronase known to partially hydrolyze the glycoproteins and the 90,000 daltons polypeptide did not change either the amount or the position of the (32)P-phosphoprotein present in the gels. The molecular weight of the (32)P-phosphoprotein is estimated to be 103,000. Pronase treatment of intact cells also did not significantly alter any of the transport parameters of the membrane such as the K pump flux, ouabain binding, or Na,K-ATPase. In contrast, treatment of ghosts with Pronase not only resulted in drastic alteration of the transport parameters but also inhibited the formation of the phosphoprotein under all conditions. Thus, while the Na:K pump is not intrinsically resistant to Pronase, those elements of the pump which are susceptible are not accessible from the outside of the cell. Further, SDS-polyacrylamide gel electrophoresis after Pronase treatment of intact cells results in a substantial increase in the purification of the phosphoprotein relative to that which was previously possible in ghosts.