Na+-H+ exchange in isolated renal brush-border membrane vesicles in response to metabolic acidosis. Kinetic effects

Chronic metabolic acidosis increased the Na+-H+ exchange activity in isolated renal brush-border membrane vesicles. Treatment altered the initial rate of Na+ uptake by increasing Vm (acidotic, 15.3 +/- 0.7 nmol of Na+ X mg-1 X 2 s-1; normal, 11.3 +/- 0.9 nmol of Na+ X mg-1 X 2 s-1), and not the apparent affinity KNa+ (acidotic, 10.2 +/- 0.5 mM; normal 10.2 +/- 0.6 mM). Metabolic acidosis resulted in the proportional increase in 1 mM Na+ uptake at every intravesicular pH measured. A positive cooperative effect on Na+ uptake was found with increased intravesicular acidity in vesicles from both normal and acidotic rats. When the data were analyzed by the Hill equation, it was found that metabolic acidosis did not change the n (acidotic, 1.33 +/- 0.13; normal, 1.43 +/- 0.07) or the K'H+ (acidotic, 0.27 +/- 0.05 microM; normal, 0.28 +/- 0.06 microM), but increased the apparent Vm (acidotic, 1.10 +/- 0.08 nmol of Na+ X mg-1 X 2 s-1; normal, 0.81 +/- 0.07 nmol of Na+ X mg-1 X 2 s-1). The uptake of Na+ in exchange for H+ in membrane vesicles from normal and acidotic animals was not influenced by membrane potential. We conclude that metabolic acidosis leads to either an increase in the number of functioning exchangers or an increase in the turnover rate of the limiting step in the exchange.     

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

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

Effect of external ATP on the plasma membrane permeability and (Na+ +K+)-ATPase activity of mouse neuroblastoma cells

1. Addition of 3.5 mM ATP to mouse neuroblastoma Neuro-2A cells results in a selective enhancement of the plasma membrane permeability for Na+ relative to K+, as measured by cation flux measurements and electro-physiological techniques. 2. Addition of 3.5 mM ATP to Neuro-2A cells results in a 70% stimulation of the rate of active K+ -uptake by these cells, partly because of the enhanced plasma membrane permeability for Na+. Under these conditions the pumping activity of the Neuro-2A (Na+ +K+)-ATPase is optimally stimulated with respect to its various substrate ions. 3. External ATP significantly enhances the affinity of the Neuro-2A (Na+ +K+)-ATPase for ouabain, as measured by direct [3H]ouabain-binding studies and by inhibition studies of active K+ uptake. In the presence of 3.5 mM ATP and the absence of external K+ both techniques indicate an apparent dissociation constant for ouabain of 2 X 10(-6)M. Neuro-2A cells contain (3.5 +/- 0.7) X 10(5) ouabain-binding sites per cell, giving rise to an optimal pumping activity of (1.7 +/- 0.4) X 10(-20) mol K+/min per copy of (Na+ +K+)-ATPase at room temperature.     

Effects of disuse on the function of fragmented sarcoplasmic reticulum of rabbit M. gastrocnemius

A preparation method has been described to obtain a relatively pure and functionally intact fragmented sarcoplasmic reticulum (SR) vesicles fraction from normal and atrophied muscles. Purified SR preparations from rabbit gastrocnemius muscle atrophied by disuse showed similar protein composition (gel electrophoresis; Laemmli 1970) and similar vanadate induced crystallization (Dux and Martonosi 1983) properties of Ca2+-ATPase as those of control preparations. In the early period of atrophy (1-2 weeks) both the Ca2+-ATPase activity and Ca2+ uptake showed a 2-3-fold increase (from 3.42 +/- 0.24 to 7.34 +/- 0.25 mumol Pi X mg-1 prot X min-1 and from 1.26 +/- 0.10 to 3.36 +/- 0.22 mumol/l Ca2+ X min-1 X mg-1 prot. respectively).     

Estradiol-induced expression of N(+)-K(+)-ATPase catalytic isoforms in rat arteries: gender differences in activity mediated by nitric oxide donors

We tested the hypothesis that previously demonstrated gender differences in ACh-induced vascular relaxation could involve diverse Na(+)-K(+)-ATPase functions. We determined Na(+)-K(+)-ATPase by measuring arterial ouabain-sensitive 86Rb uptake in response to ACh. We found a significant increase of Na+ pump activity only in aortic rings from female rats (control 206 +/- 11 vs. 367 +/- 29 nmol 86Rb/K.min(-1).g wt tissue(-1); P < 0.01). Ovariectomy eliminated sex differences in Na(+)-K(+)-ATPase function, and chronic in vivo hormone replacement with 17beta-estradiol restored the ACh effect on Na(+)-K(+)-ATPase. Because ACh acts by enhancing production of NO, we examined whether the NO donor sodium nitroprusside (SNP) mimics the action of ACh on Na(+)-K(+)-ATPase activity. SNP increased ouabain-sensitive 86Rb uptake in denuded female arteries (control 123 +/- 7 vs. 197 +/- 12 nmol 86Rb/K.min(-1).g wt tissue(-1); P < 0.05). Methylene blue (an inhibitor of guanylate cyclase) and KT-5823 (a cGMP-dependent kinase inhibitor) blocked the stimulatory action of SNP. Exposure of female thoracic aorta to the Na+/K+ pump inhibitor ouabain significantly decreased SNP-induced and ACh-mediated relaxation of aortic rings. At the molecular level, Western blot analysis of arterial tissue revealed significant gender differences in the relative abundance of catalytic isoforms of Na(+)-K(+)-ATPase. Female-derived aortas exhibited a greater proportion of alpha2-isoform (44%) compared with male-derived aortas. Furthermore, estradiol upregulated the expression of alpha2 mRNA in male arterial explants. Our results demonstrate that enhancement of ACh-induced relaxation observed in female rats may be in part explained by 1) NO-dependent increased Na(+)-K(+)-ATPase activity in female vascular tissue and 2) greater abundance of Na(+)-K(+)-ATPase alpha2-isoform in females.     

Myofibril and sarcoplasmic reticulum changes with exercise and growth

The intent of this study was to observe the effects of different treadmill running programs upon selected biochemical properties of soleus muscle from young rats. Young 10 day litter-mates were assigned to endurance (E), spring (S) and control (C) groups. Each was partitioned into either 21 or 51 day exercising groups and 10 day controls. For C the myofibril ATPase activity at 21 and 51 days were lower than 10 day activity (p less than or equal to 0.05). In the 51 day E group ATPase activity (0.378 +/- 0.009 mumol Pi X mg-1 X min-1) was greater than at 10 and 21 days (0.307 +/- 0.006 and 0.323 +/- 0.008 mumol Pi X mg-1 X min-1) (p less than or equal to 0.05). No change occurred in the S group from 10 to 21 and 51 days (p greater than or equal to 0.05). Both the 21 and 51 day S (0.318 +/- 0.011 and 0.399 +/- 0.010 mumol Pi X mg-1 X min-1) and E (0.323 +/- 0.008 and 0.378 +/- 0.009 mumol Pi X mg-1 X min-1) groups had higher activity compared to the C group (0.193 +/- 0.029 and 0.172 +/- 0.031 mumol Pi X mg-1 X min-1) (p less than or equal to 0.05). Maturation (10--51 day) resulted in a lowered sarcoplasmic reticulum (SR) yield and Ca2+ binding (p less than or equal to 0.05) while Ca2+ uptake ability did not change (p greater than or equal to 0.05). SR yield, Ca2+ binding and uptake were not altered with S training (p greater than or equal to 0.05). The E training resulted in greater Ca2+ uptake at 51 days compared to C and S (p less than or equal to 0.05), with no change in Ca2+ binding (p greater than or equal to 0.05). The data suggest that E training alters the normal development pattern of young rat soleus muscle.     

H+ transport by reconstituted gastric (H+ + K+)-ATPase

Gastric (H+ + K+)-ATPase was reconstituted into artificial phosphatidylcholine/cholesterol liposomes by means of a freeze-thaw-sonication technique. Upon addition of MgATP, active H+ transport was observed, with a maximal rate of 2.1 mumol X mg-1 X min-1, requiring the presence of 100 mM K+ at the intravesicular site. However, in the absence of ATP an H+-K+ exchange with a maximal rate of 0.12 mumol X mg-1 X min-1 was measured, which could be inhibited by the well-known ATPase inhibitors vanadate and omeprazole, giving the first evidence of a passive K+-H+ exchange function of gastric (H+ + K+)-ATPase. An Na+-H+ exchange activity was also measured, which was fully inhibited by 1 mM amiloride. Simultaneous reconstitution of Na+/H+ antiport and (H+ + K+)-ATPase could explain why reconstituted ATPase appeared less cation-specific than the native enzyme (Rabon, E.C., Gunther, R.B., Soumarmon, A., Bassilian, B., Lewin, M.J.M. and Sachs, G. (1985) J. Biol. Chem. 260, 10200-10212).     

Na+ translocation by complex I (NADH:quinone oxidoreductase) of Escherichia coli

Following on from our previous discovery of Na+ pumping by the NADH:ubiquinone oxidoreductase (complex I) of Klebsiella pneumoniae, we show here that complex I from Escherichia coli is a Na+ pump as well. Our study object was the Escherichia coli mutant EP432, which lacks the Na+/H+ antiporter genes nhaA and nhaB and is therefore unable to grow on LB medium at elevated Na+ concentrations. During growth on mineral medium, the Na+ tolerance of E. coli EP432 was influenced by the organic substrate. NaCl up to 450 mM did not affect growth on glycerol and fumarate, but growth on glucose was inhibited. Correlated to the Na+ tolerance was an increased synthesis of complex I in the glycerol/fumarate medium. Inverted membrane vesicles catalysed respiratory Na+ uptake with NADH as electron donor. The sodium ion transport activity of vesicles from glycerol/fumarate-grown cells was 40 nmol mg-1 min-1 and was resistant to the uncoupler carbonyl-cyanide m-chlorophenylhydrazone (CCCP), but was inhibited by the complex I-specific inhibitor rotenone. With an E. coli mutant deficient in complex I, the Na+ transport activity was low (1-3 nmol mg-1 min-1), and rotenone was without effect.     

Na+-dependent regulation of extramitochondrial Ca2+ by rat-liver mitochondria

The presence and significance of Na+-induced Ca2+ release from rat liver mitochondria was investigated by the arsenazo technique. Under the experimental conditions used, the mitochondria, as expected, avidly extracted Ca2+ from the medium. However, when the uptake pathway was blocked with ruthenium red, only a small rate of 'basal' release of Ca2+ was seen (0.3 nmol Ca2+ X min-1 X mg-1), in marked contrast to earlier reports on a rapid loss of sequestered Ca2+ from rat liver mitochondria. The addition of Na+ in 'cytosolic' levels (20 mM) led to an increase in the release rate by about 1 nmol Ca2+ X min-1 X mg-1. This effect was specific for Na+. The significance of this Na+-induced Ca2+ release, in relation to the Ca2+ uptake mechanism, was investigated (in the absence of uptake inhibitors) by following the change in the extramitochondrial Ca2+ steady-state level (set point) induced by Na+. A five-fold increase in this level, from less than 0.2 microM to more than 1 microM, was induced by less than 20 mM Na+. The presence of K+ increased the sensitivity of the Ca2+ homeostat to Na+. The effect of Na+ on the extramitochondrial level was equally well observed in an K+/organic-anion buffer as in a sucrose buffer. Liver mitochondria incubated under these circumstances actively counteracted a Ca2+ or EGTA challenge by taking up or releasing Ca2+, so that the initial level, as well as the Na+-controlled level, was regained. It was concluded that liver mitochondria should be considered Na+-sensitive, that the capacity of the Na+-induced efflux pathway was of sufficient magnitude to enable it to influence the extramitochondrial Ca2+ level biochemically and probably also physiologically, and that the mitochondria have the potential to act as active, Na+-dependent regulators of extramitochondrial ('cytosolic') Ca2+. It is suggested that changes of cytosolic Na+ could be a mediator between certain hormonal signals (notably alpha 1-adrenergic) and changes in this extramitochondrial ('cytosolic') Ca2+ steady state level.     

Presence of a Ca2+-sensitive CDPdiglyceride-inositol transferase in canine cardiac sarcoplasmic reticulum

Sarcoplasmic reticulum (SR) and plasma membranes from canine left ventricle were used to evaluate the presence of the enzyme CDPdiglyceride-inositol transferase in these membranes. (K+,-Ca2+)-ATPase activity, a marker for SR, was 79.2 +/- 5.0 (SE) and 11.2 +/- 2.0 mumol.mg-1.h-1 in SR and plasma membrane preparations, respectively, and (Na+,K+)-ATPase activity, a marker for plasma membranes, was 5.6 +/- 1.2 and 99.2 +/- 8.0 mumol.mg-1.h-1, respectively. Contamination of SR and plasma membrane preparations by mitochondria was estimated to be 2% and 8%, respectively, and by Golgi membranes, 0.9% and 1.8%, respectively. Transferase activity, measured at pH 6.8, was 1.32 +/- 0.04 (SE) and 0.28 +/- 0.04 nmol of [3H]phosphatidylinositol ([3H]PtdIns).mg-1.min-1 in three SR and plasma membrane preparations, respectively. The transferase activity detected in the plasma membrane preparation could be accounted for largely, but not entirely, by contaminating SR membranes. The pH optimum for the SR transferase activity was between 8.0 and 9.0; little or no activity was detectable at pH 6.3 and 5.5, the lowest pH tested. Ca2+ inhibited the enzyme, half-maximal inhibition occurring at about 10 microM Ca2+; removal of the Ca2+ by addition of ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid restored activity. No loss of [3H]PtdIns could be detected when membranes were incubated in the presence or absence of Ca2+. The Ca2+ inhibition of the transferase was noncompetitive with respect to CDP-dipalmitin while that with respect to myo-inositol was slightly noncompetitive at low [Ca2+] and became uncompetitive at higher [Ca2+].(ABSTRACT TRUNCATED AT 250 WORDS)     

Mechanism of cis- and trans-substrate interactions at the tetraethylammonium/H+ exchanger of rabbit renal brush-border membrane vesicles

The kinetic basis for trans-effects of intravesicular substrates on the uptake of the organic cation, tetraethylammonium (TEA), into rabbit renal brush-border membrane vesicles (BBMV) was studied. Preloading BBMV with 1, 2, or 4 mM TEA stimulated the initial rate of uptake and the total net accumulation of 0.1 mM [3H]TEA. The stimulatory effect of intravesicular TEA on the initial rate of uptake was a saturable function of the trans-TEA concentration, with a half-maximal effect noted at an intravesicular concentration of 0.28 mM. A 1 mM trans-concentration of TEA increased the Jmax of [3H]TEA uptake (from 4.3 to 6.8 nmol.mg-1.min-1) without affecting the apparent Kt. An outwardly directed H+ gradient also increased Jmax (to 10.7 nmol.mg-1.min-1), although the addition of an outwardly directed TEA gradient did not produce further increases in the rate of TEA uptake. External H+ acted as a competitive inhibitor of TEA uptake, and an increase in external [H+] (from 32 nM to 100 nM) produced an increase in the apparent Kt for TEA transport (from 0.12 to 0.26 mM) without affecting the Jmax. The results suggested that TEA and H+ compete for a common site or set of mutually exclusive sites on the cytoplasmic and luminal aspects of TEA/H+ exchanger in the renal brush border, and that these sites have a similar affinity for TEA.     

Taurine stimulation of isolated hamster brain Na+,K+-ATPase: activation kinetics and chemical specificity

Epileptic foci are associated with locally reduced taurine (2-aminoethanesulfonic acid) concentration and Na+,K+-ATPase (EC 3.6.1.3) specific activity. Topically applied and intraperitoneally administered taurine can prevent the development and/or spread of foci in many animal models. Taurine has been implicated as a possible cytosolic modulator of monovalent ion distribution, cytosolic "free" calcium activity, and neuronal excitability. Taurine may act in part by modulating Na+,K+-ATPase activity of neuronal and glial cells. We characterized the requirements for in vitro modulation of Na+,K+-ATPase by taurine. Normal whole brain homogenate Na+,K+-ATPase activity is 5.1 +/- 0.4 (4) mumol Pi X h-1 X mg-1 Lowry protein. Partial purification of the plasma membrane fraction to remove cytosolic proteins and extrinsic proteins and to uncouple cholinergic receptors yields a membrane-bound Na+,K+-ATPase activity of 204.6 +/- 5.8 (4) mol Pi X h-1 X mg-1 Lowry protein. Taurine activates the Na+,K+-ATPase at all levels of purification. The concentration dependence of activation follows normal saturation kinetics (K1/2 = 39 mM taurine, activation maximum = +87%). The activation exhibits chemical specificity among the taurine analogues and metabolites: taurine = isethionic acid greater than hypotaurine greater than no activation = beta-alanine = methionine = choline = leucine. Taurine can act as an endogenous activator/modulator of Na+,K+-ATPase. Its action is mediated by a membrane-bound protein.     

Identification of two molecular forms of (Na+,K+)-ATPase in rat adipocytes. Relation to insulin stimulation of the enzyme

Two molecular forms of the (Na+,K+)-ATPase catalytic subunit have been identified in rat adipocyte plasma membranes using immunological techniques. The similarity between these two forms and those in brain (Sweadner, K. J. (1979) J. Biol. Chem. 254, 6060-6067) led us to use the same nomenclature: alpha and alpha(+). The K0.5 values of each form for ouabain (determined by inhibition of phosphorylation of the enzyme from [gamma-32P]ATP) were 3 X 10(-7)M for alpha(+) and 1 X 10(-5)M for alpha. These numbers correlate well with the K0.5 values for the two ouabain-inhibitable components of 86Rb+/K+ pumping in intact cells (1 X 10(-7) M and 4 X 10(-5)M). Quantitation of the Na+ pumps in plasma membranes demonstrated a total of 11.5 +/- 0.2 pmol/mg of membrane protein, of which 8.5 +/- 0.3 pmol/mg, or 75%, was alpha(+). Insulin stimulation of 86Rb+/K+ uptake in rat adipocytes was abolished by ouabain at a concentration sufficient to inhibit only alpha(+)(2-5 X 10(-6)M). Immunological techniques and ouabain inhibition of catalytic labeling of the enzyme from [gamma-32P]ATP demonstrated that alpha(+) was present in skeletal muscle membranes as well as in adipocyte membranes, but was absent from liver membranes. Since insulin stimulates increased Na+ pump activity in adipose and muscle tissue but not in liver, there is a correlation between hormonal regulation of (Na+,K+)-ATPase and the presence of alpha(+). We propose that alpha(+) is the hormonally-sensitive version of the enzyme.     

Calcium accumulation and release by the sarcoplasmic reticulum of digitonin-lysed adult mammalian ventricular cardiomyocytes.

We have developed a model for characterizing calcium handling by the intact cardiac sarcoplasmic reticulum (SR) that yields data consistent with both mathematical simulations of in situ SR Ca2+ uptake and deduced behavior of the Ca2(+)-induced Ca2+ efflux channels in mechanically skinned single cardiac cells. In Na(+)-based media (37 degrees C, pH 7.2, 50 mM Pi, 10 mM MgATP, pMg 3.3, 10 mM phosphocreatine), SR 45Ca2+ uptake by digitonin-lysed rat myocytes as a function of free [Ca2+] peaked at pCa 6.2, declined until pCa 5.6 and increased again at lower pCa. When Ca2(+)-induced Ca2+ efflux was inhibited with 30 microM ruthenium red and 10 mM procaine, uptake was saturable with a Vmax of 160 +/- 5 nmol.min-1.mg-1, K0.5 of 500 nM free [Ca2+] and slope factor of 1.6. In K(+)-based media, maximum Pi- and oxalate-supported uptake increased to 220 and 260 nmol.min-1.mg-1, respectively. Without phosphocreatine, 45Ca2+ uptake declined under all conditions; this was correlated with a decrease in ATP/ADP. Vmax for 45Ca2+ uptake was increased 20% in hyperthyroid myocytes but depressed 30% in myocytes from heart failure-prone rats. In canine myocytes, Vmax was the same as in normal rat cells, but K0.5 was 830 nM. Without efflux inhibitors, ryanodine caused a concentration-dependent decline in net Pi-supported 45Ca2+ uptake at pCa 6.3 (K0.5 = 1 microM), while 10 microM ryanodine depressed uptake at all pCa between 7.2 and 5.6. Ruthenium red/procaine fully reversed this effect.     

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

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

Partial purification of (Ca2+ + Mg2+)-dependent ATPase from pig smooth muscle and reconstitution of an ATP-dependent Ca2+-transport system.

(CaMg)ATPase [(Ca2+ + Mg2+)-dependent ATPase] was partially purified from a microsomal fraction of the smooth muscle of the pig stomach (antrum). Membranes were solubilized with deoxycholate, followed by removal of the detergent by dialysis. The purified (CaMg)ATPase has a specific activity (at 37 degrees C) of 157 +/- 12.1 (7)nmol.min-1.mg-1 of protein, and it is stimulated by calmodulin to 255 +/- 20.9 (7)nmol.min.mg-1. This purification of the (CaMg)ATPase resulted in an increase of the specific activity by approx. 18-fold and in a recovery of the total enzyme activity of 55% compared with the microsomal fraction. The partially purified (CaMg)ATPase still contains some Mg2+-and (Na+ + K+)-dependent ATPase activities, but their specific activities are increased relatively less than that of the (CaMg)ATPase. The ratios of the (CaMg)ATPase to Mg2+- and (Na+ + K+)-dependent ATPase activities increase from respectively 0.14 and 0.81 in the crude microsomal fraction to 1.39 and 9.07 in the purified preparation. During removal of the deoxycholate by dialysis, vesicles were reconstituted which were capable of ATP-dependent Ca2+ transport.     

Relationship between gill Na+,K+-activated ATPase activity and osmotic stress in the plecopteran nymph, Paragnetina media

A relationship between osmotic stress and gill Na+,K+-activated ATPase was observed in plecopteran nymphs acclimated to diluted creek water and a hypertonic medium. An increase of 84% in diluted creek water is presumably related to an active uptake of sodium ions from the hypotonic medium. Whereas a 21% decrease in the enzyme activity may be related to the morphological changes in the specialized cells in the gills. The Na+,K+-activated ATPase activity was also compared with the Malpighian tubules and the rectum. The highest ATPase activity of 32.6 +/- 2 mumoles Pi mg protein-1 30 min-1 was observed in the Malpighian tubules. The activity in the gills (19 +/- 1.2 mumoles Pi mg-1 30 min-1) was slightly lower than the rectum. Since the ATPase activity in the gills is quite high, the gills can be considered to play an active role in hyperosmotic regulation in plecopteran nymphs.     

Properties and molecular cloning of Ca2+/H+ antiporter in the vacuolar membrane of mung bean.

Kinetic and molecular properties of the Ca2+/H+ antiporter in the vacuolar membrane of mung bean hypocotyls were examined and compared with Ca2+-ATPase. Ca2+ transport activities of both transporters were assayed separately by the filtration method using vacuolar membrane vesicles and 45Ca2+. Ca2+ uptake in the presence of ATP and bafilomycin A1, namely Ca2+-ATPase, showed a relatively low Vmax (6 nmol.min-1.mg-1 protein) and a low Km for Ca2+. The Ca2+/H+ antiporter activity driven by H+-pyrophosphatase showed a high Vmax (25 nmol.min-1.mg-1) and a relatively high Km for Ca2+. The cDNA for mung bean Ca2+/H+ antiporter (VCAX1) codes for a 444 amino-acid polypeptide. Two peptide-specific antibodies of the antiporter clearly reacted with a 42-kDa protein from vacuolar membranes and a cell lysate from a Escherichia coli transformant in which VCAX1 was expressed. These observations directly demonstrate that a low-affinity, high-capacity Ca2+/H+ antiporter and a high-affinity Ca2+-ATPase coexist in the vacuolar membrane. It is likely that the Ca2+/H+ antiporter removes excess Ca2+ in the cytosol to lower the Ca2+ concentration to micromolar levels after stimuli have increased the cytosolic Ca2+ level, the Ca2+-ATPase then acts to lower the cytosolic Ca2+ level further.     

Acetylene Reduction by Symbiosomes and Free Bacteroids from Broad Bean (Vicia faba L.) Nodules (Role of Oxalate)

We report the presence of oxalate in the organic acid fraction of broad bean (Vicia faba L.) nodule cytosol. Using both high-performance liquid chromatography and enzymic assays, high levels of oxalate were detected (70.4 [plus or minus] 2.4 mM). To study the potential role of oxalate as an energy-yielding substrate for nitrogenase activity, free bacteroids were isolated from nodules and found to oxidize oxalate in support of C2H2 reduction under O2 tensions that were lower than those required to oxidize succinate, another dicarboxylate commonly detected in legume nodules. Symbiosomes of broad bean, isolated for the first time from amide-producing nodules, were provided with [14C]oxalate and found to have uptake kinetics with a lower affinity [Km(oxalate) = 330 [mu]M] than that for free bacteroids [Km(oxalate) = 130 [mu]M]. In anaerobic preparations of symbiosomes supplied with purified oxyleghemoglobin, O2 consumption was stimulated by oxalate from 20.2 [plus or minus] 0.8 nmol O2 min-1mg-1 protein to 24.5 [plus or minus] 1.1 nmol O2 min-1 mg-1 protein but always remained lower than the rate of O2 consumption in free bacteroids (32.2 [plus or minus] 1.4 nmol O2 min-1 mg-1 protein). Under these conditions, C2H2 reduction activity was 9.7 [plus or minus] 0.8 and 15.1 [plus or minus] 0.9 nmol C2H4 min-1 mg-1 protein for symbiosomes and bacteroids, respectively. These data support the suggestion that oxalate may play a role as a carbon substrate in support of N2 fixation in broad bean nodules.     

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.