THE BIOCHEMISTRY OF SODIUM TRANSPORT

The connexion between active cation transport and metabolism in erythrocytes

1. A study has been made of the dependence on the concentrations of internal Na(+) and external K(+) of lactate and phosphate production in human erythrocytes. 2. Lactate production was stimulated by Na(+) and K(+) but only when they were internal and external respectively. The stimulation was counteracted by ouabain. The production of phosphate was affected in the same way. 3. There is a quantitative correlation between these effects and those previously found for cation movements and the membrane adenosine triphosphatase. 4. It is concluded that the rate of energy production in glycolysis is partly controlled by the magnitude of active transport; the extent of this regulation is shown to vary from 25 to 75% of a basal rate that is independent of active transport. 5. The activity of the membrane adenosine triphosphatase was also compared with rates of Na(+) and K(+) transport. The latter were varied by altering the concentrations of internal Na(+) and external K(+), and by inhibiting with ouabain. 6. A threefold variation of active transport rate was accompanied by a parallel change in the membrane adenosine-triphosphatase activity. The results show a constant stoicheiometry for the number of ions moved/mol. of ATP hydrolysed, independent of the electrochemical gradient against which the ions were moved. 7. Calculations show that the amount of ATP hydrolysed would provide enough energy for the osmotic work. The results are discussed in relation to possible mechanisms for active transport.

Adenosine-triphosphatase activity and nicotinamide nucleotide coenzymes in the parotid gland of the young lamb and adult sheep

1. The activity of a Mg(2+)-dependent Na(+)-plus-K(+)-activated adenosine triphosphatase and the concentrations of nicotinamide nucleotide coenzymes have been measured in the immature parotid glands of young lambs and in the actively secreting glands of adult sheep. 2. The activity of the adenosine triphosphatase increased during development and attained relatively high levels in the mature secreting gland. 3. A high ([NAD]+[NADH(2)])/([NADP]+[NADPH(2)]) ratio (approx. 10:1) was observed in the parotid glands of lambs and sheep. 4. The high concentrations of NAD and the very low concentrations of NADPH(2) have been discussed in relation to metabolic activity, the activity of the Na(+)-plus-K(+)-activated adenosine triphosphatase and the secretion of saliva by the parotid gland.

Effect of sodium on cellular calcium transport in rat kidney

The influence of extracellular Na (Na0) on cellular Ca transport and distribution was studied in rat kidney slices. Calcium efflux from prelabeled slices was depressed when Na0 was completely replaced by choline or tetraethylammonium (TEA) ions and it was markedly stimulated when Na was reintroduced in a Na-free medium. However, reducing Na0 (with choline or TEA as substituting ions) did not increase the total slice 40Ca, their total exchangeable Ca pool, or the 40Ca or 45Ca of mitochondria isolated from these slices. Kinetic analyses of steady-state 45Ca desaturation curves showed that reducing Na0 depressed the exchange of Ca across the plasma membrane, slightly decreased the cytosolic Ca pool, but did not significantly affect the mitochondrial Ca pool and Ca cycling. Ouabain (10(-3)M) which should reduce the Na gradient across the plasma membrane had no effect on calcium distribution and transport. These results suggest that in kidney cells low Na0 depresses Ca influx as well as Ca efflux; there may be an interaction between Na and Ca at a possible carrier located in the plasma membrane, but there is no Na/Ca exchange as described in several excitable tissues.

Solvent effects on ouabain binding to the (Na+,K+)-ATPase of rat brain

The effects of the solvents deuterated water (2H2O) and dimethyl sulfoxide (Me2SO) on [3H]ouabain binding to (Na+,K+)-ATPase under different ligand conditions were examined. These solvents inhibited the type I ouabain binding to the enzyme (i.e., in the presence of Mg2+ + ATP + Na+). In contrast, both solvents stimulated type II (i.e., Mg2+ + Pi-, Mg2+-, or Mn2+-dependent) binding of the drug. The solvent effects were not due to pH changes in the reaction. However, pH did influence ouabain binding in a differential manner, depending on the ligands present. For example, changes in pH from 7.05 to 7.86 caused a drop in the rate of binding by about 15% in the presence of Mg2+ + Na+ + ATP, 75% in the Mg2+ + Pi system, and in the presence of Mn2+ an increase by 24% under similar conditions. Inhibitory or stimulatory effects of solvents were modified as various ligands, and their order of addition, were altered. Thus 2H2O inhibition of type I ouabain binding was dependent on Na+ concentration in the reaction and was reduced as Na+ was elevated. Contact of the enzyme with the Me2SO, prior to ligands for type I binding, resulted in a greater inhibition of ouabain binding than that when enzyme was exposed to Na+ + ATP first and then to Me2SO. Likewise, the stimulation of type II binding was greater when appropriate ligands acted on enzyme prior to addition of the solvent. Since Me2SO and 2H2O inhibit type I ouabain binding, it is proposed that this reaction is favored under conditions which promote loss of H2O, and E1 enzyme conformation; the stimulation of type II ouabain binding in the presence of the solvents suggests that this type of binding is favored under conditions which promote the presence of H2O at the active enzyme center and E2 enzyme conformation. This postulation of a role of H2O in modulating enzyme conformations and ouabain interaction with them is in concordance with previous observations.

Effects of polyamines on partial reactions of membrane (Na+ + K+)-ATPase

Spermine and spermidine inhibit the (Na+ + K+)-ATPase (ATP phosphohydrolase, EC 3.6.1.3) reaction so that the effect increases as the ionic content due to Na+ and K+ in the reaction is reduced. Several other amines inhibit (Na+ + K+)-ATPase to varying degress and methylglyoxal-bis-(guanylhydrazone) was the most potent inhibitor among those tested. The inhibition by polyamines of the ATPase is uncompetitive with respect to Mg2+ and ATP activation of the reaction. Various naturally occurring polyamines and other amines inhibited Na+ activation of (Na+ + K+)-ATPase as well as Na+-dependent phosphoenzyme formation in an apparently competitive manner with respect to Na+. Likewise, K+-activation of (Na+ + K+)-ATPase as well as K+-p-nitrophenyl phosphatase was inhibited in an apparently competitive manner with respect to K+. Both the cation charge and structure (e.g., aliphatic chain length) may contribute to the inhibitory effects of the amines; however, Na+ sites appear to be more sensitive to cation charge than the aliphatic chain length of the amine, whereas the opposite appears to be true for K+ sites. The results do not indicate a specific effect of polyamines on (Na+ + K+)-ATPase or its partial reactions.

Blockage of depolarization-induced mitogenesis in CNS neurons by 5-fluoro-2'-deoxyuridine

Experiments designed to provide further evidence, at the basic metabolic level, that true mitogenesis and mitotic activity are being induced in CNS neurons in response to sustained ionic depolarization were conducted. The ability of 5-fluoro-2'-deoxyuridine (FUdR), a well-studied inhibitor of normal mitogenesis in naturally proliferating cells, to block induction of DNA synthesis (and subsequent nuclear division) in culture-matured neurons depolarized with ouabain was ascertained, as well as the ability of exogenously supplied thymidine to permit effective bypass of such blockage. Observations of the sequence of intracellular morphological changes induced by ouabain were also made, along with a determination of the alterations in this sequence introduced by FUdR. The results indicate that ouabain mediated depolarization rapidly induces and/or activates the key mitogenic enzyme thymidylate synthetase in the mitotically quiescent neurons, along with all other mitogenesis-specific enzymes required for DNA synthesis and nuclear division. A probable mechanism by which such mitogenic induction may proceed is elaborated. The early morphological changes observed correlate well with the early time-sequence of mitogenic metabolic events, while development of the later changes appears to be dependent upon the progress of mitogenesis activity. The results support the possibility that CNS neurons of adult origin may also be induced to initiate normal mitogenesis by appropriately imposed depolarization treatments.

Calcium metabolism in Ehrlich Ascites tumour cells

Ehrlich ascites tumour cells are able, under the proper experimental conditions, to extrude a substantial amount of Ca2+ from the intracellular space. The Ca2+ extrusion mechanism, probably located at the plasma membrane level, appears to be similar to that found in red blood cells. It is energy-dependent and both respiration and glycolysis are able to drive it. The use of some inhibitors and uncouplers, besides showing that this activity is different from that linked to the mitochondrial Ca2+ pump which acts in the opposite direction, proposes some speculations on the energy compartmentation in the Ehrlich ascites tumour cells.

Interrelationships of sodium transport and carbon dioxide production by the toad bladder: response to changes in mucosal sodium concentration, to vasopressin and to availability of metabolic substrate

Active sodium transport and CO2 production were measured simultaneously in toad bladders mounted in membrane chambers. The rate of sodium transport was varied by changing the concentration of sodium in the mucosal bath (substitution with choline), by adding vasopressin, by adding metabolic substrates and by adding malonate, and the ratio of the change of sodium transport and CO2 production was determined Mean values for deltaNa/deltaCO2 (equiv/mole) were: Na in equilibrium choline 18.3 +/- 1.1; vasopressin 15.5 +/- 2.8; and pyruvate (corrected for the increment in "nontransport" CO2) 15.4 +/- 3.5. Based on previously determined values for the respiratory quotient (R.Q.), calculated mean values for deltaNa/deltaO2 ranged between 15.5 and 18.5 equiv/mole. It appears that basal metabolism does not contribute to metabolism supporting sodium transport when the rate of sodium transport is varied. "Transport" metabolism appears much more responsive to changes in the availability of endogenous and exogenous substrates than does "nontransport" metabolism. We conclude that "transport" and "nontransport" metabolism are functionally separated in the toad bladder.

Regulation of ion permeabilities of isolated rat liver cells by external calcium concentration and temperature

Regulation of ion transport through the plasma membrane was studied on single cell suspensions of hepatocytes, obtained after perfusion of rat liver with collagenase/hyaluronidase solution. Steady-state intracellular K and Na contents were shown to be markedly dependent on external Ca concentration and temperature, the sum of both ion concentrations remaining nearly constant. In contrast, steady-state intracellular chloride content was found to be independent of external Ca concentration, but dependent on temperature. Using the constant field relations, the passive permeabilities PK and PCl for potassium and chloride, respectively, were derived from the experimental data. At temperatures at and above 37 degrees C, with increasing external Ca concentration, PK, exhibits a sharp decrease at about 10(-4)M. In contrast, PCl at 37 degrees C was found to be independent of Ca concentration within experimental error. Earth alkali ions other than Ca, show marked but different effects on PK if compared at equal concentrations. Preincubation of the cells with cholesterol leads to a broadening of the dependence of PK on external Ca concentration. The above results, as well as those on the dependence of PK on external Ca concentration obtained by other authors, could be quantitatively described by a theoretical model of the plasma membrane proposed earlier. This model postulates regulatory binding sites, which cooperatively undergo a cation exchange of divalent cations by K+ ions from the external medium if the cation composition of the latter is altered.

Na+-dependent phosphorylation of the rat brain (Na+ + K+)-ATPase. Possible non-equivalent activation sites for Na+

The steady state levels of Na+-dependent phosphoenzyme (E-P) in the (Na+ + K+)-ATPase (EC 3.6.1.3) of rat brain, obtained from a time course study of phosphoenzyme formation at 4 degrees C, were dependent on the concentration of Na+ in the reaction and were maximal in the presence of 64 mM Na+. The plot of phosphoenzyme vs. Na+ concentration gave a curve which on conversion to a double reciprocal plot (1/E-P vs. 1/Na+) gave a line with two breaks, yielding apparently three linear segments. This may be taken to indicate the presence of multiple Na+ sites for the formation of the phosphoenzyme. To test this hypothesis further, the following approach was taken. By making the assumption that the phosphoenzyme may represent bound Na+, it was possible to subject the data to rigorous multiple-site analysis by utilizing steady-state binding equations described by Klotz and Hunston (1971) (Biochemistry 10, 3065-3069), and by Scatchard (1949) (Ann. N.Y. Acad. Sci. 51, 660-672). The analysis of the data by these methods suggests that there may be three non-equivalent Na+ activation sites for the formation of Na+-dependent phosphoenzyme in the (Na+ + K+)-ATPase. The estimated intrinsic association constants (Ka) for activation by Na+ at each of the three sites were 3.4, 0.295, and 0.025 mM-1, respectively.

Time course and zonal variations of ischemia-induced myocardial cationic electrolyte derangements

Myocardial cationic electrolytes were determined at regular time intervals up to 24 hours after coronary artery ligation in the dog. Replicate electrolyte ratios were computed for different areas of the heart at each time interval. For purposes of statistical analysis, ratios from two border areas and four areas remote from the infarct were pooled as values for ZONE B and ZONE N, respectively, and compared with those from the infarct proper, ZONE I. Ischemia-induced tissue Mg++/Ca++ changes paralleled those of K+/Na+ with respect to time course and zonal variations. In ZONE I, both K+/Na+ and Mg++/Ca++ fell precipitously during the first hour, and the falls became more gradual thereafter, approac hing those of extracellular fluid at 24 hours. Changes in ZONE B, which appeared normal histologically, followed a similar downward trend but differed in magnitude from those in ZONE I (P smaller than 0.01). Changes in ZONE N were small but did not always overlap values in sham-operated dogs. It was concluded that lowered tissue K+/Na+ and Mg++/Ca++ were sensitive, but not specific, indices of myocardial ishemia, and multiple samplings of ionic ratios were essential for proper interpretation of ischemia-induced myocardial electrolyte derangements.

Calcium-binding properties and ATPase activities of rat liver plasma membranes

Plasma membranes from rat liver purified according to the procedure of Neville bind calcium ions by a concentration-dependent, saturable process with at least two classes of binding sites. The higher affinity sites bind 45 nmol calcium/mg membrane protein with a K(D) of 3 microM. Adrenalectomy increases the number of the higher affinity sites and the corresponding K(D). Plasma membranes exhibit a (Na(+)-K(+))-independent-Mg(2+)-ATPase activity which is not activated by calcium between 0.1 microM and 10 mM CaCl(2). Calcium can, with less efficiency, substitute for magnesium as a cofactor for the (Na(+)-K(+))-independent ATPase. Both Mg(2+)- and Ca(2+)-ATPase activities are identical with respect to pH dependence, nucleotide specificity and sensitivity to inhibitors. But when calcium is substituted for magnesium, there is no detectable membrane phosphorylation from [gamma-(32)P] ATP as it is found in the presence of magnesium. The existence of high affinity binding sites for calcium in liver plasma membranes is compatible with a regulatory role of this ion in membrane enzymic mechanisms or in hormone actions. Plasma membranes obtained by the procedure of Neville are devoid of any Ca(2+)-activated-Mg(2+)-ATPase activity indicating the absence of the classical energy-dependent calcium ion transport. These results would suggest that the overall calcium-extruding activity of the liver cell is mediated by a mechanism involving no direct ATP hydrolysis at the membrane level.

Turnover of protein-bound phosphorylserine in membrane preparations from ox brain catalysed by intrinsic kinase and phosphatase activity

1. The turnover of protein-bound phosphorylserine in preparations of membrane fragments from ox brain cortex was studied. 2. Turnover was considered to arise from the action of intrinsic protein kinases and phosphatases on a membrane protein or proteins. 3. Properties of the kinase system were studied by measuring the rate of incorporation of (32)P from gamma-labelled ATP into protein-bound phosphorylserine isolated from partial acid hydrolysates of membrane proteins. 4. Properties of the phosphatase system were studied by observing the rate of loss of (32)P from membrane preparations pre-labelled with [(32)P]ATP. 5. Net phosphorylation and dephosphorylation of membrane protein was observed during incubation of membrane preparations with and without ATP. 6. The rate of turnover was about 4nmol of P/h per mg of protein at 20 degrees C; dephosphorylation was considered to be the rate-limiting step.