Downhill Movement Research Articles

Sodium-coupled transporters for Krebs cycle intermediates.

Krebs cycle intermediates such as succinate, citrate, and alpha-ketoglutarate are transferred across plasma membranes of cells by secondary active transporters that couple the downhill movement of sodium to the concentrative uptake of substrate. Several transporters have been identified in isolated membrane vesicles and cells based on their functional properties, suggesting the existence of at least three or more Na+/dicarboxylate cotransporter proteins in a given species. Recently, several cDNAs, called NaDC-1, coding for the low-affinity Na+/dicarboxylate cotransporters have been isolated from rabbit, human, and rat kidney. The Na+/dicarboxylate cotransporters are part of a distinct gene family that includes the renal and intestinal Na+/sulfate cotransporters. Other members of this family include a Na(+)- and Li(+)-dependent dicarboxylate transporter from Xenopus intestine and a putative Na+/dicarboxylate cotransporter from rat intestine. The current model of secondary structure in NaDC-1 contains 11 transmembrane domains and an extracellular N-glycosylated carboxy terminus.

The growth dynamics of energetic cluster impact films

Dense and smooth thin films can be produced by the deposition of energetic clusters onto a solid surface. Roughnes, induced by random deposition of the clusters, is strongly suppressed by a downhill smoothing process. Molecular dynamic simulations of the impact of copper clusters onto tilted copper surfaces demonstrate that a downhill particle current proportional to the local slope of the surface is initiated. In the long wavelength limit the evolution of the surface profile is governed by the Edwards-Wilkinson equation. One parameter in this equation is related to the strength of the downhill movement due to the cluster impact and can be determined from the simulations. From the solution of the spatially Fourier transformed growth equation the power spectrum is calculated, which is in agreement with atomic force microscopy measurements for copper films.

Bioenergetics of neurotransmitter transport.

Neurotransmitter transporters are essential components in the recycling of neurotransmitters released during neuronal activity. These transporters are the targets for important drugs affecting mood and behavior. They fall into at least four gene families, two encoding proteins in the plasma membrane and two in the synaptic vesicle membrane, although the known vesicular transporters have not all been cloned. Each of these transporters works by coupling the downhill movement of small ions such as Na+, Cl-, K+, and H+ to the uphill transport of neurotransmitter. Plasma membrane transporters move the transmitter into the cytoplasm by cotransport with Na+. Many transporters also couple Cl- cotransport to transmitter influx and these all belong to the NaCl-coupled family, although within the family the coupling stoichiometry can vary. Transporters for glutamate couple influx of this excitatory amino acid to Na+ and H+ influx and K+ efflux. Transporters in synaptic vesicles couple H+ efflux to neurotransmitter transport from the cytoplasm to the vesicle lumen.

Electric field pulses induce reversible shape transformation of human erythrocytes.

Electric field pulses > 2-3 kV cm-1, long known to induce membrane poration and fusion of erythrocytes as well as to enhance the transbilayer mobility of phospholipids and to perturb aminophospholipid asymmetry, are shown to induce, at 0 degree C, transformation of the discocytic cells into echinocytes and spheroechinocytes. The extent of transformation increases with strength, duration and number of pulses. Its time course is biphasic, a major rapid phase (t/2 approximately 5 s) being followed by a minor one, lasting for 2-3 h. Shape transformation goes along with the exofacial exposure of phosphatidylserine (PS), detected by FITC-annexin V binding and quantified by a calibration curve established via externally inserted dilauroylphosphatidylserine. Incubation of these echinocytes at 37 degrees C leads to a rapid recovery of the discocytic shape followed by slower formation of stomatocytes. Shape recovery is temperature dependent (Ea approximately 100 kJ/mol), and can be impaired by depletion of ATP or Mg++ and by addition of vanadate or fluoride. Shape recovery and stomatocyte formation go along with a rapid loss of annexin binding in about 45% of the cells while the rest maintains its binding capacity. In the presence of vanadate, annexin binding increases in all cells. The results are discussed in the light of the bilayer couple concept of erythrocyte shape and the enhanced transverse mobility of phospholipids. Echinocyte formation is most likely caused by the reorientation of endofacial aminophospholipids to the outer leaflet of the bilayer. Shape recovery and stomatocyte formation probably result from a continuous reinternalization of PS via the ATP dependent aminophospholipid translocase, but may also be supported by downhill movement of PC to the inner leaflet and by other yet unidentified processes.

Coupling as a way of life.

stating that any species (S) of ion or molecule capable of f low ( J) across a biological membrane may do so under the influence of several categories of driving force: that due to thermal agitation acting on a concentration difference S, that due to an electric field ( ), that due to water flow ( JW) and drag, that due to chemical/physical interaction with other transiting ions or molecules ( JA, JB, ...) and that due to chemical coupling with metabolic reactions (JR). This formalism neatly solved a major conceptual problem of the time, i.e. how to define (primary) active transport in an unequivocal and still useful way: as endergonic transport due to coupling with JR alone, where the coupled exergonic reaction would provide the driving energy. [In this version of the Kedem equation, Rs is the resistance to movement of the solute, S; and RSW, RSA, R SB..., are the operational resistances of the membrane to the coupled flows.] A logical spin-off of this exercise was another useful definition, that of secondary active transport: i.e. transport of S coupled to JW, JA, JB, etc.), wherein S moves up its electrochemical gradient (usually concentrated into cytoplasm) at the expense of downhill movement of W, A, B, etc. Passive transport was taken by default as being due to S, or , with both of these resultant flows being exergonic and requiring no other input of energy.

Cation antiports of animal plasma membranes.

ABSTRACT Both organic and inorganic solutes can be transported across biological membranes against their electrochemical gradient, by coupling to the downhill movement of another species. In animal cells, the underlying mechanism most often involves symport (cotransport) or antiport (countertransport) with either sodium or hydrogen ions. The energy invested in the uphill translocation of solutes is ultimately provided by hydrolysis of ATP by Na+-or H+-transporting ATPases and is transiently stored as potential energy in the form of an electrochemical gradient of these ions.

Downslope movements at shallow depths related to cyclic pore-pressure changes

Slow, shallow ground movements in a slope near Yellowknife caused excessive tilting of timber piles that supported an engineering structure. To avoid damage to the structure, the pile foundations had to be replaced by rigid concrete piers that were designed to resist the forces of the moving soil mass. Downhill movements were rather slow and, during an initial inspection, were indicated only by soil that was pushed up against a series of piles on their uphill sides, while gaps had formed on their downhill sides. No open cracks or bulging was observed on the slope. A stability analysis indicated that the slope was not in a state of limit equilibrium. To obtain a better understanding of the creep movements in the slope and their effect on the rigid concrete piers, extensive instrumentation was carried out after the construction of the piers. This included slope indicators, piezometers, thermistors, and total-pressure cells against one of the concrete piers. In addition, a triaxial testing program was undertaken in which the effect of cyclic pore-water pressure changes on the long-term deformations of the shallow clay layer was investigated. From the data collected in the field and laboratory, it could be concluded that (i) tilting of the original timber piles was caused by downslope movements related to cyclic pore-water increases; (ii) the lateral soil movements increased almost linearly with depth from 2 m below the ground surface, with no indication of a slip surface; and (iii) the pressures exerted by the moving soil mass against the rigid concrete piers within the soil mass were equal to the passive resistance activated within the moving soil mass. Key words : soil creep, slope movements, soil pressures, pore-water pressures, freezing pressures, permafrost, cyclic loading.

Comparison of the renal sodium‐D‐glucose cotransporter in marine organisms

AbstractA phylogenetic comparison of the structural and functional properties of the sodium‐D‐glucose cotransporter from a variety of marine species is presented. Found in the apical membrane of epithelial cells of kidney and intestine, this transport protein mediates the uphill movement of glucose coupled to the downhill movement of sodium ions. Monoclonal antibodies to the rabbit cotransporter react with a 78,000 Da protein in renal brush border membranes from skate, dogfish, and flounder, but not toadfish (which lacks functional expression of the cotransporter). Amplification of cotransporter DNA sequences by the polymerase chain reaction correlates well with the antibody results. Additional molecular studies of the sodium‐D‐glucose cotransporter promise to elucidate further the structure‐function relationship, particularly with respect to species‐specific stoichio‐metries. © 1993 Wiley‐Liss, Inc.

Diurnal and Nocturnal Habitat Utilisation by the Whiptail Wallaby, Macropus Parryi.

Diurnal and nocturnal habitat use was determined from walk transect counts in a 40 000 ha study area in SE Queensland containing 3 land systems: (a) alluvial flats mainly clear of natural vegetation; (b) hills extending from the alluvial flats with open forest; and (c) plateaux with grassy and shrubby open forest. Habitat utilization was examined in relation to 3 vegetation variables (shrub density, live tree density and dead tree density) and 3 topographic variables (altitude, aspect and slope). Utilization was most consistent between day and night for live tree density and slope, being biased toward areas of moderate live tree density and against flat areas at both times. A preference for areas with moderate shrub density was more pronounced during the day than at night. Areas with moderate dead tree density were strongly preferred over areas with low dead tree density during the day, but at night there was no strong bias for or against areas with dead trees. M. parryi avoided very low altitudes at all times. Utilization of higher areas varied day and night, a diurnal preference for higher altitudes being ameliorated by some apparent downhill movement to lower slopes at night. A preference during the day for north and west aspects was not evident at night.

Electrogenic sodium‐calcium exchange in cultured embryonic chick heart cells.

1. The membrane potential (Em) of cultured chick embryonic heart cells depolarized to -36 mV after inhibition of the Na+-K+ pump by 0.1 mM-ouabain in a [K+]o of 24 mM: this was accompanied by a rise in Na+ content of approximately 65% in 3 min. Lowering [Na+]o to 27 mM then caused a fall in Na+ content, a rise in Ca2+ content and a small hyperpolarization of approximately 5 mV. The fall in Na+ content indicated a movement of Na+ which was in the opposite direction to the Na+ electrochemical gradient (a countergradient movement). 2. In the presence of 10 mM-Cs+ or 1 mM-Ba2+ the hyperpolarization was approximately 10 or approximately 30 mV, respectively. A 30 mV hyperpolarization took Em negative to the reversal potentials for K+, and Cl- as measured by ion-selective micro-electrodes. 3. The decay of the intracellular Na+ activity alpha iNa, in an [Na+]o of 27 mM followed a simple exponential time course (time constant, 36 s). The initial rate depended on the value to which [Na+]o was lowered in a manner suggesting a simple competitive inhibition of the exchange by external Na+. 4. The low-[Na+]o hyperpolarization was unaffected by amiloride (0.1 or 1 mM) or verapamil (20 microM). Both La3+ (1 mM) and Mn2+ (20 mM) blocked the hyperpolarization sufficiently to prevent Em hyperpolarizing negative to the reversal potentials for K+, Na+ and Cl-. 5. Re-establishing [Na+]o caused a rise in Na+ content and a countergradient drop in Ca2+ content. The effects of verapamil (20 microM), amiloride (0.1 and 1 mM), dichlorobenzamil (0.1 mM), quinidine (1 mM), Mn2+ (20 mM) and La3+ (1 mM) were tested on the movements of Na+ and Ca2+ both during exposure to an [Na+]o of 27 mM and on re-establishing [Na+]o. The only consistent and substantial effects were the attenuation by La3+ and Mn2+ and Ca2+ movements during exposure to an [Na+]o of 27 mM. However, neither La3+ nor Mn2+ affected the movements of Na+ and Ca2+ on re-establishing [Na+]o. 6. We conclude that cultured embryonic chick heart cells contain a Na+-Ca2+ exchange evidenced by the ability to cause movements of Na+ and Ca2+ which are counter to their respective electrochemical gradient and which are accompanied by downhill movements of the counter ion.(ABSTRACT TRUNCATED AT 400 WORDS)

Driving forces for the uphill transport of amino acids into epidermal brush border membrane vesicles of the sea anemone, Anemonia sulcata (Cnidaria, anthozoa)

1. 1. The transport of amino acids into membrane vesicles prepared from epidermal tentacle tissue of the sea anemone, Anemonia sulcata, depends on an electrochemical potential difference caused, e.g. by sodium chloride gradients. 2. 2. Potassium or choline chloride gradients energized the transport less effectively than sodium chloride gradients. Both Na +-ions and Cl −-ions were required for the amino acid transport. 3. 3. The uphill transport of amino acids along the downhill movement of driver ions (sodium chloride gradient conditions) was characterized by an overshoot; under sodium chloride equilibrium conditions, however, an accumulation of amino acids within the vesicles could not be measured. 4. 4. Potassium diffusion potentials in combination with valinomycin indicated that hyperpolarization (vesicle inside negative) and hypopolarization (vesicle inside positive) enhanced or depressed the accumulation of amino acids within the vesicles. 5. 5. Being at the phylogenetic base of the Eumetazoa, cnidarians show characteristics for the transmembrane transport of amino acids comparable to those established for vertebrates.

An example of a peat flow near Prince Rupert, British Columbia

Peat flows, bog flows, or bog bursts consist of a rapid downhill movement of masses of saturated peat. Although this process has been documented from peatlands in other parts of the world, the slope movement described here is the first to be reported from Canadian peatlands. The peat flow took place on the east coast of Kaien Island, near Prince Rupert, British Columbia, and was initiated by a slump in a peat spoil pile. It involved the sudden mobilization of a strip of in situ peat 210 m long and approximately 20 m wide. The peat was fibrous, rich in roots, and had a moisture content of approximately 240%. The flow demonstrates the high potential mobility of natural peat covers and the role of undrained loading in effecting movement of slopes as low as 5°. Key words: peat, flow slide, peat flow, northeast British Columbia.

Soil movements on permafrost slopes near Fairbanks, Alaska

Many north-facing slopes in the Yukon–Tanana Uplands of Alaska show signs of downhill movements in the form of hummocky surface and leaning trees. Measurements of movements and pore pressures were made at several sites in the Caribou–Poker Creek Research Watershed. It was found that, on slopes with inclinations near 30°, most of the movements occurred immediately after thaw when pore pressures were high.To evaluate the slope stability, the soil strength was measured by direct shear tests. The strength of the moss layer and tree roots was evaluated by performing tension tests on the roots and the moss–root complex. Results of stability analyses show that local failures involving individual wedges are likely. The displacements associated with a wedge slide would result in a hummock or step. The strength of the moss–root complex was found to be a significant factor in the stability of the wedge and of the step. Key words: moss, roots, permafrost, pore pressure, shear strength, slope stability, thaw.

Ki+-Nao+ countertransport in erythrocytes of patients with essential hypertension.

1. We describe in this paper a new ouabain-insensitive pathway for Na+ and K+ in human erythrocytes. K+ efflux was measured in cells loaded by the p-chloromercuribenzene-sulphonate (PCMBS) procedure to contain approximately equal amounts of Na+ and K+. K+ efflux was stimulated by external Na+ in the presence of ouabain and frusemide. Na+-stimulated K+ efflux was 0.35 +/- 0.12 (mmol h-1 l-1 of cells) in eight normal subjects and 0.64 +/- 0.13 in 13 patients with essential hypertension. 2. The Na+-stimulated K+ efflux was not observed in cells loaded in the presence of EGTA. This inhibition by EGTA suggests that K+ efflux is dependent on intracellular calcium. The Ki+-Nao+ countertransport of hypertensive patients was also inhibited by EGTA. The elevated K+-Na+ countertransport of hypertensive patients could be due to a higher intracellular Ca2+ content (Ca2i+) or to an increased affinity for Cai2+. The relationship of this pathway to the Gardos effect is not clear since Na+-stimulated K+ efflux occurs without metabolic depletion or inhibition of the Ca2+ pump. As a tentative hypothesis, we relate the Ca2+-dependent downhill movement of K+ and Na+ to the Ca2+-dependent channels described in muscle and nerve, but other hypotheses cannot be excluded at this stage of our research.

Downhill movements of the slopes at the Idrija ore deposit : Sovinc, I Proc 4th Congress International Society for Rock Mechanics, Montreux, 2–8 Sept 1979, Vol 1, P737–743. Publ Rotterdam, A A Balkema, 1979

Downhill movements of the slopes at the Idrija ore deposit : Sovinc, I Proc 4th Congress International Society for Rock Mechanics, Montreux, 2–8 Sept 1979, Vol 1, P737–743. Publ Rotterdam, A A Balkema, 1979

Mechanism of stimulation by epinephrine of active transepithelial Cl transport in isolated frog cornea.

In the isolated frog cornea, the effects of 0.1mm epinephrine were measured on both the transepithelial and intracellular electrical parameters. Epinephrine increased the short-circuit current (Isc) and transepithelial electrical conductance (gt) by 176 and 96%, respectively. The effective electromotive driving force for active transepithelial Cl transport (ECl) was 45 mV and agrees with the value forECl calculated by a different technique in the isolated rabbit corneal epithelium (Klyce, S.D., Wong, R.K.S., 1977,J. Physiol. (London)266: 777). With respect to the tear-side bathing solution, epinephrine caused the intracellular potential difference of shortcircuited frog corneas to decrease from −54 to −50 mV (P>0.05). The fractional resistance of the apical membrane {F(Ro)=(Ro/Ro+Ri)} whereRo andRi represent the resistances of the apical and basolateral membranes, respectively, decreased from 0.38±0.06 to 0.23±0.03. Using these values ofF(Ro) and the cellular conductances, the calculated Cl resistances ofRo andRi decreased 4.3- and 2.3-fold, respectively. However, the value forECl calculated from the intracellular electrical measurements (48 mV) did not appear to change since this value was in close agreement with the value forECl calculated from the effects of epinephrine on the transepithelial electrical parameters. Thus, the effects of epinephrine onIsc andgt can be accounted for by increases in the Cl conductance of both the apical and basolateral membranes. Epinephrine caused the potential difference across the basolateral membrane to hyperpolarize by 9 mV. All of these results are consistent with the notion that the steps in transepithelial Cl transport include uphill movement into the cell across the basolateral membrane followed by downhill movement across the apical membrane into the tear-side bathing solution.

Mechanism of active chloride secretion by shark rectal gland: role of Na-K-ATPase in chloride transport.

The isolated rectal gland of Squalus acanthias was stimulated to secrete chloride against an electrical and a chemical gradient when perfused in vitro by theophylline and/or dibutyryl cyclic AMP. Chloride secretion was depressed by ouabain which inhibits Na-K-ATPase. Thiocyanate and furosemide also inhibited chloride secretion but ethoxzolamide, a carbonic anhydrase inhibitor, did not. Chloride transport was highly dependent on sodium concentration in the perfusate. The intracellular concentration of chloride averaged 70-80 meq/liter in intact glands, exceeding the level expected at electrochemical equilibrium and suggesting active transport of chloride into the cell. These features suggest a tentative hypothesis for chloride secretion by the rectal gland in which the uphill transport of chloride into the cytoplasm is coupled through a membrane carrier to the downhill movement of sodium along its electrochemical gradient. The latter is maintained by the Na-K-ATPase pump while chloride is extruded into the duct by electrical forces.

Ouabain-insensitive salt and water movements in duck red cells. I. Kinetics of cation transport under hypertonic conditions.

Duck red cells in hypertonic media experience rapid osmotic shrinkage followed by gradual reswelling back toward their original volume. This uptake of salt and water is self limiting and demands a specific ionic composition of the external solution. Although ouabain (10(-4)M) alters the pattern of cation accumulation from predominantly potassium to sodium, it does not affect the rate of the reaction, or the total amount of salt or water taken up. To study the response without the complications of active Na-K transport, ouabain was added to most incubations. All water accumulated by the cells can be accounted for by net salt uptake. Specific external cation requirements for reswelling include: sufficient sodium (more than 23 mM), and elevated potassium (more than 7 mM). In the absence of external potassium cells lose potassium without gaining sodium and continue to shrink instead of reswelling. Adding rubidium to the potassium- free solution promotes an even greater loss of cell potassium, yet causes swelling due to a net uptake of sodium and rubidium followed by chloride. The diuretic furosemide (10(-3)M) inhibits net sodium uptake which depends on potassium (or rubidium), as well as inhibits net sodium uptake which depends on sodium. As a result, cell volume is stabilized in the presence of this drug by inhibition of shrinkage, at low, and of swelling at high external potassium. The response has a high apparent energy of activation (15-20 kcal/mol). We propose that net salt and water movements in hypertonic solutions containing ouabain are mediated by direct coupling or cis-interaction, between sodium and potassium so that the uphill movement of one is driven by the downhill movement of the other in the same direction.

Cell permeability, sodium transport, and the hypertensive process in the rat.

The tail artery of the spontaneously hypertensive rat (SHR) (Carworth Farms), excised rapidly and immersed immediately in cold (2 degrees C) Li-substituted physiologic salt solution (LiPSS), continues to exchange cell Na+ and K+ for Li+, this exchange is negligible to the control (Carworth Farms normotensive) CFN). In the incubated artery at 37 degrees C, when the vascular smooth muscle cell is slack, the leakiness of the cell membrane in the SHR is more than offset by increased Na+ pumping activity, so that cell Na+ is subnormal. A high precision technique with ion-specific electrodes was developed to follow the passive downhill and active uphill phases of Na+-K+ exchange in the perfused artery exposed to K+-free physiologic salt solution (K+-free PSS) followed by physiological salt solution (PSS). The exchange was found to be fully reversible and sufficiently equimolar to be definable in terms of movements of K+ alone. The rates of ionic movement across the vascular smooth muscle cell were found to be about 6 times faster for the vessel perfused at low pressure (less than 3 mm Hg) than for the slack incubated artery. The rate of passive downhill movement was significantly accelerated in the mature SHR compared with CFN, and the net active transport activity much enhanced. Similar changes were seen as early as 3 weeks after treatment with DOCA and were pronounced at 8 weeks. It is proposed that conditions favoring a sustained accumulation of Na+ in the vascular smooth muscle cell are countered by an enhanced synthesis of transport protein, of contractile protein, and of paracellular matrix protein which progressively restructure the wall.

Calcium-calcium and calcium-strontium exchange across the membrane of human red cell ghosts.

Stationary and nonstationary state45Ca fluxes as well as Sr−Ca exchange movements were studied in energy-depleted human erythrocyte ghosts at different intra-and extracellular Ca concentrations. Influx and efflux followed the kinetics of a closed two-compartment system. The influx and efflux rate constants (k in andk out, respectively, fractions of total extra- or intracellular45Ca that move in one direction per unit time) were similar in magnitude. They decreased with increasing Ca concentration on the cisside and increased with increasing Ca concentration on the trans-side of the membrane. Hence, the fluxes in both directions were characterized by saturation kinetics and appeared to be partially caused by an exchange diffusion mechanism. In the presence of a moderate inward (up to 8mm) or outward (up to 2mm) Ca concentration gradient, kin andk out did not vary in the course of an experiment and did not differ significantly from rates which were measured under stationary state conditions. Extracellular Sr induced an outward transport of intracellular Ca against the concentration gradient (counter-transport). The resulting inward Ca concentration gradient (maximal inside-to-outside concentration ratio as 1 to 3) persisted since extra- and intracellular Sr did not equilibrate. Analogous results were obtained studying45Ca−40Ca countertransport. In net flow experiments Ca−Sr exchange proved to occur on a one-for-one basis. Ca−Sr exchange was additive to the noncoupled Ca and Sr net downhill movements. The experimental results suggest that a specific ATP-independent Ca transfer system exists in the erythrocyte membrane which acts symmetrically on the two sides of the membrane and is restricted to a tightly coupled one-for-one exchange diffusion.