86Rb+ fluxes in Chinese hamster ovary cells as a function of membrane cholesterol content

Influence of 26-hydroxycholesterol on the composition and function of gel-filtered platelets

The influence of 26-hydroxycholesterol (26-OH-CHOL) on the structure and function of gel-filtered rat platelets, as a model membrane, was studied in vitro. Its influence on structure was determined by a fatty acid and a phospholipid analysis of the platelet lipids and on function by the cytoplasmic calcium concentration of the platelets exposed to increasing concentrations of 26-OH-CHOL for various periods of time. The intracellular free calcium (Ca2+)i of the gel-filtered rat platelets was monitored by a fluorescent probe (quin 2) after incubation in a 37 degrees C water bath with 1 mM Ca2+ and 20 microM quin 2/AM. The presence of 26-OH-CHOL in the incubation media changed both the phospholipid composition and the mixed fatty acid composition in the membrane and increased the intracellular free Ca2+ level of the platelets. As the incubation of platelets with cholesterol (CHOL) or esterified 26-OH-CHOL did not increase intracellular Ca2+ levels, these results indicate that the hydrophilic free 26-hydroxy group in 26-OH-CHOL may have influenced the enzymes catalyzing the synthesis of the phospholipid in the platelet membrane so as to allow it to become more and more "leaky" to Ca2+. Such a fundamental change in membrane structure and function may be responsible for the development of atherosclerosis in the intimal layer of the coronary arteries.

25-Hydroxysterols increase the permeability of liposomes to Ca2+ and other cations

25-Hydroxycholesterol and 25-hydroxy vitamin D-3 increased the permeability of liposomes to Ca2+ measured by the arsenazo III encapsulation technique. This effect was sensitive to the lipid composition of the membrane, with changes that decreased the motional freedom of phospholipid acyl chains decreasing Ca2+ permeability. The greatest permeability was observed with the zwitter-ionic phospholipids, phosphatidylcholine and phosphatidylethanolamine, whereas the acidic phospholipids, phosphatidylinositol and phosphatidylserine, depressed Ca2+ permeability. The effect was not specific for Ca2+. Other divalent cations were translocated in the order Mn2+ greater than Mg2+ = Ca2+ much greater than Sr2+ = Ba2+. The permeability of liposomes to the monovalent cation, Na+, was also substantially increased. The effect did not appear to be due to ionophoretic properties of the sterols, and it is suggested that perturbation of the membranes by the polar 25-hydroxyl group may play a role in increasing membrane permeability.

Sodium and potassium transport in trout (Salmo gairdneri) erythrocytes

The principal pathways of Na+ and K+ transport in trout erythrocytes have been characterized. Approximately 50% of K+ influx in steady-state erythrocytes was inhibited by ouabain (1 mM) and 46% by furosemide (1 mM). Furosemide-sensitive K+ influx was a saturable function of external K+ concentration with a Km of 25 mM. This flux component was also inhibited by SITS (4-acetamido-4'-isothiocyanatostilbene-2'2-disulphonate) (concentration required for 50% inhibition, I50 = 7.6 X 10(-6)M) and by the removal of external Cl-. An increase in cell volume stimulated furosemide-sensitive K+ influx and cell shrinkage inhibited this flux. K+ efflux was mainly furosemide-sensitive (64% of total). This pathway was unaffected by variations in extracellular K+ concentration and is therefore not exchange diffusion. However, it was affected by variations in cell volume in a similar way to the furosemide-sensitive K+ influx. Na+ influx was only slightly sensitive to furosemide (13% of total) but this component was very sensitive to changes in cell volume; decreased cell volume increased Na+ influx whilst increased cell volume inhibited Na+ influx. Furosemide-sensitive K+ influx was unaffected by variations in external Na+ concentration. Similarly, furosemide-sensitive Na+ influx was unaffected by variations in external K+ concentration. This indicates that the passive influxes of Na+ and K+ were not coupled, in contrast to the situation in avian erythrocytes. The opposite effects of cell volume upon passive Na+ and K+ fluxes are in good agreement with the net movements of these cations during volume regulation in erythrocytes of the flounder (Cala, 1977) and the toadfish (Lauf, 1982).

Membrane regulation of the Na+,K+-ATPase during the neuroblastoma cell cycle: correlation with protein lateral mobility

The pumping activity of the plasma membrane-bound Na+,K+-ATPase shows considerable variation during the cell cycle of mouse neuroblastoma Neuro-2A cells. Addition of external ATP at millimolar concentrations, which selectively enhances the plasma membrane permeability of Neuro-2A cells for sodium ions, stimulates the Na+,K+-ATPase pumping activity at all phases of the cell cycle from a factor of 1.05 in mitosis up to 2.2 in G1 phase. Determination of the number of Na+,K+-ATPase copies per cell by direct 3H-ouabain binding studies in the presence of external ATP shows a gradual increase in the number of pump sites on passing from mitosis to the late S/G2-phase by approximately a factor of 2. From these data the pumping activity per copy of Na+,K+-ATPase, optimally stimulated with respect to its various substrate ions, has been determined during the various phases of the cell cycle. This optimally stimulated pumping activity per enzyme copy, which is a reflection of the physicochemical state of the plasma membrane, is high in mitosis, almost twofold lower in early G1 phase, and increases gradually again during the other phases of the cell cycle. This shows that the observed regulation of Na+,K+-ATPase activity during the cell cycle is caused by a combination of three independent factors--namely variation in intracellular substrate availability (Na+), changes in number of enzyme copies per cell, and modulation of the plasma membrane environment of the protein molecules. The modulation of the optimal pumping activity per enzyme copy shows a good correlation (rho = 0.96) with the known modulation of protein lateral mobility during the cell cycle, such that a high protein lateral mobility correlates with a low enzyme activity. It is concluded that changes in plasma membrane properties take place during the Neuro-2A cell cycle that result in changes in the rate of protein lateral diffusion and Na+,K+-ATPase activity in directly correlated way.

Effect of fatty acids on plasma membrane lipid dynamics and cation permeability in neuroblastoma cells

In this study the effects of experimental modifications of plasma membrane lipid lateral mobility on the electrical membrane properties and cation transport of mouse neuroblastoma cells, clone Neuro-2A, have been studied. Short-term supplementation of a chemically defined growth medium with oleic acid or linoleic acid resulted in an increase in the lateral mobility of lipids as inferred from fluorescence recovery after photobleaching of the lipid probe 3,3'-dioctadecylindocarbocyanide iodide. These changes were accompanied by a marked depolarization of the membrane potential from -51 mV to -36 mV, 1.5 h after addition, followed by a slow repolarization. Tracer flux studies, using 86Rb+ as a radioactive tracer for K+, demonstrated that the depolarization was not caused by changes in (Na+ + K+)-ATPase-mediated K+ influx or in the transmembrane K+ gradient. The permeability ratio (PNa/PK), determined from electrophysiological measurements, however, increased from 0.10 to 0.27 upon supplementation with oleic acid or linoleic acid. This transient rise of PNa/PK was shown by 24Na+ and 86Rb+ flux measurements to be due to both an increase of the Na+ permeability and a decrease of the K+ permeability. None of these effects occurred upon supplementation of the growth medium with stearic acid.

Characterization of the concanavalin A-induced increase in lymphocyte cell membrane permeability by furosemide

The effect of ConA on passive 42K+ fluxes in human lymphocytes was analysed by treatment with furosemide and ouabain. Furosemide has no effect on net K+ transport and pump flux. The component of passive efflux of 42K+ which depends on K+-K+ exchange is inhibited by furosemide; its mechanism is discussed in detail. The 42K+ uptake in lymphocytes consists of 1. an ouabain-sensitive pump influx, 2. an ouabain-insensitive leak influx and 3. a furosemide-sensitive K+-K+-exchange. Only the first two components of 42K+ uptake are stimulated by ConA.

K+ influx components in ascites cells: the effects of agents interacting with the (Na+ + K+)-pump

Several agents known to interact with the (Na+ + K+)-pump were tested for their effects on the components of steady-state K+ flux in ascites cells. 86Rb+ was used as a tracer for K+, and influx was differentiated into a ouabain-inhibitable "pump" component, a Cl--dependent and furosemide-sensitive "exchange" component, and a residual "leak" flux. All agents tested (ouabain, quercetin, oligomycin, phosphate) affected both the "pump" flux and the Cl--linked flux. These findings suggest a linkage between the activity of the Na/K ATPase and the Cl--dependent K+ exchange flux. In the discussion we point out that the mechanism of this linkage could be direct; e.g., Cl--dependent exchange may represent a mode of operation of the Na/K ATPase. However, data from this and other systems tend to suggest an indirect linkage between the Na+ pump and a KCl symporter, perhaps via a change in the level of intracellular ATP.