Determination of reflection coefficients for various ions and neutral molecules in sarcoplasmic reticulum vesicles through osmotic volume change studied by stopped flow technique

High-frequency Contactless Sensor for the Detection of Heparin-Induced Thrombocytopenia Antibodies via Platelet Aggregation

Heparin-induced thrombocytopenia (HIT), a severe autoimmune disorder, occurs in patients undergoing heparin therapy. The presence of platelet-activating antibodies against platelet factor 4/Heparin in the blood confirms patients suffering from HIT. The most widely used methods for HIT diagnosis are immunoassays but the results only suit to rule out HIT as the assays provide only around 50% specificity. To confirm HIT, samples with positive results in immunoassays are retested in functional assays (>98% specificity) that track platelet-activating antibodies via platelet aggregation. However, the protocols in functional assays are either time-consuming (due to the requirement of the detection of serotonin release) or require highly trained staff for the visualization of platelets. Here, we applied a cheap and easy-to-use contactless sensor, which employs high-frequency microwaves to detect the changes in the resonant frequency caused by platelet aggregation/activation. Analysis of change in conductivity and permittivity allowed us to distinguish between HIT-like (KKO) and non-HIT-like (RTO) antibodies. KKO caused a stronger reduction of conductivity of platelet samples than RTO. Our results imply that the high-frequency contactless sensor can be a promising approach for the development of a better and easier method for the detection of HIT.

Conducting and voltage-dependent behaviors of potassium ion channels reconstituted from diaphragm sarcoplasmic reticulum: comparison with the cardiac isoform

Sarcoplasmic reticulum (SR) K+ channels from canine diaphragm were studied upon fusion of longitudinal and junctional membrane vesicles into planar lipid bilayers (PLB). The large-conductance cation selective channel (gamma(max) = 250 pS; Km = 33 mM) displays long-lasting open events which are much more frequent at positive than at negative voltages. A major subconducting state about 45% of the fully-open state current amplitude was occasionally observed at all voltages. The voltage-dependence of the open probability displays a sigmoid relationship that was fitted by the Boltzmann equation and expressed in terms of thermodynamic parameters, namely the free energy (delta Gi) and the effective gating charge (Zs): delta Gi = 0.27 kcal/mol and Zs = -1.19 in 250 mM potassium gluconate (K-gluconate). Kinetic analyses also confirmed the voltage-dependent gating behavior of this channel, and indicate the implication of at least two open and three closed states. The diaphragm SR K+ channel shares several biophysical properties with the cardiac isoform: g = 180 pS, delta Gi = 0.75 kcal/mol, Zs = -1.45 in 150 mM K-gluconate, and a similar sigmoid P(o)/voltage relationship. Little is known about the regulation of the diaphragm and cardiac SR K+ channels. The conductance and gating of these channels were not influenced by physiological concentrations of Ca2+ (0.1 microM-1 mM) or Mg2+ (0.25-1 mM), as well as by cGMP (25-100 microM), lemakalim (1-100 microM), glyburide (up to 10 microM) or charybdotoxin (45-200 nM), added either to the cis or to the trans chamber. The apparent lack of biochemical or pharmacological modulation of these channels implies that they are not related to any of the well characterized surface membrane K+ channels. On the other hand, their voltage sensitivity strongly suggests that their activity could be modulated by putative changes in SR membrane potential that might occur during calcium fluxes.

Evidence for a glycerol pathway through aquaporin 1 (CHIP28) channels

Permeabilities to glycerol and small non-electrolytes of three Aquaporin 1 CHIP (AQP1) water channels were measured in AQP1 cRNA-injected Xenopus laevis oocytes and in human AQP1 channels reconstituted in proteoliposomes. By an "osmotic" swelling assay, significant increases of ethylene glycol, glycerol and 1,3-propanediol apparent permeability coefficients (P'solutes) were found in oocytes expressing human, rat and frog AQP1. p-Chloromercuribenzene sulphonate (pCMBS) and CuSO4 inhibited, by 95% and 58% respectively, apparent glycerol permeability (P'gly) in oocytes expressing human AQP1. pCMBS inhibition was reversed by beta-mercaptoethanol and CuSO4 inhibition was partly reversed by the Cu(2+)-binding peptide Gly-Gly-His. Tritiated glycerol uptakes confirmed the augmented P'gly value of AQP1 cRNA-injected oocytes. In contrast, no increases of urea, meso-erythritol, D-or L-threitol, xylitol and mannitol uptakes were detected. Stopped-flow light scattering experiments performed with human AQP1 proteoliposomes also revealed a much greater increase of P'gly than did those with protein-free liposomes; the initial rate of proteoliposomes also swelling was inhibited by 96.2% with HgCl2 and by 72.5% with CuSO4. In AQP1 cRNA-injected oocytes and in proteoliposomes, the value of the glycerol reflection coefficient was 0.74-0.80, indicating that water and glycerol share the same pathway. All these results provide strong evidence that water and certain small solutes permeate the AQP1 channels expressed at the surface of X. laevis oocytes or reconstituted in proteoliposomes. The urea exclusion suggests that the selectivity of the AQP1 channels not only depends on the size of the solutes but probably also on their flexibility and their ability to form H-bonds.

Human spermatozoa glycerol permeability and activation energy determined by electron paramagnetic resonance

The permeability of human spermatozoa to glycerol and its activation energy were determined using electron paramagnetic resonance (EPR) techniques. EPR was used to monitor the aqueous cell volume change vs. time during the glycerol permeation process using the aqueous spin label 15N-tempone and the membrane impermeable broadening agent potassium trioxalatochromiate (chromium oxalate). The permeation process was completed in tens of seconds, requiring the use of a stopped-flow methodology. The glycerol permeability coefficient (Pg) was determined by fitting a simple theoretical model to the experimental data. The permeabilities of human spermatozoa in 1 molar and 2 molar glycerol at 20 degrees C are (10.3 +/- 0.3).10(-4) cm/min (mean +/- S.D.) and (6.0 +/- 1.4).10(-4) cm/min, respectively. The permeabilities of human spermatozoa in 2 molar glycerol at 30, 20, 10, and 0 degrees C are (8.3 +/- 1.3).10(-4) cm/min, (6.0 +/- 1.4).10(-4) cm/min, (2.1 +/- 0.4).10(-4) cm/min, and (1.1 +/- 0.3).10(-4) cm/min, respectively. The activation energy (Ea) for glycerol permeation between 30 degrees C and 0 degrees C was found to be 11.6 kcal/mol.

Single chloride-selective channel from cardiac sarcoplasmic reticulum studied in planar lipid bilayers

The behavior of single Cl- channel was studied by fusing isolated canine cardiac sarcoplasmic reticulum (SR) vesicles into planar lipid bilayers. The channel exhibited unitary conductance of 55 pS (in 260 mM Cl-) and steady-state activation. Subconductance states were observed. Open probability was dependent on holding potentials (-60 to +60 mV) and displayed a bell-shaped relationship, with probability values ranging from 0.2 to 0.8 with a maximum at -10 mV. Channel activity was irreversibly inhibited by DIDS, a stilbene derivative. Time analysis revealed the presence of one time constant for the full open state and three time constants for the closed states. The open and the longer closed time constants were found to be voltage dependent. The behavior of the channel was not affected by changing Ca2+ and Mg2+ concentrations in both chambers, nor by adding millimolar adenosine triphosphate, or by changing the pH from 7.4 to 6.8. The presence of sulfate anions decreased the unit current amplitude, but did not affect the open probability. These results reveal that at the unitary level the cardiac SR anion-selective channel has distinctive as well as similar electrical properties characteristic of other types of Cl- channels.

Determination of solute reflection coefficients in kidney brush-border membrane vesicles by light scattering: influence of the refractive index

Solute reflection coefficients sigma of cell membrane vesicles or liposomes are commonly determined by comparison of the water flow induced by a gradient of the studied solute and that of a reference molecule using light scattering techniques. However, variations in scattered light which are mainly related to change in vesicle volume are also influenced by the refractive index of the surrounding medium. Therefore comparing kinetics of vesicle shrinkage induced by hyperosmotic solutions which have different refractive indexes might lead to an under or over estimation of sigma. We determined sigma NaCl in rat kidney brush-border membrane vesicles by two different approaches using mannitol, a poorly permeant molecule, as reference. (1) The refractive index of the hyperosmotic NaCl solution was adjusted to that of mannitol by addition of polyvinyl pyrrolidone (Mr 40,000), without a significant increase in osmolality. Thereby the change in scattered light intensity induced by osmotic vesicle shrinkage due to gradients of NaCl and mannitol were comparable and led to a sigma NaCl value close to one instead of the previously published value of 0.53. (2) The reflection coefficient was calculated from the lifetime of vesicle shrinkage which is not refractive index-dependent. Again sigma NaCl was not different from one. These results suggest that the water proteic pathways found in the luminal membrane of proximal tubules are not shared by salts.

Physiological role and selectivity of the in situ potassium channel of the sarcoplasmic reticulum in skinned frog skeletal muscle fibers

The role of K+ as a counterion during Ca2+ release from the sarcoplasmic reticulum (SR) has been investigated. An optical technique using the Ca2+-sensitive dye antipyrylazo III monitored Ca2+ release from skinned (sarcolemma removed) muscle fibers of the frog. Skinned fibers were used since the removal of the sarcolemma allows direct access to the SR membrane. Releases were stimulated by caffeine, which activates Ca2+ release directly by binding to a receptor on the SR. Two different methods were used to decrease the SR K+ conductance so that its effect on Ca2+ release could be assessed: (a) the SR K+ channel blocker, 1,10-bis-quanidino-n-decane (bisG10) was used to eliminate current pathways and (b) substitution of the impermeant ion choline for K+ was used to decrease charge carriers. Both bisG10 and choline substitution caused a concentration-dependent decrease in the Ca2+ release rate. Therefore we conclude that K+ is an important counterion for Ca2+ during its release from the SR. The selectivity of the in situ SR K+ channel to several monovalent cations was determined by substituting them for K+ and comparing their effect on Ca2+ release. The substituted ions were expected to affect Ca2+ release in proportion to their ability to support a counterion flux, which is, in turn, a function of their relative conductance through the SR K+ channel. The selectivity sequence determined by these experiments was K+ = Rb+ = Na+ greater than Cs+ greater than Li+ greater than choline.

Temperature-dependent osmotic permeability in glycoprotein containing liposomes

The osmotic water outflow of large multilamellar liposomes containing alpha 1-acid glycoprotein was measured at a temperature near the lipid's phase transition temperature. The liposomes were formed from a mixture of DPPC, cholesterol and glycoprotein in molar ratios 100:20:1, by continuous sucrose density gradient centrifugation. These liposomes captured 35% of the radiolabeled glycoprotein. The temperature-dependent experiments showed that near phase transition temperature the initial rate of water outflow increased drastically in comparison with glycoprotein free liposomes incubated in buffer containing glycoprotein. We suggested that eventual a channel mechanism may be involved due to spontaneous incorporation of glycoprotein into the bilayer.

Osmotic water permeability in glycoprotein containing liposomes

The kinetics of osmotic water permeability in proteoliposomes containing alpha 1-acid glycoprotein was investigated by means of stopped-flow spectrophotometry. A biphasic time-course of scattered light with time was registered. The rate constants calculated from fits to an exponential function in the first phase were proportional to the final medium osmolarity. The apparent second order rate constants Kapp (Osm-1 sec-1) were determined at different glycoprotein concentrations in the original mixture for preparation of proteoliposomes. The value of Kapp at lipid:glycoprotein weight ratio = 1 was plotted in Arrhenius coordinates. The calculated activation energy for water permeation through the lipid bilayer suggests that eventual channel mechanism may be involved due to the presence of glycoprotein molecule in the liposomes.

Osmotic water permeabilities of brush border and basolateral membrane vesicles from rat renal cortex and small intestine

The osmotic water permeability Pf of brush border (BBM) and basolateral (BLM) membrane vesicles from rat small intestine and renal cortex was studied by means of stopped-flow spectrophotometry. Scattered light intensity was used to follow vesicular volume changes upon osmotic perturbation with hypertonic mannitol solutions. A theoretical analysis of the relationship of scattered light intensity and vesicular volume justified a simple exponential approximation of the change in scattered light intensity. The rate constants extracted from fits to an exponential function were proportional to the final medium osmolarity as predicted by theory. For intestinal membranes, computer analysis of optical responses fitted well with a single-exponential treatment. For renal membranes a double-exponential treatment was needed, implying two distinct vesicle populations. Pf values for BBM and BLM preparations of small intestine were equal and amount to 60 microns/sec. For renal preparations, Pf values amount to 600 microns/sec for the fast component, BBM as well as BLM, and to 50 (BBM) and 99 (BLM) microns/sec for the slow component. The apparent activation energy for water permeation in intestinal membranes was 13.3 +/- 0.6 and in renal membranes 1.0 +/- 0.3 kCal/mole, between 25 and 35 degrees C. The mercurial sulfhydryl reagent pCMBS inhibited completely and reversibly the high Pf value in renal brush border preparations. These observations suggest that in intestinal membranes water moves through the lipid matrix but that in renal plasma membranes water channels may be involved. From the high Pf values of renal membrane vesicles a transcellular water permeability for proximal tubules can be calculated which amounts to approximately 1 cm/sec. This value allows for an entirely transcellular route for water flow during volume reabsorption.

Potassium efflux from single skinned skeletal muscle fibers

The efflux of 42K from single, skinned (sarcolemma removed) skeletal muscle fibers has been determined. Isotope washout curves are kinetically complex and can be fit as the sum of three exponentials, including a fast component (k = 0.25 s-1) with a pool size equivalent to 91% of the fiber volume, an intermediate component (k = 0.08 s-1) equivalent to 6% of the fiber volume, and a slow component (k = 0.008 s-1) equivalent to 0.5% of fiber volume. Only the intermediate kinetic component is significantly affected by pretreatment of fibers with detergent. Efflux curves from detergent-treated fibers could be fit as the sum of two exponentials with coefficients and rate constants comparable to those of the fast and slow component of washout of untreated controls. Similarly the washout of [14C]sucrose can be described as the sum of two exponentials. We conclude that the intermediate component of 42K washout results from the movement of ions from a membrane bound space within the skinned fiber. Because of its relative volume, the sarcoplasmic reticulum seems to be a reasonable choice as a structural correlate for this component. Our estimate of the potassium permeability for the sarcoplasmic reticulum (SR) based on the efflux data is 10(-7) cm/s. This value is less than previous estimates from isolated preparations.

Water permeability through biological membranes by isotopic effects of fluorescence and light scattering

A light-scattering technique used to measure the water permeability across closed biomembranes is described, which is based on the different indices of refraction of D2O and H2O. This transient technique is compared with a similar method using D2O-sensitive fluorophores in the intravesicular space. The results of both techniques are equivalent although the signal-to-noise ratio favors the light-scattering or turbidity experiment. The light-scattering method is only applicable to larger particles (no point-scatterers) and is easily extended to biological objects. Data on the H2O/D2O exchange across membranes of ghosts from human erythrocytes suggest two mechanisms: the D2O and H2O permeation through the membrane and a slower D2O-induced conformational change of membraneous proteins.

Calcium release from sarcoplasmic reticulum of normal and dystrophic mice

Contraction of skeletal muscle is triggered by release of calcium from the sarcoplasmic reticulum. In this study, highly purified normal and dystrophic mouse sarcoplasmic reticulum vesicles were compared with respect to calcium release characteristics. Sarcoplasmic reticulum vesicles were actively loaded with calcium in the presence of an ATP-regenerating system. Calcium fluxes were followed by dual wavelength spectrophotometry using the metallochromic indicators antipyrylazo III and arsenazo III, and by isotopic techniques. Calcium release from sarcoplasmic reticulum vesicle was elicited by (a) changing the free calcium concentration of the assay medium (calcium-induced calcium release); (b) addition of a permeant anion to the assay medium, following calcium loading in the presence of a relatively impermeant anion (depolarization-induced calcium release); (c) addition of the lipophilic anion tetraphenylboron (TPB-) to the assay medium and (d) using specific experimental conditions, i.e. high phosphate levels and low magnesium (spontaneous calcium release). Drugs known to influence Ca2+ release were shown to differentially affect the various types of calcium release. Caffeine (10 mM) was found to enhance calcium-induced calcium release from isolated sarcoplasmic reticulum. Ruthenium red (20 microM) inhibited both calcium-induced calcium release and tetraphenylboron-induced calcium release, and partially inhibited spontaneous calcium release and depolarization-induced calcium release. Local anesthetics inhibited spontaneous calcium release in a time-dependent manner, and inhibited calcium-induced calcium release instantaneously, but did not inhibit depolarization-induced calcium release. Use of pharmacological agents indicates that several types of calcium release operate in vitro. No significant differences were found between normal and dystrophic sarcoplasmic reticulum in calcium release kinetics or drug sensitivities.

Caffeine-induced calcium release from isolated sarcoplasmic reticulum of rabbit skeletal muscle

The essential conditions for the Ca2+ releasing action of caffeine from isolated sarcoplasmic reticulum (SR) of rabbits were evaluated by an investigation into the effects of Ca2+, Mg2+, MgATP2-, and ATP concentration, ionic strength, and degree of loading. The heavy fraction (4,500 X g) of the reticulum was used. Except for the study on degree of loading, 0.2 mg protein X ml-1 SR was loaded actively with 0.02 mM 45CaCl2, resulting in greater than 90 nmol X mg protein -1 at steady state, and then the effects of various parameters with or without (control) caffeine were tested. It was found that (1) caffeine induces a transient, dose-dependent release of Ca2+, (2) the absolute amount of Ca2+ released by caffeine increases with the Ca2+ load of the SR, (3) increasing the ionic strength (mu) from 0.09 to 0.3 lowers the threshold concentration of caffeine, (4) the SR is refractory to a repeated challenge by a caffeine concentration causing maximal effect, (5) caffeine-induced Ca2+ release increases with increasing (a) external Ca2+ concentrations up to 5 microM total Ca2+ (or 3 microM free Ca2+) and (b) free ATP concentrations up to 0.45 mM, and (6) caffeine-induced Ca2+ release is not affected by changes of either the Mg2+ or the MgATP2- concentration.

Effect of osmotic gradient on the physical properties of membrane lipids in liposomes

Osmotic swelling of multilamellar liposomes was shown to produce structural changes in liposomal membranes. Creation of transmembrane osmotic gradients results in an increased lateral mobility of membrane incorporated hydrophobic probe, pyrene. This is accompanied by some decrease in order parameter of membrane phospholipid polar head group regions. The perturbations are more marked in hydrophobic than in polar regions of the membranes. It is suggested that some functional changes in biomembranes arising upon osmotic swelling may be associated with structural alterations similar to those observed in liposomes.

Mechanism of calcium release from skeletal sarcoplasmic reticulum

Ca2+ -induced Ca2+ release at the terminal cisternae of skeletal sarcoplasmic reticulum was demonstrated using heavy sarcoplasmic reticulum vesicles. Ca2+ release was observed at 10 mum Ca2+ in the presence of 1.25 mm free Mg2+ and was sensitive to low concentrations of ruthenium red and was partially inhibited by valinomycin. These results suggest that the Ca2+ -induced Ca2+ release is electrogenic and that an inside negative membrane potential created by the Ca2+ flux opens a second channel that releases Ca2+. Results in support of this formulation were obtained by applying a Cl- gradient or K+ gradient to sarcoplasmic reticulum vesicles to initiate Ca2+ release. Based on experiments the following hypothesis for the excitation-contraction coupling of skeletal muscle was formulated. On excitation, small amounts of Ca2+ enter from the transverse tubule and interact with a Ca2+ enter from the transverse tubule and interact with a Ca2+ receptor at the terminal cisternae and cause Ca2+ release (Ca2+ -induced Ca2+ release). This Ca2+ flux generates an inside negative membrane potential which opens voltage-gated Ca2+ channels (membrane potential-dependent Ca2+ release) in amounts sufficient for contraction.

Calcium release and ionic changes in the sarcoplasmic reticulum of tetanized muscle: an electron-probe study

Approximately 60-70% of the total fiber calcium was localized in the terminal cisternae (TC) in resting frog muscle as determined by electron-probe analysis of ultrathin cryosections. During a 1.2 s tetanus, 59% (69 mmol/kg dry TC) of the calcium content of the TC was released, enough to raise total cytoplasmic calcium concentration by approximately 1 mM. This is equivalent to the concentration of binding sites on the calcium-binding proteins (troponin and parvalbumin) in frog muscle. Calcium release was associated with a significant uptake of magnesium and potassium into the TC, but the amount of calcium released exceeded the total measured cation accumulation by 62 mEq/kg dry weight. It is suggested that most of the charge deficit is apparent, and charge compensation is achieved by movement of protons into the sarcoplasmic reticulum (SR) and/or by the movement of organic co- or counterions not measured by energy dispersive electron-probe analysis. There was no significant change in the sodium or chlorine content of the TC during tetanus. The unchanged distribution of a permeant anion, chloride, argues against the existence of a large and sustained transSR potential during tetanus, if the chloride permeability of the in situ SR is as high as suggested by measurements on fractionated SR. The calcium content of the longitudinal SR (LSR) during tetanus did not show the LSR to be a major site of calcium storage and delayed return to the TC. The potassium concentration in the LSR was not significantly different from the adjacent cytoplasmic concentration. Analysis of small areas of I-band and large areas, including several sarcomeres, suggested that chloride is anisotropically distributed, with some of it probably bound to myosin. In contrast, the distribution of potassium in the fiber cytoplasm followed the water distribution. The mitochondrial concentration of calcium was low and did not change significantly during a tetanus. The TC of both tetanized and resting freeze-substituted muscles contained electron-lucent circular areas. The appearance of the TC showed no evidence of major volume changes during tetanus, in agreement with the estimates of unchanged (approximately 72%) water content of the TC obtained with electron-probe analysis.

Ion movement accompanied by calcium uptake of sarcoplasmic reticulum vesicles studied through the osmotic volume change by the light scattering method

The volume change of sarcoplasmic reticulum vesicles was induced by Ca2+ uptake. This volume change was measured by the light-scattering method. When vesicles were shrunk beforehand under the condition that anions are more permeable than cations, they swelled during Ca2+ uptake due to the concomitant incorporation of anions. On the contrary, they shrank with Ca2+ uptake due to the extrusion of cations under the condition that cations are more permeable than anions. From the analysis of the volume change it was concluded that all ions other than Ca2+ were transported passively in order to neutralize the membrane potential generated by the Ca2+ pump. These results support the idea that the Ca2+ pump is electrogenic. By using this technique, it became possible to measure the fast Ca2+ uptake rate. The dependence of the Ca2+ uptake rate on the Ca2+ concentration suggests that the site at which Ca2+ inhibits Ca2+ uptake is located inside the vesicle. From the osmotic response of the vesicles, the intravesicular concentration of free Ca2+ was estimated to be about 15 mM, when Ca2+ was fully taken up under the physiological condition.