Kinetics of a Ca-2+-triggered membrane aggregation reaction of phospholipid membranes

Ca-2+ and other divalent cations can trigger aggregation of phospholipid vesicles containing phosphatidic acid or phosphatidylserine. The reaction, which can be detected by an increase in light scattering, has a critical dependence on the Ca-2+ concentration, with a threshold near 4 mM Ca-2+. This is the concentration for half-saturation of the polar head groups and for full neutralization of the membrane surface charge. The aggregation proceeds as a "polymerization" reaction, eventually forming such large aggregates that the vesicles precipitate. The stopped-flow rapid mixing technique was used to study the vesicle dimerization reaction which is the first step in the overall aggregation process. Vesicle dimerization resulted in a doubling of light scattering and had a vesicle concentration-dependent time constant (t1/2) which varied between 0.4 and 2.0s under the conditions of the study. Analysis of the dependence of the reaction amplitude and l/t 1/2 on the concentrations of vesicles and Ca-2+ showed that the Ca-2+ binding is fast, and that the dimerization proceeds by a mechanism in which the vesicles first collide to form an encounter complex followed by a slower conversion of the encounter complex to a stable complex. For phosphatidic acid vesicles, about 200-700 collisions are necessary to achieve a stable dimer. The rate-limiting step in the overall reaction is thus the transformation of the encounter complex into a stable complex, requiring 0.5 and 1.0 ms. The above-mentioned results are relatively insensitive insensitive to the type of divalent cation or to the choice of negatively charged lipid (phosphatidic acid or phosphatidylserine). Evidence is given that the stable complex is effected by Ca-2+-mediated salt bridges between the two membranes and that the rate constant of the transformation step derives from the statistics of the distribution and the rate of redistribution of Ca-2+-occupied polar head groups on the membrane surfaces. The relevance of these results to the problem of Ca-2+-induced fusion of biological membranes is discussed.

Platelet microparticles and calcium homeostasis in acute coronary ischemias

Elevation of free cytoplasmic calcium is the common pathway of platelet activation, leading to shape change, shedding of platelet microparticles (PMP), aggregation, and secretion of internal granules, including expression of CD62p on the surface. Platelet activation is well documented in unstable angina (UA) and acute myocardial infarction (MI). We investigated the following markers of platelet activation in 55 patients undergoing coronary angiography for suspected CAD: free cytoplasmic calcium, [Ca2+]cyt, PMP, CD62p expression, and platelet/leukocyte (P/L) interaction. [Ca2+]cyt was measured by Fluo-3 and the other measurements were by flow cytometry. Patients were classified into three groups: unstable angina (UA, n = 11), recent myocardial infarction (MI, n = 11), and patient controls (CTL, n = 33). Blood was drawn before infusion of heparin through femoral lines at the time of catheterizaton for assays. (

RESULTS

(1) PMP values were significantly higher in both UA and MI than in CTL, P < 0.05. There was no difference between UA and MI. (2) P/L interaction was significantly elevated only in UA, P < 0.05. (3) CD62p expression on free platelets did not differ significantly between any of the three groups. (4) The resting [Ca2+]cyt, thrombin-induced Ca2+ influx, and release of Ca2+ from internal stores were all significantly higher in platelets from the combined patient group (UA + MI) than in the patient control group, P < 0.001

CONCLUSIONS

Results on calcium hemostasis and PMP were significantly different in patients with acute coronary syndromes than those with stable angina or no coronary ischemia; this may reflect underlying pathophysiology of acute coronary ischemia. P/L interaction was higher only in the UA group, suggesting a role of leukocytes in UA.

High and low affinity Ca2+ binding to the sarcoplasmic reticulum: use of a high-affinity fluorescent calcium indicator

The fluorescent calcium indicator, calcein, has been used as a high-affinity indicator of Ca2+ in the aqueous phase at physiological pH in the study of high-affinity calcium binding to sarcoplasmic reticulum (SR). The binding constant of the indicator at physiological pH is 10(3)-10(4) M-1 and increases with increasing pH. The binding mechanism of the indicator with Ca2+ and Mg2+ is described. Application of calcein as an aqueous indicator of Ca2+ binding to the SR at room temperature has revealed two classes of binding sites: one with high capacity and low affinity (ca. 820 nmol/mg protein, Kd = 1.9 mM), and another with low capacity and higher affinity (ca. 35 nmol/mg protein, Kd = 17.5 micronM). The divalent cation specificity of the low-affinity site is low and Ca2+/Mg2+ specificity of the high-affinity site is high. Quantitative studies of the bindings indicate that the high-affinity site residues in the Ca2+ ATPase (carrier) protein and represents complexation in the active site of the carrier and that the low-affinity site residues in the nonspecific acidic binding proteins. The contribution of Donnan equilibrium effects to the measured binding is shown to be insignificant. Stopped flow kinetic studies of Ca2+ passive binding with calcein and arsenazo III dyes have demonstrated that the binding to high-affinity site is very fast and that the overall binding reaction with the low-affinity site is slow, with a time course of about 4 s. Our analysis has shown that at least part of the low-affinity acidic proteins are within the SR matrix and that Ca2+ can reach them only by transversing the membrane via the Ca2+ carrier (Ca2+ ATPase). A model of the SR is proposed that accounts for several functional properties of the organelle in terms of its known protein composition and topological organization.

Evidence for a role of phosphatidyl ethanolamine as a modulator of membrane-membrane contact

Phosphatidyl ethanolamine (PE) is shown to be effective in producing membrane aggregation. The aggregation of PE and PE/PC (phosphatidyl choline) mixed vesicles was studied as a function of pH and cation composition of the medium. The kinetics and equilibria were studied in stopped-flow rapid mixing experiments, in which PE vesicles prepared at pH 9.2 were "jumped" to pH 7. H+ ions protonate PE- and promote vesicle aggregation in a cooperative fashion. Vesicles containing PC have a decreased tendency to aggregate compared to pure PE vesicles. The apparent rate constant for aggregation was about two orders of magnitude below that for diffusion controlled aggregation and was virtually the same for PE and PE/PC mixed vesicles. A theoretical description of equilibrium for vesicle aggregation is developed in terms of three parameters: the equilibrium constant for the protonation of PE (KA), the equilibrium constant for aggregation (Keq) and the number of PE molecules in an effective area that the two vesicles must interact in order to aggregate (Neff). These parameters are compared with values and trends expected for electrostatic calculations based on dipolar repulsion and short-range binding, to which hydrogen bonding may contribute. The results are interpreted in a self-consistent fashion to indicate: (i) that PE and PC mix randomly, (ii) that head-to-tail binding occurs between PE(PC) molecules on apposing vesicles, (iii) that electrostatic screening accounts for the decrease in KA as a function of the molar fraction of PC per vesicle, (iv) that the PE must be 90% protonated before aggregation can occur, and (v) that for all the lipid systems we considered, the point at which the extent of dimerization is half maximal is close to the physiological pH, indicating that PE may have a regulatory effect in the aggregation of biological systems.

Fluorescence study of the divalent cation-transport mechanism of ionophore A23187 in phospholipid membranes

The mechanism for transport of divalent cations across phospholipid bilayers by the ionophore A23187 was investigated. The intrinsic fluorescence of the ionophore was used in equilibrium and rapid-mixing experiments as an indicator of ionophore environment and complexation with divalent cations. The neutral (protonated) form of the ionophore binds strongly to the membrane, with a high quantum yield relative to that in the aqueous phase. The negatively charged form of the ionophore binds somewhat less strongly, with a lower quantum yield, and does not move across the membrane. Complexation of the negatively charged form with divalent cations was measured by the decrease in fluorescence. An apparent rate constant (kapp) for transport of the ionophore across the membrane was determined from the rate of fluorescence changes observed in stopped-flow rapid kinetic experiments. The variation of kapp was studied as a function of pH, temperature, ionophore concentration, membrane lipid composition, and divalent cation concentration and type. Analysis and comparison with equilibrium constants for protonation and complexation show that A23187 and its metal:ionophore complexes bind near the membrane-water interface in the lipid polar-head region. The interfacial reactions occur rapidly, compared with the transmembrane reactions, and are thus in equilibrium during transport. The transport cycle can be described as follows: a 1:1 complex is formed between the membrane bound A23187-(Am-) and the aqueous divalent cation with dissociation constant K1 approximately 4.6 x 10(-4) M. This is in equilibrium with a 1:2 (metal:ionophore) complex (K2 approximately 3.0 x 10(-4) [ionophore/lipid]) that is responsible for transporting the divalent cations across the membrane. The rate constant for translocation of the 1:2 complex is 0.1-0.3 s-1. Dissociation of the complex of the trans side and protonation occur rapidly. The rate constant for translocation of H+ . A23187- is 28 s-1. A theory is presented that is capable of reproducing the kinetic data at any calcium concentration. The cation specificity for ionophore complex transport (kapp), determined at low ionophore concentration for a series of divalent cations, was found to be proportional to the equilibrium constant for 1:1 complexation. The order of ion specificity for these processes was found to be Ca2+ greater than Mg2+ greater Sr2+ greater than Ba2+. Interactions with Na+ were not observed. Maximal values of kapp were observed for vesicles prepared from pure dimyristoyl phosphatidylcholine. Inclusion of phosphatidyl ethanolamine, phosphatidic acid, or dipalmatoyl phosphatidylcholine resulted in lower values of kapp. Calcium transport by A23187 is compared with that of X537A, and it is shown that the former is 67-fold faster. The difference in rates is due to differences in the ability of each ionophore to form a 1:2 complex from a 1:1 complex.

Cyclic GMP increases the rate of the calcium extrusion pump in intact human platelets but has no direct effect on the dense tubular calcium accumulation system

Sodium nitroprusside (SNP) and other agents that elevate cGMP levels are known to inhibit the aggregation of human platelets. Published data suggest that cGMP attenuation of agonist-induced Ca2+ transients is involved in this effect. The present study shows that elevation of cGMP increases the rate of the Ca2+ extrusion pump located in the plasma membrane (PM) but does not have a direct effect on the Ca2+ accumulating pump of the dense tubules (DT). The study verifies that SNP can specifically elevate the cGMP level in the platelet. The kinetics of the Ca2+ extrusion system were studied in situ in platelets overloaded with the cytoplasmic Ca2+ indicator quin2 according to a published protocol developed in this laboratory. Elevation of cGMP by means of (10 microM) SNP increased the Vm of the Ca(2+)-ATPase pump by 63%, without affecting its Km (66-80 nM) or Hill coefficient (1.6-1.8). Dibutyryl-cGMP (Bt2-cGMP), preincubated for 45 min at 1 mM, increased the Vm by a factor of 2.2 +/- 0.4. The experiments did not give any indication that SNP or Bt2-cGMP change the rate of the Na+/Ca2+ exchanger which makes a minor contribution to Ca2+ extrusion in the studied [Ca2+]cyt range. The rate constant for passive leakage of Ca2+ across the PM was increased by 32 +/- 4% by SNP and 90 +/- 34% by Bt2-cGMP. The net result is that the free Ca2+ in the cytoplasm ([Ca2+]cyt) at 'rest' is lowered from control values of 112 nM to 89 nM or 80 nM, respectively. The kinetics of Ca2+ uptake by the dense tubules were determined in situ using the fluorescence of chlorotetracycline (CTC) according to protocols developed in this laboratory. Analysis showed that SNP and Bt2-cGMP had no effect on the Vm or Km of the dense tubular pump, and did not affect the rate constant for passive leakage. The agents did decrease resting [Ca2+]dt by 25% or 30%, respectively, but this result can be explained purely in terms of the reduced [Ca2+]cyt. The effects of cGMP (vs. cAMP) on the PM and DT pumps are closely correlated with reported effects of cGMP/cAMP induced phosphorylation of a protein of the molecular weight of the PM pump and a 22 kDa activator of the DT pump. Cyclic AMP increases the rate of both the PM and the DT pumps, whereas cGMP increases the rate of the PM pump only.(ABSTRACT TRUNCATED AT 400 WORDS)

Cyclic AMP stimulates Ca(2+)-ATPase-mediated Ca2+ extrusion from human platelets

The effect of cAMP on active Ca2+ extrusion across the plasma membrane of intact human platelets was studied using quin2, a fluorimetric indicator of free Ca2+ in the cytoplasmic compartment ([Ca2+]cyt). Elevations of cAMP were achieved by incubation with dibutyryl-cAMP or by forskolin, which was found to selectively elevate cAMP without affecting cGMP levels. Progress curves of Ca2+ extrusion from quin2-overloaded platelets were measured. The rate vs. [Ca2+]cyt characteristic was calculated as previously described (Johansson, J.S. and Haynes, D.H. (1988) J. Membr. Biol. 104, 147-163). Forskolin, at a maximally effective concentration of 10 microM, was shown to stimulate Ca2+ extrusion by increasing by a factor of 1.6 +/- 0.5 the Vm of a saturable component, previously identified with a Ca(2+)-Mg(2+)-ATPase located in the plasma membrane. Neither the Km (80 nM) or Hill coefficient (1.7 +/- 0.3) of the Ca(2+)-ATPase was affected. Forskolin had no effect on the linear, non-saturable component of extrusion (previously identified with a Na+/Ca2+ exchanger) over the [Ca2+]cyt range examined (50-1500 nM). Dibutyryl-cAMP (Bt2-cAMP, 1 mM) stimulated the Ca(2+)-Mg(2+)-ATPase component of Ca2+ extrusion by a factor of 2.0 +/- 0.6. Separate experiments showed that 10 microM forskolin reduces the resting [Ca2+]cyt from 112 nM to 96 nM. Mathematical analysis showed that this can be accounted for by the above-mentioned increase in Vm of the pump, countered by a 37-74% increase in the rate constant for passive Ca2+ leakage across the plasma membrane. The results suggest two mechanisms by which prostacyclin-induced elevation of cAMP inhibits platelet aggregation: (a) lowering of resting [Ca2+]cyt and (b) increasing the rate of Ca2+ extrusion after the initial influx or triggered release event.

Thrombin-induced calcium movements in platelet activation

The thrombin-induced Ca2+ fluxes and their coupling to platelet aggregation of the human platelet were studied using quin2 as a measure of the cytoplasmic Ca2+ concentration [( Ca2+]cyt) and chlorotetracycline (CTC) as a measure of internally sequestered Ca2+. Evidence is given that the CTC fluorescence change is proportional to the free internal Ca2+ concentration in the dense tubular lumen. The intracellular quin2 concentration was 1 mM and analysis showed that it did not perturb the processes reported herein. The value of [Ca2+]cyt at rest and during thrombin activation was analyzed in terms of Ca2+ influx, Ca2+ release, Ca2+ sequestration, and Ca2+ extrusion. Influx was distinguished from internal release by removing extracellular Ca2+ 1 min before thrombin activation. In the presence of 2 mM external Ca2+, the thrombin-induced Ca2+ influx accounts for most of the increase in [Ca2+]cyt (over 80%). Thrombin-induced Ca2+ influx and release have somewhat different EC50 values (0.17 U/ml vs. 0.35 U/ml). The contribution of influx can be inhibited by verapamil, bepridil and Cd2+ (IC50 values of 19 microM, 2 microM and 50 microM). The influx results were analyzed in terms of a thrombin-activated channel. Indomethacin pretreatment experiments suggest that activation of the arachidonic pathway accounts for approx. 50% of the influx-related [Ca2+]cyt elevation. Elevation of [Ca2+]cyt by intracellular release is not inhibited by verapamil or Cd2+ but is inhibited by bepridil with a high IC50 (25 microM). It is only 15-20% inhibited by indomethacin and is thus not dependent on thromboxane A2 formation. The release reaction does not require Ca2+ influx. The rate of thrombin-activated platelet aggregation is shown to have an approximately fourth-power dependence on [Ca2+]cyt with an apparent Km of 0.4 microM. Comparisons of aggregation rates of the partially thrombin-activated vs. fully thrombin-activated, partially verapamil-inhibited conditions suggest that this dependence on [Ca2+]cyt is the major determinant of the aggregation behavior. Analysis shows that calcium influx is the major pathway for elevating [Ca2+]cyt by thrombin when physiological concentrations of external Ca2+ are present.

Stimulation of dense tubular Ca2+ uptake in human platelets by cAMP

Elevation of intracellular cAMP is shown to increase the rate (V) and maximal extent of Ca2+ uptake by the dense tubules in intact human platelets. Elevation of [cAMP] was accomplished by preincubation with the adenylate cyclase activator forskolin or with dibutyryl-cAMP (Bt2-cAMP). The free concentration of Ca2+ in the dense tubular lumen ([Ca2+]dt) was monitored using the fluorescence of chlorotetracycline (CTC) according to protocols developed in this laboratory. The free cytoplasmic Ca2+ concentration ([Ca2+]cyt) was monitored in parallel experiments with quin2. Both [Ca2+]cyt and [Ca2+]dt were analyzed in terms of competition between pump and leak mechanisms in the plasma membrane (PM) and dense tubular membrane (DT). When platelets are incubated in media with approx. 1 microM external Ca2+, [Ca2+]cyt is approx. 50 nM and [Ca2+]dt is very low. When 2 mM external Ca2+ is added, [Ca2+]cyt rises to approx. 100 nM and the process of dense tubular Ca2+ uptake can be resolved. Forskolin (10 microM) and Bt2-cAMP increase the rate of dense tubular Ca2+ uptake (V) to 2.1 +/- 0.60 and 1.70 +/- 40 times control values (respectively). The agents also increase the final [Ca2+]dt to 1.70 +/- 0.21 and 1.72 +/- 0.60 times control values (respectively). Titrations with ionomycin (Iono) showed that the increase was due to an increase in the Vm of the dense tubular Ca2+ pump. With [Iono] = 500 nM, [Ca2+]cyt was raised to greater than or equal to 1.0 microM and Vm of the dense tubular pump was elicited. (At [Iono] = 1.0 microM, the final [Ca2+]dt values were degraded 15% due to shunting of Ca2+ uptake.) Analysis showed that forskolin (10 microM) and Bt2-cAMP (1 mM) increase the Vm by a factors of 1.56 +/- 40 and 1.56 +/- 40, respectively. Analysis showed that neither agent changed the Km of the pump significantly from its control value of 180 nM. Neither agent changed the rate constant for passive leakage of Ca2+ across the DT membrane (1.7 min-1).

Measurement of surface potential and surface charge densities of sarcoplasmic reticulum membranes

The binding of the anionic fluorescent probe 1-anilino-8-naphthalene-sulfonate (ANS-) was used to estimate the surface potential of fragmented sarcoplasmic reticulum (SR) derived from rabbit skeletal muscle. The method is based on the observation that ANS- is an obligatory anion whose equilibrium constant for binding membranes is proportional to the electrostatic function of membrane surface potential, exp(e psi o/kT), where psi o is the membrane surface potential, e is the electronic charge, and kT has its usual meaning. The potential measured is characteristic of the ANS- bindings of phosphatidylcholine head groups and is about one-third as large as the average surface potential predicted by the Gouy-Chapman theory. At physiological ionic strength the surface potentials, measured by ANS-, referred to as the aqueous phase bathing the surface, were in the range -10 to -15 mV. This was observed for the outside and inside surfaces of the Ca2+-ATPase-rich fraction of the SR and for both surfaces of the SR fraction rich in acidic Ca2+ binding proteins. The inside and outside surfaces were differentiated on the basis of ANS- binding kinetics observed in stopped-flow rapid mixing experiments. A mechanism by which changes in Ca2+ concentration could give rise to an electrostatic potential across the membrane and possibly result in changes in Ca2+ permeability. The dependence of the surface potential on the monovalent ion concentration in the medium was used together with the Gouy-Chapman theory to determine the lower limits for the surface charge density for the inside and outside surfaces of the two types of SR. Values for the Ca2+-ATPase rich SR fraction were between 2.9 X 10(3) and 3.8 X 10(3) esu/cm2, (0.96 X 10(-6) and 1.26 X 10(-6) C/cm2) with no appreciable transmembrane asymmetry. A small amount of asymmetry was observed in the values for the inside and outside surfaces of the fraction rich in acidic binding proteins which were ca. 6.6 X 10(3) and ca. 2.2 X 10(3) esu/cm2 (2.2 X 10(-6) and 0.73 X 10(-6) C/cm). The values could be accounted for by the known composition of negatively-charged phospholipids in the SR. The acidic Ca2+ binding proteins were shown to make at most a small contribution to the surface charge, indicating that their charge must be located at least several tens of A from the membrane surface. The experiments gave evidence for a Donnan effect on the K+ distribution in the fraction rich in acidic binding proteins. This could be accounted for by the known concentration of acidic binding proteins in this SR fraction. The equilibrium constant for ANS- was shown to be more sensitive to changes in the divalent cation concentration than to changes in the monovalent cation concentration, as predicted by the Gouy-Chapman theory. Use of these findings together with the stopped-flow rapid mixing techniques constitutes a method for rapid and continuous monitoring of changes in ion concentrations in the SR lumen.

Intracellular calcium storage and release in the human platelet. Chlorotetracycline as a continuous monitor

The calcium-sensitive fluorescent probe chlorotetracycline was used to monitor calcium movement in human platelets. The chlorotetracycline fluorescence signal is a linear measure of the level of free calcium in the dense tubules and in the mitochondria, with probe sensitivity in the millimolar range. Experiments perturbing the system with the calcium ionophore A23187 shows that the level of free internal calcium in the organelle depends upon the cytoplasmic level, which, in turn, depends upon the passive permeability of the plasma membrane. Chlorotetracycline in the cytoplasmic compartment does not respond to changes in the cytoplasmic calcium concentration, which is held in the micromolar to submicromolar range by an extrusion system. The calcium concentration in the cytoplasmic compartment can be directly manipulated by the calcium ionophore A23187 and is measured in parallel experiments with Quin 2, a high-affinity indicator. The calcium transport systems of the organelles are shown to be less susceptible to short circuit by A23187. Analysis shows that mitochondrial uptake is slow (t 1/2 = 20 minutes), produces a large increase in chlorotetracycline fluorescence, and is inhibited by sodium azide plus oligomycin. Uptake by the dense tubules is more rapid (t 1/2 = 2 minutes), produces a smaller increase in chlorotetracycline fluorescence, is inhibited by trifluoperazine, and is less sensitive to A23187. The Km is estimated as 1 microM or lower. Studies show that the chlorotetracycline technique is useful for the monitoring of calcium uptake and release by the platelet organelles, and suggests that the Quin 2/chlorotetracycline technique will be useful as a diagnostic of both physiological and pathological activation mechanisms.

Deliberate quin2 overload as a method for in situ characterization of active calcium extrusion systems and cytoplasmic calcium binding: application to the human platelet

The objectives of the title were accomplished by a four-step experimental procedure followed by a simple graphical and mathematical analysis. Platelets are (i) overloaded with the indicator quin2 to cytoplasmic concentrations of 2.9 mM and (ii) are exposed to 2 mM external Ca2+ and 1.0 microM ionomycin to rapidly achieve cytoplasmic Ca2+ ([Ca2+]cyt) of ca. 1.5 microM. (iii) The external Ca2+ is removed by EGTA addition, and (iv) the active Ca2+ extrusion process is then monitored as a function of time. Control experiments show that the ionophore shunts dense tubular uptake and does not contribute to the Ca2+ efflux process during phases iii-iv and that the extrusion process is sensitive to metabolic inhibitors. The progress curves for the decline of quin2 fluorescence (resulting from active Ca2+ extrusion) were analyzed as a function of [Ca2+]cyt using a mathematical model involving the probability that an exported Ca2+ was removed from a quin2 complex (vs. a cytoplasmic binding element). The observed rates of decline of quin2 fluorescence at a particular [Ca2+]cyt are dependent upon (i) the absolute rate of the extrusion system (a function of its Km, Vm and Hill coefficient (n)), (ii) the intrinsic Ca2+ buffer capacity of the cytoplasm (a function of the total site concentration ([B]T) and its Kd) and (iii) the buffer capacity of the intracytoplasmic quin2 (a function of its concentration and Kd). The contribution of (iii) was known and varied and was used to determine (ii) and (i) as a function of [Ca2+]cyt. The Ca2+ binding data were verified by 45Ca2+ experimentation.(ABSTRACT TRUNCATED AT 250 WORDS)