Cation binding to submitochondrial particles

Mathematical model of mitochondrial ionic homeostasis: three modes of Ca2+ transport

Mitochondria play an important role in regulation of Ca2+ homeostasis in a cell. Here we present a mathematical model of mitochondrial ion transport and use this model to analyse different modes of Ca2+ uptake by mitochondria. The model includes transport of H+, Ca2+, K+, inorganic phosphate and oxidative substrates across the inner mitochondrial membrane harboring permeability transition pore (PTP). The detailed description of ion fluxes is based on the experimental ion kinetics in isolated mitochondria. Using the model we show that the kinetics of Ca2+ uptake by mitochondria is regulated by the total amount of Ca2+ in the system and the rate of Ca2+ infusion. Varying these parameters we find three different modes of ion transport. When the total amount of Ca2+ is below 140 nmol Ca2+/mg protein, all available Ca2+ is accumulated in the matrix without activation of the PTP. Between 140 and 160 nmol Ca2+/mg protein, accumulation of Ca2+ generates periodic opening and closure of the PTP and oscillations of ion fluxes. Higher levels of Ca2+ (> 160 nmol Ca2+/mg protein) result in a permanently open PTP, membrane depolarization and loss of small ions from the matrix. We show that in the intermediate range of Ca2+ concentrations the rate of Ca2+ infusion regulates the PTP state, so that slow Ca2+ infusion does not lead to PTP opening, while fast Ca2+ infusion results in an oscillatory state.

Characterization of two Mg2+ transporters in sealed plasma membrane vesicles from rat liver

The plasma membrane of mammalian cells possesses rapid Mg2+ transport mechanisms. The identity of Mg2+ transporters is unknown, and so are their properties. In this study, Mg2+ transporters were characterized using a biochemically and morphologically standardized preparation of sealed rat liver plasma membranes (LPM) whose intravesicular content could be set and controlled. The system has the advantages that it is not regulated by intracellular signaling machinery and that the intravesicular ion milieu can be designed. The results indicate that 1) LPM retain trapped intravesicular total Mg2+ with negligible leak; 2) the addition of Na+ or Ca2+ induces a concentration- and temperature-dependent efflux corresponding to 30-50% of the intravesicular Mg2+; 3) the rate of flux is very rapid (137.6 and 86.8 nmol total Mg2+ . micrometer -2 . min-1 after Na+ and Ca2+ addition, respectively); 4) coaddition of maximal concentrations of Na+ and Ca2+ induces an additive Mg2+ efflux; 5) both Na+- and Ca2+-stimulated Mg2+ effluxes are inhibited by amiloride, imipramine, or quinidine but not by vanadate or Ca2+ channel blockers; 6) extracellular Na+ or Ca2+ can stimulate Mg2+ efflux in the absence of Mg2+ gradients; and 7) Mg2+ uptake occurs in LPM loaded with Na+ but not with Ca2+, thus indicating that Na+/Mg2+ but not Ca2+/Mg2+ exchange is reversible. These data are consistent with the operation of two distinct Mg2+ transport mechanisms and provide new information on rates of Mg2+ transport, specificity of the cotransported ions, and reversibility of the transport.

Minimal model of beta-cell mitochondrial Ca2+ handling

We develop a simplified, but useful, mathematical model to describe Ca2+ handling by mitochondria in the pancreatic beta-cell. The model includes the following six transport mechanisms in the inner mitochondrial membrane: proton pumping via respiration and proton uptake by way of the F1Fzero-ATPase (adapted from D. Pietrobon and S. Caplan. Biochemistry 24: 5764-5778, 1985), a proton leak, adenine nucleotide exchange, the Ca2+ uniporter, and Na+/Ca2+ exchange. Each mechanism is developed separately into a kinetic model for the rate of transport, with parameters taken from experiments on isolated mitochondrial preparations. These mechanisms are combined in a modular fashion first to describe state 4 (nonphosphorylating) and state 3 (phosphorylating) mitochondria with mitochondrial NADH and Ca2+ concentrations as fixed parameters and then to describe Ca2+ handling with variable mitochondrial Ca2+ concentration. Simulations are compared to experimental measurements and agree well with the threshold for Ca2+ uptake, measured mitochondrial Ca2+ levels, and the influence of Ca2+ on oxygen uptake. In the absence of Ca2+ activation of mitochondrial dehydrogenases, the simulations predict a significant reduction in the rate of production of ATP that involves a "short circuit" via Ca2+ uptake through the uniporter. This effect suggests a potential role for mitochondrial Ca2+ handling in determining the ATP-ADP ratio in the pancreatic beta-cell.

The effect of lidocaine on myocardial ischemia with asanguinous reperfusion. An in vitro study

The effect of lidocaine on the ischemic nor-mothermic rat heart was studied in a Langendorff preparation. Ventricular fibrillation, total retrograde coronary flow and effluent lactate concentration were monitored in preischemia (control), ischemia (20 min) and reperfusion (20 min). Myocardial metabolites were determined in specimens excised at termination of reperfusion. Six hearts were infused with lidocaine in Ringer solution at onset of ischemia (group A) and six with only Ringer solution (group B). Sinus rhythm proceeded directly to diastolic arrest after 17 sec in group A, while all group B hearts showed ventricular fibrillation before arrest at 174 sec. Effluent lactate concentration was reduced in group A during the first 10 min of ischemia, but not subsequently. After 10 min of reperfusion, coronary flow was reduced by 12% in group A and 20% in group B. ATP was higher and ADP, AMP and IMP were lower in group A than in group B after 20 min of reperfusion. Creatine phosphate showed no intergroup difference, but creatine was higher in group B. Cardiac arrest with lidocaine thus reduced lactate formation during ischemia and lessened high-energy phosphate depletion after reperfusion.

Estimation of the pH gradient and donnan potential in de-energized heart mitochondria

The transmembrane pH gradient maintained by nonrespiring, uncoupled heart mitochondria has been estimated using the distribution of methylamine and of 5,5-dimethyl-2,4-oxazolidinedione (DMO) and compared with the delta pH reported by the fluorescent probe 2,7-biscarboxyethyl-5(6)-carboxyfluorescein (BCECF). Under these conditions the protonmotive force approaches zero and the membrane potential (delta psi) should equal 59 delta pH (P. Mitchell and J. Moyle (1969) Eur. J. Biochem. 7, 471-484). The delta pH reported by DMO corresponds closely to that estimated by BCECF and is consistent with a Donnan potential of no greater than about -30 mV (interior negative) for nonenergized mitochondria in a sucrose medium. This potential appears to result from the presence of immobile negative charges in the matrix and is eliminated by addition of 10 to 25 mM KCl. Measurements of delta pH using the methylamine and of delta tsi using the distribution of 42K+ in the presence of valinomycin result in an apparent overestimation of these parameters due to binding of these components to negative sites on the membrane. Increasing ionic strength decreases this contribution of surface potential, but significant binding can still be detected in 100 mM KCl. These studies suggest that 42K+ (or 86Rb+) is far from an ideal probe for measuring delta tsi in respiring mitochondria and may significantly overestimate this parameter, especially in sucrose media.

Respiration-dependent uptake and extrusion of Mg2+ by isolated heart mitochondria

It has been known for some time that isolated heart mitochondria can both take up and extrude Mg2+ by respiration-dependent, uncoupler-sensitive processes. A re-examination of these reactions reveals that the respiration-dependent uptake of Mg2+ can be quite rapid and efficient and is apparently preceded by a passive binding to the inner membrane. The rate of Mg2+ uptake can exceed 30 ng ion/min/mg protein at an efficiency of about 1 ng ion Mg2+ accumulated per ng atom O2 consumed. Passive binding and respiration-dependent accumulation of Mg2+ are strongly inhibited by K+ and other monovalent cations and the uptake reaction is further decreased by the presence of ATP or ADP. Under conditions approaching those faced by mitochondria in situ (state 3 respiration in a KCl medium) the rate of Mg2+ uptake, as estimated from 28Mg2+ distribution, is no more than 0.25 ng ion/min/mg. When heart mitochondria are suspended in a Mg2+-free medium, a slow, respiration-dependent Mg2+ efflux is seen. This reaction is quite insensitive to external K+ and otherwise shows an inhibitor profile markedly different from that of the Mg2+ accumulation reaction. Neither the uptake nor the loss of Mg2+ is inhibited by ruthenium red or diltiazem. These reactions therefore appear unrelated to those involved in the uptake and release of Ca2+. It is concluded that heart mitochondria have separate pathways available for Mg2+ uptake and release.

The internal concentration of K+ in isolated rat liver mitochondria

A simple osmotic method has been developed to determine the internal K+ concentration of mitochondria by determining the concentration of external K+ at constant osmotic pressure at which metabolically inhibited mitochondria neither shrink nor swell. This concentration has been found to correspond to approx. 80-85 mM in freshly isolated mitochondria and considerably lower after additional centrifugation procedures. Since mitochondria are in osmotic equilibrium with the suspending medium (in this case, 0.32 osmolal), and K+ is the primary exchangeable internal ion, a significant proportion of the internal osmotic pressure must be exerted by the sucrose. Results for experiments determining internal K+ after centrifuging mitochondria at various G values confirm the reports of Sitaramam et al. (Sitaraman, V. and Sarma, M.K.J. (1981) Proc. Natl. Acad. Sci. USA 78, 3441-3445 and Sambasivarao, D. and Sitaramam, V. (1983) Biochim. Biophys. Acta 722, 256-270) that centrifugation induces the entry of sucrose in mitochondria isolated in a sucrose medium.

The intramitochondrial volume measured using sucrose as an extramitochondrial marker overestimates the true matrix volume determined with mannitol

The matrix volume of isolated liver and heart mitochondria has been estimated at various osmolarities and in various osmotic supports using 36Cl- and [14C]sucrose, D-mannitol, D-3-methoxyglucose and choline as extramitochondrial markers. The use of 3-methoxyglucose was only possible at 0 degree C since it entered mitochondria at physiological temperatures. All extramitochondrial markers used gave linear plots of apparent matrix volume against the reciprocal of the osmolarity, but the slope of this plot was greater when sucrose was used than with the other extramitochondrial markers. When extrapolated to infinite osmolarity the mean matrix volume was zero when mannitol was used, but about 0.6 microliter/mg of protein for sucrose and Cl- and -0.4 microliter/mg of protein when choline was used. At physiological osmolarity (about 330 m-osmol) the mean matrix volume of de-energized liver mitochondria in KCl medium estimated using mannitol was 0.46 microliter/mg of protein, whereas that obtained using sucrose was 1.68 microliters/mg of protein. Values in mannitol, choline and sucrose media were similar when mannitol but not sucrose was used as extramitochondrial marker. It is argued that the 3H2O/[14C]mannitol space more accurately reflects the true mitochondrial matrix volume than does the 3H2O/[14C]sucrose space. The consequences of this for measurements of the protonmotive force and the intramitochondrial concentration of metabolites are discussed.

Isolation of a fraction with Ca2+ ionophore properties from rat liver mitochondria

Isolation of a small protein with properties of a Ca2+ ionophore from calf heart mitochondria has recently been reported [A. Y. Jeng and A. E. Shamoo, 1980, J. Biol. Chem. 255, 6897, 6904]. We have isolated a fraction with similar physical and chemical properties from rat liver mitochondria. In particular, the hepatic preparation is able to bind Ca2+ with high affinity in such a fashion that the resultant complex is soluble in a hydrophobic phase. It will also transport Ca2+ through a stirred organic phase (Pressman cell). Interaction of the liver preparation with Ca2+ is sensitive to inhibitors of mitochondrial Ca2+ uptake. The hepatic preparation contains both protein and lipid components. The phospholipid components were identified and the behavior of a similar mixture of commercially available phospholipids was compared to that of the ionophore fraction from rat liver mitochondria. All of the Ca2+ binding properties of the rat liver preparation could be mimicked by the lipids. In a preliminary experiment, reduction of the phospholipid content of the preparation to less than one lipid phosphate per protein molecule (assuming a molecular weight of 3000 by analogy with the calf heart case) resulted in a protein that was unable to bind Ca2+. We, therefore, suggest that the ability of the preparation to interact with Ca2+ is due to the constituent phospholipids. Measurements of phospholipid-Ca2+ interactions in the model systems and under the conditions of low (microM) Ca2+ and phospholipid concentration utilized here demonstrated an affinity for Ca2+ (Ks approximately 1 microM) and a cation selectivity that have not previously been reported.

Effects of pentobarbitone on 45Ca2+ transport by rat brain mitochondria

The effects of pentobarbitone on the transport of 45Ca2+ by rat brain mitochondria were studied, using the Ruthenium Red-EGTA quench technique. In the presence of succinate and inorganic phosphate, mitochondria rapidly accumulate 45Ca2+. Pentobarbitone (0.1-1.0 mM) stimulates the initial rate of Ca2+ transport. In contrast, pentobarbitone (1 mM) did not affect the NaCl (50 mM)-induced efflux of 45Ca2+ from mitochondria. Dibucaine (60 micro M), a clinically used local anaesthetic, inhibits both 45Ca2+ uptake an efflux. The results suggest that barbiturate stimulation of mitochondrial Ca2+ uptake may, in combination with effects on other Ca2+ sequestering processes, contribute to the inhibitor of transmitter release observed at a number of synapses.

The transport of cations in mitochondria

The present paper has reviewed several factors related to ion transport and examined the properties of cation transport in mitochondria. The analysis suggests that: (1) The concept that a metabolically dependent electrical potential across the mitochondrial membrane plays a role in determining ion fluxes and steady-state concentrations is not justified and the data indicate that such exchanges are generally electroneutral. (2) Generally, the influx and efflux of an ion proceed by the same mechanism with at least one exception. (3) There are indications that some of the steps in transport are common to several cations. (4) The idea that carrier or ionophoric molecules are involved in cation transport has been examined in some detail together with the possible involvement of some known mitochondrial components. In particular, a model has been introduced in which local charge imbalances produced by H+ fluxes serve as the driving force of transport. The molecules of the complex are arranged in series in a tripartite arrangement including a filter or gate, a nonselective channel and an H+-transferring portion linked to either electron transport or the ATPase. Parts of this model have been introduced by other investigators. Models in which different portions of channels have differing functions have been proposed previously for other transport systems.

Mitochondrial calcium transport and calcium-activated phospholipase in porcine malignant hyperthermia

The interaction of Ca2+ with mitochondria isolated from longissismus dorsi, a predominantly white skeletal muscle, of normal and malignant hyperthermia pigs was investigated using tightly-coupled preparations. Arrhenius plots of mitochondrial Ca2+ -stimulated respiration for succinate oxidation of malignant hyperthermia pigs showed a transition temperature (Tt) of 26.31 +/- 0.80 degrees C (n = 5), which was decreased by spermine to 15.41 +/- 0.69 degrees C (n = 3), a value very similar to that for normal pigs. No difference in either the Tt or in the activation energy (Ea) was observed between the two types of pigs when ADP was used instead of Ca2+. Mitochondria of malignant hyperthermia pigs were uncoupled at 40 degrees C by exogenous Ca2+ at 1221 +/- 301 (n = 9) nmol Ca2+ per mg proteinn during succinate oxidation and the uncoupled mitochondria showed large amplitude swelling. Both the Ca2+ -induced uncoupling and swelling were prevented by bovine serum albumin and by the phospholipase inhibitors, spermine and tetracaine. In contrast, mitochondria of normal pigs were still tightly coupled even after a total addition of 2313 +/- 287 (n = 5) nmol Ca2+ per mg protein and retained the original condensed configuration in the presence or absence of spermine and tetracaine. Mitochondria of malignant hyperthermia pigs contained significantly (P less than 0.001) higher quantities of endogenous Ca2+ and showed a significantly (P less than 0.001) faster FCCP-induced endogenous Ca2+ efflux rate than normal when monitored spectroscopically with murexide. No significant difference was observed in either the rate of exogenous Ca2+ uptake or in the extent of Ca2+ accumulated in the aerobic steady state during succinate oxidation between the two types of pigs. The rate of mitochondrial Ca2+ efflux of malignant hyperthermia pigs during anaerobiosis was about twice that of normal. Experimental evidence suggests that mitochondria from musculi longissimus dorsi of malignant hyperthermia pigs contained a Ca2+ -stimulated phospholipase A2 (EC 3.1.1.4, phosphatide 2-acylhydrolase), and that this enzyme if present in mitochondria of normal pigs is either latent or in very low concentration. The significance of the Ca2+ -stimulated phospholipase A2 and its association with the enhanced rate of glycolysis in porcine malignant hyperthermia syndrome and in the post-mortem formation of the pale, soft and exudative conditions observed in white skeletal muscles of malignant hyperthermia pigs is discussed.

The action of Nupercaine on calcium efflux from rat liver mitochondria

1. Nupercaine inhibits the Ca2+ efflux from rat liver mitochondria observed in the presence of Ruthenium Red, 50% inhibition being obtained at 80 microM-Nupercaine. 2. Neither the Ruthenium Red-stimulated efflux nor its inhibition by Nupercaine can be directly attributed to effects on mitochondrial stability. 3. Nupercaine perturbs the steady-state external Ca2+ concentration in the absence of Ruthenium Red to an extent that is explicable in terms of the inhibition of Ca2+ efflux. 4. Various factors that are likely to be involved in determining steady-state extra-mitochondrial Ca2+ concentrations are discussed.

E. coli endotoxin shock in the dog; treatment with lidocaine or indomethacin

1 Dogs treated with lidocaine (1 mg kg-1 h-1) or indomethacin (1.5 mg/kg) before and after an LD60 dose (1 mg/kg) of E. coli endotoxin survived for at least 72 h. 2 Although all dogs in both treated groups survived, only those treated with indomethacin were significantly protected against the fall in the arterial blood pressure 1 to 2 min following endotoxin administration. 3 Endotoxin increased the plasma prostaglandin F2alpha (PGF2alpha) concentration in the control and lidocaine-treated groups, however, no increase was observed with indomethacin treatment. 4 Neither lidocaine nor indomethacin alone had any significant effect on the parameters measured in this model. 5 Following the administration of endotoxin, lidocaine-treated animals had significantly decreased plasma fibrinogen concentrations when compared to the other groups. 6 This study suggests that lidocaine, a local anaesthetic and a drug widely used for cardiac arrhythmias, might offer protection in endotoxin shock.

Efflux of magnesium and potassium ions from liver mitochondria induced by inorganic phosphate and by diamide

Addition to rat liver mitochondria of 2 mM inorganic phosphate or 0.15 mM diamide, a thiol-oxidizing agent, induced an efflux of endogenous Mg2+ linear with time and dependent on coupled respiration. No net Ca2+ release occurred under these conditions, while a concomitant release of K+ was observed. Mg2+ efflux mediated either by Pi or low concentration of diamide was completely prevented by EGTA, Ruthenium red, and NEM. These reagents also inhibited the increased rate of state 4 respiration induced both by Pi and diamide. At higher concentrations (0.4 mM), diamide induced an efflux of Mg2+ which was associated also with a release of endogenous Ca2+. Under these conditions EGTA completely prevented Mg2+ and K+ effluxes, while they were only partially inhibited by Ruthenium red and NEM. It is assumed that Mg2+ efflux, occurring at low diamide concentrations or in the presence of phosphate, is dependent on a cyclic in-and-out movement of Ca2+ across the inner mitochondrial membrane, in which the passive efflux is compensated by a continuous energy linked reuptake. This explains the dependence of Mg2+ efflux on coupled respiration, as well as the increased rate of state 4 respiration. The dependence of Mg2+ efflux on phosphate transport is explained by the phosphate requirement for Ca2+ movement.

Interactions between prostaglandins and calcium: the importance of bell-shaped dose-response curves

Biological responses to PGs show two basic forms of dose/response relationship, plateau and bell-types. Although bell-shaped dose/response curves are well documented their possible occurrence is almost always ignored in the design and interpretation of experiments on PGs and related substances. This may lead to serious errors, several types of which are described. The ignoring of a well-documented phenomenon may take place because there is no accepted hypothesis which attempts to explain the bell-type curves. A hypothesis is proposed which accounts for both plateau and bell type responses. It is developed primarily with respect to PG-calcium interactions but may be applicable to some PG-cyclic nucleotide interactions as well. The model leads to precise predictions which can be experimentally tested in many systems.

Roles of prostaglandins suggested by the prostaglandin agonist/antagonist actions of local anaesthetic, anti-arrhythmic, anti-malarial, tricyclic anti-depressant and methyl xanthine compounds. Effects on membranes and on nucleic acid function

We have recently found that local anaesthetic, anti-malarial, anti-arrhythmic, tricyclic antidepressant and methyl xanthine compounds behave as prostaglandin antagonists at drug concentrations readily attainable in human body fluids. We have also found that various hormones, including prolactin, vasopressin and angiotensin when present in physiological concentrations can be potent stimulators of prostaglandin synthesis. Cortisol has no effect on basal prostaglandin systhesis but in physiological concentrations is able to reverse the effects of other hormones. The drugs are thought to act mainly by interfering with either calcium interactions with membranes, or with cyclic nucleotide synthesis or degradation or with nucleic acid function. We propose that prostaglandins play crucial roles at all three points. The hypothesis leads to new approaches to many aspects of cell regulation, to the functions of many systems, especially the neuromuscular, cardiovascular, renal and respiratory systems. It also has profound implications for drug design and evaluation and for the understanding of many clinical disorders.

Effect of Mg2+ and spermine on the kinetics of Ca2+ transport in rat-liver mitochondria

Plots relating the initial rate of mitochondrial Ca2+ transport to the Ca2+ concentration (kinetic plots) have a hyperbolic shape in a Ca2+ concentration range of 2.5-100 muM as measured in sucrose or KCl media. In the presence of Mg2+ or a polyamine spermine, which both are competitive inhibitors of Ca2+ binding to low affinity sites at the membrane surface, the shape of the plots becomes sigmoidal. At higher concentrations of these agents linear kinetic plots are obtained as measured in a sucrose medium. In a KCl medium the sigmoidality of the kinetic plots is enhanced by an increase in the Mg2+ or spermine concentration. It is suggested that Mg2+ and spermine affect the kinetics of Ca2+ transport by interfering with Ca2+ binding to low affinity sites of the membrane surface and that the binding of Ca2+ to these sites is the first step of the mitochondrial Ca2+ transport.

Effect of inhibitors on the sigmoidicity of the calcium ion transport kinetics in rat liver mitochondria

The kinetic plot (initial rate of Ca2+ transport versus concentration) of mitochondrial Ca2+ transport is hyperbolic in a sucrose medium. The plot becomes sigmoidal in the presence of competitive inhibitors of Ca2+ binding to low affinity sites of the membrane surface such as Mg2+ and K+. The plot also becomes sigmoidal in the presence of Ba2+. Ba2+ is a competitive inhibitor of both Ca2+ transport and Ca2+ binding to the low affinity sites. The 5i for the inhibition of Ca2+ transport by Ba2+ increases in the presence of K+ and Mg2+, which suggests a competition for the low affinity sites between the cations. The plot is still hyperbolic in the presence of La3+, which inhibits Ca2+ transport competitively. Ruthenium red which is a pure non-competitive inhibitor of mitochondrial Ca2+ transport, does not affect the shape of the kinetic plot. These results indicate that the surface potential, which depends on the ions bound to the low affinity sites, determines whether the kinetics of Ca2+ uptake in mitochondria is sigmoidal or hyperbolic.

Chloroquine, quinine, procaine, quinidine, tricyclic antidepressants, and methylxanthines as prostaglandin agonists and antagonists

Chloroquine, quanine, procaine, quinidine, clomipramine, theophylline, and caffeine have been shown to be strong prostaglandin antagonists and weak agonists. The antagonist effect is clearly demonstrable at concentrations reached in human plasma when the drugs are used therapeutically. This suggests that prostaglandins are important in several situations in which their role has hitherto been unsuspected. New approaches to the development of prostaglandin antagonists and new uses for established drugs are indicated. In a preliminary study chloroquine has been successfully used to close patent ductus arteriosus in three infants.

Osmotic behavior and permeability of osmotically lysed mitochondria

Experiments were carried out with water-treated isolated rat liver mitochondria (mitochondria ghosts) previously studied by Caplan and Greenawalt (Caplan, A.I., and J.W. Greenawalt. 1966. J. Cell Biol. 31:455-472) and Vasington and Greenawalt (Vasington, F., and J. Greenawalt. 1968. J. Cell Biol. 39:661-675). The ghosts have permeability properties and osmotic behavior comparable to those of isolated mitochondria. Although they have lost most of their internal contents, they must have resealed. Four properties were found which have not been previously described in systems derived from biological membranes: (a) an osmotic behavior in the virtual absence of internal components. (b) a self-arranging property in the formation of invaginations corresponding in morphology to the cristae. The results suggest that the assembly of the molecular components of the inner membrane is sufficient to specify the morphology. Hence the surface area to volume ratio of the vesicles may specify the presence or absence of cristae-like folds. (c) an increase in the permeability of the membranes to sucrose in the presence of iso-osmotic concentrations of sucrose. (d) an independence of the light transmitted by suspensions of the vesicles from the refractive index of the external medium. This observations run counter to the general previous experience with either mitochondria or liposomes.