Spin label studies on rat liver and heart plasma membranes: effects of temperature, calcium, and lanthanum on membrane fluidity

Electron Spin Resonance Evaluation of Buccal Membrane Fluidity Alterations by Sodium Caprylate and L-Menthol

The ability of sodium caprylate and l-menthol to fluidize phospholipid bilayers composed of lipids simulating the buccal epithelium was investigated using electron spin resonance (ESR) to evaluate the action of these agents as permeation enhancers. 5-Doxyl stearic acid (5-DSA) and 16-doxyl stearic acid (16-DSA) were used as spin labels to identify alterations in membrane fluidity near the polar head groups or inner acyl regions of the lipid bilayer, respectively. The molecular motion of both 5-DSA and 16-DSA showed increased disorder near the polar and inner hydrophobic regions of the bilayer in the presence of sodium caprylate suggesting fluidization in both the regions, which contributes to its permeation enhancing effects. L-menthol decreased the order parameter for 16-DSA, showing membrane fluidization only in the inner acyl regions of the bilayer, which also corresponded to its weaker permeation enhancing effects. The rapid evaluation of changes in fluidity of the bilayer in the presence of potential permeation enhancers using ESR enables improved selection of effective permeation enhancers and enhancer combinations based on their effect on membrane fluidization.

Biopolymer-Lipid Bilayer Interaction Modulates the Physical Properties of Liposomes: Mechanism and Structure

This study was conducted to elucidate the conformational dependence of the modulating ability of chitosan, a positively charged biopolymer, on a new type of liposome composed of mixed lipids including egg yolk phosphatidylcholine (EYPC) and nonionic surfactant (Tween 80). Analysis of the dynamic and structure of bilayer membrane upon interaction with chitosan by fluorescence and electron paramagnetic resonance techniques demonstrated that, in addition to providing a physical barrier for the membrane surface, the adsorption of chitosan extended and crimped chains rigidified the lipid membrane. However, the decrease in relative microviscosity and order parameter suggested that the presence of chitosan coils disturbed the membrane organization. It was also noted that the increase of fluidity in the lipid bilayer center was not pronounced, indicating the shallow penetration of coils into the hydrophobic interior of bilayer. Microscopic observations revealed that chitosan adsorption not only affected the morphology of liposomes but also modulated the particle aggregation and fusion. Especially, a number of very heterogeneous particles were visualized, which tended to confirm the role of chitosan coils as a "polymeric surfactant". In addition to particle deformation, the membrane permeability was also tuned. These findings may provide a new perspective to understand the physiological functionality of biopolymer and design biopolymer-liposome composite structures as delivery systems for bioactive components.

Modulating effect of lipid bilayer-carotenoid interactions on the property of liposome encapsulation

Liposomes have become an attractive alternative to encapsulate carotenoids to improve their solubility, stability and bioavailability. The interaction mechanism of carotenoid with lipid bilayer is one of the major concerns in improving the delivery efficiency of liposomes. In this study, the microstructure and carotenoid encapsulation efficiency of liposomes composed of native phospholipid (egg yolk phosphatidylcholine, EYPC) and nonionic surfactant Tween 80 were investigated by atomic force microscopy, dynamic light scattering, and Raman spectroscopy, respectively. Subsequently, the effects of carotenoid incorporation on the physical properties of liposomal membrane were performed by Raman spectroscopy, fluorescence polarization, and electron paramagnetic resonance. Results showed that the incorporation of carotenoids affected the liposomes morphology, size and size distribution to various extents. Analysis on the Raman characteristic peaks of carotenoids revealed that lutein exhibited the strongest incorporating ability into liposomes, followed by β-carotene, lycopene, and canthaxanthin. Furthermore, it was demonstrated that carotenoids modulated the dynamics, structure and hydrophobicity of liposomal membrane, highly depending on their molecular structures and incorporated concentration. These modulations were closely correlated with the stabilization of liposomes, including mediating particle aggregation and fusion. These findings should guide the rationale designing for liposomal encapsulation technology to efficiently deliver carotenoids in pharmaceutics, nutraceuticals and functional foods.

Membrane organization and cell fusion during mating in fission yeast requires multipass membrane protein Prm1

The involvement of Schizosaccharomyces pombe prm1(+) in cell fusion during mating and its relationship with other genes required for this process have been addressed. S. pombe prm1Δ mutant exhibits an almost complete blockade in cell fusion and an abnormal distribution of the plasma membrane and cell wall in the area of cell-cell interaction. The distribution of cellular envelopes is similar to that described for mutants devoid of the Fig1-related claudin-like Dni proteins; however, prm1(+) and the dni(+) genes act in different subpathways. Time-lapse analyses show that in the wild-type S. pombe strain, the distribution of phosphatidylserine in the cytoplasmic leaflet of the plasma membrane undergoes some modification before an opening is observed in the cross wall at the cell-cell contact region. In the prm1Δ mutant, this membrane modification does not take place, and the cross wall between the mating partners is not extensively degraded; plasma membrane forms invaginations and fingers that sometimes collapse/retract and that are sometimes strengthened by the synthesis of cell-wall material. Neither prm1Δ nor prm1Δ dniΔ zygotes lyse after cell-cell contact in medium containing and lacking calcium. Response to drugs that inhibit lipid synthesis or interfere with lipids is different in wild-type, prm1Δ, and dni1Δ strains, suggesting that membrane structure/organization/dynamics is different in all these strains and that Prm1p and the Dni proteins exert some functions required to guarantee correct membrane organization that are critical for cell fusion.

Thoracic organ preservation

Clinical heart transplantation began in December 1967 when Cristiaan Barnard performed the first human to human heart transplant on a 57 year old man with ischaemic heart disease, in Cape Town. This ushered in a bout of enthusiastic heart transplantations world-wide over the subsequent few years which soon waned as the problems of acute rejection and infection became apparent to those who had embarked on this venture without fully understanding the complications. The importance of a well functioning donor heart cannot be overemphasized. Early donor heart failure accounts for approximately 26% of the deaths of heart transplant recipients today and there is also a steep rise in acute mortality associated with storage times in excess of two hours (9.8% less than 2 hours rising to 17.6 greater than 4 hours), although satisfactory function has been reported in a few hearts stored for up to 6 hours. Careful selection and meticulous management of the donor, followed by optimal storage, are therefore essential to a satisfactory outcome. There is evidence that some of the problems of organ preservation are related to metabolic changes in the donor consequent upon brain death and recent ongoing studies by our own group show some benefit from hormone replacement therapy in the donor. There are essentially two major approaches to the problem of organ storage; metabolic inhibition resulting in reduced substrate requirements, and the supply of metabolic requirements, or a combination of both. Although nonperfusion methods currently predominate, the simplicity of these methods are overshadowed by the short safe time interval which they allow and the variable functional quality which results. The author believes that perfusion preservation methods will predominate in the future and may also allow expansion of the donor pool by whole donor and/or ex vivo thoracic organ resuscitation.

Plasmalogen-derived lysolipid induces a depolarizing cation current in rabbit ventricular myocytes

Plasmalogen rather than diacyl phospholipids are the preferred substrate for the cardiac phospholipase A2 (PLA2) isoform activated during ischemia. The diacyl metabolite, lysophosphatidylcholine, is arrhythmogenic, but the effects of the plasmalogen metabolite, lysoplasmenylcholine (LPLC), are essentially unknown. We found that 2.5 and 5 micromol/L LPLC induced spontaneous contractions of intact isolated rabbit ventricular myocytes (median times, 27.4 and 16.4 minutes, respectively) significantly faster than lysophosphatidylcholine (>60 and 37.8 minutes, respectively). Whole-cell recordings revealed that LPLC depolarized the resting membrane potential from -83.5+/-0.2 to -21.5+/-1.0 mV. Depolarization was due to a guanidinium toxin-insensitive Na+ influx. The LPLC-induced current reversed at -18.5+/-0.9 mV and was shifted 26.7+/-4.2 mV negative by a 10-fold reduction of bath Na+ (Na+/K+ permeability ratio, approximately 0.12+/-0.06). In contrast, block of Ca2+ channels with Cd2+ and reducing bath Cl failed to affect the current. The actions of LPLC were opposed by lanthanides. Gd3+ and La3+ were equally effective inhibitors of the LPLC-induced current and equally delayed the onset of spontaneous contractions. However, the characteristics of lanthanide block imply that Gd3+-sensitive, poorly selective, stretch-activated channels were not involved. Instead, the data are consistent with the view that lanthanides increase phospholipid ordering and may thereby oppose membrane perturbations caused by LPLC. Plasmalogens constitute a significant fraction of cardiac sarcolemmal choline phospholipids. In light of their subclass-specific catabolism by phospholipase A2 and the present results, it is suggested that LPLC accumulation may contribute to ventricular dysrhythmias during ischemia.

Cholesterol redistribution within human platelet plasma membrane: evidence for a stimulus-dependent event

The fluorescent analog NBD-phosphatidylethanolamine and the analogs of cholesterol NBD-cholesterol and cholestatrienol were used to study the distribution of these lipids within the plasma membrane bilayer of human platelets. The probes were incorporated into platelets using phosphatidylcholine donor vesicles. The distribution of NBD lipid and of cholestatrienol in the platelet plasma membrane bilayer was followed by quenching with dithionite and TNBS, respectively. The t1/2 of cholestatrienol incorporation into platelet membranes was 39 min, and approximately 65% of the probe was quenched by addition of TNBS. When platelets were exposed to collagen or to ADP, a portion of the probe became inaccessible to quenching. Within 2 min of stimulation by collagen (10 micrograms/mL), the percentage of cholestatrienol fluorescence quenched by TNBS decreased to 45%. The fluorescent probe was not found to be associated either with the intracellular membranes or in the extracellular media after collagen stimulation. Similar data were obtained with NBD-cholesterol, but the decrease in accessibility of this probe to quenching was considerably slower. The redistribution of endogenous membrane cholesterol was also measured using cholesterol oxidase. Exposure of platelets to collagen decreased the accessibility of endogenous membrane cholesterol to enzymatic oxidation with cholesterol oxidase. Taken together, the foregoing observations are consistent with the stimulus-dependent translocation of cholesterol out of the outer monolayer. Coincident with the redistribution of cholesterol is the reciprocal movement of NBD-phosphatidylethanolamine into the outer monolayer. In the presence of the chaotropic agents urea and guanidine HCl, the movement of cholestatrienol upon collagen stimulation was prevented, but the redistribution of NBD-phosphatidylethanolamine was still detected. We propose that cholesterol translocates to the inner platelet monolayer following collagen stimulation, but the possibility that the sterol moves laterally within the outer membrane monolayer cannot be rigorously excluded.

Effect of magnesium and calcium on myocardial protection by cardioplegic solutions

The cardioprotective effect of the magnesium and calcium content of hyperkalemic cardioplegic solutions was investigated using isolated rat hearts subjected to hypothermic ischemic arrest. Hearts were arrested for 180 minutes at 20 degrees C by administering a 3-minute infusion of cardioplegic solution containing various concentrations of magnesium and calcium. Treatment groups received solution with either 0, 8, or 16 mmol/L magnesium. For each of these magnesium concentrations, 0.1, 0.6, or 1.2 mmol/L calcium was also present in the solution. At each concentration of magnesium, the percentage recovery of aortic flow was dependent on the calcium concentration. The maximum percentage recovery of aortic flow was 67.9% +/- 2.3% (mean +/- standard error of the mean) in the Mg-free, 0.1 mmol/L Ca group, whereas it was 65.1% +/- 2.7% in the 8 mmol/L Mg, 0.1 mmol/L Ca group and 70.0% +/- 3.5% in the 16 mmol/L Mg, 0.6 mmol/L Ca group. No significant differences in the recovery of cardiac function and creatine kinase leakage were observed between the three groups. The findings suggested that the cardioprotective effect was dependent on the relative concentration of both magnesium and calcium, and that it is important to maintain an appropriate ionic balance in cardioplegic solutions.

The reciprocal protective effects of magnesium and calcium in hyperkalemic cardioplegic solutions on ischemic myocardium

The cardioprotective effects of magnesium and calcium in hyperkalemic cardioplegic solutions were investigated in isolated rat hearts. Isolated rat hearts were arrested for 30 min at 37 degrees C in cardioplegic solutions containing magnesium and calcium in varying concentrations. In St. Thomas' Hospital cardioplegic solution, the magnesium and calcium concentrations were varied as follows: Mg 0; Ca 0.1, 0.3, 0.6, 1.2 mmol/l (mM)./ Mg 4; Ca 0.1, 0.3, 0.6, 1.2 mM./ Mg 8; Ca 0.1, 0.3, 0.6, 0.9, 1.2 mM./ Mg 12; Ca 0.1, 0.6, 0.9, 1.2 mM./ Mg 16; Ca 0.1, 0.6, 1.2, 1.5 mM. For each magnesium concentration, the percentage recovery of aortic flow generated dose-response curves depending on calcium concentration. The maximum percentage recovery of aortic flow was 76.0% +/- 2.7% (mean +/- SEM) in the Mg0-Ca0.1 mM group, 77.1% +/- 2.0% in the Mg4-Ca0.3 mM group, 78.5% +/- 2.3% in the Mg8-Ca0.6 mM group, 79.8% +/- 2.4% in the Mg12-Ca0.9 mM group and 80.0% +/- 3.4% in the Mg16-Ca1.2 mM (ST solution) group. Significant difference in the recovery of aortic flow has not been observed among these groups, and furthermore, significant differences in the recovery of other parameters of cardiac function and Ck leakage have not been observed among these groups. These results suggest that the cardioprotective effects depend on the relative combination of magnesium and calcium concentrations, and that it is important to maintain an appropriate balance of magnesium and calcium in hyperkalemic cardioplegic solutions.

Myocardial cell vulnerability to exogenous phospholipase attack

Myocardial cell vulnerability to phospholipase C (PC-PLC) attack was investigated in three different preparations of rat myocardial cells: triacylglycerol (TG)-loaded, hypothermic/rewarmed and energy depleted myocytes. The attack by PC-PLC was evaluated as PC-PLC induced glycerol output due to the combined action of phospholipase C and intracellular lipases. PC-PLC induced glycerol output was significantly higher (p < 0.05) in all three myocyte preparations, compared to their respective controls. Cell morphology (% rod shaped myocytes) of TG-loaded or hypothermic/rewarmed myocytes was not different from their controls, whereas energy depleted myocytes almost exclusively were rounded up, due to hypercontraction of the myofilaments. Hypothermic/rewarmed and energy depleted myocytes showed a significantly higher release of lactate dehydrogenase (LDH), compared to their controls although the difference was much more pronounced in the latter. Finally, the cellular contents of ATP were maintained both in TG-loaded and hypothermic rewarmed myocytes, while energy depleted myocytes contained only about 25% of the normal ATP level. These results demonstrate that attack from exogenously added phospholipases can occur, not only in seriously damaged cardiac myocytes, but in myocytes with a more subtle damage as well.

Adverse effect of prearrest hypothermia in immature hearts: rate versus duration of cooling

It has been suggested that rapid cooling before the induction of arrest may be harmful to the newborn myocardium. The objective of this study was twofold: (1) to evaluate whether prearrest rapid cooling is indeed detrimental to myocardial recovery and (2) if so, to evaluate whether the adverse effect of prearrest hypothermia is dependent on the rate of cooling or the total duration of cold perfusion. After an initial stabilization period isolated Langendorff hearts (n = 5 per group) from neonatal piglets (5 to 7 days old) were randomized to four groups: group 1, 5 minutes of rapid cooling to 15 degrees C; group 2, 20 minutes of slow cooling to 15 degrees C; group 3 and group 4, rapid and slow cooling, respectively, with the addition of St. Thomas cardioplegic solution. All groups were then subjected to 2 hours of ischemia at 15 degrees C followed by 30 minutes of reperfusion at 38.5 degrees C. Post-ischemic recovery of left ventricular developed pressure was significantly greater in group 1 versus group 2 (80% +/- 3% versus 61% +/- 2%; p less than 0.05) and in the presence of cardioplegia, group 3 versus group 4 (72% +/- 3% versus 57% +/- 3%; p less than 0.05). The increase in left ventricular end-diastolic pressure was significantly less in group 1 versus group 2 (8% +/- 5% versus 33% +/- 7%; p less than 0.01). Myocardial adenosine triphosphate content recovery correlated with ventricular recovery.(ABSTRACT TRUNCATED AT 250 WORDS)

Fatty-acid spin probe interactions with erythrocyte ghosts and liposomes prepared from erythrocyte ghosts

A model for the binding of 5-nitroxide stearate, I(12.3), to human erythrocyte ghosts was developed by comparing spin probe interactions with ghosts and liposomes prepared from ghosts. At low probe/lipid (P/L less than 1/2500), I(12.3) binds to a similar class of high-affinity, noninteracting sites in both ghosts and liposomes, indicating that lipid moieties are responsible for probe uptake. Saturation occurs in both systems with increasing P/L, and, at higher loading (e.g., P/L = 1/360 for ghosts and liposomes), the probe inserts itself at initially dilute sites to form a class of low-affinity sites consisting of clusters of variable size. At still higher P/L ranges (greater than 1/100), much increased probe uptake was observed in ghosts than in liposomes, which was attributed to another class of low-affinity sites, representing nonspecific interactions of I(12.3) with membrane proteins. The nature of the spectral components and ultrafiltration experiments with ghosts labeled at high P/L indicate that both 'dilute' and 'clustered' I(12.3) are due to membrane-incorporated probe.

An electron spin resonance investigation of internal rust spot, a physiological disorder of the potato tuber

ESR spectroscopy has been used to investigate the formation of paramagnetic species during the development of internal rust spot (IRS) in the potato tuber. Production of free radicals and oxidation of metal ions such as Fe(II) and Mn(II) occur when necrotic tissue is formed. However, since IRS develops in periods of calcium stress (low calcium supply), it is suggested that the principal cause of the disorder is a loss of cell membrane integrity which is brought about by a lack of calcium. Cell senescence and the formation of necrotic tissue may then result either from increased oxygen radical production within the cell or from oxidation of metal complexes in the extracellular regions of the tissue.

Thermotropic lipid phase separation in the human immunodeficiency virus

The presence of thermodependent lipid domains in the envelope of the human immunodeficiency virus (HIV) was studied. HIV was propagated in Hut-78 cells and purified by differential-gradient centrifugation. Since the virus was highly infectious in cell culture and Western blots of detergent-inactivated HIV showed envelope proteins when exposed to sera containing anti-HIV antibodies, this viral preparation was not deficient in 'spike' or 'knob' particles. Electron spin resonance (ESR) studies of intact HIV labeled with 5-nitroxide stearate (5-NS) indicated that a temperature-dependent lipid phase separation occurs with a high onset at approx. 42 degrees C and a low onset at approx. 15 degrees C. Cooling below 42 degrees C induces 5-NS clustering. Similar phase separations with high onsets at approx. 37-38 degrees C were previously identified in 5-NS labeled human erythrocytes (cholesterol/phospholipid (C/P) molar ratio = 0.90) and cholesterol-loaded (C/P = 0.85-0.98) rat liver plasma membranes. These were attributed to a temperature-sensitive redistribution of endogenous lipid components such that 5-NS is excluded from cholesterol-rich domains and tends to reside in cholesterol-poor domains at low temperatures. Since HIV has a lipid envelope with a similarly high C/P of 0.88 (Aloia et al. (1988) Proc. Natl. Acad. Sci. USA 85, 900-904), cholesterol-rich and cholesterol-poor domains also probably exist in HIV at physiologic temperatures. The reduced stability and infectivity of HIV noted on heating above 42 degrees C may be due, in part, to the abolition of these thermodependent domains.

Optimal storage temperature and benefit of hypothermic cardioplegic arrest for long-term preservation of donor hearts: a study in the dog

Currently, for practical clinical purposes, the preservation of donor hearts is limited to about 4 h. Transplantation must be finished within this period to assure complete functional recovery upon reperfusion. From the clinical setting it is well known that hypothermia results in a better myocardial preservation during ischemia. During ischemia, rapid catabolism of high-energy phosphates (e.g., ATP and creatine phosphate) occurs. The purpose of this study was to investigate the influence of temperature during a 24-h preservation period on the rate of catabolism of ATP and on the rate of accumulation of breakdown products (ADP, AMP, adenosine, inosine, hypoxanthine, and xanthine). For this purpose, hearts were excised and stored for 24 h at 0.5 degrees, 12 degrees, or 18 degrees C. In addition, the effect of initial cardioplegic arrest was compared with simple normothermic excision of the heart followed by 24 h in cold storage. It was found that the higher the storage temperature, the higher the rate of catabolism of high-energy phosphates and, hence, after 24 h, the lower the final ATP level and the higher the level of breakdown products, mainly nucleosides. It was also found that the initial cardioplegic arrest strongly benefits the preservation of high-energy phosphates as a result of the ATP-sparing effect.

Temperature dependency of calcium-induced reperfusion injury in the isolated rat heart

The temperature dependence of Ca-induced reperfusion injury was studied in an isolated rat heart preparation. Hearts were subjected to 90 minutes of hypothermic arrest (20 degrees C) followed by 15 minutes of reperfusion at 20, 28, or 37 degrees C with a reperfusate containing various concentrations of Ca (0.1-2.55 mM). When reperfusion was started at 37 degrees C, the Ca concentration in the reperfusate significantly affected both postischemic functional recovery and creatine kinase leakage. Bell-shaped dose-response curves were observed. The optimal Ca concentration for 37 degrees C reperfusion was between 0.3 and 0.7 mM. When reperfusion was started at 20 degrees C, neither functional recovery nor creatine kinase leakage was dependent on the Ca concentration in the reperfusate. At 28 degrees C, functional recovery was not dependent on the Ca concentration, however, creatine kinase leakage was. These results indicate that Ca-induced reperfusion injury depends on the temperature of the reperfusate and that the boundary temperature of the reperfusate at which Ca-induced reperfusion injury becomes manifest seems to be near 28 degrees C.

Surgical hypothermia

Induced hypothermia is an interesting and useful adjunct to therapy in many areas of surgery and medicine. To paraphrase Professor Swan (1973), clinical hypothermia 'has a past and some promise for the future'.

Effects of sodium on the calcium paradox in rat hearts

Reduction of the Na concentration in the Ca-free perfusion solution reduces the amount of myoglobin released by the cells when Ca is readmitted if sucrose is used to replace NaCl under mild hypothermia. When salts like cholinechloride or LiCl are used instead of sucrose, no protection is seen at any temperature. The temperature threshold above which myoglobin loss sharply increases is lowered by prolonged Ca depletion or by the addition of EGTA to the Ca-free solution. Protection by sucrose does not occur in the presence of EGTA. An increase of cell Na induced by strophanthidin during the Ca depletion phase has no effect on myoglobin release. The exponential decline in twitch tension in the early phase of Ca deprivation has the same half-live (T1/2) for Ca-free solutions containing 145 mM Na or 35 mM Na (110 mM Li or choline), but its T1/2 is prolonged if sucrose is used to replace NaCl. When 5 mM EGTA is added to the Ca-free solutions, the T1/2 is shortened and is not changed by the replacement of NaCl with sucrose. The rate of washout of Ca within the first 20 s of Ca depletion has a similar time course in a normal Na or in a Li and low Na solution. In a sucrose and low Na solution the rate of the Ca efflux is reduced. The addition of EGTA increases this rate and abolishes the slowing effect of a sucrose and low Na solution. Therefore myoglobin release during the Ca paradox does not depend on the Na gradient across the sarcolemma.(ABSTRACT TRUNCATED AT 250 WORDS)

Estimation of spin probe clustering in biological membranes

An iterative spectral subtraction technique has been developed which accurately estimates the proportion of 'dilute' and 'clustered' I(12, 3) (i.e., 5-nitroxide stearate) in human erythrocyte ghosts at 37 degrees C, even if subtractant spectra free from probe-probe interactions cannot be measured due to technical limitations. Gordon et al. ((1985) J. Membrane Biol. 84, 81-95) earlier showed that I(12, 3) occupies a class of high-affinity sites in ghosts at probe/total lipid ratios (P/L) less than 1/2250. Saturation occurs with increasing probe concentration, and, at higher loading, the probe inserts itself at initially dilute sites to form membrane-bound clusters of variable size. Although this model allows determination of the dilute/clustered probe ratio, it requires subtraction of experimental spectra with a 'magnetically dilute' spectrum obtained using P/L less than 1/4600. The new methodology accurately profiles the % probe clustering in human erythrocyte ghosts over the entire P/L range, even if the lowest P/L for the subtractant spectrum contains substantial probe-probe interactions (i.e., P/L of 1/604 or 1/303). Application of either the subtraction technique in Gordon et al. (1985) or the iterative subtraction protocol described here should allow determination of probe clustering in a wide range of I(12, 3)-labeled biological membranes.

Electron spin resonance spectroscopy in the study of lymphoid cell receptors

ESR spectroscopy is a sensitive method which can be used to monitor a wide range of changes in and near the plasma membranes of lymphoid cells during such events as ligand-induced receptor patching and capping, activation and endocytosis and phagocytosis. The advantages of the technique are rapidity, sensitivity, small sample size (10(6)-10(7) cells), and nondestructive nature. Spin labels that are attached to a range of intrinsic and extrinsic molecules give information about the fluidity and polarity of the environment in which they are located. Because of the occurrence of interaction when probes are sequestered in restricted regions of the membrane, ESR spectroscopy is also a valuable technique for measuring the formation of domains in the cell membrane.

Membrane structural/functional perturbations induced by gossypol. Effects on membrane order, liposome permeability, and insulin-sensitive hexose transport

The effects of gossypol on membrane structure and membrane-associated functions were studied to explore possible reasons for the ability of gossypol to disrupt cellular processes, many of which involve intracellular and plasma membranes. The experiments reported here measured the effects of gossypol on membrane order, permeability, and hexose transport. Electron spin resonance (ESR) studies of I(12,3) nitroxide fatty acid spin-labeled unilamellar liposomes showed that exposure to 0.05 to 4 mM gossypol caused a dose-dependent increase in the polarity-corrected order parameter (S), indicating reduced motional freedom of the spin probe after exposure to gossypol. This observation is consistent with the idea that gossypol causes an ordering or "condensing" of the membrane lipid matrix. Gossypol-induced changes in order parameter in phosphatidylcholine:cholesterol liposomes varied depending on the liposome composition. Liposomes exposed to gossypol also showed increasing permeability to glycerol as the gossypol:phospholipid ratio increased up to 10 mole %. Higher concentrations of gossypol were less effective at enhancing permeability. In addition, basal and insulin-stimulated 2-deoxy-D-[3H]glucose transport were inhibited in freshly isolated rat adipocytes incubated with gossypol at 37 degrees. Half-maximal inhibition occurred at approximately 0.2 mM for uptake in both the presence and absence of 40 ng/ml insulin. Microscopic observation of the cells under low power (40 X) confirmed that diminished hexose transport was not simply due to breakage of the adipocyte plasma membrane, resulting in a decrease in intact cell population and decreased accumulation of label in the gossypol-treated cells. Gossypol produced no significant changes in numbers of intact cells or gross morphology at the concentrations tested. We suggest that ordering and increased permeability of the lipid regions of plasma and subcellular membranes may contribute to some of the toxic and pharmacologic properties of gossypol. Our results also support the idea that gossypol may exert more pronounced effects in cells that are most sensitive to variations in availability of glucose substrates for energy metabolism.

The positive chronotropic effects of Bay K-8644 and calcium as influenced by temperature

The positive chronotropic effects of the dihydropyridine calcium agonist BAY K-8644 and calcium were studied in spontaneously beating right guinea-pig atria. Within the temperature range from 22 to 37 degrees C, the following observations were made: (1) The basal pacemaker frequency increased linearly with the ambient temperature; (2) the increase in pacemaker rate produced by a maximally effective concentration of BAY K-8644 was of the same magnitude as that produced by calcium; (3) the increases in pacemaker rate produced by BAY K-8644 and by calcium were directly proportional to the increase in temperature above 22 degrees C; and (4) at 37 degrees C, the apparent ED50 for the positive chronotropic effect of calcium was 2.3 X 10(-3) M and of BAY K-8644 was 0.7 X 10(-7) M. At temperatures below 22 degrees C, irregular pacemaking activity or complete arrest of pacemaking activity frequently occurred. The data are in strong support of the assumption that BAY K-8644 elicits its positive chronotropic effect by increasing the availability of calcium ions.

Modulation by n-alkanols of rat cardiac adenylate cyclase activity

n-Alkanols (from methanol to decanol) have a biphasic effect on rat cardiac adenylate cyclase either basal or stimulated by GTP, GppNHp, NaF or hormones (isoproterenol, glucagon, secretin) in the presence of GTP. At high concentration, all the enzyme activities are inhibited. At low concentration, adenylate cyclase activity is either unchanged or potentiated depending on both the stimulus and the alkanols involved. Potentiation is due to an increase of maximum velocity with no change in the activation constant of the enzyme. Basal activity is unchanged as well as the isoproterenol- and glucagon-stimulated enzyme. The secretin-stimulated enzyme is potentiated. It is the guanyl nucleotide regulatory protein-mediated stimulation of adenylate cyclase which is mainly affected. An attempt was made to relate these effects on adenylate cyclase with physical parameters of the alkanols (partition coefficient). From the data obtained as a function of the alkanol chain-length and of temperature on the adenylate cyclase stimulated by GTP, GppNHp, NaF and permanently activated, it is concluded that the increase in efficacy observed in the presence of alkanol is due to an interaction with the protein moeity particularly with the guanyl nucleotide regulatory protein.

Temperature-dependent inotropic action of A23187 on guinea-pig ventricular muscles

The effects of the Ca2+ ionophore, A23187, on the contraction and membrane action potential of the isolated guinea-pig papillary muscle were examined at various temperatures (30-16 degrees C) and compared to those of isoprenaline and a high calcium medium. A23187 caused a marked positive inotropic effect with a significant prolongation of the action potential duration at an early repolarization phase but not a late repolarization phase at normal temperature (30 degrees C). Such an inotropic effect was completely abolished at low temperature (16 degrees C) where a marked positive inotropic effect of isoprenaline (5 X 10(-8) M) and a high calcium medium (6.2 mM) still remained. These results suggest that the cardiac responsiveness to A23187 was sensitive to a low temperature at which a membrane lipid phase transition may occur.

Effect of nifedipine in hypothermic cardioplegia: a phosphorus-31 nuclear magnetic resonance study

The ability of nifedipine to enhance myocardial protection was assessed on isolated perfused rat hearts subjected to 180 min of hypothermic (20 degrees C), global ischemia, followed by 45 min of normothermic reperfusion. Intracellular pH, ATP, Pi and phosphocreatine content were serially measured at 4 min intervals by phosphorus-31 nuclear magnetic resonance spectroscopy and correlated with simultaneously recorded hemodynamic parameters. Addition of nifedipine (0.075 mumol/l and 0.5 mumol/l) to Saint Thomas' cardioplegic solution reduced Pi accumulation during ischemic arrest and increased phosphocreatine levels during reperfusion. Post-ischemic functional recovery was not improved at a drug concentration of 0.075 mumol/l and was depressed at 0.5 mumol/l. These results clearly show that the presence of nifedipine in Saint Thomas' cardioplegic solution does not provide significant additional myocardial protection under hypothermic conditions.

Effects of extracellular calcium on calcium transport during hyperthermia of tumor cells

The effects of different concentrations of extracellular ion calcium on the transport of calcium by tumor cells have been studied by means of the uptake of radiocalcium. Tumor cells incubated at 45 degrees C take up 4-10 times the amount of radioactivity incorporated by cells incubated at 37 degrees C. The difference is still greater (up to 100 times) for the intracellular incorporation as assessed by elimination of the membrane-bound calcium by EGTA treatment. The possible mechanisms involved in this differential behavior are discussed.

Sensitivity of adipocyte basal and insulin-stimulated hexose transport to the membrane lipid structure

A series of anesthetic alcohols inhibited basal and insulin-stimulated 2-deoxy-D-[1-14C]glucose transport in adipocytes over total alcohol concentration ranges that cause local anesthesia of rat sciatic nerve. The relative potencies of the inhibition caused by the alcohols increased in the following order: methanol less than ethanol less than propanol less than butanol less than benzyl alcohol less than hexanol less than octanol. The inhibition was reversible and correlated well with the known partitioning of the alcohols into lipids of biological membranes. Adipocyte membranes were labeled with the 5-nitroxide stearate spin probe to investigate the effects of the alcohols on the dynamic structure of membrane lipids of the adipocyte. The alcohols increased the membrane "fluidity", and the relative concentration dependence of the effects closely paralleled that noted from methanol to octanol in transport studies. Alcohols from methanol to hexanol caused inhibition of hexose transport at molar potencies comparable to that observed for membrane disordering. This suggests that hydrophobic regions of the transporter and its lipid environment are perturbed by a comparable mechanism for each alcohol. The cholesterol-complexing polyene antibiotic filipin inhibited hexose transport and influenced the mobility of lipid domains sampled with the nitroxide cholestane, cholesterol-like spin probe. The data are consistent with the concept that the membrane structural/functional effects are mediated by formation of 1:1 cholesterol:filipin complexes. Alcohols and filipin inhibited inherent transporter activity and perturbed the membrane lipid structure without dramatically diminishing transport stimulation by insulin above basal. The specific organization of membrane lipids (particularly cholesterol) may provide an essential environment for optimal transport system activity.

Spin probe clustering in human erythrocyte ghosts

A model has been developed for 5-nitroxide stearate, I(12,3), distribution in human erythrocyte ghosts which accurately predicts ESR spectral alterations observed with increased probe/total lipid (P/L) at 37 degrees C. This spin probe occupies a class of high-affinity, noninteracting sites at low loading. Saturation occurs with increasing probe concentration, and, at higher loading, the probe inserts itself at initially dilute sites to form membrane-bound clusters of variable size. No 'low' probe remains at high P/L where all I(12,3) clusters in a 'concentrated' phase. This model allows determination of the dilute/clustered probe ratio, and shows that I(12,3) segregates in erythrocytes at what might otherwise be considered low P/L (e.g., 1/359). These findings validate the earlier use of empirical parameters to estimate probe sequestration in biological membranes.

Evidence that a Ca2+ chelator and a calmodulin blocker interfere with the structure of inter-Sertoli junctions

Ca2+ dependence of tight junction structure has been well documented in cultured epithelial tissues, and regulatory mechanisms have been identified. To analyse the possible control exerted on inter-Sertoli junctions, we exposed guinea-pig seminiferous tubules to the presence of a Ca2+ chelator (EGTA) and to a calmodulin blocker (Trifluoperazine, TFP) in vitro, for times ranging from 30 to 120 min. We observed the morphology of junctional complexes and the basal cytoplasmic regions in sections and replicas. Sertoli cell response to Ca2+ depletion involved several events: retraction of cells toward the base of the tubule and a consequent stretching of the points of fusion, augmented density of the cytoplasm, and destabilization of the array of intramembrane particles. Exposure of tubules to TFP resulted in disruption of the interactions between actin filaments and membrane junctional specialization, as well as a disorganization of other cytoskeletal elements. Thus, in vitro, junction integrity appears to be related to Ca2+ level, and Ca2+ depletion apparently interferes with Ca2+ distribution inside the cell and on microfilaments involved in junction regulation. Our results do not provide direct evidence for any particular mechanism of action of TFP, but a multiple effect is evident. TFP, which affects Ca2+ regulation and membrane fluidity, probably acts indirectly on junction-associated filaments. Both the experimental conditions tested suggest a Ca2+-mediated regulatory role of microfilaments of this complex junction.

Effects of an extremely low frequency electromagnetic field on the cell division rate and plasma membrane of Paramecium tetraurelia

The eukaryotic protozoan, Paramecium, was examined as a model for effects of pulsated electromagnetic fields (PEMF) on cells. A 72-Hz PEMF similar to fields employed clinically increased cell division rates in Paramecium by 8.5%. Two calcium transport mutants of these organisms showed differential responses to the same field. Verapamil, a calcium channel blocker, abolished any effect of PEMFs on cell division rates. A fluorescent probe that is thought to sense changes in membrane potential also manifested an altered response in the PEMF-exposed cells whereas a fluorescent lipid bilayer fluidity probe produced evidence of decreased membrane fluidity in the exposed cells. An effect of PEMFs on ion transport mediated by either a direct or indirect effect on the cell membrane is suggested by these studies.

Ca2+ depletion-induced disconnection of tight junctions in isolated rat brain microvessels

Cerebral microvessels were isolated from rat brains. One part of the microvessel pellets was incubated for 25 or 90 min in Krebs-Henseleit bicarbonate buffer (KHB) at pH 7.5 (control group). The other part of the pellets was treated for the same periods of time with Ca2+-free KHB, containing 2.2 mM EGTA and 2 mM glucose (experimental group). Morphological changes of endothelial tight junctions were evaluated in 100 randomly selected interendothelial clefts from isolated cerebral microvessels of each groups by electron microscopy. Following 25 min of incubation time, either with Ca2+-containing or with Ca2+-free KHB, no significant changes of tight junctions were observed. After 90 min of incubation in Ca2+-free medium, 58% of tight junctions were altered (in 42% partial, and in 16% complete disconnection of tight junctions were found). This contrasted the control group, where only 14% of tight junctions were disconnected (12% partially and 2% completely). Our results are consistent with a role for intercellular Ca2+ in maintaining structural integrity of cerebral tight junctions.

Effects of insulin on the lipid structure of liver plasma membrane measured with fluorescence and ESR spectroscopic methods

Insulin increased the lipid order of rat and mouse liver plasma membrane domains sampled by the hydrophobic fluorescent probe 1,6-diphenyl-1,3,5-hexatriene in a concentration-dependent saturable manner. The ordering is half maximal at 5.1 X 10(-11) M and fully saturated at 1.7 X 10(-10) M insulin. Membranes prepared from obese hyperglycemic (ob/ob) mice demonstrated a right-shift in the dose-dependent ordering induced by insulin, such that ordering was half maximal at 1.2 X 10(-10) M and fully saturated at 2.0 X 10(-10) M. Insulin also increased the order of rat liver plasma membranes labeled with the cis- and trans-parinaric acid methyl esters. The ordering caused by insulin as detected with cis methyl parinarate was complete within approx. 15 min. after hormone addition at 37 degrees C, and the ordering was approximately double that observed with the trans isomer. Additional ESR experiments demonstrated that the addition of insulin increased the outer hyperfine splittings of spectra recorded from membranes labeled with the steroid-like spin labels, nitroxide cholestane and nitroxide androstane, but not the fatty acid spin probe, 5-nitroxide stearate. Studies utilizing model membrane systems strongly suggest that the 5-nitroxide stearate samples a cholesterol-poor domain of the membrane, while the steroid-like probes preferentially sample cholesterol-rich regions of the membrane. Finally, insulin-induced membrane ordering was dose-dependently inhibited by cytochalasin B in the range 1-50 microM. From these results, we conclude that (1) the ordering effect of insulin addition to isolated liver plasma membrane fractions occurs within the physiological range of hormone concentration, and the dose-response is right-shifted in membranes from 'insulin resistant' animals; (2) the relative responses of the fluorescent and spin probes suggest that the effects of insulin are confined to specific domains within the membrane matrix; and (3) the direct effects of insulin on the membranes may involve protein components having cytochalasin B binding sites.

Temperature optimum of insulin-stimulated 2-deoxy-D-glucose uptake in rat adipocytes. Correlation of cellular transport with membrane spin-label and fluorescence-label data

The effects of temperature alterations between 22 degrees C and 48 degrees C on basal and insulin-stimulated 2-deoxy-D-[1-14C]glucose uptake were examined in isolated rat adipocytes. A distinct optimum was found near physiological temperature for uptake in the presence of maximally effective insulin concentrations where insulin stimulation and hexose uptake were both conducted at each given assay temperature. Basal uptake was only subtly affected. Control and maximally insulin-stimulated cells incubated at 35 degrees C subsequently exhibited minimal temperature-sensitivity of uptake measured between 30 and 43 degrees C. The data are mostly consistent with the concept that insulin-sensitive glucose transporters are, after stimulation by insulin, functionally similar to basal transporters. Adipocyte plasma membranes were labelled with various spin- and fluorescence-label probes in lipid structural studies. The temperature-dependence of the order parameter S calculated from membranes labelled with 5-nitroxide stearate indicated the presence of a lipid phase change at approx. 33 degrees C. Membranes labelled with the fluorescence label 1,6-diphenylhexa-1,3,5-triene, or the cholesterol-like spin label nitroxide cholestane, reveal sharp transitions at lower temperatures. We suggest that a thermotropic lipid phase separation occurs in the adipocyte membrane that may be correlated with the temperature-dependence of hexose transport and insulin action in the intact cells.

Thermotropic lipid phase separations in human erythrocyte ghosts and cholesterol-enriched rat liver plasma membranes

Electron spin resonance (ESR) studies of human erythrocyte ghosts labeled with 5-nitroxide stearate, I(12,3), indicate that a temperature-dependent lipid phase separation occurs with a high onset at 38 degrees C. Cooling below 38 degrees C induces I(12,3) clustering. Similar phase separations were previously identified in human platelet and cholesterol-loaded [cholesterol/phospholipid molar ratio (C/P) = 0.85] rat liver plasma membranes [L.M. Gordon et al., 1983; J. Membrane Biol. 76; 139-149]; these were attributed to redistribution of endogenous lipid components such that I(12,3) is excluded from cholesterol-rich domains and tends to reside in cholesterol-poor domains. Further enrichment of rat liver plasma membranes to C/P ratios of 0.94-0.98 creates an "artificial" system equivalent to human erythrocyte ghosts (C/P = 0.90), using such criteria as probe flexibility, temperature dependent I(12,3) clustering; and polarity of the probe environment. Consequently, cholesterol-rich and -poor domains probably exist in both erythrocyte ghosts and high cholesterol liver membranes at physiologic temperatures. The temperature dependence of cold-induced hypertonic lysis of intact human erythrocytes was examined by incubating cells in 0.9 M sucrose for 10 min at 1 degree C intervals between 9 and 46 degrees C (Stage 1), and then subjecting them to 0 degrees C for 10 min (Stage 2). Plots of released hemoglobin are approx. sigmoidal, with no lysis below 18 degrees C and maximal lysis above 40 degrees C. The protective effect of low temperatures during Stage 1 may be due to the formation of cholesterol-rich domains that alter the bilayer distribution and/or conformation of critical membrane-associated proteins.

Temperature dependence of verapamil action

Ca2+-tolerant ventricular myocytes from adult rats were electrically stimulated. The maximal contraction frequency (fm) was determined at different temperatures. In drug-free Tyrode solution, fm follows the Arrhenius equation from 7 to 39.5 degrees C. However, verapamil introduces a discontinuity around 27 degrees C into the Arrhenius plot of fm. Above this transition temperature the calcium antagonist lowers fm more pronouncedly than below. Below, a tenfold higher concentration is needed for the same relative effect as at 37 degrees C. It is argued that this finding might be important in cardiac surgery when calcium antagonists are used for cardioplegia at deep hypothermia.

Thermotropic lipid phase separations in human platelet and rat liver plasma membranes

Electron spin resonance (ESR) studies were conducted on human platelet plasma membranes using 5-nitroxide stearate, I(12,3). The polarity-corrected order parameter S and polarity-uncorrected order parameters S(T parallel) and S(T perpendicular) were independent of probe concentration at low I(12.3)/membrane protein ratios. At higher ratios, S and S(T perpendicular) decreased with increasing probe concentration while S(T parallel) remained unchanged. This is the result of enhanced radical interactions due to probe clustering. A lipid phase separation occurs in platelet membranes that segregates I(12,3) for temperatures less than 37 degrees C. As Arrhenius plots of platelet acid phosphatase activity exhibit a break at 35 to 36 degrees C, this enzyme activity may be influenced by the above phase separation. Similar experiments were performed on native [cholesterol/phospholipid ratio (C/P) = 0.71] and cholesterol-enriched [C/P = 0.85] rat liver plasma membranes. At 36 degrees C, cholesterol loading reduces I(12,3) flexibility and decreases the probe ratio at which radical interactions are apparent. The latter effects are attributed to the formation of cholesterol-rich lipid domains, and to the inability of I(12,3) to partition into these domains because of steric hinderance. Cholesterol enrichment increases both the high temperature onset of the phase separation occurring in liver membranes from 28 degrees to 37 degrees C and the percentage of probe-excluding, cholesterol-rich lipid domains at elevated temperatures. A model is discussed attributing the lipid phase separation in native liver plasma membranes to cholesterol-rich and -poor domains. As I(12,3) behaves similarly in cholesterol-enriched liver and human platelet plasma membranes, cholesterol-rich and -poor domains probably exist in both systems at physiologic temperatures.

Alterations in Ca2+ binding by and composition of the cardiac sarcolemmal membrane in chronic diabetes

Chronic streptozotocin-induced diabetes in rats was associated with a significant loss in the ability of isolated cardiac sarcolemmal membranes to bind Ca2+. Administration of insulin to the diabetic rats normalized the sarcolemmal Ca2+ binding capacity. The content of sialic acid residues, which are considered to represent a superficial Ca2+ pool in sarcolemma, was decreased in preparations from diabetic rats, and this change also was reversible upon insulin treatment of the diabetic rats. Treatment of sarcolemma with neuraminidase decreased Ca2+ binding by 37% in control preparations but had no effect on diabetic preparations. Diphosphatidylglycerol content was decreased but other acidic phospholipids such as phosphatidylinositol and phosphatidylserine, which also bind Ca2+, were not altered during diabetes. An increase in lysophosphatidylcholine and a decrease in phosphatidylethanolamine contents were observed in membranes isolated from diabetic rats. These results suggest that some alterations occur in Ca2+ binding and composition of heart sarcolemma in chronically diabetic rats and may provide further insight into the pathogenesis of diabetic cardiomyopathy.

Lipid domains in plasma membranes from rat liver

The existence of fluid and solid lipid domains in isolated rat-liver plasma membranes was evaluated using the fluorescent fatty acids trans-parinaric and cis-parinaric acid as probe molecules for solid and fluid membrane areas, respectively. The fluorescence probe 1,6-diphenyl-1,3,5-hexatriene indicated that a phase transition was present in the liver plasma membrane between 18 degrees C and 30 degrees C. At intermediate temperatures, cis-parinaric acid, which partitioned approximately equally into fluid and solid lipid areas, detected two lipid domains: the mole fractions of fluid and solid lipid domains at 24 degrees C were 0.32 and 0.68 while the mole fractions of cis-parinaric acid in each domain were 0.34 and 0.66, respectively. The dissociation constant, aqueous to membrane lipid partition coefficient, and bound to free ratio for trans-parinaric acid were 7.0 +/- 0.7 microM, 4.0 +/- 0.6 x 10(6), and 83:17, respectively. The affinity of the membrane for cis-parinaric acid was twofold lower than for trans-parinaric acid. The trans-parinaric acid partitioned preferentially into solid lipid, Ksp/f = 3.30, while the cis-parinaric acid partitioned equally between fluid and solid phases Ksp/f = 0.92. Thus, the data demonstrate the coexistence of fluid and solid domains in rat liver plasma membranes.