Phosphatidate and oxidized fatty acids are calcium ionophores. Studies employing arsenazo III in liposomes

Excitatory effect of Clostridium perfringens alpha toxin on the rat isolated aorta

Clostridium perfringens alpha toxin caused contraction of the isolated aorta of the rat in a dose-dependent manner. The contractile action caused by the toxin was inhibited or abolished by calcium antagonists such as nifedipine, verapamil and cinnarizine, or a Ca-free medium, but was not affected by phentolamine, chlorpheniramine, atropine, tetrodotoxin or a low Na medium. The toxin stimulated Ca uptake into the aorta in a dose-dependent manner. 8-N,N'-diethylaminooctyl-3,4,5-trimethoxybenzoate (TMB-8) blocked significantly both the toxin- and noradrenaline (NA)-induced contractions. Trifluoperazine (TFP) and N-(6-aminohexyl)-5-chloro-1-naphtharene sulphonamide (W-7) did not affect the contractile activity of the toxin but blocked the NA-induced contraction. The toxin also stimulated the 32P phosphate labelling of phosphatidylinositol (PI) and phosphatidic acid (PA) in the preparation. These results indicate that the toxin-induced contraction, which is different from that induced by NA, is the result of a direct action of the toxin on the aorta and is due to an increased Ca2+ permeability across the smooth muscle membrane. It is suggested that the contractile response to the toxin is associated with activation of phospholipid metabolism and enhanced entry of Ca into the aorta.

Oxidative stress impairs the function of sarcoplasmic reticulum by oxidation of sulfhydryl groups in the Ca2+-ATPase

Sarcoplasmic reticulum (SR) microsomes were oxidized by exposure to peroxydisulfate, hydrogen peroxide, or iron/ascorbate or by extended storage. The decline in Ca2+-ATPase activity, Ca2+ transport, and the increase in Ca2+ permeability which occurred under these conditions did not appear to result from lipid oxidation because these functional changes were not correlated with the amount of thiobarbituric acid-reactive lipid. Consistent with this interpretation, lipid antioxidants did not prevent the decline in SR function. This suggests that inhibition was independent of lipid oxidation. Instead, oxidation directly inhibited the Ca2+-ATPase. The decline in enzyme activity may be due to oxidation of SH groups, as suggested by the ability of reducing agents to prevent inhibition, the decline in sulfhydryl content of oxidized SR, and the ability of sulfhydryl-binding agents to inhibit Ca2+-ATPase. Inhibition was not primarily due to crosslinking of the Ca2+-ATPase, because sodium dodecyl sulfate-polyacrylamide gels of normal and oxidized SR showed that the area of the Ca2+-ATPase band was not correlated with the Ca2+-ATPase activity. Inhibition of the Ca2+-ATPase by oxidative stress is relevant to models of cellular dysfunction in which toxicity is caused by a rise in intracellular calcium.

Effect of cholesterol on Ca2+-induced aggregation of liposomes and calcium diphosphatidate membrane traversal

Sonicated cholesterol-phosphatidylcholine (PC) liposomes containing 4 mol % phosphatidic acid (PA) aggregate in 10 mM Ca2+, slowly at low molar fractions of cholesterol (up to 30%) and 15 times faster at higher concentrations; the inflection point is at ca. 35 mol % bilayer cholesterol. O-[[(Methoxyethoxy)ethoxy]ethyl]cholesterol (OH-blocked cholesterol) does not give this rate enhancement. If PC is replaced by diether PC (CO groups abolished), cholesterol does not accelerate aggregation at concentrations in the bilayer below 50 mol %. No change in Ca2+-induced aggregation rates was observed if the ester CO groups of the bridge-forming PA only were replaced by CH2 (diether PA) in liposomes containing PC and cholesterol. PA-mediated Ca2+ membrane traversal seems to be accelerated by the addition of cholesterol to the PC-PA membrane, but analysis shows that the effect is due to the bilayer condensation effect of cholesterol resulting in an increase in the surface concentration of PA and that membrane cholesterol in fact slightly reduces the rate of Ca(PA)2 traversal; OH-blocked cholesterol, however, increases this rate 3-fold. It appears that lipid OH and CO groups interact, directly or with the mediation of water, in establishing the structure of the membrane "hydrogen belts", i.e., the strata containing those hydrogen-bond donors and acceptors. Cholesterol hydroxyl above 33 mol % (saturation of a 2:1 PC/cholesterol complex?) causes a restructuring of the hydrogen belts that facilitates membrane-water-membrane dehydration, the prerequisite for liposome aggregation by trans-Ca(PA)2 formation. On the other hand, the formation of the dehydrated cis-Ca(PA)2 complex that precedes Ca2+ membrane traversal is not accelerated by presence of the cholesterol hydroxyl group.

Modification of renal cortical subcellular membrane phospholipids induced by mercuric chloride

Administration of mercuric chloride (HgCl2, 6 mg/kg) to rabbits produced renal failure, with changes in serum creatinine from 1.01 +/- 1 in controls to 6.46 +/- 0.91 mg/dl 24 hr after administration. Mitochondria isolated from HgCl2-treated rabbits exhibited alterations in acceptor control ratios, with reduction to 1.9 +/- 0.2 from 3.9 +/- 1.2 in controls. Ultrastructurally, the mitochondria showed swelling and loss of inner mitochondrial membranes. Total lipids from mitochondria of control and treated rabbits were obtained by modified Folch extraction and phospholipids analyzed by TLC. Brush border membranes and basolateral membranes were prepared from control and HgCl2-treated kidneys 2 and 24 hr after HgC12 administration. At 24 hr mitochondria showed a 36% fall in phosphatidylcholine (PC), a 36% fall in phosphatidylethanolamine (PE), and a 27% fall in cardiolipin. Brush border showed a decrease in phosphatidylserine (PS) of 29% and in PE of 40%. The basolateral membranes showed a reduction only in PE of 35%. At 2 hr post HgCl2, early changes are confined to the BBM and consist of a reduction in PE in this membrane. This changes in membrane phospholipids may be important in the functional derangements that occur at the cellular level after HgCl2 administration.

Ca2+-translocation activities of phosphatidylinositol, diacylglycerol and phosphatidic acid inferred by quin-2 in artificial membrane systems

Ca2+-translocating activities of phosphatidylinositol, diacylglycerol and phosphatidic acid were investigated in phosphatidylcholine liposomes. Using a fluorescent indicator of Ca2+ concentration, quin-2, release of encapsulated Ca2+ from egg yolk phosphatidylcholine liposomes containing 2 mol% of one of these lipids was measured at 37 degrees C. The rate of Ca2+ translocation across the liposomal membrane mediated by phosphatidic acid was about 3-fold larger than those mediated by phosphatidylinositol and diacylglycerol. The result implies that phosphatidic acid has Ca2+-ionophore activity in the agonist dependent metabolism of inositol phospholipids. The ionophoretic activity depended on the degree of unsaturation of the fatty acyl chains. The Ca2+ translocation rate was smallest in dipalmitoylphosphatidic acid, and it increased in the order of dioleoyl-, dilinoleoyl- and dilinolenoyl-phosphatidic acid. Ca2+ mobilization of a stimulated cell is discussed in the light of Ca2+-ionophore activity of phosphatidic acid converted from inositol phospholipids.

Calcium efflux from sarcoplasmic reticulum microsomes due to oxidation and sulfhydryl-binding agents

Calcium permeability of sarcoplasmic reticulum (SR) microsomes was measured after aging or after exposure to peroxydisulfate or to sulfhydryl-binding agents. Under conditions where the Ca2+-ATPase was active, the maximum net release of Ca2+ was not significantly different between control and oxidized SR. However, when calcium uptake was prevented by EGTA or apyrase, the Ca2+ permeability of oxidized microsomes was 2 to 3 times greater than control of low (10(-9), 10(-7) M) but not high (10(-6) M) levels of external calcium. The observation that vesicles preincubated with 5 mM dithiothreitol loaded up to 3 times as much calcium and had a slightly lower calcium permeability coefficient than control vesicles suggested that sulfhydryl oxidation might modulate calcium flux. This hypothesis was tested by exposing to sulfhydryl-binding agents: silver, arsenite, and p-chloromercuriphenylsulfonic acid. Sulfhydryl-binding agents initiated a rapid release of calcium from microsomes, and release was halted by dithiothreitol. Inhibition of calcium transport could not entirely account for the apparent increase in permeability because the calcium permeability of SR treated with sulfhydryl-binding agents was 5 times greater than that of SR exposed to Ca2+-ATPase inhibitors. These results suggest that oxidation may increase the calcium permeability of SR by allowing calcium loss through a channel that can be gated by sulfhydryl oxidation.

Hypothesis: control of intracellular calcium level

It is proposed that cells store calcium in the hydrogen belt of their membranes, on the cytoplasmic side, with the Ca2+ ion captive in cages formed by the phosphate and carbonyl oxygens of two acidic phospholipid molecules; for instance, phosphatidylinositol and phosphatidylserine. Evidence for the existence of such Ca-cages is adduced from the properties of the [Ca(phosphatidate)2] complex. Cytoplasmic Ca2+ concentration, approx. 10(-7) M, corresponds to the calcium cage dissociation constant. The high stability of the cages is the result of multiple hydrogen bonds between inositol and serine, or inositol and inositol. Phosphorylation of the inositol in position 4 and 5 opens the calcium cage by breaking the inter-headgroup hydrogen bonds and by introducing electrostatic and steric hindrance. This allows the escape of Ca2+ into the cytosol. The mono in equilibrium with di in equilibrium with triphosphoinositide shuttle serves as a regulator of Ca2+ concentration in the cytoplasm: phosphorylation of the lipids will raise, dephosphorylation lower the level of free Ca2+. The inositide shuttle may be linked to a stimulus-induced inositide cycle in which inositol triphosphate is generated, and to Ca(phosphatidate)2 cross-membrane transport.

Interaction of Ca2+ with cardiolipin-containing liposomes and its inhibition by adriamycin

The interaction of cardiolipin-containing, unilamellar liposomes with Ca2+ was assessed by flow dialysis in the presence of 2-100 microM 45Ca2+, using vesicles formed from phosphatidylcholine (PC) and from PC and cardiolipin in mole ratios from 16:1 to 1:1. Control (PC only) vesicles bound no detectable Ca2+. In contrast, Ca2+ binding to cardiolipin-containing vesicles was substantial and dependent on vesicle concentration. Scatchard plots for the binding were concave upward. Resolution of the data, assuming the presence of two independent classes of binding sites, indicated a high-affinity site with apparent KD = 5.57 +/- 0.48 microM (S.D.) and a second site with KD in the millimolar range. Interaction of cardiolipin-containing liposomes with Ca2+ was insensitive to monovalent cations (Na+, K+, Rb+), but was inhibited by ruthenium red much greater than La3+ greater than Mn2+ greater than Mg2+. Progressive increases in the PC: cardiolipin ratio markedly increased the apparent KD for Ca2+ at the high-affinity site. Stoichiometry of Ca2+ binding at the site passed through a maximum at a PC: cardiolipin ratio of 4:1. The potent antineoplastic agent adriamycin also inhibited the interaction of Ca2+ with cardiolipin-containing liposomes in a dose-dependent manner; effects were detected at 10 microM antibiotic. Unlike PC, adriamycin altered the stoichiometry of the high-affinity interaction but not the apparent KD. Adriamycin effects increased with pH in the range of the pKA of its amino group. These results suggest that inhibition by adriamycin may result from a mechanism other than simple competition for the charged head group of cardiolipin.

Structural considerations for calcium ionophoresis by prostaglandins

The prostaglandins PGB2, PGE2 and PGF2 alpha were found to translocate calcium in a modified Pressman cell. At pH 7.40, PGB2 was more potent than PGE2 and than PGF2 alpha. When incorporated at a 1% molar ratio in liposomes made of cholesterol and different diacyl phosphatidyl choline, prostaglandins are able to mediate a slow calcium exchange diffusion. A significant prostaglandin-mediated calcium release that depends on the lipid matrix rigidity is observable at 37 degrees but not at 22 degrees. Conformational analysis of the complex formed by two molecules of prostaglandins and one calcium atom, either at a simulated membrane-water interface or in a simulated bulk lipid phase reveals rigid complexes with great distances between hydrophilic and hydrophobic gravity centres that predict low ionophoretic properties.

Essential adaptation of the calcium influx assay into liposomes with entrapped arsenazo III for studies on the possible calcium translocating properties of acidic phospholipids

An adapted version of the Ca2+-influx assay of Weissmann et al. (Weissmann, G., Anderson, P., Serhan, C., Samuelson, E. and Goodman, E. (1980) Proc. Natl. Acad. Sci. USA 77, 1506-1510) is presented for studies on the possible ionophoretic properties of acidic phospholipids. This method is based on the use of the metallochromic dye arsenazo III enclosed in liposomal vesicles, to indicate the Ca2+ influx. An essential control is introduced to discriminate between Ca2+-arsenazo III complex formation inside the vesicles, as a consequence of Ca2+ influx, and outside the vesicles, as a consequence of arsenazo III leakage from the vesicles. Furthermore, some minor improvements are added, like the use of large unilamellar vesicles instead of multilamellar vesicles, and the use of dual wavelength spectrophotometry. Using this method, it was found that dioleoylphosphatidylcholine vesicles, containing 20 mol% dioleoylphosphatidylglycerol, were impermeable to Ca2+. In this system a selective Ca2+ permeability could be induced by the addition of the fungal Ca2+ ionophore A23187. In contrast, dioleoylphosphatidylcholine vesicles, containing 20 mol% dioleoylphosphatidic acid, incubated in the presence of Ca2+ were permeable to both Ca2+ and arsenazo III.

Transforming NRK cells with avian sarcoma virus reduces the extracellular Ca2+ requirement without affecting the calcicalmodulin requirement for the G1/S transition

NRK rat cells infected with a transformation-defective, temperature-sensitive (ts) mutant of the avian sarcoma virus could not proliferate in Ca2+-deficient medium at a nonpermissive temperature (40 degrees C) that inactivated the viral pp60v-src-transforming product and rendered the cells phenotypically untransformed. However, these arrested cells were stimulated to initiate DNA replication with little or no delay while still in the Ca2+-deficient medium, either by adding Ca2+ or calmodulin at 40 degrees C or by reducing the temperature to 36 degrees C which restored the transformed phenotype by rapidly reactivating pp60v-src. The G1/S transition triggered by restoring the transformed phenotype was suppressed by three different anticalmodulin drugs (R24571, trifluoperazine, W7). The suppression by one of these drugs, trifluoperazine, was overcome by adding calmodulin. Thus, neoplastic transformation by the avian sarcoma virus sharply reduces the extracellular Ca2+ requirement for the initiation of DNA replication without bypassing a calcicalmodulin-dependent mechanism also needed for the G1/S transition.

A mechanism for phosphoglyceride and Ca2+ transbilayer movement

The ionophoretic capabilities of phosphoglycerides (PL) have been examined by measuring their translocation via cations from aqueous dispersions into linear and cyclic hydrocarbons. The PL surveyed were phosphatidic acid (PA), phosphatidylglycerol (PG), phosphatidylethanolamine (PE), phosphatidylcholine (PC) and phosphatidylinositol (PI). Only PA displayed ionophoretic activity in single lipid dispersions with a cation selectivity order of Mn greater than Ca. PG, PE and PC, but not PI, had a synergistic affect of PA induced translocation. These PL, inactive individually or in any combination, became strong Ca2+ ionophores of variable activity in association with PA. A dimeric structure proposed for the ionophoretic species forms the basis of a mechanism for transbilayer movement of PA, PG, PE and PC which would establish an asymmetric distribution of these lipids in the two faces of the bilayer by equilibrium processes.

Calcium translocation by fatty acid derivatives in a two-phase partition model. Structure-activity relationships

The relationship between structures of fatty acid derivatives, long-chain fatty alcohols, phospholipids and their calcium-transporting activity was investigated using the two-phase model system in which 45Ca is transported from an aqueous to an immiscible organic phase. Calcium translocation by all saturated and unsaturated fatty acids was significant only at 10 mM concentrations, but minimal or negligible below 1 mM; the corresponding methyl esters and alcohols were inactive at 10 mM. Polyunsaturated fatty acid derivatives, prepared by incubation with lipoxygenase (linoleate: oxygen oxidoreductase; EC 1.13.11.12) or by autoxidation in air, showed a markedly increased potency over the parent compounds. The oxidation products of linoleic and arachidonic acids were most potent. For example, the equieffective concentrations were 10 mM for linoleic acid, 0.4 mM for its lipoxygenase metabolites and 0.094 mM for its autoxidation products. Similarly, for arachidonic acid and its derivatives, equieffective concentrations were 10, 0.104 and 0.112 mM, respectively. The potency of the autoxidized fatty acid derivatives varied with both duration of autoxidation and the specific structure. Methyloleate and oleyl alcohol remained inactive even after a prolonged oxidation, whereas methyllinoleate and linoleyl alcohol were very potent only after 4 weeks but not after 1 week autoxidation. The potency of esters and alcohols with three or more double bonds increased significantly even after a short-term autoxidation, reflecting the differences in both the rate of formation and the contribution to calcium-transporting properties of the primary and secondary oxidation products. All phospholipids tested, with the exception of phosphatidylcholine and lysophosphatidylcholine, showed considerable calcium-transporting activities at 0.01 mM or greater concentrations; some members were of similar or greater potencies than the classical calcium ionophores, X537A and A23187.

Antigenic stimulated release of arachidonic acid, lipoxygenase activity and histamine release in a cloned murine mast cell MC9

A cloned murine mast cell MC9 expresses phospholipase and lipoxygenase activity when stimulated with IgE and hapten. Addition of DNP-BSA to sensitized MC9 cells causes release of 58% of the cell histamine and 127 pmoles LTC4/10(6) cells. Prelabelling studies with [1-14C]-arachidonic acid showed that LTC4 production was proceeded by the release of arachidonic acid from membrane phospholipids. Approximately 8.7% of the cell arachidonic acid was released and half of this was converted to LTC4. The remaining radioactivity was converted to diHETES including LTB4 (15%), 5-HETE (10%), free arachidonic acid (10%), reesterified 5-HETE and arachidonic acid (8%) and prostaglandins (7%). This stimulation was dependent on hapten (DNP-BSA) and extracellular Ca++. Under identical conditions the calcium ionophore A23187 stimulated the release of 10.3% of the total cell arachidonic acid, and 51% of this was metabolized to LTC4. In addition the ionophore stimulated the release of 61% of the total cellular histamine.

Fertilization stimulates lipid peroxidation in the sea urchin egg

Arachidonic acid is rapidly taken-up by Strongylocentrotus purpuratus eggs and eventually incorporated into cellular lipids. During the first few minutes following fertilization the arachidonic acid that has not been incorporated into other lipid forms is oxidized to a hydroxy-fatty acid. In vivo, the time of arachidonic acid conversion coincides with the transient period of increased intracellular free calcium after fertilization. In vitro, this lipid peroxidizing activity has been shown to be initiated by micromolar calcium. Taken together with the presence of Ca2+-stimulated lipase, these results suggest that calcium regulates both the release of polyunsaturated fatty acids from cellular lipids and their subsequent oxidation. The physiological function of lipid hydroxides or hydroperoxides in sea urchin fertilization is unknown. A possibility is that they may be important in regulating the many membrane permeability changes occurring within minutes after fertilization.

Effect of Clostridium perfringens alpha toxin on the isolated rat vas deferens

Alpha toxin produced by Clostridium perfringens potentiated norepinephrine-evoked contraction in the isolated rat vas deferens, but itself caused no contraction within 60 min. The potentiating activity was dependent on the dose of the toxin and was quantitatively related to the phospholipase C activity of the toxin preparation.

Absence of correlation between ACh-induced Ca influx and phosphatidic acid labeling in rat uterus

Rat uterine smooth muscle was preincubated in Ca-depleted modified Locke-Ringer solution to investigate the correlation between the 32Pi incorporation into phosphatidic acid induced by acetylcholine and the contractile response to acetylcholine induced by the addition of CaCl2 (Ca influx). The results showed that in rat uterine smooth muscle under these conditions phosphatidic acid does not act as a Ca ionophore or as a trigger for opening the Ca channel.

Phosphatidic acid as a calcium ionophore in large unilamellar vesicle systems

The ionophoretic capabilities of dioleoylphosphatidic acid (DOPA) for transporting calcium across phospholipid bilayers have been investigated. Calcium uptake by large unilamellar vesicles is shown to depend on the presence of DOPA. This uptake is sensitive to the nature and concentration of calcium chelators in the vesicle interior, indicating that accumulation results from DOPA-mediated translocation of calcium across the membrane. Further, it is shown that characteristics of DOPA-mediated Ca2+ uptake are similar to those observed for the fungal calcium ionophore, A23187.

Platelet-activating factor phosphatidate, but not platelet-activating factor, is a powerful calcium ionophore in the human red cell

Platelet-activating factor (1-0-alkyl-2-acetyl-sn-glyceryl-3-phosphorylcholine, PAF) is a potent inducer of shape-change, aggregation and secretion in platelets. PAF causes a rapid increase in intracellular calcium, but has no calcium gating effect in intact lipid bilayers. Human red cells (RBC) did not metabolize either PAF or PAF-phosphatidate (PAF-PA). While PAF (10 microM) was devoid of calcium ionophoretic activity, PAF-PA (1-5 microM) stimulated calcium influx into intact human RBC. In addition, PAF-PA (1-10 microM), but not PAF (10 microM), elicited a series of satellite effects related to the rise of intracellular calcium: 1) increased efflux of intracellular potassium (Gàrdos effect); 2) alkalinization of unbuffered RBC suspensions; 3) stimulation of ATP consumption and production, and enhancement of glycolytic flux with crossover at the glyceraldehyde 3-phosphate dehydrogenase step. These effects exactly duplicate those brought about by the calcium ionophore A23187. The ionophoretic potency of PAF-PA was about half that of A23187. Approximately the same concentrations of PAF-PA as those that stimulate calcium influx into RBC elicit full aggregatory response in human platelets. It is possible that transformation of PAF into PAF-PA by the combined action of phospholipase C and diacylglycerol kinase contributes to the increase of calcium influx in platelets.

Polyphosphoinositide turnover in rat mast cells stimulated by antigen: rapid and preferential breakdown of phosphatidylinositol 4-phosphate (DPI)

The effect of antigen on the metabolism of polyphosphoinositides was investigated in sensitized rat peritoneal mast cells. Addition of antigen to rat peritoneal mast cells prelabelled with [3H]arachidonic acid resulted in a very rapid decrease in the level of phosphatidylinositol 4-phosphate (DPI) within 5 sec, which appeared to precede the breakdown of phosphatidylinositol (PI), while there was no significant decline of PI 4,5-bisphosphate (TPI). The reduced levels of these phosphoinositides returned almost to control or even slightly higher values by 300 sec in parallel with the antigen-stimulated [32P]phosphate incorporation into these lipids. This early and transient disappearance in DPI prior to that in PI was also observed in [3H]glycerol-prelabelled cells. These data suggest that DPI degradation upon stimulation by antigen in mast cells may be an initial step in the histamine release process.

Ca(phosphatidate)2 can traverse liposomal bilayers

Phosphatidic acid can act as Ca2+ cross-membrane ionophore without the necessity of previous autoxidation. The apparent PA-CA2+ dissociation constant is 3 X 10(-3), i.e., in the range of extracellular Ca2+ concentration. There is at least 100-fold preference for Ca2+ over Mg2+. Ca2+ transfer rates are proportional to the square of phosphatidic acid concentration in the bilayer. Removal of the fatty acid ester CO groups reduces the Ca2+ ferrying rate by more than 90 percent. It appears that the cation is held in a cage formed by phosphate and carbonyl oxygens of two PA molecules. In this coordination complex both Ca2+ and the phosphatidic acid headgroups are dehydrated, and the Ca(phosphatide)2 assembly becomes lipid-soluble and can traverse the bilayer.

Oxygenation products of arachidonic acid: third messengers for insulin release

Although an association between membrane phospholipid turnover and exocytotic hormone release has long been recognized, a causal relationship has not been firmly established. Recent studies suggest that glucose (and probably other insulin secretagogues) activates phospholipases and thereby releases membrane-bound arachidonic acid (AA). AA is then converted through islet 12-lipoxygenase to mediators or modulators of insulin release (tentatively identified as peroxides and epoxides of arachidonate). These products may be critical links in stimulus-secretion coupling, since blockade of either AA release or lipoxygenation abrogates insulin release induced by glucose and many other (but not all) stimuli. Cogeneration of prostaglandins from AA through the cyclooxygenase pathway may directly or indirectly modulate the formation and/or effect of lipoxygenase products. A critical role for lipoxygenase products (and possibly metabolites of AA synthesized by other pathways, such as P-450-dependent monooxygenases) may extend to many secretory cells in addition to pancreatic beta cells. The phasic release of AA described in many cells could explain the biphasic pattern of insulin release induced by glucose. Since some phospholipases and lipoxygenases are Ca++ activated, the release of AA in conjunction with its oxygenation appears to be a concerted system generating "third messengers" for hormone release.

Calcium-dependent atrial slow action potentials generated with phosphatidic acid or phospholipase D

Phosphatidic acid (PA) formed following phosphatidylinositol hydrolysis has been proposed as a necessary step in receptor-mediated Ca2+ flux. This study demonstrates that PA generates Ca2+-dependent slow action potentials (APs) in rat atrium partially depolarized with 22 mM K+. The slow response was not due to release of endogenous catecholamines or prostaglandin formation since propranolol and indomethacin failed to attenuate the PA-induced slow AP in normal and reserpinized rats. PA-induced slow APs demonstrated Ca2+-dependence such that increasing [Ca2+]o from 0.5 to 5.0 mM caused the amplitude of the slow AP to rise linearly with the logarithm of [Ca2+]o. Phospholipase D (PLD) but not phospholipase C, was able to induce a slow AP, possibly through PA formation. Adenosine attenuated the PA and PLD-induced slow response and aminophylline reversed these effects. The observation that PA and PLD generate Ca2+-dependent slow APs in depolarized rat atrium supports a role for PA mediating Ca2+ influx.

Acetylglyceryl ether phosphorylcholine (AGEPC; platelet-activating factor)-induced stimulation of rabbit platelets: correlation between phosphatidic acid level, 45Ca2+ uptake, and [3H]serotonin secretion

When 32Pi-labeled rabbit platelets were incubated with 5 X 10(-10) M 1-O-alkyl-2-acetyl-sn-glyceryl-3-phosphorylcholine (AGEPC), either in the presence or absence (0.1 mM EGTA) of added Ca2+, there was a three- to five-fold increase in the [32P]phosphatidic acid (PA) pool within 15 to 20 s. This event was followed by a gradual decrease in the [32P]PA level to near basal level in 5 min. AGEPC effected this change in [32P]PA in a characteristic dose- and time-dependent manner. Polar head group analogs of AGEPC, such as AGEDME and AGEMME, also effected an increase in PA labeling at levels comparable to those previously reported for their activity toward rabbit platelets [K. Satouchi, R. N. Pinckard, L. M., McManus, and D. J. Hanahan (1981) J. Biol. Chem. 256, 4425-4432]. Other analogs, i.e., lysoGEPC and the enantiomer, sn-1-AGEPC, which are inactive toward rabbit platelets, also showed no effect on the level of [32P]PA. The finding that the PA level in rabbit platelets could be manipulated by the addition of AGEPC, without any added Ca2+, provided an excellent model system for establishing a correlation between the uptake of Ca2+, serotonin release, and PA level. Thus, PA must be regarded as a sensitive indicator of a reaction mechanism important to the platelet response to AGEPC, and could be the focal point in promoting calcium uptake during the stimulation process.

Differences in Ca2+ mobilization induced by alpha-adrenergic agonist and phosphatidic acid in cultured hepatocytes

In an attempt to elucidate the relationship between phosphatidylinositol breakdown and alpha-adrenergic responses, effects of phosphatidic acid and phosphatidylinositol related metabolites on Ca2+ mobilization and glucose output in cultured hepatocytes were examined. Norepinephrine induced the net 45Ca2+ efflux from preloaded cells and stimulated glucose output via alpha-adrenergic receptor stimulation, whereas phosphatidic acid caused 45Ca2+ uptake to cells and did not stimulate glucose output. Myo-inositol-monophosphate, diglyceride and arachidonic acid, which are released by phosphatidylinositol breakdown, had no effect on 45Ca2+ efflux and glucose output in cells. These results suggest that phosphatidic acid and phosphatidylinositol related metabolites can not mimic the alpha-adrenergic actions in cultured hepatocytes.

Differential effects of phenylmethanesulfonyl fluoride (PMSF) on carbachol and potassium stimulated phosphoinositide turnover and contraction in longitudinal smooth muscle of guinea pig ileum

Phenylmethanesulfonyl fluoride (PMSF) (2 mM), a putative inhibitor of phosphatidylinositol-specific phospholipase C, almost completely inhibited carbachol-stimulated inositol incorporation into phosphatidylinositol (PI) of longitudinal smooth muscle of guinea pig ileum, while it had no effect on potassium-stimulated inositol incorporation. This suggests that the two stimuli may affect phosphoinositide turnover by different mechanisms, distinguishable by PMSF. In contrast to its specific inhibition of carbachol-stimulated phosphoinositide turnover, PMSF produced a transient inhibition of contraction by both carbachol and potassium. The non-selective effect of PMSF on contraction suggests that it is not the result of its inhibitory effect on phosphoinositide breakdown. PMSF (2 mM) inhibited carbachol-stimulated inositol phosphate accumulation in the presence of Li+ by only 15%-19%, indicating that PMSF inhibition of phosphoinositide turnover was not due to its inhibition of phosphoinositide phosphodiesterase, but to one or more steps following phosphoinositide breakdown.

Rheumatoid arthritis. The role of neutrophil activation

Neutrophils constitute over 90% of cells found in the synovial fluid of rheumatoid arthritis (RA) patients. Since such fluids also contain immune complexes (IgG-IgG and IgG-IgM rheumatoid factors) and complement split products (C5, C5A, DES, ARG, C3B, etc.), all of the reactants are present for a local Arthus lesion. Moreover, neutrophils from RA patients endocytose these immune complexes and complement components in vivo and in vitro. In consequence, it has been suggested that lysosomal enzymes and other mediators of inflammation released by neutrophils after uptake of immune complexes (in the bulk phase or on the surface) account, at least in part, for rheumatoid inflammation. Secretion of lysosomal hydrolases, especially neutral proteases, which provoke tissue injury and generation of reactive oxygen species (e.g. O2) is part of a stimulus-secretion response to a variety of secretagogues, including immune complexes and complement components. However, the pathways of secretion and O2 generation are stimulus-specific and can be dissected to establish cause and effect relationships by (a) kinetic analysis, (b) varying the stimulus, (c) use of impermeant reagents to block discrete responses. Neutrophils also generate products of 11-cyclo-oxygenase (e.g., PGE2, TXA2) and of the 5- and 15-lipoxygenase (mono-, di and tri-hetes, LTB4 and their isomers). However, the cyclo-oxygenase products (except TXA2) do not cause inflammation acting alone; indeed, they inhibit the function of neutrophils, platelets, macrophages and mast cells. The most potent proinflammatory agent yet identified as a product of arachidonate is LTB4. LTB4 is a potent Ca ionophore, a strong chemo-attractant, induces local inflammation, and activates neutrophils.

Phosphatidic acid and the calcium-dependent actions of gonadotropin-releasing hormone in pituitary gonadotrophs

The stimulation of luteinizing hormone (LH) release and cyclic GMP (cGMP) production in rat anterior pituitary cells by gonadotropin-releasing hormone (GnRH) are receptor mediated and calcium dependent, and have been shown to be accompanied by increased phospholipid turnover and arachidonic acid release. The incorporation of 32Pi into the total phospholipid fraction of pituitary gonadotrophs was significantly elevated by 10(-8) M GnRH, with specific increases in the labeling of phosphatidylinositol and phosphatidic acid (PA). Since PA acts as a calcium ionophore in several cell types, its effects upon calcium-mediated gonadotroph responses were compared with those elicited by GnRH. In rat pituitary gonadotrophs prepared by centrifugal elutriation, PA stimulated LH release and cGMP production by 9-fold and 5-fold, respectively. The stimulation of LH release by 30 microM PA was biphasic in its dependence on extracellular calcium concentration, rising from zero in the absence of calcium to a maximum of 10-fold at 0.5 mM Ca2+ and declining at higher calcium concentrations. In dose-response experiments, PA was 3-fold more potent at 0.5 mM Ca2+ than at 1.2 mM Ca2+. The cGMP response to PA in cultured gonadotrophs was also calcium dependent, and was progressively enhanced by increasing Ca2+ concentrations up to 1.5 mM. The ability of PA to stimulate both LH release and cGMP formation in a calcium-dependent manner suggests that endogenous PA formed in response to GnRH receptor activation could function as a Ca2+ ionophore in pituitary gonadotrophs, and may participate in the stimulation of gonadotroph responses by GnRH and its agonist analogs.

Phosphatidic acid regulates the activity of the channel-forming ionophores alamethicin, melittin, and nystatin: a 1H-NMR study using phospholipid membranes

The regulation of ion channels by phosphatidic acid (a proposed active metabolite in the phosphatidylinositol effect) was investigated using 1H-NMR spectroscopy and small unilamellar phospholipid vesicles. Transport across egg-yolk phosphatidylcholine (egg PC) and dipalmitoyl phosphatidylcholine (DPPC) vesicular membranes in the presence of the channel-forming ionophores alamethicin, melittin, and nystatin was monitored using the lanthanide probe ion Pr3+. In the absence of the ionophores, phosphatidic acid (PA) alone was found to have no ionophore properties, but in the presence of the ionophores the incorporation of 3 mol % phosphatidic acid in the bilayer markedly increased the rate of transport using melittin and nystatin, but decreased the rate using alamethicin, independent of the type of phosphatidylcholine used. The presence of PA in the bilayer also stimulated the production of lytic type channels, the extent of which were both ionophore- and lipid-dependent. These results are discussed in terms of possible molecular interactions between the PA, the individual ionophores, and type of lipid used.

Effect of 1-O-alkyl-2-acetyl-sn-glyceryl-3-phosphorylcholine on calcium fluxes by human platelet microsomes

Under conditions where optimal concentrations of arachidonic acid, phosphatidic acid, or the calcium ionophore A23187 caused release of 50-95% of calcium from preloaded platelet microsomes, basophil platelet activating factor (1-O-alkyl-2-acetyl-sn-glyceryl-3-phosphorylcholine, AGEPC) did not cause the release of calcium at concentrations as high as 2 X 10(-5) M. The failure to stimulate calcium release was not due to metabolism or inactivation of AGEPC. These results show that AGEPC is not a calcium ionophore and is unable to directly effect the release of calcium from microsomes by mechanisms other than ionophoric action. The increase in intracellular levels that occurs during AGEPC-induced platelet aggregation must be an indirect effect of the AGEPC.

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

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

Mechanisms of mediator release from neutrophils

The encounter of neutrophils with immune complexes and complement components - in the bulk phase or on a surface - leads to their secretion of lysosomal hydrolases, especially neutral proteases, which provoke tissue injury. Secretion of lysosomal enzymes and generation of reactive oxygen species (e.g., O2-. generation are stimulus-specific and can be dissected to establish cause and effect relationships by means of: a) kinetic analysis, b) variations in the stimulus, and c) use of impermeant reagents to block discrete responses. Neutrophils also generate products of 11-cyclooxygenase (e.g., PGE2, TxA2) and of the 5- and 15-lipoxygenases (mono-, di-, and tri-HETEs, LTB4, and their isomers). But the cyclooxygenase products (save TxA2) are not phlogistic by themselves: they inhibit the functions of neutrophils, platelets, macrophages, and mast cells. The most potent pro-inflammatory agent yet identified as a product of arachidonate is LTB4. LTB4 is a potent Ca ionophore, constricts airways, is a potent chemoattractant, and induces local inflammation.