Modulation by fatty acids of Ca2+ fluxes in sarcoplasmic-reticulum vesicles

Hepatic endoplasmic reticulum calcium fluxes: effect of free fatty acids and KATP channel involvement

As a common sequel to obesity, plasma and intracellular free fatty acid (FFA) concentrations are elevated and, as a consequence, manifold disturbances in metabolism may ensue. Biochemical processes in the cytosol and organelles, such as mitochondria and endoplasmic reticulum (ER), can be disturbed. In the ER, the maintenance of a high calcium gradient is indispensable for viability. In sarcoplasmic reticulum, selective FFA can induce ER stress by disrupting luminal calcium homeostasis; however, there are limited studies in hepatic microsomes. Our studies found that FFA has a noxious effect on rat hepatic microsomal calcium flux, and the extent of which depended on the number of double bonds and charge. Furthermore, insofar as the FFA had no effect on microsomal calcium efflux, their inhibitory action primarily involves calcium influx. Finally, other cationic channels have been found in hepatic ER, and evidence is presented of their interaction with the Ca²⁺ ATPase pump.

TRPV4 inhibition attenuates stretch-induced inflammatory cellular responses and lung barrier dysfunction during mechanical ventilation

Mechanical ventilation is an important tool for supporting critically ill patients but may also exert pathological forces on lung cells leading to Ventilator-Induced Lung Injury (VILI). We hypothesised that inhibition of the force-sensitive transient receptor potential vanilloid (TRPV4) ion channel may attenuate the negative effects of mechanical ventilation. Mechanical stretch increased intracellular Ca²⁺ influx and induced release of pro-inflammatory cytokines in lung epithelial cells that was partially blocked by about 30% with the selective TRPV4 inhibitor GSK2193874, but nearly completely blocked with the pan-calcium channel blocker ruthenium red, suggesting the involvement of more than one calcium channel in the response to mechanical stress. Mechanical stretch also induced the release of pro-inflammatory cytokines from M1 macrophages, but in contrast this was entirely dependent upon TRPV4. In a murine ventilation model, TRPV4 inhibition attenuated both pulmonary barrier permeability increase and pro-inflammatory cytokines release due to high tidal volume ventilation. Taken together, these data suggest TRPV4 inhibitors may have utility as a prophylactic pharmacological treatment to improve the negative pathological stretch-response of lung cells during ventilation and potentially support patients receiving mechanical ventilation.

Prevention of ventilator-induced lung edema by inhalation of nanoparticles releasing ruthenium red

The acute respiratory distress syndrome (ARDS), a devastating lung disease that has no cure, is exacerbated by life-supportive mechanical ventilation that worsens lung edema and inflammation through the syndrome of ventilator-induced lung injury. Recently, the membrane ion channel transient receptor potential vanilloid 4 (TRPV4) on alveolar macrophages was shown to mediate murine lung vascular permeability induced by high-pressure mechanical ventilation. The objective of this study was to determine whether inhalation of nanoparticles (NPs) containing the TRPV4 inhibitor ruthenium red (RR) prevents ventilator-induced lung edema in mice. Poly-lactic-co-glycolic acid NPs containing RR were evaluated in vitro for their ability to block TRPV4-mediated calcium signaling in alveolar macrophages and capillary endothelial cells. Lungs from adult C57BL6 mice treated with nebulized NPs were then used in ex vivo ventilation perfusion experiments to assess the ability of the NPs to prevent high-pressure mechanical ventilation-induced lung edema. Poly-lactic-co-glycolic acid NPs (300 nm) released RR for 150 hours in vitro, and blocked TRPV4-mediated calcium signaling in cells up to 7 days after phagocytosis. Inhaled NPs deposited in alveoli of spontaneously breathing mice were rapidly phagocytosed by alveolar macrophages, and blocked increased vascular permeability from high-pressure mechanical ventilation for 72 hours in ex vivo ventilation perfusion experiments. These data offer proof of principle that inhalation of NPs containing a TRPV4 inhibitor prevents ventilator damage for several days, and imply that this novel drug delivery strategy could be used to target alveolar macrophages in patients at risk of ventilator-induced lung injury before initiating mechanical ventilation.

3,4-Methylenedioxymethamphetamine induces a hyperthermic and hypermetabolic crisis in pigs with and without a genetic disposition for malignant hyperthermia

BACKGROUND

Clinical symptoms of acute 3,4-methylenedioxymethamphetamine (MDMA) intoxication and malignant hyperthermia have many similarities. At present, however, there is contradictory evidence concerning the malignant hyperthermia trigger potency of MDMA.

OBJECTIVE

This study was designed to investigate whether MDMA has malignant hyperthermia trigger potential and leads to malignant hyperthermia in pigs with or without a genetic predisposition to the condition. In addition, the therapeutic effectiveness of a new dantrolene sodium suspension was examined.

DESIGN

Experimental study, using an animal model of Piétrain pigs.

SETTINGS

Institute for Research in Operative Medicine, University of Witten/Herdecke, Hospital Cologne Merheim, Cologne, Germany, October 2006 to February 2007. Trigger-free anaesthesia was performed on seven malignant hyperthermia-susceptible and six malignant hyperthermia-normal Piétrain pigs, and cumulative doses of MDMA were administered to each animal.

INTERVENTIONS

After achieving predefined malignant hyperthermia criteria, standardised therapy was initiated; dantrolene sodium suspension (5 mg kg(-1)) was administered and the injection was repeated after 24 min.

MAIN OUTCOME MEASURES

The malignant hyperthermia trigger potency of MDMA was analysed by monitoring pH, PaCO2 and temperature. In addition, concentrations of thyroid hormone, mitochondrial uncoupling protein 3, noradrenaline and free fatty acids during administration of MDMA and dantrolene sodium suspension were analysed.

RESULTS

MDMA administration led to fulminant hypermetabolic and hyperthermic responses in malignant hyperthermia-susceptible and malignant hyperthermia-normal pigs, with significant decreases in pH (susceptible: pH 7.21 ± 0.11, normal: pH 7.21 ± 0.07), severe hypercapnia (susceptible: paCO2 10.3 ± 3.5 kPa, normal: paCO2 9.8 ± 1.7 kPa), and hyperthermia (susceptible: 40.6 ± 2.0°C, normal: 40.1 ± 0.4°C). There were no significant differences in changes in clinical and laboratory variables between groups. The dantrolene therapy regimen was effective in treating the MDMA-induced metabolic crises.

CONCLUSION

MDMA is not a classic trigger for the development of malignant hyperthermia reactions in pigs. MDMA intoxication leads to severe, long-lasting hyperthermia and hypermetabolism in both malignant hyperthermia-susceptible and hyperthermia-normal pigs, with life-threatening malignant hyperthermia-like symptoms which are responsive to supportive treatment and dantrolene sodium suspension.

Thermal tolerance of contractile function in oxidative skeletal muscle: no protection by antioxidants and reduced tolerance with eicosanoid enzyme inhibition

Mechanisms for the loss of muscle contractile function in hyperthermia are poorly understood. This study identified the critical temperature, resulting in a loss of contractile function in isolated diaphragm (thermal tolerance), and then tested the hypotheses 1) that increased reactive oxygen species (ROS) production contributes to the loss of contractile function at this temperature, and 2) eicosanoid metabolism plays an important role in preservation of contractile function in hyperthermia. Contractile function and passive force were measured in rat diaphragm bundles during and after 30 min of exposure to 40, 41, 42 or 43 degrees C. Between 40 and 42 degrees C, there were no effects of hyperthermia, but at 43 degrees C, a significant loss of active force and an increase in passive force were observed. Inhibition of ROS with the antioxidants, Tiron or Trolox, did not inhibit the loss of contractile force at 43 degrees C. Furthermore, treatment with dithiothreitol, a thiol (-SH) reducing agent, did not reverse the effects of hyperthermia. A variety of global lipoxygenase (LOX) inhibitors further depressed force during 43 degrees C and caused a significant loss of thermal tolerance at 42 degrees C. Cyclooxygenase (COX) inhibitors also caused a loss of thermal tolerance at 42 degrees C. Blockage of phospholipase with phospholipase A(2) inhibitors, bromoenol lactone or arachidonyltrifluoromethyl ketone failed to significantly prevent the loss of force at 43 degrees C. Overall, these data suggest that ROS do not play an apparent role in the loss of contractile function during severe hyperthermia in diaphragm. However, functional LOX and COX enzyme activities appear to be necessary for maintaining normal force production in hyperthermia.

Mechanism of modulation of the plasma membrane Ca(2+)-ATPase by arachidonic acid

The intracellular level of long chain fatty acids controls the Ca(2+) concentration in the cytoplasm. The molecular mechanisms underlying this Ca(2+) mobilization are not fully understood. We show here that the addition of low micromolar concentrations of fatty acids directly to the purified plasma membrane Ca(2+)-ATPase enhance ATP hydrolysis, while higher concentration decrease activity, exerting a dual effect on the enzyme. The effect of arachidonic acid is similar in the presence or absence of calmodulin, acidic phospholipids or ATP at the regulatory site, thereby precluding these sites as probable acid binding sites. At low arachidonic acid concentrations, neither the affinity for calcium nor the phosphoenzyme levels are significantly modified, while at higher concentrations both are decreased. The action of arachidonic acid is isoenzyme specific. The increase on ATP hydrolysis, however, is uncoupled from calcium transport, because arachidonic acid increases the permeability of erythrocyte membranes to calcium. Oleic acid has no effect on membrane permeability while linoleic acid shows an effect similar to that of arachidonic acid. Such effects might contribute to the entry of extracellular Ca(2+) following to fatty acid release.

Brown adipose tissue Ca2+-ATPase: uncoupled ATP hydrolysis and thermogenic activity

In this report a sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) was identified in rats brown adipose tissue. Electrophoretic analysis of brown fat microssomal protein yields a 110-kDa band that is reactive to SERCA 1 antibody but is not reactive to SERCA 2 antibodies. Nevertheless, the kinetics properties of the brown fat SERCA differ from the skeletal muscle SERCA 1 inasmuch they manifest a different Ca2+ affinity and a much higher degree of ATPase/Ca2+ uncoupling. A SERCA enzyme is not found in white fat. Fatty acids promoted Ca2+ leakage from brown fat vesicles. The heat released during ATP hydrolysis was -24.7 kcal/mol when a Ca2+ gradient was formed across the vesicles membrane and -14.4 kcal/mol in the absence of a gradient. The data reported suggest that in addition to storing Ca2+ inside the endoplasmic reticulum, the Ca2+-ATPase may represent a source of heat production contributing to the thermogenic function of brown adipose tissue.

Effect of indomethacin on force responses and sarcoplasmic reticulum function in skinned skeletal muscle fibers and cytosolic [Ca2+] in myotubes

In this study, the effects of phospholipase A2 (PLA2) inhibitors on excitation-contraction coupling (ECC) and sarcoplasmic reticulum (SR) function were examined in skinned extensor digitorum longus (EDL) muscle fibers of the rat. The nonspecific PLA2 inhibitor indomethacin (200 microM) significantly increased the peak (approximately 2-fold, P = 0.02) and the width (approximately 6-fold, P = 0.008) of depolarization-induced force responses (DIFRs) elicited in the fibers (n = 4). Exposure of the skinned EDL fibers to indomethacin (200 microM) (n = 7) and another PLA2 inhibitor quinacrine (200 microM) (n = 5) resulted in the return of large DIFRs after use-dependent rundown. However, aristolochic acid (100 microM), an inhibitor of secretory PLA2, failed to return DIFRs after rundown. Indomethacin did not protect against the loss of DIFRs induced by exposure to elevated myofibrillar [Ca2+]. Indomethacin (200 microM) produced a small but significant increase in the Ca2+ sensitivity of the contractile apparatus of skinned EDL fibers and the maximum force production. Indomethacin (200 microM) also had significant effects on SR function, increasing SR Ca2+ loading in the skinned fibers (117.2 +/- 3.0% of controls, P = 0.0008, n = 8) and inducing intracellular Ca2+ release in isolated intact flexor digitorum brevis (FDB) fibers (n = 7) and C2C12 myotubes (n = 6). These data suggest that intracellular PLA2 may be an important modulator of ECC in skeletal muscle.

Arachidonic acid inhibition of muscarinic receptor-mediated nitric oxide production occurs at the level of calcium mobilization in Chinese hamster ovary cells

Strong evidence supports that nitric oxide (NO) alters cell signaling pathways involving arachidonic acid (AA). Little is known, however, about the reciprocal modulation of nitrergic pathways by AA. The effects of exogenous AA on signal transduction of M1 muscarinic acetylcholine receptors were investigated in a model system of stably transfected Chinese hamster ovary cells. AA concentration-dependently inhibited the effects of carbachol in producing NO (IC50 = 191 microM) but did not alter inositol phosphate production or M1 receptor binding. AA inhibited both carbachol-induced transient and sustained increase in intracellular calcium concentration ([Ca2+]i; IC50 = 11 and 12 microM, respectively). Furthermore, AA-induced increase in [Ca2+]i cross-desensitizes with thapsigargin, but AA does not inhibit Ca(2+)-ATPase activity. These data support the concept that AA concentration-dependently inhibits receptor-mediated NO production at the level of calcium mobilization.

Activation of TRPV4 channels (hVRL-2/mTRP12) by phorbol derivatives

We have studied activation by phorbol derivatives of TRPV4 channels, the human VRL-2, and murine TRP12 channels, which are highly homologous to the human VR-OAC, and the human and murine OTRPC4 channel. 4alpha-Phorbol 12,13-didecanoate (4alpha-PDD) induced an increase in intracellular Ca(2+) concentration, [Ca(2+)](i), in 1321N1 cells stably transfected with human VRL-2 (hVRL-2.1321N1) or HEK-293 cells transiently transfected with murine TRP12, but not in nontransfected or mock-transfected cells. Concomitantly with the increase in [Ca(2+)](i), 4alpha-PDD activated an outwardly rectifying cation channel with an Eisenman IV permeation sequence for monovalent cations that is Ca(2+)-permeable with P(Ca)/P(Na) = 5.8. Phorbol 12-myristate 13-acetate also induced an increase in [Ca(2+)](i) but was approximately 50 times less effective than 4alpha-PDD. EC(50) for Ca(2+) increase and current activation was nearly identical (pEC(50) approximately 6.7). Similar effects were observed in freshly isolated mouse aorta endothelial cells which express TRP12 endogenously. By using 4alpha-PDD as a tool to stimulate TRP12, we showed that activation of this channel is modulated by [Ca(2+)](i); an increase in [Ca(2+)](i) inhibits the channel with an IC(50) of 406 nm. Ruthenium Red at a concentration of 1 microm completely blocks inward currents at -80 mV but has a smaller effect on outward currents likely indicating a voltage dependent channel block. We concluded that the phorbol derivatives activate TRPV4 (VR-OAC, VRL-2, OTRPC4, TRP12) independently from protein kinase C, in a manner consistent with direct agonist gating of the channel.

Pharmacological modulation of monovalent cation currents through the epithelial Ca2+ channel ECaC1

1. The recent identification of the epithelial Ca(2+) channel, ECaC1, represents a major step forward in our knowledge of renal Ca(2+) handling. ECaC1 constitutes the rate-limiting apical Ca(2+) entry mechanism of active, transcellular Ca(2+) reabsorption. This unique highly selective Ca(2+) channel shares a low but significant homology with transient receptor potential (TRP) channels and vanilloid receptors (VR). 2. We have studied the pharmacological modulation of currents through ECaC1 heterologously expressed in HEK 293 cells. Monovalent cation currents were measured by use of the whole cell patch clamp technique in cells dialysed with 10 mM BAPTA or 10 mM EGTA to prevent the fast Ca(2+) dependent inactivation of ECaC1. 3. Several modulators were tested, including inorganic cations, putative store-operated Ca(2+) entry (SOC) blockers, the vanilloid receptor (VR-1) blocker capsazepine, protein tyrosine kinase blockers, calmodulin antagonists and ruthenium red. 4. Ruthenium red and econazole appeared to be the most effective inhibitors of currents through ECaC1, with IC(50) values of 111 nM and 1.3 microM, respectively, whereas the selective SOC inhibitor, SKF96365, was nearly ineffective. 5. The divalent cation current block profile for ECaC1 is Pb(2+)=Cu(2+) >Zn(2+) >Co(2+) >Fe(2+) with IC(50) values between 1 and approximately 10 microM. 6. In conclusion, ECaC activity is effectively inhibited by various compounds including ruthenium red, antimycotic drugs and divalent cations, which might be useful tools for pharmacological manipulation and several disorders related to Ca(2+) homeostasis could benefit from such developments.

Effect of polymer molecular weight and addition of calcium stearate on response of MG63 osteoblast-like cells to UHMWPE particles

Periprosthetic osteolysis and implant loosening is associated with the presence of ultrahigh molecular weight polyethylene (UHMWPE) wear debris particles. Osteoblast phenotypic expression in vitro is affected by UHMWPE particles, suggesting that bone formation may also be affected by wear debris. Here we tested the hypothesis that the response of osteoblasts to UHMWPE can be modified by changes in UHMWPE particle chemistry. We used four different commercially available preparations of GUR UHMWPE particles to determine if chemical composition (+/- Ca-stearate) or polymer molecular weight (3.1-4.2 million or 5.4-6.5 million g/mol) modulates osteoblast response. Particles were characterized by size distribution, morphology, and number of particles added to the culture medium. They had an average equivalent circle diameter ranging from 0.46-1.26 microm. MG63 cell response was assessed by measuring cell number, cellular and cell layer alkaline phosphatase, and prostaglandin E2 (PGE2) production. There were dose-dependent effects of the particles on cell response. Cell number and PGE, production were increased, while alkaline phosphatase specific activity was decreased. In addition, there was a marked difference between cultures treated with particles containing Ca-stearate and as a function of polymer molecular weight. Particles of higher molecular weight caused a greater stimulation of proliferation and inhibition of alkaline phosphatase than particles of lower molecular weight. The presence of Castearate exerted a more pronounced depression of osteoblast phenotype as well as a significantly greater increase in PGE2 release by the cells. The present study shows that chemical composition and polymer molecular weight of UHMWPE are capable of modulating osteoblast response to particles. The results suggest that osteoblast differentiation is inhibited by UHMWPE particles, whereas cell proliferation and PGE2 production are stimulated. This may have direct effects on osteoblasts and bone formation, but also paracrine effects on cells of the monocytic lineage inducing bone resorption and promoting inflammation which may lead to aseptic loosening. The present results suggest that the cellular events in aseptic loosening may be modulated or even accelerated by changes in the composition of the UHMWPE used to fabricate implants.

Energy interconversion by the sarcoplasmic reticulum Ca2+-ATPase: ATP hydrolysis, Ca2+ transport, ATP synthesis and heat production

The sarcoplasmic reticulum of skeletal muscle retains a membrane bound Ca2+-ATPase which is able to interconvert different forms of energy. A part of the chemical energy released during ATP hydrolysis is converted into heat and in the bibliography it is assumed that the amount of heat produced during the hydrolysis of an ATP molecule is always the same, as if the energy released during ATP cleavage were divided in two non-interchangeable parts: one would be converted into heat, and the other used for Ca2+ transport. Data obtained in our laboratory during the past three years indicate that the amount of heat released during the hydrolysis of ATP may vary between 7 and 32 Kcal/mol depending on whether or not a transmembrane Ca2+ gradient is formed across the sarcoplasmic reticulum membrane. Drugs such as heparin and dimethyl sulfoxide are able to modify the fraction of the chemical energy released during ATP hydrolysis which is used for Ca2+ transport and the fraction which is dissipated in the surrounding medium as heat.

Ca(2+) release and heat production by the endoplasmic reticulum Ca(2+)-ATPase of blood platelets. Effect of the platelet activating factor

Different sarco/endoplasmic reticulum Ca(2+)-ATPases isoforms are found in blood platelets and in skeletal muscle. The amount of heat produced during ATP hydrolysis by vesicles derived from the endoplasmic reticulum of blood platelets was the same in the absence and presence of a transmembrane Ca(2+) gradient. Addition of platelets activating factor (PAF) to the medium promoted both a Ca(2+) efflux that was arrested by thapsigargin and an increase of the yield of heat produced during ATP hydrolysis. The calorimetric enthalpy of ATP hydrolysis (DeltaH(cal)) measured during Ca(2+) transport varied between -10 and -12 kcal/mol without PAF and between -20 and -24 kcal/mol with 4 microM PAF. Different from platelets, in skeletal muscle vesicles a thapsigargin-sensitive Ca(2+) efflux and a high heat production during ATP hydrolysis were measured without PAF and the DeltaH(cal) varied between -10 and -12 kcal/mol in the absence of Ca(2+) and between -22 up to -32 kcal/mol after formation of a transmembrane Ca(2+) gradient. PAF did not enhance the rate of thapsigargin-sensitive Ca(2+) efflux nor increase the yield of heat produced during ATP hydrolysis. These findings indicate that the platelets of Ca(2+)-ATPase isoforms are only able to convert osmotic energy into heat in the presence of PAF.

Fatty acid-mediated calcium sequestration within intracellular calcium pools

Intracellular Ca2+ pools play an essential role in generating Ca2+ signals. The heterogeneity of intracellular Ca2+ pools reflects the complex and dynamic character of the endoplasmic reticulum within which they reside. Translocation of Ca2+ between distinct subcompartments of the endoplasmic reticulum is mediated by a sensitive and specific GTP-activated process involving formation of reversible communicating junctions (Rys-Sikora, K. E., Ghosh, T. K., and Gill, D. L. (1994) J. Biol. Chem. 269, 31607-31613). In the presence of palmitate at 10 microM or above, this GTP-activated mechanism mediates substantial Ca2+ accumulation within a specific Ca2+-pumping pool. The fatty acid- and GTP-dependent accumulation of Ca2+ was highly chain length-specific; pentadecanoate (C15) and palmitate (C16) were equally effective, whereas fatty acids of shorter or longer chain length were either marginally effective or devoid of effect. Fatty acids with one or more unsaturated carbons were without effect, regardless of chain length. Palmitate-induced Ca2+ accumulation was immediately terminated with 2 microM palmitoyl-CoA, a blocker of the GTP-activated Ca2+-translocating mechanism. The anion transport inhibitor 4, 4'-diisothiocyanostilbene-2,2'-disulfonic acid completely prevented both palmitate- and oxalate-mediated GTP-dependent Ca2+ accumulation, with EC50 approximately 30 microM. Ca2+ sequestered in the presence of palmitate and GTP could be immediately and completely released by A23187, whereas the sequestered Ca2+ was remarkably resistant to release induced by inositol 1,4,5-trisphosphate (InsP3). In contrast, oxalate-sequestered Ca2+ within the same pool could be effectively released by either ionophore or InsP3. The results indicate that fatty acids are specifically transported into the lumen of a subset of Ca2+ pools, wherein they mediate substantial sequestration of Ca2+ in a distinct membrane-associated substate that is not readily releasable by opened InsP3-sensitive Ca2+ channels.

Modulation of ACh receptor currents by arachidonic acid

The present study investigated the effects of arachidonic acid on Torpedo (alpha beta gamma delta) and neuronal nicotinic acetylcholine (ACh) receptors (chick alpha7; rat alpha7, alpha3 beta2, alpha3 beta4, alpha4 beta2, and alpha4 beta4). Arachidonic acid (10 microM) depressed currents through normal Torpedo ACh receptors during treatment and afterward, persistently (>/=30 min) potentiated the currents. The potentiation was blocked by the selective protein kinase C (PKC) inhibitor, GF109203X or PKC inhibitor peptide (PKCI). The depression was not inhibited by any protein kinase inhibitor examined here, but greater in Ca2+-free extracellular solution. Arachidonic acid also potentiated currents through mutant Torpedo ACh receptors lacking PKC phosphorylation sites at Ser333 on the alpha subunit and Ser377 on the delta subunit without depression, but otherwise, it depressed currents through mutant receptors replacing of each Ser by negatively charged amino acid residue, possibly that mimics PKC phosphorylation of the receptors. These results suggest that the depression was due to the direct blocking effect on Ca2+-modulatory sites, which was accelerated under conditions of the receptors phosphorylated by PKC, and that the potentiation was caused by PKC activation, independently of PKC phosphorylation of the receptors. Arachidonic acid reduced currents through chick alpha7 receptors by a mechanism independent of protein kinase activation. In contrast, arachidonic acid potentiated currents through rat alpha7, alpha3 beta2, alpha4 beta2, and alpha4 beta4 receptors, perhaps by the same mechanism as the potentiation observed in Torpedo ACh receptors, although it had no effect on rat alpha3 beta4 receptors. The results of the present study thus demonstrate that arachidonic acid exerts diverse actions on nicotinic ACh receptors by different mechanisms.

Uncoupling of Ca2+ transport ATPase in muscle and blood platelets by diacylglycerol analogues and cyclosporin A antagonism

The possibility that diacylglycerol analogues might have a wider spectrum of intracellular targets than the well-known protein kinase C was investigated with vesicles containing the Ca2+-ATPase derived from the dense tubular system in platelets and from the sarcoplasmic reticulum of skeletal muscle. The diacylglycerol analogues PMA and 1-oleoyl-2-acetyl-rac-glycerol (OAG) inhibited Ca2+ accumulation by these vesicles, an effect that was antagonized by cyclosporin A. The inhibitory activity of PMA and OAG resulted from the uncoupling of the Ca2+-ATPase, characterized by a pronounced inhibition of Ca2+ uptake accompanied by a discrete decrease in ATPase activity and by the inhibition of the enzyme's phosphorylation by Pi, leading to both a decrease in ATP synthesis and an enhancement of Ca2+ efflux. The inhibition of Ca2+ uptake by PMA was found to decrease as the Ca2+ concentration in the medium was raised from 0.1 to 10.0 microM. This was observed with muscle, but not with platelet vesicles. In contrast, the ability of cyclosporin A to antagonize the inhibition of Ca2+ uptake by PMA also increased when the Ca2+ concentration in the medium was raised from 0.1 to 10.0 microM, but this was observed with both muscle and platelet vesicles. The fact that phospholipase C activity and products from the inositol metabolism have been described as localized in regions of the sarcoplasmic reticulum where Ca2+-ATPase and Ca2+ channels are found suggests a possible physiological role for these products in the regulation of cytosolic Ca2+ levels.

Regulation of intracellular calcium release channel function by arachidonic acid and leukotriene B4

Arachidonic acid has been shown to affect the intracellular calcium concentration in many cell types (1-5), but the target of this regulation was unclear. Here we show that two types of intracellular calcium release channel, the inositol 1,4,5-trisphosphate-gated channel (IP3R) and the ryanodine receptor (RyR) are modulated in an opposing manner by arachidonic acid and its product leukotriene B4 (LTB4). The IP3R was inhibited by arachidonic acid (Ki = 27 nM), whereas the RyR was unaffected by this compound. In contrast, 100 nM LTB4 fully activated the RyR but did not influence the IP3R. The concerted action of arachidonic acid and LTB4 could provide specific mobilization of stored calcium by terminating IP3-induced release and activating the RyR/calcium release channel by its newly identified agonist.

Inhibition of mitochondrial calcium uptake slows down relaxation in mitochondria-rich skeletal muscles

Isolated fibres from various muscles were skinned mechanically in oil. From a Ca2+-loaded micropipette, local applications of Ca2+ were made. These produced a limited contraction which relaxed spontaneously. The time-course of sarcomere shortening and re-lengthening was recorded by microcinephotography. Application of Ruthenium Red, a potent and specific inhibitor of Ca2+ uptake by mitochondria, did not affect the contraction-relaxation cycles of typical glycolytic white fibres (frog sartorius, pigeon breast). By contrast, Ruthenium Red greatly slowed down the relaxation rate in mitochondria-rich fibres (rat soleus and rabbit masseter). In these fibres, Ca2+ uptake by mitochondria seems to play an active role in promoting relaxation.

Sarco/endoplasmic reticulum Ca2+-ATPase isoforms: diverse responses to acidosis

The effects of acidic pH on the kinetics of Ca2+-ATPase isoforms from intracellular membranes of skeletal muscle, cardiac muscle, cerebellum and blood platelets were studied. At neutral pH, all four Ca2+-ATPase isoforms exhibited similar Ca2+-concentration requirements for half-maximal rates of Ca2+ uptake and ATP hydrolysis. A decrease in the pH from 7.0 to 6.0 promoted a decrease in both the apparent affinity for Ca2+ [increasing half-maximal activation (K0.5)] and the maximal velocity (Vmax) of Ca2+ uptake. With skeletal muscle vesicles these effect were 5 to 10 times smaller than those observed with all the other isoforms. Acidification of the medium from pH 7.0 to 6.5 caused the release of Ca2+ from loaded vesicles and a decrease in the amount of Ca2+ retained by the vesicles at the steady state. With the vesicles derived from skeletal muscle these effects were smaller than for vesicles derived from other tissues. The rate of passive Ca2+ efflux from skeletal and cardiac muscle vesicles, loaded with Ca2+ and diluted in a medium containing none of the ligands of Ca2+-ATPase, was the same at pH 7.0 and 6.0. In contrast, the rate of Ca2+ efflux from cerebellar and platelet vesicles increased 2-fold after acidification of the medium. The effects of DMSO, Mg2+ with Pi and arsenate on the rate of Ca2+ efflux varied among the different preparations tested. The differences became more pronounced when the pH of the medium was decreased from 7.0 to 6.0. It is proposed that the kinetic differences among the Ca2+-ATPase isoforms may reflect different adaptations to cellular acidosis, such as that which occurs during ischaemia.

Substrate effects on sarcolemmal permeability in the normoxic and hypoxic perfused rat heart

OBJECTIVES

Based on the hypothesis that provision of glucose is good and fatty acids are bad for the ischaemic myocardium, the aims of this study were to determine i) the effects of different substrates on sarcolemmal permeability during normoxia, low-flow hypoxia (HLF) and reperfusion, ii) whether increased membrane permeability is associated with ultrastructural damage and increased influx of Ca2+ into cells and iii) whether changes in membrane permeability correlate with myocardial function and high energy phosphate metabolism.

METHODS

The isolated rat heart subjected to HLF was used as model of global ischaemia, and sarcolemmal permeability assessed by release of LDH from and influx of lanthanum and Ca2+ into myocardial tissue. Myocyte structural injury was also evaluated quantitatively, and mechanical activity was monitored throughout the experimental protocol.

RESULTS

Regardless of the substrate used, HLF caused a 80-90% and 20-40% reduction in myocardial oxygen uptake and coronary flow rate, respectively. Palmitate (0.5 mM conjugated to 0.1 mM albumin) or substrate-free perfusion caused ultrastructural damage and loss of normal sarcolemmal integrity during both normoxia and HLF. Although reperfusion reversed injury in some cells, in general, myocytes exhibited myofibrillar contracture, while membrane integrity recovered to some extent, as indicated by reduced lanthanum influx. Intracellular Ca2+ increased significantly upon reperfusion. Mechanical function as well as tissue high energy phosphates were significantly depressed during both HLF and reperfusion. Glucose, on the other hand, protected against ischaemia-induced structural damage and loss of sarcolemmal integrity. Reperfusion in these experiments resulted in almost complete recovery of normal morphology, ultrastructure and sarcolemmal integrity, while intracellular Ca2+ remained unchanged. Mechanical function and tissue high energy phosphates were significantly higher in glucose-perfused hearts than in palmitate-perfused or substrate-free hearts. Glucose was also able to attenuate the harmful effects of palmitate on myocardial ultrastructure, membrane integrity, mechanical function, energy metabolism and prevented Ca2+ overloading during reperfusion.

CONCLUSION

The results provide new evidence for the protective role of glucose during myocardial ischaemia and reperfusion. Although the exact mechanism of the beneficial actions of glucose remains to be established, the results suggest that glycolytic flux and thus glycolytically derived ATP protect against ischaemic damage via preservation of membrane integrity.

Ethanol has different effects on Ca(2+)-transport ATPases of muscle, brain and blood platelets

The effects of ethanol on different sarco/endoplasmic reticulum Ca(2+)-transport ATPases (SERCAs) were studied. In sarcoplasmic reticulum vesicles, ethanol concentrations varying from 5 to 20% promoted a progressive inhibition of Ca2+ uptake, enhancement of Ca2+ efflux, activation of the ATPase activity, increase of the enzyme phosphorylation by ATP and inhibition of enzyme phosphorylation by P1. The effects of ethanol on Ca2+ uptake and Ca2+ efflux were antagonized by Mg2+, P(i) and spermine. The increased efflux promoted by ethanol was antagonized by Ca2+ and thapsigargin. In brain and platelet vesicles a biphasic effect of ethanol was observed, so that activation occurred at low concentrations (5-10%) and inhibition at higher concentrations. The activation was not observed with the use of n-propanol and n-butanol. Different from the situation in sarcoplasmic reticulum, the decrease of the Ca2+ uptake in brain and platelet vesicles was associated with an inhibition of the ATPase activity. Mg2+ and P(i) antagonized the enhancement of Ca2+ efflux and the inhibition of Ca2+ uptake promoted by ethanol. However, thapsigargin and Ca2+ did not arrest the Ca2+ efflux promoted by ethanol in brain and platelet preparations. These results suggest that, in sarcoplasmic reticulum vesicles, ethanol uncouples the pump, promoting its activity as a Ca2+ channel. The SERCA isoform found in skeletal muscle has different properties from the isoforms found in brain and blood platelets.

Complementary effects of paeoniflorin and glycyrrhizin on intracellular Ca2+ mobilization in the nerve-stimulated skeletal muscle of mice

Effects of paeoniflorin (PF) and glycyrrhizin (GLR), contained in paeony and licorice roots, respectively, on contractile and non-contractile Ca2+ mobilization were examined by measuring the Ca(2+)-aequorin luminescence (Ca2+ transients) of the nerve-stimulated skeletal muscle of mice in the presence of neostigmine (0.3 microM). PF (0.1-1 mM) prolonged the duration of non-contractile Ca2+ transients, which may induce the desensitization of nicotinic acetylcholine receptor, but did not affect contractile Ca2+ transients. GLR (0.3-1 mM) depressed contractile Ca2+ transients without affecting non-contractile transients. These results suggest that PF and GLR may have complementary effects on intracellular Ca2+ mobilization to block the neuromuscular transmission.

Ligand-gated channel of the sarcoplasmic reticulum Ca2+ transport ATPase

In resting muscle, cytoplasmic Ca2+ concentration is maintained at a low level by active Ca2+ transport mediated by the Ca2+ ATPase from sarcoplasmic reticulum. The region of the protein that contains the catalytic site faces the cytoplasmic side of the membrane, while the transmembrane helices form a channel-like structure that allows Ca2+ translocation across the membrane. When the coupling between the catalytic and transport domains is lost, the ATPase mediates Ca2+ efflux as a Ca2+ channel. The Ca2+ efflux through the ATPase channel is activated by different hydrophobic drugs and is arrested by ligands and substrates of the ATPase at physiological pH. At acid pH, the inhibitory effect of cations is no longer observed. It is concluded that the Ca2+ efflux through the ATPase may be sufficiently fast to support physiological Ca2+ oscillations in skeletal muscle, that occur mainly in conditions of intracellular acidosis.

The Ca(2+)-ATPase isoforms of platelets are located in distinct functional Ca2+ pools and are uncoupled by a mechanism different from that of skeletal muscle Ca(2+)-ATPase

Vesicles derived from the dense tubular system of platelets possess a Ca(2+)-ATPase that can use either ATP or acetyl phosphate as a substrate. In the presence of phosphate as a precipitating anion, the maximum amount of Ca2+ accumulated by the vesicles with the use of acetyl phosphate was only one-third of that accumulated with the use of ATP. Vesicles derived from the sarcoplasmic reticulum of skeletal muscle accumulated equal amounts of Ca2+ regardless of the substrate used. When acetyl phosphate was used in platelet vesicles, the transport of Ca2+ was inhibited by Na+, Li+, and K+; in sarcoplasmic reticulum vesicles, only Na+ caused inhibition. When ATP was used as substrate, the different monovalent cation had no effect on either sarcoplasmic reticulum or platelet vesicles. The catalytic cycle of the Ca(2+)-ATPase is reversed when a Ca2+ gradient is formed across the vesicle membrane. The stoichiometry between active Ca2+ efflux and ATP synthesis was one in platelet vesicles and two in sarcoplasmic reticulum vesicles. The coupling between ATP synthesis and Ca2+ efflux in sarcoplasmic reticulum vesicles was abolished by arsenate regardless of whether the vesicles were loaded with Ca2+ using acetyl phosphate or ATP. In platelets, uncoupling was observed only when the vesicles were loaded using acetyl phosphate. In both sarcoplasmic reticulum and platelet vesicles, the effect of arsenate was antagonized by thapsigargin (2 microM), micromolar Ca2+ concentrations, P(i) (5-20 mM), and MgATP (10-100 microM). Trifluoperazine also uncoupled the platelet Ca2+ pump but, different from arsenate, this drug was effective in vesicles that were loaded using either ATP or acetyl phosphate. Trifluoperazine enhanced Ca2+ efflux from both sarcoplasmic reticulum and platelet vesicles; thapsigargin, Ca2+, Mg2+, or K+ antagonized this effect in sarcoplasmic reticulum but not in platelet vesicles. The data indicate that the Ca(2+)-transport isoforms found in sarcoplasmic reticulum and in platelets have different kinetic properties.

Structure and function of ryanodine receptors

Membrane depolarization, neurotransmitters, and hormones evoke a release of Ca2+ from intracellular Ca(2+)-storing organelles like the endoplasmic reticulum and, in muscle, the sarcoplasmic reticulum (SR). In turn, the released Ca2+ serves to trigger a variety of cellular responses. The presence of Ca2+ pumps to replenish intracellular stores was described more than 20 years ago. The presence of Ca2+ channels, like the ryanodine receptor, which suddenly release the organelle-stored Ca2+, is a more recent finding. This review describes the progress made in the last five years on the structure, function, and regulation of the ryanodine receptor. Numerous reports have described the response of ryanodine receptors to cellular ions and metabolites, kinases and other proteins, and pharmacological agents. In many cases, comparative measurements have been made using Ca2+ fluxes in SR vesicles, single-channel recordings in planar bilayers, and radioligand binding assays using [3H]ryanodine. These techniques have helped to relate the activity of single ryanodine receptors to global changes in the SR Ca2+ permeability. Molecular information on functional domains within the primary structure of the ryanodine receptor is also available. There are at least three ryanodine receptor isoforms in various tissues. Some cells, such as amphibian muscle cells, express more than a single isoform. The diversity of ligands known to modulate gating and the diversity of tissues known to express the protein suggest that the ryanodine receptor has the potential to participate in many types of cell stimulus-Ca(2+)-release coupling mechanisms.