Imaging of changes in sarcoplasmic reticulum [Ca(2+)] using Oregon Green BAPTA 5N and confocal laser scanning microscopy

We describe experiments in which the low affinity indicator Oregon Green BAPTA 5N was used to record the spatially resolved changes in [Ca(2+)] from intracellular stores in rat gastric myocytes. Cells were loaded with the membrane permeant form of the indicator and imaged using a confocal scanning laser microscope. In doubly stained cells the Oregon Green signal colocalized with BIODIPY 558/568 Brefeldin A, a label for the endo/sarcoplasmic reticulum (SR) and Golgi apparatus. Oregon Green BAPTA 5N was calibrated in gastric myocytes, giving an in situ K(d) of 90 microM. The resting free [Ca(2+)] within the SR averaged 65 microM. A reversible decrease in Oregon Green fluorescence was observed on bath application of Inositol triphosphate (IP(3)) (10 microM) to permeabilized cells. Similar changes were also observed when cyclopiazonic acid (5 microM) was applied to intact myocytes, again with recovery of store [Ca(2+)] following drug washout. Identical patterns of Ca(2+) depletion were seen when caffeine (1 microM) and carbachol (10 microM) were applied sequentially to the same cells, suggesting that activation of ryanodine and IP(3)-sensitive channels can result in the release of Ca(2+) from the same regions of the SR.

Bax and Bak promote apoptosis by modulating endoplasmic reticular and mitochondrial Ca2+ stores

Alterations in intracellular Ca(2+) homeostasis and cytochrome c release from mitochondria have been implicated in the regulation of apoptosis, but the relationship between these events remains unclear. Here we report that enforced expression of either Bax or Bak via adenoviral gene delivery results in the accumulation of the proteins in the endoplasmic reticulum (ER) and mitochondria, resulting in early caspase-independent BCL-2-sensitive release of the ER Ca(2+) pool and subsequent Ca(2+) accumulation in mitochondria. The inhibition of ER-to-mitochondrial Ca(2+) transport with a specific inhibitor of mitochondrial Ca(2+) uptake attenuates cytochrome c release and downstream biochemical events associated with apoptosis. Bax and Bak also directly sensitize mitochondria to cytochrome c release induced by immediate emptying of ER Ca(2+) pool. Our results demonstrate that the effects of the "multidomain" proapoptotic BCL-2 family members Bak and Bax involve direct effects on the endoplasmic reticular Ca(2+) pool with subsequent sensitization of mitochondria to calcium-mediated fluxes and cytochrome c release. These effects modulate the kinetics of cytochrome c release and apoptosis.

Activation of poly(ADP-ribose) polymerase contributes to development of doxorubicin-induced heart failure

Activation of the nuclear enzyme poly(ADP-ribose) polymerase (PARP) by oxidant-mediated DNA damage is an important pathway of cell dysfunction and tissue injury in conditions associated with oxidative stress. Increased oxidative stress is a major factor implicated in the cardiotoxicity of doxorubicin (DOX), a widely used antitumor anthracycline antibiotic. Thus, we hypothesized that the activation of PARP may contribute to the DOX-induced cardiotoxicity. Using a dual approach of PARP-1 suppression, by genetic deletion or pharmacological inhibition with the phenanthridinone PARP inhibitor PJ34, we now demonstrate the role of PARP in the development of cardiac dysfunction induced by DOX. PARP-1+/+ and PARP-1-/- mice received a single injection of DOX (25 mg/kg i.p). Five days after DOX administration, left ventricular performance was significantly depressed in PARP-1+/+ mice, but only to a smaller extent in PARP-1-/- ones. Similar experiments were conducted in BALB/c mice treated with PJ34 or vehicle. Treatment with a PJ34 significantly improved cardiac dysfunction and increased the survival of the animals. In addition PJ34 significantly reduced the DOX-induced increase in the serum lactate dehydrogenase and creatine kinase activities but not metalloproteinase activation in the heart. Thus, PARP activation contributes to the cardiotoxicity of DOX. PARP inhibitors may exert protective effects against the development of severe cardiac complications associated with the DOX treatment.

Leukocytes extravasation in acute homocysteinemic rats

Homocysteine may promote atherogenesis and thrombogenesis by enhancing leukocyte-endothelium interactions. We explored this hypothesis in an acute hyperhomocysteinemia rat model, which was created by a continuous venous homocysteine infusion (4 ml/h/kg body weight) with 2.5 and 10 mg/ml D,L-homocysteine upto 90 min. Venous homocysteine levels were monitored periodically and varied 65-276 micromol/l, a range observed frequently in homocysteinemic and homocystinuric patients. We measured hemodynamic parameters in mesentery by intravital microscopy in rats infused with homocysteine (N=5 for each dose) and saline (N=7). Homocysteine infusion for 90 min did not change the mean carotid arterial blood pressure, velocity of red blood cells and rolling leukocyte flux. However at the dose of 10 mg/ml the venular wall shear rate was reduced to 66-69% of the pre-infusion value (P<0.05). The leukocyte rolling velocity decreased to 78-82% (P<0.05). The number of leukocytes adhering to the venular wall increased 2.4-fold (P<0.05), and the leukocyte extravasation increased 4.7-fold (P<0.001). Each of these effects was time-dependent and homocysteine dose-dependent. But none were observed in saline infused rats. In conclusion, while homocysteine infusion did not change hemodynamic parameters, it significantly enhanced dose-dependent leukocyte-endothelium interactions, which may contribute to homocysteine induced endothelial dysfunction.

Endothelial dysfunction in aging animals: the role of poly(ADP-ribose) polymerase activation

Recent work has demonstrated the production of reactive oxygen and nitrogen species in the vasculature of aging animals. Oxidant induced cell injury triggers the activation of nuclear enzyme poly(ADP ribose) polymerase (PARP) leading to endothelial dysfunction in various pathophysiological conditions (reperfusion, shock, diabetes). Here we studied whether the loss of endothelial function in aging rats is dependent upon the PARP pathway within the vasculature. Young (3 months-old) and aging (22 months-old) Wistar rats were treated for 2 months with vehicle or the PARP inhibitor PJ34. In the vehicle-treated aging animals there was a significant loss of endothelial function, as measured by the relaxant responsiveness of vascular rings to acetylcholine. Treatment with PJ34, a potent PARP inhibitor, restored normal endothelial function. There was no impairment of the contractile function and endothelium-independent vasodilatation in aging rats. Furthermore, we found no deterioration in the myocardial contractile function in aging animals. Thus, intraendothelial PARP activation may contribute to endothelial dysfunction associated with aging.

Ca2+ marks: miniature calcium signals in single mitochondria driven by ryanodine receptors

Propagation of cytosolic [Ca(2+)] ([Ca(2+)](c)) signals to the mitochondria is believed to be supported by a local communication between Ca(2+) release channels and adjacent mitochondrial Ca(2+) uptake sites, but the signaling machinery has not been explored at the level of elementary Ca(2+) release events. Here, we demonstrate that [Ca(2+)](c) sparks mediated by ryanodine receptors are competent to elicit miniature mitochondrial matrix [Ca(2+)] signals that we call "Ca(2+) marks." Ca(2+) marks are restricted to single mitochondria and typically last less than 500 ms. The decay of Ca(2+) marks relies on extrusion of Ca(2+) from the mitochondria through the Ca(2+) exchanger, whereas [Ca(2+)](c) sparks decline primarily by diffusion. Mitochondria also appear to have a direct effect on the properties of [Ca(2+)](c) sparks, because inhibition of mitochondrial Ca(2+) uptake results in an increase in the frequency and duration of [Ca(2+)](c) sparks. Thus, a short-lasting opening of a cluster of Ca(2+) release channels can yield activation of mitochondrial Ca(2+) uptake, and the competency of mitochondrial Ca(2+) handling may be an important determinant of cardiac excitability through local feedback control of elementary [Ca(2+)](c) signals.

Inhibitory actions of the selective serotonin re-uptake inhibitor citalopram on HERG and ventricular L-type calcium currents

Using whole-cell patch clamp recording of heterologous HERG-mediated currents in transfected mammalian cells, we observed that the selective serotonin re-uptake inhibitor citalopram blocks HERG with an IC(50) of 3.97 microM. This is slightly less potent than fluoxetine in our system (IC(50) of 1.50 microM). In isolated guinea pig ventricular cardiomyocytes citalopram inhibited L-type calcium current (I(Ca,L)). The voltage dependence of I(Ca,L) inactivation in the presence of 100 microM citalopram was shifted significantly leftward. As a result, the I(Ca,L) 'window' in citalopram was found to be (a) smaller and (b) leftward-shifted compared to control. The effects of citalopram on both calcium current amplitude and the I(Ca,L) 'window' may help to explain citalopram's good cardiac safety profile, given its propensity to block HERG at excessive dosages.

The antidepressant drug fluoxetine is an inhibitor of human ether-a-go-go-related gene (HERG) potassium channels

Fluoxetine is a commonly prescribed antidepressant compound. Its action is primarily attributed to selective inhibition of the reuptake of serotonin (5-hydroxytryptamine) in the central nervous system. Although this group of antidepressant drugs is generally believed to cause fewer proarrhythmic side effects compared with tricyclic antidepressants, serious concerns have been raised by case reports of tachycardia and syncopes associated with fluoxetine treatment. To determine the electrophysiological basis for the arrhythmogenic potential of fluoxetine, we investigated the effects of this drug on cloned human ether-a-go-go-related gene (HERG) potassium channels heterologously expressed in Xenopus oocytes using the two-microelectrode voltage-clamp technique. We found that fluoxetine blocked HERG channels with an IC(50) value of 3.1 microM. Inhibition occurred fast to open channels with very slow unbinding kinetics. Analysis of the voltage dependence of block revealed loss of inhibition at membrane potentials greater than 40 mV, indicating that channel inactivation prevented block by fluoxetine. No pronounced changes in electrophysiological parameters such as voltage dependence of activation or inactivation, or inactivation time constant could be observed, and block was not frequency-dependent. This is the first study demonstrating that HERG potassium channels are blocked by the selective serotonin reuptake inhibitor fluoxetine. We conclude that HERG current inhibition might be an explanation for the arrhythmogenic side effects of this drug.

The role of poly(ADP-ribose) polymerase activation in the development of myocardial and endothelial dysfunction in diabetes

Patients with diabetes exhibit a high incidence of diabetic cardiomyopathy and vascular complications, which underlie the development of retinopathy, nephropathy, and neuropathy and increase the risk of hypertension, stroke, and myocardial infarction. There is emerging evidence that the activation of the nuclear enzyme poly(ADP-ribose) polymerase (PARP) importantly contributes to the development of endothelial dysfunction in a streptozotocin-induced model of diabetes. We investigated the role of PARP activation in the pathogenesis of cardiac dysfunction in streptozotocin-induced and genetic (nonobese diabetic) models of diabetes in rats and mice. Development of diabetes was accompanied by hyperglycemia, cardiac PARP activation, a selective loss of endothelium-dependent vasodilation in the thoracic aorta, and an early diastolic dysfunction of the heart. Treatment with a novel potent phenanthridinone-based PARP inhibitor, PJ34, starting 1 week after the onset of diabetes, restored normal vascular responsiveness and significantly improved cardiac dysfunction, despite the persistence of severe hyperglycemia. The beneficial effect of PARP inhibition persisted even after several weeks of discontinuation of the treatment. Thus, PARP activation plays a central role in the pathogenesis of diabetic cardiovascular (cardiac as well as endothelial) dysfunction. PARP inhibitors may exert beneficial effects against the development of cardiovascular complications in diabetes.

Losing heart: the role of apoptosis in heart disease--a novel therapeutic target?

Cardiovascular disease is a leading cause of death worldwide. In recent years it has emerged that loss of myocardial cells may be a major pathogenic factor. Cell death can occur in a destructive, uncontrolled manner via necrosis or by a highly regulated programmed cell suicide mechanism termed apoptosis. As cell death in conditions such as heart failure and myocardial infarction does not always follow a typically apoptotic pathway, it remains to be established whether it occurs by apoptosis, necrosis, or a novel uncharacterized mechanism combining aspects of both types of cell death. Apoptotic pathways have been well studied in nonmyocytes and it is thought that similar pathways exist in cardiomyocytes. These pathways include death initiated by ligation of membrane-bound death receptors or death initiated by release of cytochrome c from mitochondria. Increasing evidence supports the existence of these pathways and their regulators in the heart. These regulators include inhibitors of caspases, which are the key enzymes of apoptosis, the Bcl-2 family of proteins, growth factors, stress proteins, calcium, and oxidants. It is hoped that a better understanding of the pathways of apoptosis and their regulation may yield novel therapeutic targets for cardiovascular disease.

Calcium signalling and removal mechanisms in myenteric neurones

To characterize further the Ca2+ signalling mechanisms of myenteric neurones, we studied the effect of thapsigargin, a blocker of the Ca2+-store ATPase, and the mechanisms involved in restoring the intracellular Ca2+ concentration ([Ca2+]i) after activation. Thapsigargin (5 x 10(-6) mol L(-1)) induced an oscillatory [Ca2+]i response in 86.6% of the neurones (n=276), which was blocked by the removal of extracellular Ca2+ and by omega-conotoxin MVIIA (5 x 10(-7) mol L(-1)). The IP3-blocker, 2-aminoethyl-diphenyl-borate (75 x 10(-6) mol L(-1)), blocked or reduced the responses in 74.5% of the neurones. The oscillatory responses induced by the depletion of Ca2+ stores suggest that myenteric neurones might recruite N-type Ca2+ channels as a refill mechanism. Thapsigargin pretreatment increased the amplitude, the upstroke and duration of the K+-induced [Ca2+]i responses. Mitochondrial blockers (rotenone and antimycin/oligomycin) also prolonged the responses, but without affecting the amplitude. Furthermore, it was found that for high [Ca2+]i, the thapsigargin-sensitive Ca2+ uptake was crucial, while mitochondrial blockade affected the Ca2+ uptake over a wide range of concentrations. The Ca2+-sequestering components might also have been compensating for each other, as most drugs only delayed and not inhibited Ca2+ removal.

Potential clinical applications of poly(ADP-ribose) polymerase (PARP) inhibitors

Poly(ADP-ribose) polymerases (PARPs) are defined as cell signaling enzymes that catalyze the transfer of ADP-ribose units from NAD(+)to a number of acceptor proteins. PARP-1, the best characterized member of the PARP family, that presently includes six members, is an abundant nuclear enzyme implicated in cellular responses to DNA injury provoked by genotoxic stress (oxygen radicals, ionizing radiations and monofunctional alkylating agents). Due to its involvement either in DNA repair or in cell death, PARP-1 is regarded as a double-edged regulator of cellular functions. In fact, when the DNA damage is moderate, PARP-1 participates in the DNA repair process. Conversely, in the case of massive DNA injury, elevated PARP-1 activation leads to rapid NAD(+)/ATP consumption and cell death by necrosis. Excessive PARP-1 activity has been implicated in the pathogenesis of numerous clinical conditions such as stroke, myocardial infarction, shock, diabetes and neurodegenerative disorders. PARP-1 could therefore be considered as a potential target for the development of pharmacological strategies to enhance the antitumor efficacy of radio- and chemotherapy or to treat a number of clinical conditions characterized by oxidative or NO-induced stress and consequent PARP-1 activation. Moreover, the discovery of novel functions for the multiple members of the PARP family might lead in the future to additional clinical indications for PARP inhibitors.

Reversal of endocardial endothelial dysfunction by folic acid in homocysteinemic hypertensive rats

The role of L- and D-isomers of homocysteine (Hcy) in vascular versus endocardial endothelial (EE) remodeling and function is not well understood. The hypothesis is that Hcy decreases EE cell density by activating matrix metalloproteinase (MMP) and by inducing left ventricular hypertrophy (LVH) in homocysteinemic hypertensive rats (HHR). And L- and D-isomers of Hcy have differential effects in vessel and myocardium. We used: 1) spontaneously hypertensive rats (SHR) in which endogenous total homocyst(e)ine (tHcy) levels are moderately high (18 micromol/L); 2) control age- and sex-matched normotensive Wistar rats (NWR) in which tHcy levels are normal (4 micromol/L); to create hyperhomocyst(e)inemia, 32 mg/day Hcy was administered for 12 weeks in 3) SHR (SHR-H), and in 4) NWR (NWR-H) rats; 5) endogenous tHcy levels were reduced (from 18 to 12 micromol/L) in SHR by folic acid administration (SHR-F). Plasma tHcy levels were measured by HPLC and spectrophometric methods. The MMP activity, measured by zymography, is increased by chronic Hcy administration, and folic acid treatment decreases MMP activity. The collagen and transforming growth factor-beta1 (TGF-beta1), measured by reverse transcriptase-polymerase chain reaction, are increased by Hcy. Folic acid treatment decreases collagen expression and increases TGF-beta1. In vivo LV function was measured in anesthetized rats by a catheter in the left ventricle. The partial decrease in tHcy levels and no change in arterial pressure in SHR after folic acid administration, suggested that folic acid decreases one of the L- or D-isomer of Hcy, which is not responsible for an increase in arterial pressure, but may be responsible for myocardial dysfunction. The chronic Hcy administration decreases EE function in NWR and SHR. The treatment of folic acid in SHR improves LVH and EE function. Folic acid improves cardiac remodeling and EE function by decreasing one of the D- or L-isomer of Hcy and by decreasing MMP activity in HHR. These results may suggest a differential role of L- and D-isomers in vascular versus cardiac remodeling.

Inhibition of protein kinase Cbeta prevents impaired endothelium-dependent vasodilation caused by hyperglycemia in humans

The bioavailability of nitric oxide is decreased in animal models and humans with diabetes mellitus. Hyperglycemia, in particular, attenuates endothelium-dependent vasodilation in healthy subjects. In vitro and in vivo animal studies implicate activation of protein kinase Cbeta as an important mechanism whereby hyperglycemia decreases endothelium-derived nitric oxide. Accordingly, this study tested the hypothesis that inhibition of protein kinase Cbeta would prevent impairment of endothelium-dependent vasodilation in healthy humans exposed to hyperglycemia. This study was a randomized, double-blind, placebo-controlled, crossover trial. Healthy subjects were treated with an orally active, selective, protein kinase Cbeta inhibitor, LY333531, or matching placebo once a day for 7 days before vascular function testing. Forearm blood flow was measured using venous-occlusion, strain-gauge plethysmography. Endothelium-dependent vasodilation was measured via incremental brachial artery administration of methacholine chloride (0.3 to 10 microg/min) during euglycemia and after 6 hours of hyperglycemic clamp. The forearm blood flow dose-response curve to methacholine was significantly attenuated by hyperglycemia after placebo treatment (P=0.009 by ANOVA, euglycemia versus hyperglycemia) but not after treatment with LY333531. Inhibition of protein kinase Cbeta prevents the reduction in endothelium-dependent vasodilation induced by acute hyperglycemia in healthy humans in vivo. These findings suggest that hyperglycemia impairs endothelial function, in part, via protein kinase Cbeta activation.

Cutting to the chase: calpain proteases in cell motility

Calpains are a large family of intracellular proteases whose precise and limited cleavage of specific proteins might be an integral regulatory aspect of signaling pathways. This intriguing mechanism for transducing biochemical and biophysical information from the external milieu seems to operate during cell motility. The two first described and ubiquitous isoforms, mu-calpain and M-calpain, have been implicated in enabling cell spreading by modifying adhesion sites and in promoting locomotion of adherent cells by facilitating rear-end detachment. Recent elucidation of the molecular structure of calpain opens the door for understanding how these pluripotential signal proteins are regulated to help govern migration. Armed with this knowledge, the precise roles of calpains in inflammation, wound repair and tumor progression can be ascertained and offer novel therapeutic targets.

Diet-induced mild hyperhomocysteinemia and increased salt intake diminish vascular endothelial function in a synergistic manner

OBJECTIVES

We investigated the influence of hyperhomocysteinemia and high salt intake on sodium handling, oxidative state, vascular endothelial function and blood pressure in a rat model.

METHODS

Eight-week-old male Sprague-Dawley rats were divided into subgroups and maintained for 4 weeks prior to experimentation on either control chow containing 0.36% methionine and 0.5% NaCl; or one of the following modified diets containing either 0.7% methionine, 8% NaCl or 0.7% methionine + 8% NaCl. Sodium handling, homocysteine metabolism, lipid profile, NO synthesis, oxidative state, blood pressure and relaxation to acetylcholine of carotid rings were evaluated and compared.

RESULTS

Diet-induced mild hyperhomocysteinemia (plasma homocysteine levels 1.4-fold higher than control), by itself, had no significant influence on sodium excretion, vascular endothelial function and blood pressure. Increased salt intake had no influence on homocysteine metabolism, vascular endothelial function and blood pressure. The coexistence of mild hyperhomocysteinemia and high salt intake significantly diminished vascular endothelial function (rmax to acetylcholine; control chow 83.2 +/- 6.2%, 0.7% methionine diet 74.7 +/- 3.9%, 8% NaCl diet 85.1 +/- 4.6%, 0.7% methionine + 8% NaCl diet 57.9 +/- 6.6%) but manifested no rise in blood pressure. No significant difference in oxidative state was observed in this analysis.

CONCLUSIONS

Diet-induced mild hyperhomocysteinemia, the extent of which is comparable with the levels that are associated with a predisposition to common atherosclerotic diseases, was found to induce vascular endothelial dysfunction only when accompanied by high salt intake.

A single dose of the serotonin neurotransmission agonist paroxetine enhances motor output: double-blind, placebo-controlled, fMRI study in healthy subjects

Since serotonin (5-HT) stimulates motor function, pharmacological potentiation of 5-HT neurotransmission may improve motor function in healthy subjects and, possibly, recovery in post-stroke patients. Indeed, fluoxetine, a selective serotonin reuptake inhibitor (SSRI), increased activation in executive motor areas of healthy subjects as fenozolone, a releaser of monoamines (including noradrenaline, dopamine, and serotonin) from intracellular stores. This study is intended to test the hypothesis that paroxetine can likewise modulate brain motor activity in a dose-dependent manner in healthy subjects. In a double-blind counterbalanced study, six subjects underwent functional MRI examinations on three sessions 1 week apart (E1, E2, and E3) at the time of peak plasma concentrations (5 h after drug intake, i.e., either 20 or 60 mg of paroxetine or placebo) with a complex sequential opposition task. Rest and activation alternated in a block design. During activation, subjects performed, with the right hand, a 1-Hz-paced task that alternated two fist closings with a sequential opposition task. Paroxetine elicited effects similar to those reported for fluoxetine; notable changes were hyperactivation in the contralateral S1/M1, and posterior SMA and widespread hypoactivation of basal ganglia and cerebellum. There was an inverse correlation between dose and effect: significantly greater effects were observed with the 20-mg dose compared with 60 mg. Paroxetine dose-dependently modulates activation of the entire motor pathway in a way that favors motor output. Thus, a single dose of the SSRI paroxetine reorganized motor processing.

Nucleolin is a calcium-binding protein

We have purified a prominent 110-kDa protein (p110) from 1.6 M NaCl extracts of rat liver nuclei that appears to bind Ca2+. p110 was originally identified by prominent blue staining with 'Stains-All' in sodium dodecyl sulfate-polyacrylamide gels and was observed to specifically bind ruthenium red and 45Ca2+ in nitrocellulose blot overlays. In spin-dialysis studies, purified p110 saturably bound approximately 75 nmol Ca2+/mg protein at a concentration of 1 mM total Ca2+ with half-maximal binding observed at 105 microM Ca2+. With purification, p110 became increasingly susceptible to proteolytic (likely autolytic) fragmentation, although most intermediary peptides between 40 and 90 kDa retained "Stains-All", ruthenium red, and 45Ca2+ binding. N-terminal sequencing of intact p110 and a 70-kDa autolytic peptide fragment revealed a strong homology to nucleolin. Two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE)/IEF revealed autolysis produced increasingly acidic peptide fragments ranging in apparent pI's from 5.5 for intact p110 to 3.5 for a 40 kDa peptide fragment. Intact p110 and several peptide fragments were immunostained with a highly specific anti-nucleolin antibody, R2D2, thus confirming the identity of this protein with nucleolin. These annexin-like Ca2+-binding characteristics of nucleolin are likely contributed by its highly acidic argyrophilic N-terminus with autolysis apparently resulting in largely selective removal of its basic C-terminal domain. Although the Ca2+-dependent functions of nucleolin are unknown, we discuss the possibility that like the structurally analogous HMG-1, its Ca2+-dependent actions may regulate chromatin structure, possibly during apoptosis.

Mitochondria regulate the amplitude of simple and complex calcium oscillations

In a mathematical model for simple calcium oscillations [Biophys. Chem. 71 (1998) 125], it has been shown that mitochondria play an important role in the maintenance of constant amplitudes of cytosolic Ca(2+) oscillations. Simple plausible rate laws for Ca(2+) fluxes across the inner mitochondrial membrane have been used in this model. Here we show that it is possible to use the same rate laws as a plug-in element in other existing mathematical models and obtain the same effect on amplitude regulation. This result appears to be universal, independent of the type of model and the type of Ca(2+) oscillations. We demonstrate this on two models for spiking Ca(2+) oscillations [J. Biol. Chem. 266 (1991) 11068; Cell Calcium 14 (1993) 311] and on two recent models for bursting Ca(2+) oscillations; one of them being a receptor-operated model [Biophys. J. 79 (2000) 1188] and the other one being a store-operated model [BioSystems 57 (2000) 75].

VDAC-dependent permeabilization of the outer mitochondrial membrane by superoxide induces rapid and massive cytochrome c release

Enhanced formation of reactive oxygen species (ROS), superoxide (O2*-), and hydrogen peroxide (H2O2) may result in either apoptosis or other forms of cell death. Here, we studied the mechanisms underlying activation of the apoptotic machinery by ROS. Exposure of permeabilized HepG2 cells to O2*- elicited rapid and massive cytochrome c release (CCR), whereas H2O2 failed to induce any release. Both O2*- and H2O2 promoted activation of the mitochondrial permeability transition pore by Ca2+, but Ca2+-dependent pore opening was not required for O2*--induced CCR. Furthermore, O2*- alone evoked CCR without damage of the inner mitochondrial membrane barrier, as mitochondrial membrane potential was sustained in the presence of extramitochondrial ATP. Strikingly, pretreatment of the cells with drugs or an antibody, which block the voltage-dependent anion channel (VDAC), prevented O2*--induced CCR. Furthermore, VDAC-reconstituted liposomes permeated cytochrome c after O2*- exposure, and this release was prevented by VDAC blocker. The proapoptotic protein, Bak, was not detected in HepG2 cells and O2*--induced CCR did not depend on Bax translocation to mitochondria. O2*--induced CCR was followed by caspase activation and execution of apoptosis. Thus, O2*- triggers apoptosis via VDAC-dependent permeabilization of the mitochondrial outer membrane without apparent contribution of proapoptotic Bcl-2 family proteins.

Mitochondrial modulation of calcium signaling at the initiation of development

Fertilization triggers cytosolic Ca(2+) oscillations that activate mammalian eggs and initiate development. Extensive evidence demonstrates that Ca(2+) is released from endoplasmic reticulum stores; however, less is known about how the increased Ca(2+) is restored to its resting level, forming the Ca(2+) oscillations. We investigated whether mitochondria also play a role in activation-associated Ca(2+) signaling. Mitochondrial dysfunction induced by the mitochondrial uncoupler FCCP or antimycin A disrupted cytosolic Ca(2+) oscillations, resulting in sustained increase in cytosolic Ca(2+), followed by apoptotic cell death. This suggests that functional mitochondria may participate in sequestering the released Ca(2+), contributing to cytosolic Ca(2+) oscillations and preventing cell death. By centrifugation, mouse eggs were stratified and separated into fractions containing both endoplasmic reticulum and mitochondria and fractions containing endoplasmic reticulum with no mitochondria. The former showed Ca(2+) oscillations by activation, whereas the latter exhibited sustained elevation in cytosolic Ca(2+) but no Ca(2+) oscillations, suggesting that mitochondria take up released cytosolic Ca(2+). Further, using Rhod-2 for detection of mitochondrial Ca(2+), we found that mitochondria exhibited Ca(2+) oscillations, the frequency of which was not different from that of cytosolic Ca(2+) oscillations, indicating that mitochondria are involved in Ca(2+) signaling during egg activation. Therefore, we propose that mitochondria play a crucial role in Ca(2+) signaling that mediates egg activation and development, and apoptotic cell death.

Human mast cell and airway smooth muscle cell interactions: implications for asthma

Asthma is characterized by inflammation, hyperresponsiveness, and remodeling of the airway. Human mast cells (HMCs) play a central role in all of these changes by releasing mediators that cause exaggerated bronchoconstriction, induce human airway smooth muscle (HASM) cell proliferation, and recruit and activate inflammatory cells. Moreover, the number of HMCs present on asthmatic HASM is increased compared with that on nonasthmatic HASM. HASM cells also have the potential to actively participate in the inflammatory process by synthesizing cytokines and chemokines and expressing surface molecules, which have the capacity to perpetuate the inflammatory mechanisms present in asthma. This review specifically examines how the mediators of HMCs have the capacity to modulate many functions of HASM; how the synthetic function of HASM, particularly through the release and expression of stem cell factor, has the potential to influence HMC number and activation in an extraordinarily potent and proinflammatory manner; and how these interactions between HMCs and HASM have potential consequences for airway structure and inflammation relevant to the disease process of asthma.

Participation of mitochondria in calcium signalling in the exocrine pancreas

This minireview is an attempt to put together some of the recent advances regarding the implications of mitochondria in Ca2+ homeostasis. Although the main role of this cytoplasmic organelle is ATP supply to the cell, during the past years strong evidence has been accumulated supporting an active role of these organelles in Ca2+ handling by the cell. The discovery of mitochondrial specific fluorescent dyes has permitted the study of these organelles within living cells. Due to its ubiquitous localisation within the cytosol, mitochondria would play an important role in the modulation of the subcellular patterns of Ca2+ signalling, and therefore would act as modulators of Ca2+-dependent cellular processes.

Modern views on an ancient chemical: serotonin effects on cell proliferation, maturation, and apoptosis

Evolutionarily, serotonin existed in plants even before the appearance of animals. Indeed, serotonin may be tied to the evolution of life itself, particularly through the role of tryptophan, its precursor molecule. Tryptophan is an indole-based, essential amino acid which is unique in its light-absorbing properties. In plants, tryptophan-based compounds capture light energy for use in metabolism of glucose and the generation of oxygen and reduced cofactors. Tryptophan, oxygen, and reduced cofactors combine to form serotonin. Serotonin-like molecules direct the growth of light-capturing structures towards the source of light. This morphogenic property also occurs in animal cells, in which serotonin alters the cytoskeleton of cells and thus influences the formation of contacts. In addition, serotonin regulates cell proliferation, migration and maturation in a variety of cell types, including lung, kidney, endothelial cells, mast cells, neurons and astrocytes). In brain, serotonin has interactions with seven families of receptors, numbering at least 14 distinct proteins. Of these, two receptors are important for the purposes of this review. These are the 5-HT1A and 5-HT2A receptors, which in fact have opposing functions in a variety of cellular and behavioral processes. The 5-HT1A receptor develops early in the CNS and is associated with secretion of S-100beta from astrocytes and reduction of c-AMP levels in neurons. These actions provide intracellular stability for the cytoskeleton and result in cell differentiation and cessation of proliferation. Clinically, 5-HT1A receptor drugs decrease brain activity and act as anxiolytics. The 5-HT2A receptor develops more slowly and is associated with glycogenolysis in astrocytes and increased Ca(++) availability in neurons. These actions destabilize the internal cytoskeleton and result in cell proliferation, synaptogenesis, and apoptosis. In humans, 5-HT2A receptor drugs produce hallucinations. The dynamic interactions between the 5-HT1A and 5-HT2A receptors and the cytoskeleton may provide important insights into the etiology of brain disorders and provide novel strategies for their treatment.

Mitochondrial [Ca(2+)] oscillations driven by local high [Ca(2+)] domains generated by spontaneous electric activity

Mitochondria take up calcium during cell activation thus shaping Ca(2+) signaling and exocytosis. In turn, Ca(2+) uptake by mitochondria increases respiration and ATP synthesis. Targeted aequorins are excellent Ca(2+) probes for subcellular analysis, but single-cell imaging has proven difficult. Here we combine virus-based expression of targeted aequorins with photon-counting imaging to resolve dynamics of the cytosolic, mitochondrial, and nuclear Ca(2+) signals at the single-cell level in anterior pituitary cells. These cells exhibit spontaneous electric activity and cytosolic Ca(2+) oscillations that are responsible for basal secretion of pituitary hormones and are modulated by hypophysiotrophic factors. Aequorin reported spontaneous [Ca(2+)] oscillations in all the three compartments, bulk cytosol, nucleus, and mitochondria. Interestingly, a fraction of mitochondria underwent much larger [Ca(2+)] oscillations, which were driven by local high [Ca(2+)] domains generated by the spontaneous electric activity. These oscillations were large enough to stimulate respiration, providing the basis for local tune-up of mitochondrial function by the Ca(2+) signal.

Prostaglandin H synthase and vascular function

Prostaglandin H synthase (PGHS) is a rate-limiting enzyme in the production of prostaglandins and thromboxane, which are important regulators of vascular function. Under normal physiological conditions, PGHS-dependent vasodilators (such as prostacyclin) modulate vascular tone. However, PGHS-dependent vasoconstriction (mediated by thromboxane and/or its immediate precursor, PGH(2)) predominates in some vascular pathologies (eg, systemic hypertension, diabetes, cerebral ischemia, and aging). This review will discuss the role of PGHS-dependent modulation of vascular function in a number of vascular beds (systemic, pulmonary, cerebral, and uterine) with an emphasis on vascular pathophysiology. Moreover, the specific contributions of the different isoforms (PGHS-1 and PGHS-2) are discussed. Understanding the role of PGHS in vascular function is of particular importance because they are the targets of the commonly used nonsteroidal antiinflammatory drugs (NSAIDs), which include aspirin and ibuprofen. Importantly, with the advent of specific PGHS-2 inhibitors for treatment of conditions such as chronic inflammatory disease, it is an opportune time to review the data regarding PGHS-dependent modulation of vascular function.

Selective serotonin-reuptake inhibitor antidepressants increase the risk of falls and hip fractures in elderly people by inhibiting cardiovascular ion channels

Surprising results from recently published retrospective studies show that the use of new selective serotonin-reuptake inhibitor antidepressants (SSRIs), similarly to the older tricyclic antidepressants (TCAs), increases the risk of falls and hip fractures among elderly people.The mechanism whereby antidepressants increase this risk is complex and may include orthostatic hypotension, arrhythmias, sedation and confusion. The increased risk of falls and hip fractures with the use of TCAs is not surprising considering their well-known cardiovascular, anticholinergic and antihistaminergic side-effects. But the increased risk of falls with SSRIs is highly unexpected since these drugs are believed to be free from the disadvantages of TCAs. We hypothesized that the new SSRI antidepressants may also have cardiovascular effects similarly to the older TCA compounds, which may be an explanation for the increased rate of falls and hip fractures. The experimental and clinical evidence in support of this hypothesis are discussed.

Altered K(+) channel gene expression in diabetic rat ventricle: isoform switching between Kv4.2 and Kv1.4

Expression of voltage-gated K(+) channels encoding the K(+) independent transient outward current in the streptozocin-induced diabetic (DM) rat ventricle was studied to determine the basis for slowed cardiac repolarization in diabetes mellitus. Although hypertrophy was not detected in diabetic rats at 12 wk after streptozocin treatment, ventricular Kv4.2 mRNA levels decreased 41% relative to nondiabetic controls. Kv1.4 mRNA levels increased 179% relative to controls, whereas Kv4.3 mRNA levels were unaffected. Immunohistochemistry and Western blot analysis of the diabetic heart showed that the density of the Kv4.2 protein decreased, whereas Kv1.4 protein increased. Thus isoform switching from Kv4.2 to Kv1.4 is most likely the mechanism underlying the slower kinetics of transient outward K(+) current observed in the diabetic ventricle. Brain Kv1.4, Kv4.2, or Kv4.3 mRNA levels were unaffected by diabetes. Myosin heavy chain (MHC) gene expression was altered with a 32% decrease in alpha-MHC mRNA and a 259% increase in beta-MHC mRNA levels in diabetic ventricle. Low-dose insulin-like growth factor-II (IGF-II) treatment during the last 6 of the 12 wk of diabetes (DM + IGF) protected against these changes in MHC mRNAs despite continued hyperglycemia and body weight loss. IGF-II treatment did not change K(+) channel mRNA levels in DM or control rat ventricles. Thus IGF treatment may prevent some, but not all, biochemical abnormalities in the diabetic heart.

Effects of norfluoxetine, the major metabolite of fluoxetine, on the cloned neuronal potassium channel Kv3.1

The effects of fluoxetine and its major metabolite, norfluoxetine, were studied using the patch-clamp technique on the cloned neuronal rat K(+) channel Kv3.1, expressed in Chinese hamster ovary cells. In whole-cell recordings, fluoxetine and norfluoxetine inhibited Kv3.1 currents in a reversible concentration-dependent manner, with an IC(50) value and a Hill coefficient of 13.11+/-0.91 microM and 1.33+/-0.08 for fluoxetine and 0.80+/-0.06 microM and 1.65+/-0.08 for norfluoxetine at +40 mV, respectively. In inside-out patches, norfluoxetine applied to the cytoplasmic surface inhibited Kv3.1 with an IC(50) value of 0.19+/-0.01 microM. The inhibition of Kv3.1 currents by both drugs was characterized by an acceleration in the apparent rate of current decay, without modification of the activation time course and with relatively fewer effects on peak amplitude. The degree of inhibition of Kv3.1 by norfluoxetine was voltage-dependent. The inhibition increased steeply between 0 and +30 mV, which corresponded with the voltage range for channel opening. In the voltage range positive to +30 mV, inhibition displayed a weak voltage dependence, consistent with an electrical distance delta of 0.31+/-0.05. The association (k(+1)) and dissociation (k(-1)) rate constants for norfluoxetine-induced inhibition of Kv3.1 were 21.70+/-3.39 microM(-1) s(-1) and 14.68+/-3.94 s(-1), respectively. The theoretical K(D) value derived by k(-1)/k(+1) yielded 0.68 microM. Norfluoxetine did not affect the ion selectivity of Kv3.1. The reversal potential under control conditions was about -85 mV and was not affected by norfluoxetine. Norfluoxetine slowed the deactivation time course, resulting in a tail crossover phenomenon when the tail currents, recorded in the presence and absence of norfluoxetine, were superimposed. The voltage dependence of steady-state inactivation was not changed by the drug. Norfluoxetine produced use-dependent inhibition of Kv3.1 at a frequency of 1 Hz and slowed the recovery from inactivation. It is concluded that at clinically relevant concentrations, both fluoxetine and its major metabolite norfluoxetine inhibit Kv3.1, and that norfluoxetine directly inhibits Kv3.1 as an open channel blocker.

mAKAP and the ryanodine receptor are part of a multi-component signaling complex on the cardiomyocyte nuclear envelope

The physical association of regulatory enzymes and ion channels at relevant intracellular sites contributes to the diversity and specificity of second messenger-mediated signal transduction in cells. mAKAP is a scaffolding protein that targets the cAMP-dependent protein kinase A and phosphodiesterase type 4D3 to the nuclear envelope of differentiated cardiac myocytes. Here we present data that the mAKAP signaling complex also includes nuclear envelope-resident ryanodine receptors and protein phosphatase 2A. The ryanodine receptor is the major cardiac ion channel responsible for calcium-induced calcium release from intracellular calcium ion stores. As demonstrated by a combination of immunohistochemistry and tissue fractionation, mAKAP is targeted specifically to the nuclear envelope, whereas the ryanodine receptor is present at both the sarcoplasmic reticulum and nuclear envelope intracellular membrane compartments. At the nuclear envelope, a subset of cardiac ryanodine receptor is bound to mAKAP and via the association with mAKAP may be regulated by protein kinase A-mediated phosphorylation. By binding protein kinase A and ryanodine receptor, mAKAP may serve as the scaffold for a cAMP- and calcium ion-sensitive signaling complex.

Extracellular NAD(+) induces calcium signaling and apoptosis in human osteoblastic cells

ADP-ribosyl cyclase/CD38 is a bifunctional enzyme that catalyzes at its ectocellular domain the synthesis from NAD(+) (cyclase) and the hydrolysis (hydrolase) of the calcium-mobilizing second messenger cyclic ADP ribose (cADPR). Furthermore, CD38 mediates cADPR influx inside a number of cells, thereby inducing Ca(2+) mobilization. Intracellularly, cADPR releases Ca(2+) from ryanodine-sensitive pools, thus activating several Ca(2+)-dependent functions. Among these, the inhibition of osteoclastic-mediated bone resorption has been demonstrated. We found that HOBIT human osteoblastic cells display ADP-ribosyl cyclase activity and we examined the effects of CD38 stimulation on osteoblasts function. Extracellular NAD(+) induced elevation of cytosolic calcium due to both Ca(2+) influx from the extracellular medium and Ca(2+) release from ryanodine-sensitive intracellular stores. Culturing these cells in the presence of NAD(+) caused a complete growth arrest with a time-dependent decrease of cell number and the appearance of apoptotic nuclei. The first changes could be observed after 24 h of treatment and became fully evident after 72-96 h. We propose a role of extracellular NAD(+) in bone homeostatic control.

Ion homeostasis and apoptosis

Alterations in the transmembrane gradients of several physiological ions may influence programmed cell death. In particular, recent data suggest that increases in intracellular calcium may either promote or inhibit apoptosis, depending on the level, timing and location, whereas loss of intracellular potassium promotes apoptosis.

Influence of ATP-sensitive K+ channel modulation on the mechanical properties of diabetic myocardium

ATP-sensitive K+ (KATP) channels are therapeutic targets for hypertension and diabetes. KATP channel opening elicits vasorelaxation and myocardial protection, whereas its closing stimulates insulin secretion. The cardiac KATP conductance is believed altered under diabetes. This study was to evaluate the influence of KATP channel openers and blocker on myocardial contractile dysfunction in diabetes. Adult rats were made diabetic with streptozotocin (55 mg/kg) and maintained for eight weeks. Contractile properties were studied using isolated papillary muscles in the absence or presence of KATP channel openers (BRL 38227 and pinacidil) and KATP blocker (glyburide). Experimental diabetes led to hyperglycemia, reduced growth, cardiac hypertrophy and hepatomegaly. Mechanical properties exhibited prolonged duration and reduced velocity of both contraction and relaxation in diabetic myocardium, characteristic of diabetic cardiomyopathy. Acute exposure to both KATP channel openers induced concentration-dependent negative inotropic effects (NIE) on myocardial contraction. The magnitude of the NIE was similar between the normal and diabetic groups and was fully reversible upon washout for BRL 38227 although not for pinacidil. Both KATP channel openers depressed the velocity of contraction and relaxation, whereas exerted no effect on the duration of contraction and relaxation, in myocardium from both groups. Acute exposure to glyburide, a KATP channel blocker, failed to alter any of the mechanical parameters measured. These data suggest that acute modulation of KATP channel with channel opener or blocker had little influence on diabetic cardiomyopathy, at least in the setting of multicellular preparations.

Propagation of the apoptotic signal by mitochondrial waves

Generation of mitochondrial signals is believed to be important in the commitment to apoptosis, but the mechanisms coordinating the output of individual mitochondria remain elusive. We show that in cardiac myotubes exposed to apoptotic agents, Ca2+ spikes initiate depolarization of mitochondria in discrete subcellular regions, and these mitochondria initiate slow waves of depolarization and Ca2+ release propagating through the cell. Traveling mitochondrial waves are prevented by Bcl-x(L), involve permeability transition pore (PTP) opening, and yield cytochrome c release, caspase activation and nuclear apoptosis. Mitochondrial Ca2+ uptake is critical for wave propagation, and mitochondria at the origin of waves take up Ca2+ particularly effectively, providing a mechanism that may underlie selection of the initiation sites. Thus, apoptotic agents transform the mitochondria into an excitable state by sensitizing PTP to Ca2+. Expansion of the local excitation by mitochondrial waves propagating through the whole cell can be especially important in activation of the apoptotic machinery in large cells.

Stress hormone-related psychopathology: pathophysiological and treatment implications

Stress is commonly associated with a variety of psychiatric conditions, including major depression, and with chronic medical conditions, including diabetes and insulin resistance. Whether stress causes these conditions is uncertain, but plausible mechanisms exist by which such effects might occur. To the extent stress-induced hormonal alterations (e.g., chronically elevated cortisol levels and lowered dehydroepiandrosterone [DHEA] levels) contribute to psychiatric and medical disease states, manipulations that normalize these hormonal aberrations should prove therapeutic. In this review, we discuss mechanisms by which hormonal imbalance (discussed in the frameworks of "allostatic load" and "anabolic balance") might contribute to illness. We then review certain clinical manifestations of such hormonal imbalances and discuss pharmacological and behavioural treatment strategies aimed at normalizing hormonal output and lessening psychiatric and physical pathology.

Selected contribution: NO released to flow reduces myogenic tone of skeletal muscle arterioles by decreasing smooth muscle Ca(2+) sensitivity

To clarify the contribution of intracellular Ca(2+) concentration ([Ca(2+)](i))-dependent and -independent signaling mechanisms in arteriolar smooth muscle (aSM) to modulation of arteriolar myogenic tone by nitric oxide (NO), released in response to increases in intraluminal flow from the endothelium, changes in aSM [Ca(2+)](i) and diameter of isolated rat gracilis muscle arterioles (pretreated with indomethacin) were studied by fluorescent videomicroscopy. At an intraluminal pressure of 80 mmHg, [Ca(2+)](i) significantly increased and myogenic tone developed in response to elevations of extracellular Ca(2+) concentration. The Ca(2+) channel inhibitor nimodipine substantially decreased [Ca(2+)](i) and completely inhibited myogenic tone. Dilations to intraluminal flow (that were inhibited by N(omega)-nitro-L-arginine methyl ester) or dilations to the NO donor S-nitroso-N-acetyl-DL-penicillamine (that were inhibited by the guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one) were not accompanied by substantial decreases in aSM [Ca(2+)](i). 8-Bromoguanosine cGMP and the cGMP-specific phosphodiesterase inhibitor zaprinast significantly dilated arterioles yet elicited only minimal decreases in [Ca(2+)](i). Thus flow-induced endothelial release of NO elicits relaxation of arteriolar smooth muscle by a cGMP-dependent decrease of the Ca(2+) sensitivity of the contractile apparatus without substantial changes in the pressure-induced level of [Ca(2+)](i).

Serotonin reuptake inhibitors fluoxetine and citalopram relax intestinal smooth muscle

Selective serotonin reuptake inhibitor antidepressants (SSRIs) exert depressant effects on cardiac myocytes and vascular smooth muscle cells by inhibiting Ca2+ channels. We hypothesized that the SSRIs fluoxetine and citalopram affect the contractile activity of intestinal smooth muscle by interfering with Ca2+ entry and (or) signaling pathways. The effects of fluoxetine and citalopram on contractions of guinea-pig ileum longitudinal muscle-myenteric plexus preparations (LMMP) were compared with the effects of the voltage-operated Ca2+ channel inhibitors nifedipine and diltiazem. In a concentration-dependent manner, nifedipine, diltiazem, fluoxetine, and citalopram elicited relaxation of LMMPs contracted by electrical field stimulation (EC50 values of 4 x 10(-7) M, 1.4 x 10(-6) M, 1.4 x 10(-5), and 6.8 x 10(-6) M, respectively). Nifedipine, diltiazem, fluoxetine, and citalopram also relaxed LMMPs contracted with a depolarizing concentration of KCl (48 mM; EC50 values of 1.8 x 10(-8) M, 1.4 x 10(-7) M, 3.7 x 10(-6) M, and 6.3 x 10(-6), respectively), a response that could be reversed by increasing the extracellular Ca2+ concentration (2.5-30 mM). These data suggest that fluoxetine and citalopram elicit relaxation of intestinal smooth muscle, likely by inhibiting Ca2+ channel(s). This effect may be of clinical importance.

Energized mitochondria increase the dynamic range over which inositol 1,4,5-trisphosphate activates store-operated calcium influx

In eukaryotic cells, activation of cell surface receptors that couple to the phosphoinositide pathway evokes a biphasic increase in intracellular free Ca2+ concentration: an initial transient phase reflecting Ca2+ release from intracellular stores, followed by a plateau phase due to Ca2+ influx. A major component of this Ca2+ influx is store-dependent and often can be measured directly as the Ca2+ release-activated Ca2+ current (I(CRAC)). Under physiological conditions of weak intracellular Ca2+ buffering, respiring mitochondria play a central role in store-operated Ca2+ influx. They determine whether macroscopic I(CRAC) activates or not, to what extent and for how long. Here we describe an additional role for energized mitochondria: they reduce the amount of inositol 1,4,5-trisphosphate (InsP3) that is required to activate I(CRAC). By increasing the sensitivity of store-operated influx to InsP3, respiring mitochondria will determine whether modest levels of stimulation are capable of evoking Ca2+ entry or not. Mitochondrial Ca2+ buffering therefore increases the dynamic range of concentrations over which the InsP3 is able to function as the physiological messenger that triggers the activation of store-operated Ca2+ influx.

Homocysteine reduces smooth muscle [Ca2+]i and constrictor responses of isolated arterioles

Chronic elevation of plasma homocysteine concentration has been shown to be associated with impaired vascular function. The acute direct effect of homocysteine on the tone and vasoactive responses of arterioles and the possible underlying mechanisms, however, have not yet been elucidated. Thus arterioles were isolated from gracilis muscle of rats (d: approximately 130 microm) and their diameter was measured by videomicroscopy. Homocysteine (10(-6)-10(-4) M) elicited dose-dependent dilation of arterioles (maximum: 44+/-6% at 10(-4) M). The dilation was not affected by the presence of the nitric oxide synthase inhibitor Nomega-nitro-L-arginine methyl ester or by removal of the endothelium, or the free radical scavenger catalase and superoxide dismutase, or the K+ channel inhibitors glibenclamide, 4-aminopyridine, or tetraethyl ammonium. Incubation of vessels with homocysteine (10(-4) M, 20 min) did not affect dilations to acetylcholine or sodium nitroprusside, whereas it significantly decreased constrictions to norepinephrine (at 10(-6) M; control: 57+/-7%, homocysteine: 21+/-5%) and to the thromboxane A2 analogue U46619 (at 10(-8) M: control: 44+/-3%, homocysteine: 20+/-4%). Homocysteine (10(-4) M), similar to the voltage-operated Ca2+ channel inhibitor nitrendipine (10(-8) M), significantly decreased the arteriolar smooth muscle [Ca2+]i as assessed by changes in the fura-2 ratiometric signal (R(Ca), -6+/-1% and -24+/-3%, respectively). These data suggest that in isolated arterioles homocysteine decreases pressure-induced tone and responses to vasoconstrictor agents, likely by altering Ca2+ signaling of arteriolar smooth muscle.

Effect of fluoxetine on intracellular Ca2+ levels in bladder female transitional carcinoma (BFTC) cells

The effect of the antidepressant fluoxetine on Ca2+ signaling in cultured cells was largely unknown. The effect of various concentrations of fluoxetine on [Ca 2+] i in populations of bladder female transitional cancer (BFTC) cells was evaluated by using fura-2 as a Ca2+ probe. Fluoxetine increased [Ca 2+] i concentration dependently (20-100 microM) with an EC50 value of 30 microM. The response was inhibited by 50-60% on extracellular Ca2+ removal. In Ca2+ -free medium, pretreatment with 1 microM thapsigargin (an inhibitor of the endoplasmic reticulum Ca2+ pump) abolished 50 microM fluoxetine-induced Ca2+ release; whereas pretreatment with fluoxetine did not alter the thapsigargin-induced Ca2+ response. Addition of 3 mM Ca2+ increased [Ca 2+] i after pretreatment with 50 microM fluoxetine in Ca2+ -free medium, suggestive of fluoxetine-induced capacitative Ca2+ entry. Suppression of inositol 1,4,5-trisphosphate formation by 2 microM U73122 (a phospholipase C inhibitor) did not affect 50 microM fluoxetine-induced Ca2+ release. Collectively, this study shows that fluoxetine increased [Ca 2+] i in bladder cancer cells in a concentration-dependent fashion, by releasing Ca2+ from thapsigargin-sensitive Ca2+ stores in an IP3-independent manner, and by inducing Ca2+ influx from extracellular medium.

Impaired homocysteine metabolism and atherothrombotic disease

Based on recent retrospective, prospective, and experimental studies, mild to moderate elevation of fasting or postmethionine-load plasma homocysteine is accepted as an independent risk factor for cardiovascular disease and thrombosis in both men and women. Hyperhomocysteinemia results from an inhibition of the remethylation pathway or from an inhibition or a saturation of the transsulfuration pathway of homocysteine metabolism. The involvement of a high dietary intake of methionine-rich animal proteins has not yet been investigated and cannot be ruled out. However, folate deficiency, either associated or not associated with the thermolabile mutation of the N(5,10)-methylenetetrahydrofolate reductase, and vitamin B(6) deficiency, perhaps associated with cystathionine beta-synthase defects or with methionine excess, are believed to be major determinants of the increased risk of cardiovascular disease related to hyperhomocysteinemia. Recent experimental studies have suggested that moderately elevated homocysteine levels are a causal risk factor for atherothrombotic disease because they affect both the vascular wall structure and the blood coagulation system. The oxidant stress that results from impaired homocysteine metabolism, which modifies the intracellular redox status, might play a central role in the molecular mechanisms underlying moderate hyperhomocysteinemia-mediated vascular disorders. Because folate supplementation can efficiently reduce plasma homocysteine levels, both in the fasting state and after methionine loading, results from further prospective cohort studies and from on-going interventional trials will determine whether homocysteine-lowering therapies can contribute to the prevention and reduction of cardiovascular risk. Additionally, these studies will provide unequivocal arguments for the independent and causal relationship between hyperhomocysteinemia and atherothrombotic disease.

Sorting of calcium signals at the junctions of endoplasmic reticulum and mitochondria

Calcium signal transmission between endoplasmic reticulum (ER) and mitochondria is supported by a local [Ca(2+)] control that operates between IP(3)receptor Ca(2+)release channels (IP(3)R) and mitochondrial Ca(2+)uptake sites, and displays functional similarities to synaptic transmission. Activation of IP(3)R by IP(3)is known to evoke quantal Ca(2+)mobilization that is associated with incremental elevations of mitochondrial matrix [Ca(2+)] ([Ca(2+)](m)). Here we report that activation of IP(3)R by adenophostin-A (AP) yields non-quantal Ca(2+)mobilization in mast cells. We also show that the AP-induced continuous Ca(2+)release causes relatively small [Ca(2+)](m)responses, in particular, the sustained phase of Ca(2+)release is not sensed by the mitochondria. Inhibition of ER Ca(2+)pumps by thapsigargin slightly increases IP(3)-induced [Ca(2+)](m)responses, but augments AP-induced [Ca(2+)](m)responses in a large extent. In adherent permeabilized cells exposed to elevated [Ca(2+)], ER Ca(2+)uptake fails to affect global cytosolic [Ca(2+)], but attenuates [Ca(2+)](m)responses. Moreover, almost every mitochondrion exhibits a region very close to ER Ca(2+)pumps visualized by BODIPY-FL-thapsigargin or SERCA antibody. Thus, at the ER-mitochondrial junctions, localized ER Ca(2+)uptake provides a mechanism to attenuate the mitochondrial response during continuous Ca(2+)release through the IP(3)R or during gradual Ca(2+)influx to the junction between ER and mitochondria.

Who develops severe or fatal adverse drug reactions to selective serotonin reuptake inhibitors?

OBJECTIVE

To ascertain the risk factors associated with the development of severe and fatal adverse drug reactions (ADRs) or intentional overdoses by patients on selective serotonin reuptake inhibitors (SSRIs).

METHODS

We undertook a data analysis of Health Canada's database of Adverse Drug Reactions to SSRIs from 1986 to 1996, as well as a literature review.

RESULTS

Among the complete ADR reports in the SSRI database, there were no sex differences in occurrences of all ADRs (n = 1011), severe ADRs (n = 295), drug-drug interactions (n = 312), deaths (n = 87), or intentional overdoses (n = 79), when sex differences in prescription practices were considered. There were no differences in ADR rates among different SSRIs. The most common cause of death among patients taking SSRIs was intentional overdose (n = 65, 74.7%). This was reported in 47 (72.3%) women and 18 (27.7%) men. The most common drugs taken with SSRIs in patients who died of intentional overdoses were benzodiazepines, tricyclic antidepressants (TCAs), narcotics, alcohol, and diphenhydramine. Patients who had severe or fatal ADRs were more likely to be taking an SSRI with 2 or more other drugs, including alcohol. Drug combinations that included another CYP-450 drug were especially problematic. A total of 129 cardiovascular ADRs were reported, most of which were severe. These included rhythm disturbances, blood pressure perturbations, and chest pain or angina. Cardiovascular ADRs most often occurred with concomitant drug use of benzodiazepines, TCAs, histamine H2 antagonists, lithium, and calcium channel blockers. There were 3 deaths from malignant neuroleptic syndrome unassociated with intentional overdose.

CONCLUSIONS

SSRIs are relatively safe when their widespread use is compared with the prevalence of ADRs. SSRIs may, however, be associated with ADRs, and even death, following intentional overdose or when taken with 2 or more other drugs or alcohol (particularly another drug metabolized by CYP-450). Physicians prescribing SSRIs need to consider drug-drug interactions and carefully monitor patients with severe affective disorders, comorbid medical conditions (especially cardiovascular disease), alcohol abuse, or a history of overdosing.

Evidence against oxidative stress as mechanism of endothelial dysfunction in methionine loading model

Endothelial dysfunction reflects reduced nitric oxide (NO) bioavailability due to either reduced production, inactivation of NO, or reduced smooth muscle responsiveness. Oral methionine loading causes acute endothelial dysfunction in healthy subjects and provides a model in which to study mechanisms. Endothelial function was assessed using flow-mediated dilatation (FMD) of the brachial artery in humans. Three markers of oxidative stress were measured ex vivo in venous blood. NO responsiveness was assessed in vascular smooth muscle and platelets. Oral methionine loading induced endothelial dysfunction (FMD decreased from 2.8 +/- 0.8 to 0.3 +/- 0.3% with methionine and from 2.8 +/- 0.8 to 1.3 +/- 0.3% with placebo; P < 0.05). No significant changes in measures of plasma oxidative stress or in vascular or platelet sensitivity to submaximal doses of NO donors were detected. These data suggest that oxidative stress is not the mechanism of endothelial dysfunction after oral methionine loading. Furthermore, the preservation of vascular and platelet NO sensitivity makes a signal transduction abnormality unlikely.

Calcium channel blockers and pain therapy

This review focuses on the advances in the development of N-type calcium channel blockers as analgesic agents over the last 2 years. Firstly, it highlights the clinical progress with SNX-111 (Ziconotide; Elan Pharmaceuticals, Smithfield, RI) and then secondly, it outlines the various approaches being taken by researchers to design orally active, selective, small molecule modulators without the perceived disadvantages associated with SNX-111.

Inositol 1,4,5-trisphosphate (IP3)-mediated Ca2+ release evoked by metabotropic agonists and backpropagating action potentials in hippocampal CA1 pyramidal neurons

We examined the properties of [Ca(2+)](i) changes that were evoked by backpropagating action potentials in pyramidal neurons in hippocampal slices from the rat. In the presence of the metabotropic glutamate receptor (mGluR) agonists t-ACPD, DHPG, or CHPG, spikes caused Ca(2+) waves that initiated in the proximal apical dendrites and spread over this region and in the soma. Consistent with previously described synaptic responses (Nakamura et al., 1999a), pharmacological experiments established that the waves were attributable to Ca(2+) release from internal stores mediated by the synergistic effect of receptor-mobilized inositol 1,4, 5-trisphosphate (IP(3)) and spike-evoked Ca(2+). The amplitude of the changes reached several micromoles per liter when detected with the low-affinity indicators fura-6F, fura-2-FF, or furaptra. Repetitive brief spike trains at 30-60 sec intervals generated increases of constant amplitude. However, trains at intervals of 10-20 sec evoked smaller increases, suggesting that the stores take 20-30 sec to refill. Release evoked by mGluR agonists was blocked by MCPG, AIDA, 4-CPG, MPEP, and LY367385, a profile consistent with the primacy of group I receptors. At threshold agonist concentrations the release was evoked only in the dendrites; threshold antagonist concentrations were effective only in the soma. Carbachol and 5-HT evoked release with the same spatial distribution as t-ACPD, suggesting that the distribution of neurotransmitter receptors was not responsible for the restricted range of regenerative release. Intracellular BAPTA and EGTA were approximately equally effective in blocking release. Extracellular Cd(2+) blocked release, but no single selective Ca(2+) channel blocker prevented release. These results suggest that IP(3) receptors are not associated closely with specific Ca(2+) channels and are not close to each other.