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G Vassort

Publications and source records attributed to G Vassort.

At least 73 records · Page 4Linked to original sources
PubMed1991

Dual action of prajmalium on the Ca currents in frog isolated cardiomyocytes.

The effects of N-n-propylajmaline (prajmalium) on the Na and Ca currents of single frog atrial and ventricular cells were studied by means of the whole-cell patch-clamp technique. Prajmalium (10(-9) to 10(-6) M) depressed the Na current (INa) in a dose- and use-dependent manner. In the same range of concentrations, prajmalium induced a dual effect on the high (ICaL) and low (ICaT) threshold Ca currents (the latter being only present in atrial cells). At a low concentration (10(-9) M), prajmalium increased both Ca currents while at high concentrations (10(-6) M) it depressed them. Prajmalium appeared very potent on ICaT although this current is generally reported to be barely sensitive to agonists and drugs. The action of the drug was also accompanied by a shortening in the half-time of inactivation of the Ca currents and a slight hyperpolarizing shift of their availability curves. The increase in ICaL by prajmalium was not prevented by prazosin (10(-7) M) nor by propranolol (10(-6) M), and it was also observed after ICaL had been fully stimulated by isoproterenol (10(-7)M). Nifedipine (10(-6) M), however, was able to prevent or block the prajmalium-induced increase in ICaL. Some similarities between the actions of prajmalium and dihydropyridine agonists on Ca currents are discussed.

Animals↗
PubMed1991

Extracellular MgATP activates the Cl-/HCO3- exchanger in single rat cardiac cells.

1. The effect of extracellular MgATP on cytosolic pH (pHi) was investigated in single rat cardiac cells loaded with the pH-sensitive probe Snarf-1. 2. Basal pHi in HEPES-buffered solution (containing 4.4 mM-NaHCO3) was 7.08. MgATP induced a transient acidification followed by an alkalinization. The latter is prevented by ethylisopropylamiloride (EIPA) and has been attributed to the activation of the Na+/H+ antiport. The MgATP-induced acidification reached a maximal value of 0.42 +/- 0.03 pH units (U pH). It was concentration dependent with a K0.5 of 2.6 microM-MgATP. This acidification was also observed with the same magnitude in the presence of the more physiological Krebs-bicarbonate buffer but was greatly reduced in nominally HCO3-free HEPES. 3. The MgATP-induced acidification was prevented by 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS), probenecid and ethacrynic acid but not by bumetanide. It was dependent upon the external chloride concentration. The K0.5[Cl-] was 9 mM and the maximal acidification required 60 mM-Cl-. 4. MgATP accelerated the recovery from an alkalinization triggered by a pulse of NH4Cl. The nucleotide also facilitated the efflux of HCO3- when the cell was switched from a Krebs-bicarbonate buffer gassed with 5% CO2 to an HEPES buffer. 5. The acidification was only evoked by MgATP and its poorly hydrolysable analogues but not by the other nucleotides (ADP, GTP (guanosine triphosphate), CTP (cytidine triphosphate) UTP (urodine triphosphate), ITP (inositol triphosphate) nor by adenosine. It required the presence of Mg2+ ions. 6. These results provide evidence that MgATP activates the Cl-/HCO3- exchanger and that this activation accounts for the acidification. Such an activation could not be related to the P1- or the P2-purinergic receptors since it requires triphosphate adenylic compounds and Mg2+ ions. This leads us to suggest the existence of a putative P3-type of purinergic receptor.

Adenosine Triphosphate↗
PubMed1990

Effects of purinergic stimulation on the Ca current in single frog cardiac cells.

Ca current (ICa) was measured by whole-cell voltage clamp in single cells isolated from frog ventricle, in which the Na current was inhibited by tetrodotoxin (0.3 microM) and K currents were blocked by substituting K with 120 mM intracellular and 20 mM extracellular Cs. The influence of stimulation by ATP (0.1-100 microM) was assessed in the presence of propranolol (1 microM) or pindolol (0.1 microM), prazozin (0.1 microM) and atropine (10 microM). ATP, in the micromolar range, had two types of effect. Like other P1-purinoagonists, it antagonized the increase in ICa elicited by beta-adrenostimulation. When added alone, 1 microM ATP could increase ICa up to twofold. An increase in ICa was also observed even after it had been maximally enhanced by intracellularly applied cAMP (50 microM). Voltage dependence and kinetics of ICa were not affected. These effects were considered to be related to P2-purinoceptor activation. At higher ATP concentrations the increase in ICa was less; at 100 microM, ATP reduced ICa. The ATP-induced increase in ICa was prevented by internal perfusion of the cells with GDP [beta-S] or neomycin, respectively, to block signal transduction to phospholipase C or its phosphodiesterase activity on the polyphosphoinositides. We conclude that P2-purinoceptor stimulation increases the Ca current in frog ventricular cells by a pathway that might involve phosphoinositide turnover.

Adenosine Triphosphate↗
PubMed1990

Mechanism of extracellular ATP-induced depolarization in rat isolated ventricular cardiomyocytes.

Adenosine triphosphate (ATP) is released during neural stimulation and cardiac hypoxia and several mechanisms of its action have been reported in different tissues. ATP stimulates P1 and P2 purinergic receptors; it also activates receptor-operated channels and increases membrane permeability to small ions. In single rat ventricular cells under whole-cell patch-clamp, a stepwise application of ATP in the micromolar range affects the resting potential and membrane currents through an entirely novel mechanism of action which involves several steps. Extracellular ATP induces an inward current and depolarization of the cell, leading to automaticity. The inward current is non-specific for cations, its reversal potential is around -5 mV. The conductance change evoked by ATP is suppressed by 4,4-diisothiocyanostilbene 2,2-disulphonic acid (DIDS) and low-chloride media and is prolonged by adding intracellular bicarbonate. These effects are specific for ATP in the presence of magnesium and are not evoked by a non-hydrolysable analogue of ATP or in the presence of vanadate. Other nucleotides are ineffective. We propose that ATP hydrolysis activates the chloride/bicarbonate (Cl-/HCO3-) exchanger. The induced local acidification could then increase intracellular free calcium and as a consequence, increases the sarcolemmal conductance. Thus, a sudden release of ATP in pathological conditions would induce a depolarization which could generate ventricular arrhythmias.

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗
PubMed1990

Inhibition by alcohols, halothane and chloroform of the Ca current in single frog ventricular cells.

Changes in the Ca current, ICa, evoked by ethanol, long chain alcohols (hexanol, octanol and dodecanol) and two halogenated anesthetics (chloroform and halothane) were studied in single isolated frog ventricular cells using the whole cell patch clamp technique. Each of these general anesthetics induced a concentration-dependent decrease in ICa which was usually followed by a partial recovery during long application (5 min). However, an increase in ICa was observed with low ethanol concentrations. The potency of the alcohols was related to their oil-water partition coefficient in that the longer the carbon chain the greater the reduction in ICa. For example, to reduce ICa to half its control amplitude required 300 mM ethanol but only 0.3 microns dodecanol. The kinetics of ICa were also altered by exposure to the alcohols. The halogenated derivatives also decreased ICa and altered the kinetics of ICa, but in a different way to the alcohols. The complex and different alterations of ICa are probably the consequence of multifactorial effects induced by the general anesthetics. Previous suggestions have included alterations in the membrane lipid bilayer, membrane surface charge and hydrophobic pockets on channel proteins. We suggest that the increase in Cai may also participate to these effects.

Action Potentials↗
PubMed1990

Calcium current in single cells isolated from normal and hypertrophied rat heart. Effects of beta-adrenergic stimulation.

The L-type calcium current was investigated in normal and hypertrophied rat ventricular myocytes as a possible cause of the action potential lengthening that has been reported during hypertrophy. Regulation of the calcium current (ICa) by a beta-adrenergic agonist (isoproterenol) was also analyzed since beta-agonist-induced positive inotropy is less marked in hypertrophied heart. Left ventricular hypertrophy was induced by stenosis of the abdominal aorta. For recording ICa, the whole-cell patch-clamp technique was used. Potassium currents were suppressed by replacing K+ ions with Cs+ ions in both the extracellular and intracellular media, and sodium current was blocked by 50 microM tetrodotoxin. The Ca2+ current was larger in hypertrophied cells (2.2 +/- 0.6 nA [n= 31]) than in normal cells (1.2 +/- 0.5 nA [n = 33]). However, if one relates ICa amplitude to the cell membrane area, as estimated by membrane capacitance measurement, no significant difference was observed in current density (8.5 +/- 2.5 pA/pF [n = 31] and 8.3 +/- 2.1 pA/pF [n = 33] in hypertrophied and in normal cells, respectively). In both cell types, ICa displayed the same voltage and time dependence. When expressed as a percentage, the maximal increase in ICa amplitude that was obtained with 100 nM isoproterenol was less in hypertrophied cells (+78%) than in normal cells (+120%). The sensitivity of ICa to beta-adrenergic stimulation was not modified: EC50 was 3.8 nM for hypertrophied cells and 4.8 nM for normal cells. Forskolin and cyclic AMP were as effective in both cell types. Stimulation of ICa by beta-adrenergic agonist was decreased in agreement with a reduced number of binding sites of beta-agonists and/or an altered coupling of the G-proteins.

Action Potentials↗
PubMed1990

Neurohormonal control of calcium sensitivity of myofilaments in rat single heart cells.

To investigate the changes in the properties of cardiac contractile proteins due to neurohormonal stimulation, different agonists were applied to single cells isolated from rat ventricle. Cells were then rapidly skinned by Triton X-100, and force was recorded after gluing the cells to a strain gauge. The skinned cells had mechanical properties very similar to those described for thin trabeculas. Tension-pCa relations were highly reproducible from one cell to another, with sarcomere length fixed at 2.1 microns. The application of alpha 1-adrenergic and muscarinic agonists, which increase the turnover of phosphatidylinositol, for 5 minutes before skinning the cells increased the sensitivity of the myofilaments to calcium, as indicated by a leftward shift of the tension-pCa relation, whereas beta-adrenergic stimulation induced a rightward shift. The increase in calcium sensitivity was also evoked by protein kinase C activators such as 1,2-dioctanoylglycerol and phorbol 12-myristate 13-acetate but not by protein kinase C itself or by purinergic agonists, although the latter also increased the turnover of phosphatidylinositol. Incubation of the skinned cells with phosphatase reversed the alterations in calcium sensitivity induced by previous agonist stimulation of the intact cells. In conclusion, this study demonstrates a potentially influential mechanism for the physiological regulation of cardiac muscle contractility.

Actin Cytoskeleton↗
PubMed1990

The mechanism of positive inotropy induced by adenosine triphosphate in rat heart.

When applied extracellularly in the micromolar range, ATP and related compounds induced a positive inotropy in the rat papillary muscle. This was also true in the rat auricle after pertussis toxin treatment. Then, in both tissues, ATP further increased the contraction after a maximal beta-adrenergic stimulation. The increase in contractile force could be related to the increase in the calcium current. The L-type calcium current was measured by whole-cell patch-clamp recording in single cells isolated from the rat ventricle after the sodium and potassium currents were inhibited by tetrodotoxin and cesium, respectively. When added alone, 10 microM ATP increased the calcium current by 60%. Adenosine 5'-O-(3-thiotriphosphate) was also able to increase calcium current. Adenosine was much less effective, and GTP, UTP, CTP, and ITP were without effect. A similar increase in calcium current was observed when ATP was added in addition to a maximal stimulation by a beta-adrenergic agonist or after internal perfusion with cyclic AMP. However, this increase was preceded by a transient decrease whose origin could not be attributed to a P1-purinergic agonistic effect of ATP. The transient decrease was not elicited by adenosine or in a magnesium-free HEPES solution and was not suppressed after pertussis toxin treatment. This effect appeared related to the variations in the holding current also observed upon ATP application. Together with vasodilation, ATP and adenine compounds induced positive inotropy. The latter effect could be attributed in part to the increase in calcium current and was independent of cyclic AMP. Both effects are complementary with the beta-adrenergic stimulation and can help healthy cells to compensate the failing zone from which ATP could be released.

1-Methyl-3-isobutylxanthine↗
PubMed1989

Inhibition of ICa in single frog cardiac cells by quinidine, flecainide, ethmozin, and ethacizin.

The effects of four class I antiarrhythmic compounds on the Ca current (ICa), recorded with whole cell patch clamp in single isolated frog ventricular cells, were compared. Na and K currents were blocked by tetrodotoxin and Cs. Quinidine and flecainide induced an apparent tonic block with a 50% effective dose (ED50) at 10 and 20 microM, respectively; there was no clear use-dependent inhibition. Ethmozin and ethacizin, two phenothiazine derivatives, exhibited both tonic and use-dependent inhibition. Ethacizin was at least 10 times more potent than ethmozin; at 1 microM, it induced a 15% tonic block and 5, 35, and 42% use-dependent block at 0.125, 1, and 2 Hz, respectively. These compounds appeared only 3- to 10-fold less efficient on the ICa than on the Na current recorded in parallel experiments, with flecainide showing the largest different potency. All four compounds shifted the availability curves by a few millivolts toward hyperpolarization, had a clear voltage-dependent inhibition, and slowed reactivation, the latter effect being more marked with less negative holding potential. Consequently, the absence of use-dependence inhibition with quinidine and flecainide could be the consequence of a very fast association of the two compounds with the open channels that would be complete during the 200-ms depolarizing pulse. Since arrhythmias are frequently associated with tissue depolarization and can be related to Ca-dependent action potential and slow conduction, the inhibition of the ICa reported above should account, in part, both for the antiarrhythmic and the negative inotropic effects of these compounds.

Animals↗
PubMed1989

Ethacizin blockade of calcium channels: a test of the guarded receptor hypothesis.

The effect on calcium channels of the sodium channel antagonist, ethacizin, was studied in isolated frog ventricular cells using the whole cell voltage-clamp methodology. Ethacizin was found to block inward calcium current in a frequency-, voltage-, and concentration-dependent manner. The frequency-dependent blocking properties were modeled by considering the drug interaction with a voltage-dependent mixture of calcium channels harboring either an accessible or an inaccessible binding site. With repetitive stimulation, the pulse-to-pulse reduction in peak current is shown to be exponential, with a rate linearly related to the interstimulus interval and the drug concentration. Observed frequency- and concentration-dependent blocks were consistent with the predictions of the model, and mixture-specific rate constants were estimated from these data. The negligible shift in channel inactivation and the reduction of apparent binding and unbinding rates with more polarized membrane potentials imply the active moiety of ethacizin blocks open channels and is trapped within the channel at resting membrane potentials. The binding rate at 0 mV is similar to that observed in studies of interactions of other open channel blocking agents with voltage- and ligand-gated channels.

Animals↗
PubMed1988

Functional state of myofibrils, mitochondria and bound creatine kinase in skinned ventricular fibers of cardiomyopathic hamsters.

Functional states of cardiac contractile apparatus and mitochondria were studied in hereditary cardiomyopathic hamsters (CHF 146) and control golden hamsters using cardiac fibers skinned by two different techniques. The Triton X-100 skinned fibers obtained from diseased animals of 175 to 200 days old, or from control animals, demonstrated the same resting and maximal Ca-activated tensions, the same stiffness, the same rate of tension recovery after quick stretch; the fibers from cardiomyopathic animals differed only by a slightly increased calcium sensitivity. Functional activity of myofibrillar creatine kinase in cardiomyopathy was decreased as indicated by a smaller shift in the pMgATP/rigor tension curve to lower [MgATP] in the presence of phosphocreatine and by a slower rate of the tension recovery after quick stretch in the presence of phosphocreatine and ADP (without ATP). The saponin-skinned fibers allow evaluation of the respiration properties of the total tissue mitochondria. Data obtained in the preparations isolated from diseased animals of two ages (75 to 100 and 175 to 200 days) showed that the ratio of maximal ADP-stimulated respiration rate to the respiration rate in the absence of ADP (an analog of respiration control index) was unchanged in myopathy as compared with age-matched controls. However stimulation of respiration after an addition of creatine at submaximal ADP concentration was observed to be respectively 1.45 times and 3.5 times less in the preparations from younger and older myopathic animals as compared with their respective controls, thus indicating the impairment of functional coupling between mitochondrial creatine kinase reaction and oxidative phosphorylation. These results suggest that hereditary cardiomyopathy is associated with alterations in myocardial creatine kinase system, while myofilaments and mitochondria preserve their basic functional properties.

Adenosine Diphosphate↗
PubMed1988

Halothane, enflurane, and isoflurane decrease calcium sensitivity and maximal force in detergent-treated rat cardiac fibers.

This study was designed to test the hypothesis that the volatile anesthetics directly affect cardiac contractile proteins. For this purpose, the effects of various anesthetic doses of halothane, enflurane, and isoflurane on myocardial calcium sensitivity and maximal calcium-activated force were examined in rat cardiac fibers skinned with Triton X-100. In this preparation, all membranes are chemically destroyed, and the sarcoplasmic reticulum is not functional. The three anesthetics shifted the pCa/tension curves (pCa = -log10[Ca2+]) toward higher calcium concentrations and decreased pCa for half-maximum activation (pCa50) in a dose-dependent and reversible fashion without changing the slope of this relationship (Hill coefficient). No differences between agents were observed at equipotent anesthetic concentrations. In addition, the three anesthetics decreased both maximal activated tension and tension at half-maximal activation in a dose-dependent fashion. Both the decrease in calcium sensitivity and the decrease in maximum activated tension may contribute to the negative inotropic effects of these agents. The relative importance of such effects compared with the other mechanisms of action remains to be determined, however.

Animals↗
PubMed1988

ATP and other adenine compounds increase mechanical activity and inositol trisphosphate production in rat heart.

1. The effects of adenosine 5'-triphosphate (ATP) and other adenine compounds were examined on rat papillary and right ventricular muscles in the presence of 10 microM-propranolol, 10 microM-atropine and 0.1 microM-prazosin or 10 microM-phentolamine. 2. Adenosine, adenosine 5'-monophosphate (AMP), adenosine 5'-diphosphate (ADP), ATP and alpha,beta-methylene ATP (APCPP) produced small positive inotropic effects, sometimes preceded by transient negative effects. 3. 8-Phenyltheophylline (8-PT), a P1-purinoceptor antagonist antagonized the negative effects and increased the positive inotropy induced by ATP and adenosine. 4. In the presence of APCPP, a P2-purinergic agonist, ATP had only negative inotropic effects. 5. Adenosine and ATP increased inositol 1, 4, 5- and inositol 1, 3, 4-trisphosphate as well as inositol mono- and bisphosphate formation. Maximal effects were obtained at concentrations of 0.5 mM. 6. APCPP increased inositol phosphate formation while 8-PT did not prevent the effects of adenosine and ATP. 7. It is suggested that P2-purinoceptor activation induces both a positive inotropy and an increase in inositol-lipid metabolism in rat ventricular muscles.

Adenine Nucleotides↗
PubMed1988

Sustained function of normoxic hearts depleted in ATP and phosphocreatine: a 31P-NMR study.

A model of high-energy phosphate depletion was developed in the normoxic isovolumic rat heart perfused with acetate, 2-deoxy-D-glucose (2DG), and insulin. Intracellular phosphorylation of 2DG abstracts phosphorus from its normal pathways. This results in a decrease of high-energy phosphates without any increase in Pi. During the first 15 min of 2DG phosphorylation, the changes in ATP, Pi, and intracellular pH (pHi) were slight, and work was unaltered, although phosphocreatine (PCr) concentration dropped by 50%. After 45 min, the heart reached a new steady state characterized by a drastic reduction in both PCr and ATP: PCr was 15% of control, and in most hearts ATP became invisible on the nuclear magnetic resonance (NMR) spectra. Nevertheless, the heart still developed 65% of its original systolic pressure, whereas diastolic pressure was unchanged. Oxygen consumption per unit work remained constant during 2DG perfusion. This is, to our knowledge, the first experimental model of sustained cardiac contractility at such low contents of both ATP and PCr. However, our results are compatible with present knowledge of the cytosolic energy transfer by PCr and of the control of force in myofilaments.

Adenine Nucleotides↗
PubMed1987

Role of creatine kinase in force development in chemically skinned rat cardiac muscle.

The influence of phosphocreatine in the presence or absence of MgATP and MgADP was studied in Triton X-100-treated thin papillary muscles and ventricular strips of the rat heart. The pCa/tension relationships, the pMgATP/tension relationships, and the tension responses to quick length changes were analyzed. The results show three major consequences of the reduction of the phosphocreatine concentration in the presence of millimolar concentrations of the MgATP. (a) The resting tension and the maximal Ca2+-activated tension were increased, and the pCa/tension relationship was shifted toward higher pCa values and its steepness was decreased; these effects were enhanced by the inclusion of MgADP. (b) The time constant of tension recoveries after quick stretches applied during maximal activation was increased, while the extent of these recoveries was decreased. (c) The study of pMgATP/tension relationships in low Ca concentrations showed that the decrease in phosphocreatine induced a shift toward higher MgATP values with no changes in maximal rigor tension or the slope coefficient; these effects were increased by the increase in MgADP and were independent of the preparation diameter. Thus, modifications of the apparent Ca sensitivity and resting and maximal tension when phosphocreatine is decreased seem to be due to an increasing participation of rigor-like or slowly cycling cross-bridges spending more time in the attached state. These results suggest that endogenous creatine kinase is able to ensure maximal efficiency of myosin ATPase by producing a local high MgATP/MgADP ratio.

Adenosine Diphosphate↗
PubMed1987

Reversible MM-creatine kinase binding to cardiac myofibrils.

Skinned rat papillary muscles and purified preparations of rat cardiac myofibrils were used to study the nature of the interaction of creatine kinase with cardiac myofibrils. High activity of creatine kinase (2 IU/mg protein in fibers and 0.9 IU/mg in purified myofibrils) was due mostly to reversibly bound enzyme. This activity could be removed and rebound. The process of creatine kinase rebinding was characterized by apparent Km value of 0.14 mg/ml (approximately equal to 2 X 10(6) M). Rebinding of creatine kinase to cardiac myofibrils restored the phenomenon of functional compartmentation of adenine nucleotides in myofibrillar space and restored the ability of phosphocreatine to decrease the rigor tension in the presence of MgADP. The physiological experiments with quick length changes showed that rebinding of creatine kinase to skinned papillary muscle also restored Ca sensitivity, increased maximal tension development, decreased stiffness, and restored the tension recovery after quick length changes in muscle under condition of inhibition of endogenous creatine kinase by 1-fluoro-2,4-dinitrobenzene. It is concluded that creatine kinase reversibly bound to cardiac myofibrils is involved in the energy supply for cardiac contraction.

Adenine Nucleotides↗
PubMed1986

Inositol phosphate production following alpha 1-adrenergic, muscarinic or electrical stimulation in isolated rat heart.

A possible participation of polyphosphoinositide metabolism in the excitation-contraction coupling in heart was investigated. Isolated rat ventricles prelabelled with myo-[2-3H]inositol were stimulated by conditions that increase mechanical activity. Both noradrenaline and carbachol increased the basal level of IP3, IP2 and IP by the activation of alpha 1-adrenergic and muscarinic receptors, respectively. Electrical stimulation accelerated inositol lipid degradation by phospholipase C thus enhancing the IP3 level as compared to quiescent ventricles. It is proposed that IP3 may be involved in excitation-contraction coupling in cardiac tissue.

Animals↗