PubMed Health⌕ Search

Biomedical subjects

J Nargeot

Publications and source records attributed to J Nargeot.

At least 91 records · Page 5Linked to original sources
PubMed1991

Properties and Modulation of Ca channels in adult human atrial cells.

Ca-channel currents have been investigated in single cells isolated from adult human atrium using the whole-cell patch clamp technique. Ca-channel currents are activated at voltage positive to -40 mV, peak between -10 and 0 mV and inactivate with a slow decay when Ba2+ ions (5 mM) are used as charges carrier. These properties correspond to those of the high voltage activated, DHP-sensitive, (L-type) Ca channel. No low voltage activated (T-type) currents have been evidenced. The present work also provides the first report about the modulation of Ca channels in adult human atrial cells by beta-adrenergic agonists and dihydropyridines (agonists and antagonists). Electrophysiological and pharmacological properties of these Ca channels are qualitatively similar to those of the L-type Ca currents recorded from cardiac animal cells. However, at a physiological calcium concentration (2 mM), basal Ca currents are often very small or even absent but are revealed following the addition of the dihydropyridine (DHP) agonist Bay K 8644. Whether the decrease of the basal Ca current amplitude may be related to the chronic pretreatment of the patients by Ca channel blockers or to the pathology is discussed.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed1991

Modulation of Ca currents in isolated frog atrial cells studied with photosensitive probes. Regulation by cAMP and Ca2+: a common pathway?

We have studied the regulation of cardiac Ca current by intracellular cyclic AMP (cAMP) and Ca2+, using photosensitive, caged compounds and the whole-cell, patch-clamp technique in isolated frog atrial cells. Although both low voltage activated (LVA) and high voltage activated (HVA) Ca channels were found to be present in these cells, only the HVA Ca currents were sensitive to modulation by isoproterenol or dihydropyridines (DHPs). The application of extracellular isoproterenol, as well as the photorelease of intracellular cAMP or Ca2+ at micromolar and submicromolar concentrations, respectively, had no effect on LVA Ca currents. In contrast, these agents: (i) increased the amplitude of currents through HVA channels, carried by either Ca2+ or Ba2+ with a similar time-course, (ii) slowed the decay of the current when Ba2+ was the permeating ion, and (iii) modulated the agonist effect of the DHP Bay-K 8644. The strong similarities between the effects of cAMP and Ca2+ suggest that both of these intracellular messengers might eventually lead to the phosphorylation of HVA Ca channels. It is possible that Ca-dependent phosphorylation of the channels may account for the potentiation of Ca current induced by repetitive stimulation.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed1990

Functional expression of Ca2(+)-mobilizing alpha-thrombin receptors in mRNA-injected Xenopus oocytes.

alpha-Thrombin (TH) initiates a program of intracellular events that lead to DNA replication in quiescent CCL39 Chinese hamster lung fibroblasts via membrane receptors that have yet to be characterized at a molecular level. Functional TH receptors were expressed in Xenopus laevis oocytes following injection of poly(A)+ RNA from TH-responsive CCL39 cells; their presence was demonstrated by TH-stimulated 45Ca2+ efflux or Ca2(+)-dependent Cl- channel activation. In voltage clamp experiments on microinjected oocytes a Ca2(+)-activated Cl- current was detected in response to TH (0.2-10 U/ml). The TH response was blocked by a specific TH inhibitor, and potentiated by addition of FGF or intracellular injection of GTP-gamma-S.

Animals↗
PubMed1990

Cyclic-AMP-dependent phosphorylation modulates the stereospecific activation of cardiac Ca channels by Bay K 8644.

Voltage-gated Ca channels have been reported to be regulated by membrane potential, phosphorylation and binding of specific agonists or antagonists such as dihydropyridines. We report here evidence that cyclic AMP (cAMP) modulates the activation of Ca-channel current by the dihydropyridine agonist Bay K 8644. Bay K 8644 (racemate) alone induces a primary voltage-dependent, potentiating effect on peak current amplitude and accelerates the current decay. In contrast, in the presence of cAMP activators, we observed a striking slowing of the decay in addition to the increase in peak current. The agonist (-)-Bay K 8644, but not the antagonist (+)-Bay K 8644, when applied in combination with cAMP, forskolin or isoproterenol, mimics the effect of the racemate. We have interpreted the results presented here in respect of a cAMP-dependent modulation of Bay K 8644 effects on cardiac Ca-channel currents. It may open the new perspective that dephosphorylated and phosphorylated Ca channels have distinct pharmacology.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed1990

Zinc has opposite effects on NMDA and non-NMDA receptors expressed in Xenopus oocytes.

Pharmacological characterization of Zn2+ effects on glutamate ionotropic receptors was investigated in Xenopus oocytes injected with rat brain mRNA, using a double microelectrode, voltage-clamp technique. At low concentration, Zn2+ inhibited NMDA currents (IC50 = 42.9 +/- 1.3 microM) and potentiated both AMPA (EC50 = 30.0 +/- 1.2 microM) and desensitized kainate responses (EC50 = 13.0 +/- 0.1 microM). At higher concentrations, Zn2+ inhibited non-NMDA responses with IC50 values of 1.3 +/- 0.1 mM and 1.2 +/- 0.3 mM for AMPA and kainate, respectively. The potentiation of AMPA or quisqualate currents by Zn2+ was more than 2-fold, whereas that of the kainate current was only close to 30%. This potentiating effect of Zn2+ on AMPA current modified neither the affinity of the agonist for its site nor the current-voltage relationship. In addition, 500 microM Zn2+ differentially affected NMDA and non-NMDA components of the glutamate-induced response. The possible physiological relevance of Zn2+ modulation is discussed.

Animals↗
PubMed1990

Characterization of voltage-dependent calcium channels expressed in Xenopus oocytes injected with mRNA from rat heart.

1. The properties of voltage dependent cardiac Ca channels expressed in Xenopus laevis oocytes after injection of mRNA from rat heart were investigated using the double-microelectrode voltage-clamp technique. 2. Endogenous Ba current (IBa,E) and expressed cardiac Ba current (IBa,C) were studied at various external concentrations of barium (Ba2+). These two entities could be distinguished by their amplitude and their pharmacology. IBa,C was more sensitive to the inorganic Ca channel blocker manganese (Mn2+). The contaminant IBa,E presented properties of voltage dependence identical to IBa,C, but was negligible in the presence of a low external Ba2+ concentration (2 mM). 3. In 2 mM-Ba2+, IBa,C activated at -35 mV, peaked at -14 mV, and reversed at +26 mV. Steady-state inactivation properties, in consideration of the half-inactivation potential of -35 mV, were also typical of L-type Ba currents. However, the decay of IBa,C was very slow (time constant of inactivation near 600 ms). No evidence for the expression of cardiac transient Ca channels (T-type) was found. 4. IBa,C was enhanced after exposure to the 1,4-dihydropyridine (DHP) agonist Bay K 8644. The enhancement of IBa,C was voltage dependent (maximum at -30 +/- 5 mV) and associated with a slowing in current decay. Current-voltage and concentration-response curves obtained for various Ba2+ concentrations revealed an antagonism between external Ba2+ and the 1,4-DHP agonist Bay K 8644. Similar results were found using the (-)Bay K 8644 pure agonist isomer. 5. We conclude that oocytes injected with mRNA from rat heart expressed only the high threshold, long-lasting or L-type Ca channels. The availability of expressed L-type Ca channels for quantitative pharmacological studies using low Ba2+ concentration has been demonstrated.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed1990

Two pathways for Ca2+ channel gating differentially modulated by physiological stimuli.

In cardiac muscle, Ca2+ entry through voltage-gated Ca2+ channels plays an important role in the generation of action potentials and in the development of tension. Although it had been assumed that there was a single type of cardiac Ca2+ channel, recent studies reveal that multiple Ca2+ channel types coexist in some myocardial cells. Here, we report that macroscopic Ca2+ current (ICa) waveforms in isolated adult rat ventricular myocytes comprise two kinetically distinct components; these are referred to here as ICa (fc) and ICa (sc) to denote the fast and slow components, respectively, of ICa decay. In contrast to findings in other cells, the properties of ICa (fc) and ICa (sc) suggest the presence of two pathways for gating of a single type of high-threshold Ca2+ channel rather than two distinct Ca2+ channel types. In addition, gating via ICa (fc) and ICa (sc) is regulated by changes in membrane potential and stimulation frequency. Hyperpolarized potentials and low stimulation frequencies reveal preferential activation via ICa (fc); depolarized potentials and high stimulation frequencies, in contrast, favor activation via ICa (sc). After exposure to beta-adrenergic agonists or the Ca2+ agonist BAY K 8644, peak ICa amplitudes increase owing to the preferential augmentation of ICa (fc); beta-agonists and BAY K 8644 also increase ICa (sc), albeit to a smaller extent than ICa (fc). Thus, in addition to voltage- and frequency-dependent regulation, the two pathways for Ca2+ channel gating are modulated differentially by beta-adrenergic and Ca2+ channel agonists.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed1989

Electrophysiological expression of endothelin and angiotensin receptors in Xenopus oocytes injected with rat heart mRNA.

Functional endothelin and angiotensin receptors have been expressed in Xenopus oocyte following the microinjection of rat heart mRNA. Under voltage clamp conditions, application of these peptides clearly induced oscillatory Ca2+-activated chloride currents in a dose-dependent manner. In addition, no direct modulation of expressed or native cardiac Ca channels was observed.

Angiotensin II↗
PubMed1989

Augmentation of cardiac calcium current by flash photolysis of intracellular caged-Ca2+ molecules.

The entry of calcium ions into cells through voltage-activated Ca2+ channels in the plasma membrane triggers many important cellular processes. The activity of these channels is regulated by several hormones and neurotransmitters, as well as intracellular messengers such as Ca2+ itself (for examples, see refs 1-9). In cardiac muscle, myoplasmic Ca2+ has been proposed to potentiate Ca2+ influx, although a direct effect of Ca2+ on these channels has not yet been demonstrated. Photosensitive 'caged-Ca2+' molecules such as nitr-5, however, provide powerful tools for investigating possible regulatory roles of Ca2+ on the functioning of Ca2+ channels. Because its affinity for Ca2+ is reduced by irradiation, nitr-5 can be loaded into cells and induced to release Ca2+ with a flash of light. By using this technique we found that the elevation of intracellular Ca2+ concentration directly augmented Ca2+-channel currents in isolated cardiac muscle cells from both frog and guinea pig. The time course of the current potentiation was similar to that seen with beta-adrenergic stimulation. Thus Ca2+ may work through a similar pathway, involving phosphorylation of a regulatory Ca2+-channel protein. This mechanism is probably important for the accumulation of Ca2+ and the amplification of the contractile response in cardiac muscle, and may have a role in other excitable cells.

Animals↗
PubMed1989

A specific quisqualate agonist inhibits kainate responses induced in Xenopus oocytes injected with rat brain RNA.

Electrophysiological recording was used to study non-N-methyl-D-aspartate (NMDA) excitatory amino acid (EAA) receptors after injection of rat brain ribonucleic acid (RNA) in Xenopus laevis oocytes. Quisqualate (QA) induced two types of responses, a smooth one and an oscillatory one. These responses are probably mediated by the ionotropic (QAi, a cationic channel) and the metabotropic (QAp, a newly discovered receptor coupled to phospholipase C) QA receptors respectively. alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) only induced a smooth inward current suggesting that it acts only on QAi. Kainate (KA) also induced a smooth inward current, the maximal KA response being 10-fold higher than the maximal AMPA. AMPA inhibited the KA response in a dose-dependent and competitive manner. Amongst various complex hypotheses the simplest to explain these results would be that KA and AMPA both activate the same receptor-channel complex, AMPA inducing a smaller response than KA.

Animals↗
PubMed1989

Are Ba2+ and Sr2+ ions transported by the Na+-Ca2+ exchanger in frog atrial cells?

Ba2+ and Sr2+ ions are widely used to replace Ca2+ ions for the study of Ca2+ channel currents in electrophysiological experiments. Using the double sucrose gap technique, we investigated the effects of Sr2+ and Ba2+ ions on the Na+ Ca2+ exchange activity in frog atrial fibres where it is the major relaxation mechanism. With either Sr2+ or Ba2+ ions instead of Ca2+ in the extracellular bath, Na-free contractures reversibly developed but with different kinetics. Voltage clamp experiments showed that the tonic tension recorded in the presence of Sr2+ or Ba2+ was markedly increased following the addition of monensin, a Na+ ionophore known to increase the intracellular Na+ activity. In Na-free solutions (Li-substituted), it was possible to induce contractures by substituting Sr2+ or Ba2+ ions for extracellular Ca2+. These contractures could be relaxed by reintroducing Na+ or Ca2+ ions in the extracellular medium. Taken together, these results suggest that Sr2+ and Ba2+ ions can interact with the Na+-Ca2+ exchange mechanism and potentially participate not only in Na+-cation but also in Ca2+-cation exchanges on either side of the sarcolemmal membrane.

Action Potentials↗
PubMed1988

Effects of the Ca-antagonist nicardipine on K+ currents and Na+-Ca2+ exchange in frog atrial fibres.

This paper concerns the specificity of nicardipine, a calcium antagonist from the dihydropyridine class which is used in the treatment of cardiovascular disorders. It is well established that in cardiac cells dihydropyridines inhibit the calcium current (Isi) and the correlated phasic tension. In this study we demonstrate that nicardipine, in the concentration range which blocks Isi (KD = 1 microM) also decreases the amplitude of the potassium-delayed current (KD = 3 microM) in frog atrial fibres. Moreover, tonic tension, which has been reported to be directly dependent on the Na+-Ca2+ exchange, was also reduced by nicardipine and the time course of the onset of both contraction and relaxation was significantly slowed. These results indicate that on depolarized membranes, dihydropyridines probably affect voltage-dependent mechanisms with a high threshold of activation that are unrelated to Ca2+ channels, such as potassium-delayed current and Na+-Ca2+ exchange.

Animals↗
PubMed1987

Direct evidence for the modulation of human platelet cytosolic free Ca2+ by intracellular cyclic AMP produced with a photoactivatable derivative.

We have previously reported that intraplatelet "cyclic AMP jumps" produced with newly synthesized photoactivatable cyclic AMP analogue, inhibited washed rat platelet aggregation and serotonin release as induced by thrombin. Using the same approach on human platelets, thrombin-induced platelet aggregation was dose-dependently inhibited only when a flash was delivered. The mechanism of action of intraplatelet cyclic AMP as resulting from photolysis could be by controlling the level of cytosolic Ca2+. In order to test this hypothesis, the same protocol was used on human platelets preloaded with the internal Ca2+ fluorescent indicator, Quin 2, we found that the extent and the rate of the rise of the cytosolic Ca2+ induced by thrombin were dramatically decreased, in the presence of the photoactivatable cyclic AMP, only following photoirradiation. In addition, the flashes were produced, in the presence of photoactivatable cyclic AMP, after the thrombin-induced rise of internal Ca2+ had reached its peak. In these conditions, photoirradiation caused a rapid fall in fluorescence. These experiments provide the first direct evidence that intracellular cyclic AMP is involved in the control of platelet cytosolic Ca2+ by inhibition of its mobilization and by stimulation of its sequestration.

Aminoquinolines↗
PubMed1987

Tension activation and relaxation in frog atrial fibres. Evidence for direct effects of divalent cations (Ca2+, Sr2+, Ba2+) on contractile proteins and Na-Ca exchange.

The effect of alkali-earth cations (Ca2+, Sr2+, Ba2+) on the excitation-contraction coupling events of the frog atrial fibres were studied using a double mannitol gap voltage clamp technique coupled with a mechano-electric transducer. Photoremoval of the suppressive effect of nifedipine on the calcium channels allowed to obtain rapid transient Ca2+, Sr2+ or Ba2+ ions current jumps. The effect on the amplitude of the associated contraction was proportional to the current jumps. These results together with the correlation established between the estimated increase in the internal concentration of divalent cations and the amplitude of the phasic tension suggest that the essential source of divalent cations for activation of contraction is the extracellular space. Also Ba2+ ions reduced the tonic tension and strongly slowed the relaxation of the phasic component whereas Sr2+ exhibited smaller effects. Sr2+ ions could be more efficient than Ba2+ ions in substituting for Ca2+ ions in the Na+-Ca2+ exchange mechanism known to regulate these two mechanical events. The conclusions are that the order of effectiveness of these ions (Ca2+ greater than Sr2+ greater than Ba2+) is the same with regard to transarcolemmal exchange for Na+ ions, presumed uptake by a "second relaxing system", activation of contraction, and inactivation of the slow inward current.

Action Potentials↗
PubMed1987

Electrophysiological analysis of the action of nifedipine and nicardipine on myocardial fibers.

The effects of nifedipine and nicardipine, 2 dihydropyridines (DHP) used in the treatment of cardiovascular disorders, were compared in frog atrial fibers. Rapid photolysis of nifedipine with a single UV flash (1-ms duration) reversed the block, allowing comparison of effects of both drugs on the same preparation, and manipulation of the calcium channel on a millisecond timescale. The results show that inhibition of the action potential (AP) and slow inward current (Isi) is more pronounced with nifedipine than with nicardipine. Concentration-effect relationships confirm that nicardipine (IC50 = 1 microM) is less potent than nifedipine (IC50 = 0.2 microM) in blocking cardiac calcium channels. Both DHP block calcium channels in the closed state at the resting potential, inducing a large tonic block (in the absence of stimulation). An additional phasic block can be observed with nifedipine and nicardipine. A slight voltage dependence to the block is observed for both DHP, their effects being enhanced depolarization holding potentials. Rapid unblocking of calcium channels by a single light flash, presented during the decay phase of Isi, reveals a higher affinity of DHP for inactivated channels. This effect is most pronounced when inactivation is slowed by using Ba++, Sr++, or Na+ ions as the current carriers. Open channel block is also suggested. The mechanism of DHP action on calcium channels can be described according to the "modulated receptor hypothesis". These DHP exhibit an additional nonspecific effect on potassium channels. It is concluded that nicardipine is a less potent Ca++ antagonist than nifedipine in atrial fibers and that the reduction of delayed potassium current, which occurs in a similar range of concentrations to the blockade of Isi, could also be involved in its therapeutic effects.

Action Potentials↗