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Biomedical subjects

G Vassort

Publications and source records attributed to G Vassort.

At least 55 records · Page 3Linked to original sources
PubMed1995

Cl- -HCO3- exchange in developing neonatal rat cardiac cells. Biochemical identification and immunolocalization of band 3-like proteins.

The Cl- -HCO3- exchanger is the main anionic exchanger (AE) that alleviates alkaline loads in cardiac cells. We recently identified in adult ventricular cells two membrane proteins (80 and 120 kD) immunologically related to the erythroid band 3 and likely to mediate the anion exchange. In the present study, we further investigated the Cl- -HCO3- exchanger activity concomitantly with the expression and intracellular localization of the band 3-like proteins during the development of neonatal rat cardiac cells maintained in culture for 17 days. Microspectrofluorometric measurements of pHi in single cells show that neonatal rat cardiomyocytes display a fully functional DIDS-sensitive Cl- -HCO3- exchanger at early stages of development. Neither basal pHi nor the anion exchange activity changes with different stages of the culture. In Western blotting with an anti-whole erythroid band 3 antibody, we found both the 80- and the 120-kD band 3-like proteins in whole heart and cultured neonatal cardiac cells. The 80-kD protein was also recognized by an anti-AE1 antiserum, whereas the 120-kD protein was specifically detected by an anti-cardiac AE3 antibody. Thus, we propose that the proteins are encoded by two different genes, AE1 and AE3, respectively. Subcellular fractionation of isolated and cultured cardiomyocytes revealed the presence of both proteins in the membrane, nuclear, and myofibril fractions. The results obtained in biochemical experiments corroborate the confocal images of immunostained neonatal cells, which demonstrate perinuclear location of band 3-like proteins at an early stage of development and their appearance within myofilaments after cell maturation.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1994

Pharmacological profile of the ATP-mediated increase in L-type calcium current amplitude and activation of a non-specific cationic current in rat ventricular cells.

1. The pharmacological profile of the ATP-induced increase in ICa amplitude and of ATP activation of a non-specific cationic current, IATP, was investigated in rat ventricular cells. 2. The EC50 values for ICa increase and IATP activation were 0.36 microM and 0.76 microM respectively. Suramin (10 microM) and cibacron blue (1 microM) competitively antagonized both effects of ATP. 3. The rank order of efficacy and potency of ATP analogues in increasing ICa amplitude was 2-methylthio-ATP approximately ATP approximately ATP gamma S. The derivatives alpha,beta-methylene-ATP, beta,gamma-methylene-ATP and beta,gamma-imido-ATP up to 500 microM had no significant effects. 4. The rank order of efficacy of ATP analogues in activating a non-specific cationic current, IATP, was 2-methylthio-ATP > ATP >> ATP gamma S. The rank order of potency was 2-methylthio-ATP approximately ATP. The EC50 of ATP gamma S could not be determined owing to its very low efficacy. 5. The ATP analogues alpha,beta-methylene-ATP, beta,gamma-methylene-ATP and beta,gamma-imido-ATP at 500 microM did not activate IATP but acted as antagonists of activation of IATP by ATP. 6. The results suggest that the increase in ICa amplitude induced by external ATP is due to activation of P2Y-purinoceptors. 7. The mechanism of IATP activation remains to be determined before the receptor subtype involved can be deduced.

Adenosine Triphosphate↗
PubMed1994

Cardiac T-type calcium current: pharmacology and roles in cardiac tissues.

A low threshold, voltage-gated calcium current is reported in most cardiac tissues but rarely in ventricular cells. This article reports some recently described characteristics and discusses their possible pathophysiologic implications. It also reviews the alterations induced in this current by a variety of chemical agents including several neuromediators in cardiac and other tissues.

Animals↗
PubMed1994

Effect of extracellular ATP on the Na+ current in rat ventricular myocytes.

Extracellular ATP concentration can rise because of its release by nerve terminals and by damaged cells during ischemia. After the activation of P2-purinergic receptors, ATP induces a positive inotropic effect and increases the L-type Ca2+ current via activation of a Gs protein but without cAMP production. In addition, ATP shifts the voltage characteristics of Ca2+ current toward hyperpolarized potentials. If ATP produced similar effects on the Na+ current (INa), this compound should also affect cardiac excitability and conduction. Using the whole-cell patch-clamp to record INa in rat ventricular cells, we show that extracellular application of ATP induced hyperpolarizing shifts in the current-voltage relation and the availability of INa. The ED50 for the shifts in both conductance and availability was obtained with 0.7 mumol/L ATP. Maximal shifts in conductance and availability were respectively 9.7 +/- 0.6 and 10.6 +/- 0.7 mV. The leftward shift of the availability curve is responsible for the decrease of INa amplitude at less polarized holding potentials. These effects were not cholera toxin sensitive and thus cannot be attributed to activation of the Gs protein. At 100 mumol/L, ATP gamma S and alpha,beta-methylene ATP could induce shift, whereas UTP and beta,gamma-methylene ATP as well as ADP and adenosine were without effect. Thus, depending on the resting membrane potential, ATP should either enhance excitability or favor slow conduction and weaken cardiac electrical homogeneity and consequently favor arrhythmia.

Adenosine Triphosphate↗
PubMed1994

Chloride dependence of pH modulation by beta-adrenergic agonist in rat cardiomyocytes.

The effects of beta-adrenergic agonists on pHi were studied on single ventricular myocytes isolated from adult rat heart and loaded with the acetoxymethyl ester (AM) form of the pH indicator SNARF-1. In modified Krebs' solution containing 20 mmol/L HEPES and 4.4 mmol/L HCO3-, isoproterenol (1 mumol/L) caused a significant decrease of steady-state pHi from 7.20 +/- 0.02 to 7.13 +/- 0.02 (mean +/- SEM) within 2 minutes. This acidification, which was also observed in myocytes that were preloaded with the Ca2+ chelator BAPTA and superfused with nominally Ca(2+)-free solution, was blocked by propranolol as well as by the specific beta 1-antagonist CGP 20712 A but not by the beta 2-antagonist ICI 118,551. Forskolin (10 mumol/L) induced a similar reversible decrease of pHi (average decrease, 0.11 +/- 0.02 pH unit). Furthermore, adenosine (100 mumol/L) substantially attenuated the isoproterenol-induced decrease of pHi. The effect of isoproterenol was not prevented by inhibitors of the Na(+)-H+ antiport, amiloride (1 mmol/L) and 2-N,N-hexamethylene amiloride (20 mumol/L). On the other hand, blockers of Cl- transport mechanisms, DIDS (200 mumol/L) and probenecid (100 mumol/L), inhibited this acidification, Isoproterenol also failed to induce a decrease of steady-state pHi in myocytes incubated in Cl(-)-free medium. Rather, the initial rate of rise of pHi observed on removal of external Cl- ions was significantly increased in the presence of isoproterenol or dibutyryl cAMP. Because the alkalinization induced by removal of Cl- ions is mainly due to reversal of the Cl(-)-HCO3- exchanger, the augmentation of this initial rate of pHi rise directly points to a beta-adrenergic stimulation of the exchanger. Furthermore, the pHi recovery following NH4Cl exposure was accelerated by isoproterenol in the presence of probenecid, indicating that the Na(+)-HCO3- cotransport and/or the Na(+)-H+ antiport also could be activated.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine↗
PubMed1993

Effect of an antihypertensive hydrazine derivative on Ca2+ current of single frog cardiac cells.

The effects of MP 518, an acylated 2-chlorobenzylidene hydrazidone derivative with antihypertensive properties were investigated on the Ca current, ICa, recorded under whole-cell patch-clamp in single frog ventricular cells. MP 518 (1-100 microM) had no effect on ICa under control conditions. However, at 10 microM it significantly increased the beta-adrenergic stimulated ICa, an effect similar to that of isobutylmethyl-xanthine (IBMX), a non-specific phosphodiesterase inhibitor. The effects of MP 518 and IBMX were not, however, additive. This positive effect was also observed with both compounds, MP 518 and IMBX, when a submaximal dose of cyclic AMP was intracellularly perfused. In the presence of IBMX or at a high concentration (100 microM), MP 518 had a negative effect on beta-adrenergic stimulated ICa. It was thus considered that the main effect of MP 518 is an antiphosphodiesterase activity, since the increase in ICa induced by low concentrations of MP 518 could be related to inhibition of cAMP degradation; however, at higher concentrations, MP 518 antagonizes beta-adrenergic stimulation, possibly at several levels. Such an antiphosphodiesterase activity can account for the vasorelaxant effects as well as the tachycardic effects of MP 518.

1-Methyl-3-isobutylxanthine↗
PubMed1993

Energetics of ionic contracture in rat-heart papillary muscles.

Energy dissipation and the bearing of tension during ionic contracture in myocardium may not result from one and the same process. To test this, comparative indirect and direct microcalorimetry determinations were made in non-perfused tissue-cell preparations under optimal oxygenation conditions (right papillary muscles, high-pO2 superfusion, 30 degrees C) before, during and after exposure to low-Na, high-K solutions. Over a 15-min contracture plus 45-min recovery cycle, both heat production rate (E), and the indirectly determined heat production rate (EO2) which is oxygen uptake multiplied by the overall energetic equivalent of O2 for nutrient oxidations, were constantly larger than basal rates. The two 60-min time integrals of this increase in metabolic rate were equal [30.3 +/- 3.7 and 31.0 +/- 3.9 (SE) J/g muscle wet weight (n = 9) for E and EO2 respectively]. During contracture however, E exceeded EO2 by 24% (4.7 +/- 1.7 J/g), and during the recovery period EO2 exceeded E by 21% (5.4 +/- 2.6 J/g). Whereas oxidative recovery of the energy lost by the preparation during 15-min contractures was complete, after longer contractures recovery did not occur or was incomplete. In keeping with the now prevalent idea that ion--namely Ca--transport activities are maintained foremost among cellular ATP-dependent processes and consume significant amounts of energy, the present finding that in a 15-min ionic contracture myocardium incurs not only some, but the maximum oxygen debt still compatible with complete oxidative recovery suggests that contracture tension is maintained at low energy cost, essentially by slow-cycling or "rigor" bridges as in hypoxic contractures, whereas heat is mainly related to intracellular calcium homeostasis.

Animals↗
PubMed1993

Modulation of L-type Ca channel activity by P2-purinergic agonist in cardiac cells.

The mechanism of enhancement of the L-type Ca current by a P2-purinergic agonist adenosine-5'-O-(3-thiotriphosphate) (ATP gamma S) was studied by recording single channel activity from cell-attached patches on rat isolated ventricular cells using patch pipettes containing 110 mM Ba2+. The application of ATP gamma S to the patch membrane through the pipette solution did not affect single channel activity. The addition of ATP gamma S to the bath containing a depolarizing solution was ineffective due to the voltage dependence of the purinergic stimulation. Bath application of ATP gamma S (100 microM) to control 4-(2-hydroxyethyl)-1-piperazine-ethanesulphonic acid (HEPES) solution increased the amplitude of ensemble average currents both by decreasing the probability of a blank sweep occurring and by increasing the number of openings per non-blank sweep. The single channel conductance (17 pS) was not changed by ATP gamma S. Both activation and inactivation curves were shifted towards hyperpolarized potentials by about 10 mV under P2-purinergic stimulation. Since ATP gamma S increased channel activity when applied via the bath, it must be supposed that a diffusible messenger is involved.

Adenosine Triphosphate↗
PubMed1993

Alpha 1-adrenoceptor and purinoceptor agonists modulate Na-H antiport in single cardiac cells.

We investigated the effects of an alpha 1-adrenoceptor (phenylephrine) and a purinoceptor agonist (ATP), both of which accelerate the phosphoinositide turnover, on the Na-H antiport activity of rat single cardiac cells using the pH-sensitive fluorescent indicator seminaphthorhodafluor-1 (SNARF-1). Both phenylephrine, in the presence of a beta-adrenoceptor blocker, and ATP enhanced the ability of the cell to regulate its intracellular pH (pHi) after an imposed acid load. This effect was observed in HCO3-free N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES) and prevented by Na-H antiport inhibitors ethylisopropylamiloride (EIPA) or amiloride. Similar results were obtained when cells were bathed in an acidic extracellular medium. Hence, the alpha 1-adrenoceptor and purinoceptor agonists activate the Na-H antiport even when it is partially inhibited by extracellular protons. To further evaluate the effects of the two neurohormones, the rate of proton efflux was estimated as a function of the magnitude of the imposed acid load. The results indicate that the agonist-induced modulation of the Na-H antiport is caused by an acceleration of its exchange activity and by a shift of its dependence on pHi toward more alkaline pH values. The agonist-mediated stimulation of the antiport was also observed in partially depolarized cells and was not dependent on intracellular Ca. Phorbol 12-myristate 13-acetate was not able to reproduce the effects of the agonists on the Na-H antiport. Conversely, the inhibitors of protein kinase C did not prevent the activation of the antiport by the neurohormones. Thus our data suggest that neither a Ca-calmodulin-dependent kinase nor protein kinase C is responsible for the alpha 1-adrenoceptor- and purinoceptor-mediated stimulation of the antiport.

Acidosis↗
PubMed1992

Protein kinase C enhances myosin light-chain kinase effects on force development and ATPase activity in rat single skinned cardiac cells.

Many neurohormones alter the force of cardiac contraction by variations in the intracellular Ca2+ concentration. alpha 1-Adrenergic and muscarinic stimulations, rather, modify the sensitivity of contractile proteins to Ca(2+)-calmodulin-myosin light-chain kinase (MLCK) complex induces a large increase in Ca2+ sensitivity (0.14 pCa unit) of these easily accessible myofilaments. This increase is further enhanced by up to 0.19 pCa unit when protein kinase C (PKC) is added together with MLCK. Similarly, the Ca2+ ATPase activity of skinned cells in suspension is increased in the presence of MLCK and further in the presence of both kinases. 32P-labelling and SDS/PAGE show that these changes are associated with light-chain 2 (LC2) phosphorylation together with phosphorylation of troponin I and troponin T when PKC is added. Although to a smaller extent than in smooth muscle, phosphorylation of cardiac myosin LC2 may be involved in the modulation of heart contractility.

Adenosine Triphosphatases↗
PubMed1992

Phenylephrine and ATP enhance an amiloride insensitive bicarbonate-dependent alkalinizing mechanism in rat single cardiomyocytes.

To expel the excess protons generated during a cellular acidification and to fully recover basal intracellular pH (pHi), cardiac cells rely on the amiloride-sensitive Na/H antiport. We report that rat single ventricular cardiomyocytes, loaded with the fluorescent pH indicator Snarf-1 and treated with inhibitors of the Na/H antiport, amiloride or its analogues, partially restored their pHi through a bicarbonate-dependent mechanism following an acidosis (imposed by the ammonia-pulse technique). In the presence of ethylisopropylamiloride (10 microM) or amiloride (1 mM) and 25 mM bicarbonate in the extracellular solution, the average time that cells needed to recover half of their pHi, following the removal of 20 mM NH4Cl, was 3.4 min, while the rate of proton efflux was calculated to be 2.0 mM/min. The stilbene derivative, 4-4'-di-isothiocyanostilbene-2,2'-disulphonate (DIDS 200 microM), a known blocker of anion transporters, inhibited this recovery. Both phenylephrine (100 microM, 3 microM propranolol present), an alpha 1-adrenoceptor agonist, and ATP (10 microM), a purinergic agonist, significantly enhanced the rate of proton efflux that was due to this HCO3-dependent alkalinizing mechanism. Phenylephrine and ATP also shortened by three-fold the time that a myocyte needed to recover half of its initial pHi. This bicarbonate-dependent alkalinizing mechanism could provide an additional means by which cardiac cells recover their pHi from acidosis, especially under conditions in which the Na/H antiport is inhibited. Furthermore, catecholamines and ATP, which are released under various pathophysiological conditions often associated with intracellular acidosis, could play an important role in the modulation of pHi under these conditions.

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗
PubMed1992

Properties of the low threshold Ca current in single frog atrial cardiomyocytes. A comparison with the high threshold Ca current.

The properties of the low threshold Ca current (ICaT) in bullfrog (Rana catesbeiana) isolated atrial cardiomyocytes were studied using the whole-cell recording patch-clamp technique and compared with those of the high threshold Ca current (ICaL). In 91% of atrial cells we observed both ICaT and ICaL when collagenase and trypsin were used to dissociate the cells. But when pronase was used, only 30% of the cells exhibited ICaT. ICaT was never found in ventricular cells. ICaT could be investigated more easily when ICaL was inhibited by Cd ions (50 microM). Its kinetics were unchanged by substituting Ba for Ca, or in the presence of high concentrations of Ba. Both ICaT and ICaL exhibited reduced inactivation after high depolarizing prepulses. ICaT was found to be sensitive to dihydropyridines: 1 microM nifedipine decreased this current while 1 microM BAY K 8644 increased it; this occurred without significant variations in the steady-state inactivation curve. ICaT was more sensitive than ICaL to alpha 1-adrenergic and P2-purinergic stimulations, while ICaL was more sensitive to beta-adrenergic stimulation. Isoproterenol was still able to increase ICaT in the presence of high intracellular cAMP. Both currents were increased by 1 microM ouabain (although ICaL only transiently) and decreased by 10 microM ouabain. It is concluded that the two types of Ca channels can be observed in bullfrog atrial cells and that they are specifically altered by pharmacological agents and neuromediators. This may have implications for cardiac behavior.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed1992

A Gs protein couples P2-purinergic stimulation to cardiac Ca channels without cyclic AMP production.

P2-purinergic stimulation of the L-type Ca current induced by the external application of 100 microM ATP gamma S was investigated in rat ventricular cardiomyocytes using the whole-cell patch-clamp technique. The purinergic-induced increase in ICa was slow and monophasic and reached a steady state within 3 min. In contrast to beta-adrenergic stimulation, after a brief agonist application the current did not continue to increase on washout; recovery started immediately after agonist removal. The P2-purinergic increase in ICa was significantly less in the presence of GDP beta S, but it occurred much faster and was twice as large when a low dose of GTP gamma S (100 microM) was added to a GTP-containing internal medium. This suggests that the ICa increase was mediated by a G protein. Based on electrophoretic mobility and susceptibility to cholera toxin and anti-G alpha s serum, it is proposed that the G protein involved during purinergic-induced ICa stimulation is an isoform of Gs not coupled to the adenylyl cyclase, since the cyclic AMP level was unaffected. High intracellular GTP gamma S (1 mM) maximally activated ICa so that neither beta-adrenergic nor P2-purinergic agonists further increased ICa. In the absence of GTP and an ATP-regenerating system, GTP gamma S was much more potent in increasing basal ICa and supporting purinergic stimulation. This indicates that a nucleoside diphosphate kinase activity might replenish endogenous GTP; GTP exchange with GTP gamma S on the G protein was promoted by the P2-purinergic stimulation and led to a reversible and reproducible increase in ICa. In the presence of 3 mM internal ATP gamma S, the P2-purinergic stimulation was also reversible and reproducible. Moreover, under these conditions (ATP gamma S or GTP gamma S) the increase in ICa was not maintained during prolonged agonist application. Such an inhibition occurred slowly and irreversibly; it might be related to the threefold increase in cyclic GMP. In conclusion, we propose that extracellular ATP induces both a stimulatory and an inhibitory effect on ICa, probably mediated by subtypes of P2-purinergic receptors. An isoform of the Gs protein is likely to mediate the stimulation.

Adenosine Diphosphate↗
PubMed1992

Alpha 1-adrenergic effects on intracellular pH and calcium and on myofilaments in single rat cardiac cells.

1. The cellular effects of alpha 1-adrenoceptor stimulation by phenylephrine were studied in the presence of propranolol in single cells isolated from the ventricles of rat hearts. 2. Phenylephrine (10-100 microM) induced a biphasic pattern of inotropism in these cells: a transient negative followed by a sustained positive inotropic effect as usually observed in cardiac tissues. 3. In Snarf-1-loaded cells, phenylephrine induced an alkalinization. This effect was reversible on wash-out and inhibited by prazosin, an alpha 1-adrenoceptor antagonist. 4. The alpha 1-adrenoceptor-mediated increase in intracellular pH (pHi) was 0.1 pH unit in HEPES buffer containing 4.4 mM-NaHCO3 and in Krebs buffer containing 25 mM-NaHCO3. 5. The alkalinization was blocked by the Na(+)-H+ antiport blocker, ethylisopropylamiloride (EIPA). 6. The recovery from an acidosis induced by a NH4Cl pre-pulse was accelerated by phenylephrine. The phenylephrine-induced alkalinization was attributed to activation of the Na(+)-H+ antiport. 7. Despite its ability to increase pHi, phenylephrine did not alter Ca2+ current amplitude and kinetics. 8. Ca2+ transients recorded in Indo-1-loaded cells were not augmented by phenylephrine. Diastolic calcium level was decreased. 9. In single skinned cells, the Ca2+ sensitivity of the contractile proteins was increased by a pre-treatment with phenylephrine even when the alpha 1-adrenoceptor-mediated alkalinizing effect had been prevented by EIPA. 10. These results lead us to propose that the alpha 1-adrenergic-induced positive inotropic response of heart muscle could result from an increased sensitivity of the myofilaments to Ca2+ ions. This alpha 1-adrenoceptor-mediated Ca2+ sensitization could result both from an intracellular alkalinization and from a direct effect on contractile proteins.

Amiloride↗
PubMed1992

Prajmalium, an antiarrhythmic with positive inotropic effect: mechanism of action in rabbit single cardiomyocytes.

The propyl derivative of ajmaline, N-n-propylajamaline (prajmalium), is an antiarrhythmic compound that lacks the commonly reported negative inotropic effects of all others under clinical use. The present study was undertaken to establish and understand its effects at the cellular level in mammalian preparations. Electrical and mechanical activities were recorded from right ventricular strips and Na and L-type Ca currents (INa and ICaL, respectively) were recorded with the whole-cell patch-clamp technique in right ventricular myocytes freshly dissociated from rabbit hearts. Prajmalium decreased the maximal rate of depolarization of the action potential in a dose-dependent manner with an EC50 of 3 microM. This effect was use and frequency dependent. Action potential duration was increased by 1 microM prajamalium but decreased on applying higher concentrations. The force of contraction was slightly (15%) increased at 0.1 microM, not affected at all at 1 microM and depressed by 30% at 20 microM. In single cardiomyocytes maintained at negative holding potentials, INa was slightly depressed by prajmalium at 10 nM and reduced by 75% at 10 microM. ICaL was increased by 30 and 20% on applying prajmalium at 1 and 10 microM, respectively; on the other hand, ICaL was reduced by these two concentrations of prajmalium at less negative holding potentials. A higher prajmalium concentration (100 microM) decreased ICaL at all holding potentials studies and this effect was enhanced with depolarization. The increase in ICaL induced by prajmalium (1 microM) was also observed after ICaL had been fully beta-adrenergic and P2-purinergic stimulated by isoproterenol (1 microM) in the presence of IBMX (100 microM) and ATP (10 microM). It is concluded that prajmalium is able to increase ICaL in rabbit ventricular cells in a voltage-dependent manner, an effect that could account in part for the observed lack of negative inotropism of this antiarrhythmic in clinics.

Action Potentials↗
PubMed1991

Extracellular ATP-induced acidification leads to cytosolic calcium transient rise in single rat cardiac myocytes.

The origin of the increase in cytosolic free Ca2+ concentration ([Ca2+]i) induced by extracellular ATP was investigated in single isolated cardiac myocytes loaded with indo-1. The nucleotide added at a concentration of 10 microM triggers a few Ca2+ spikes, followed by a cluster of Ca2+ oscillations, increasing [Ca2+]i to around 200 nM from a basal value of 70 nM. Neither caffeine nor ryanodine affects the magnitude of the Ca2+ transient, but both shorten it by preventing the Ca2+ oscillations. This indicates that the latter must be related to the release of Ca2+ from the sarcoplasmic reticulum. Since ATP also induces cell depolarization (as shown by experiments using the potential sensitive dye bis-oxonol), the initial Ca2+ spikes were attributed to the opening of voltage-dependent Ca2+ channels. A small Ca2+ transient still remains under experimental conditions designed to prevent Ca2+ influx from external medium (low-Ca2+ high-Mg2+ medium containing La3+) and after depletion of the sarcoplasmic-reticulum Ca2+ load with caffeine. Under these conditions, when this Ca2+ transient was buffered by 1,2-bis-(O-aminophenoxy)ethane-NNN'N'-tetra-acetic acid, ATP was unable to trigger the initial Ca2+ spikes. These results indicate that ATP mobilizes Ca2+ ions from an intracellular pool other than the sarcoplasmic reticulum and that this Ca2+ release is responsible for the depolarization. The effects of ATP on [Ca2+]i share the same characteristics as the acidification simultaneously induced by the nucleotide (as shown by experiments using the pH-sensitive probe snarf-1). These ionic variations are highly specific to ATP and its hydrolysis-resistant analogues. They both require the presence of Mg2+ and Cl- ions in the extracellular medium, and they are prevented by pretreatment of the cells with 4,4'-di-isothiocyanostilbene or probenecid. These results suggest that: (1) the ATP-induced acidification leads to displacement of Ca2+ ions from or close to the internal face of sarcolemma; (2) the Ca2+ ions activate a non-specific membrane conductance responsible for the depolarization of the cells; (3) the depolarization leads to a Ca2+ influx, owing to the opening of the voltage-dependent Ca2+ channels; (4) this increase in Ca2+ triggers the release of Ca2+ from the sarcoplasmic reticulum, which is facilitated by the increase in inositol trisphosphate following P2-purinergic stimulation.

Adenosine Triphosphate↗