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

Cardiac purinergic signalling in health and disease

This review is a historical account about purinergic signalling in the heart, for readers to see how ideas and understanding have changed as new experimental results were published. Initially, the focus is on the nervous control of the heart by ATP as a cotransmitter in sympathetic, parasympathetic, and sensory nerves, as well as in intracardiac neurons. Control of the heart by centers in the brain and vagal cardiovascular reflexes involving purines are also discussed. The actions of adenine nucleotides and nucleosides on cardiomyocytes, atrioventricular and sinoatrial nodes, cardiac fibroblasts, and coronary blood vessels are described. Cardiac release and degradation of ATP are also described. Finally, the involvement of purinergic signalling and its therapeutic potential in cardiac pathophysiology is reviewed, including acute and chronic heart failure, ischemia, infarction, arrhythmias, cardiomyopathy, syncope, hypertrophy, coronary artery disease, angina, diabetic cardiomyopathy, as well as heart transplantation and coronary bypass grafts.

Cav1.4 encodes a calcium channel with low open probability and unitary conductance

When transiently expressed in tsA-201 cells, Ca(v)1.4 calcium channels support only modest whole-cell currents with unusually slow voltage-dependent inactivation kinetics. To examine the basis for this unique behavior we used cell-attached patch single-channel recordings using 100 mM external barium as the charge carrier to determine the single-channel properties of Ca(v)1.4 and to compare them to those of the Ca(v)1.2. Ca(v)1.4 channel openings occurred infrequently and were of brief duration. Moreover, openings occurred throughout the duration of the test depolarization, indicating that the slow inactivation kinetics observed at the whole-cell level are caused by sustained channel activity. Ca(v)1.4 and Ca(v)1.2 channels displayed similar latencies to first opening. Because of the rare occurrence of events, the probability of opening could not be precisely determined but was estimated to be <0.015 over a voltage range of -20 to +20 mV. The single-channel conductance of Ca(v)1.4 channels was approximately 4 pS compared with approximately 20 pS for Ca(v)1.2 under the same experimental conditions. Additionally, in the absence of divalent cations, Ca(v)1.4 channels pass cesium ions with a single-channel conductance of approximately 21 pS. Although Ca(v)1.2 opening events were best described kinetically with two open time constants, Ca(v)1.4 open times were best described by a single time constant. BayK8644 slightly enhanced the single-channel conductance in addition to increasing the open time constant for Ca(v)1.4 channels by approximately 45% without, however, causing the appearance of an additional slower gating mode. Overall, our data indicate that single Ca(v)1.4 channels support only minute amounts of calcium entry, suggesting that large numbers of these channels are needed to allow for significant whole-cell current activity, and providing a mechanism to reduce noise in the visual system.

Long-term potentiation and long-term depression induced by local application of ATP to hippocampal CA1 neurons of the guinea pig

The present study has investigated the role of ATP in the induction of synaptic plasticity, using local application of ATP by picopump administration into the stratum radiatum of guinea pig hippocampal region CA1. Excitatory postsynaptic currents (EPSCs) evoked by stimulation of Schaffer collateral/commissural afferents synapsing on CA1 pyramidal cells of hippocampal slices were monitored in voltage-clamp mode, using whole-cell recording. Brief local application of ATP (1 mM) induced an inward current, usually consisting of early- and late-phase components. Because the late-phase component of an ATP-induced current was largely inhibited by Ca2+-free solution, this component is supposed to depend on extracellular Ca2+. After local application of ATP, long-term synaptic modification of EPSCs was induced: LTP was detected in neurons exhibiting a small late Ca2+ current, while LTD was obtained from recordings showing a large late Ca2+ current in response to ATP application. There was a statistically significant correlation between the magnitude of long-term plastic changes and the size of Ca2+ currents in response to ATP application. Furthermore, there was significant difference between the average size of the Ca2+ current in the LTP group and the size in the LTD group. These results suggest that a small Ca2+ influx in response to ATP application induces LTP, whereas a large one induces LTD in guinea pig hippocampal CA1 neurons.

Ca2+ sparks in rabbit ventricular myocytes evoked by action potentials: involvement of clusters of L-type Ca2+ channels

It is not clear how many L-type Ca2+ channels (LCCs) are required to ensure that a Ca2+ spark is triggered during a normal mammalian action potential (AP). We investigated this in rabbit ventricular myocytes by examining both the properties of sparks evoked by APs and the activity of LCCs. We measured Ca2+ sparks evoked by repeated APs with pipettes containing 2 mmol/L EGTA and single LCC activity in cell-attached patches depolarized to +50 mV using pipettes containing 110 mmol/L Ba2+. With 2 mmol/L Ca2+ in the external solution, we observed sparks at the beginning of every evoked AP at numerous locations. Each spark was observed repeatedly at a fixed location and began during a limited interval after the AP peak. These sparks occurred with a probability of approximately unity. However, the chance that an LCC does not open during the interval when a spark is triggered is quite high ( approximately 0.13). Therefore, because single channels open with a probability significantly lower than 1, more than one LCC must be available to ensure that sparks are triggered with a probability of approximately unity. We conclude that it is likely that a cluster of LCCs is involved in gating a cluster of ryanodine receptors at the beginning of an AP.

Stimulation of cardiac L-type calcium channels by extracellular ATP

The co-release of ATP with norepinephrine from sympathetic nerve terminals in the heart may augment adrenergic stimulation of cardiac Ca(2+) channel activity. To test for a possible direct effect of extracellular ATP on L-type Ca(2+) channels, single channels were reconstituted from porcine sarcolemma into planar lipid bilayers so that intracellular signaling pathways could be controlled. Extracellular ATP (2-100 microM) increased the open probability of the reconstituted channels, with a maximal increase of approximately 2.6-fold and an EC(50) of 3.9 microM. The increase in open probability was due to an increase in channel availability and a decrease in channel inactivation rate. Other nucleotides displayed a rank order of effectiveness of ATP > alpha,beta-methylene-ATP > 2-methylthio-ATP > UTP > adenosine 5'-O-(3-thiotriphosphate) >> ADP; adenosine had no effect. Several antagonists of P2 receptors had no impact on the ATP-dependent increase in open probability, indicating that receptor activation was not required. These results suggest that extracellular ATP and other nucleotides can stimulate the activity of cardiac L-type Ca(2+) channels via a direct interaction with the channels.

Adenosine 5'-triphosphate: a P2-purinergic agonist in the myocardium

ATP, besides an intracellular energy source, is an agonist when applied to a variety of different cells including cardiomyocytes. Sources of ATP in the extracellular milieu are multiple. Extracellular ATP is rapidly degraded by ectonucleotidases. Today ionotropic P2X(1--7) receptors and metabotropic P2Y(1,2,4,6,11) receptors have been cloned and their mRNA found in cardiomyocytes. On a single cardiomyocyte, micromolar ATP induces nonspecific cationic and Cl(-) currents that depolarize the cells. ATP both increases directly via a G(s) protein and decreases Ca(2+) current. ATP activates the inward-rectifying currents (ACh- and ATP-activated K(+) currents) and outward K(+) currents. P2-purinergic stimulation increases cAMP by activating adenylyl cyclase isoform V. It also involves tyrosine kinases to activate phospholipase C-gamma to produce inositol 1,4,5-trisphosphate and Cl(-)/HCO(3)(-) exchange to induce a large transient acidosis. No clear correlation is presently possible between an effect and the activation of a given P2-receptor subtype in cardiomyocytes. ATP itself is generally a positive inotropic agent. Upon rapid application to cells, ATP induces various forms of arrhythmia. At the tissue level, arrhythmia could be due to slowing of electrical spread after both Na(+) current decrease and cell-to-cell uncoupling as well as cell depolarization and Ca(2+) current increase. In as much as the information is available, this review also reports analog effects of UTP and diadenosine polyphosphates.

Distribution of purinergic P2X receptors in the rat heart

The distribution of P2X purinergic receptor subtypes has been determined in relation to nerve varicosities in the rat heart with immunohistochemistry. Large clusters (about 1 microm diameter) of co-localised and sometimes co-extensive P2X1 and P2X3 receptors were found at sites of tyrosine hydroxylase (TH) positive axon varicosities in the atrium and the ventricle. Varicosities that were labelled with antibodies to the synaptic vesicle epitope SV2 were frequently labelled also with antibodies to P2X3, P2X5 and P2X6 but not always with antibodies to P2X1. Especially prominent were large numbers of small clusters (about 400 nm diameter) of co-localised P2X2 and P2X5 receptors on the sarcolemma unrelated to nerves at all. During development the 1 day-old heart possessed an abundance of co-localised P2X2 and P2X5 small receptor clusters on the sarcolemma. These observations are discussed in relation to the role of purinergic receptors in the mammalian heart.

Cardiac ionic currents and acute ischemia: from channels to arrhythmias

The aim of this review is to provide basic information on the electrophysiological changes during acute ischemia and reperfusion from the level of ion channels up to the level of multicellular preparations. After an introduction, section II provides a general description of the ion channels and electrogenic transporters present in the heart, more specifically in the plasma membrane, in intracellular organelles of the sarcoplasmic reticulum and mitochondria, and in the gap junctions. The description is restricted to activation and permeation characterisitics, while modulation is incorporated in section III. This section (ischemic syndromes) describes the biochemical (lipids, radicals, hormones, neurotransmitters, metabolites) and ion concentration changes, the mechanisms involved, and the effect on channels and cells. Section IV (electrical changes and arrhythmias) is subdivided in two parts, with first a description of the electrical changes at the cellular and multicellular level, followed by an analysis of arrhythmias during ischemia and reperfusion. The last short section suggests possible developments in the study of ischemia-related phenomena.

Effects of beta2-adrenergic stimulation on single-channel gating of rat cardiac L-type Ca2+ channels

Cardiac L-type Ca2+ channels can be stimulated by activation of beta2-adrenoceptors. We intended to determine how the gating behavior at the single-channel level (cell-attached configuration) is affected after selective stimulation of beta2-adrenoceptors. Rat cardiomyocytes were exposed to zinterol, a beta2-agonist (n = 7), isoproterenol (n = 6), a nonselective agonist, 8-bromo-cAMP (n = 6), and a combination of isoproterenol and ICI-118551 (n = 8), a selective beta2-receptor antagonist, or isoproterenol and CGP-20712A, a beta1-selective antagonist (n = 7). In all groups the ensemble-average current and the availability of the channels to open on depolarization were increased in a similar fashion. In addition, the open probability (Po) within active sweeps was elevated. However, zinterol exerted this effect in a unique manner. It elevated Po not by shortening closed times but solely by reducing active sweeps with very low Po and a short burst duration. All zinterol effects were abolished by ICI-118551 (n = 5) and mimicked by isoproterenol plus CGP-20712A (n = 7). We conclude that beta2-adrenoceptor activation of L-type channels differs qualitatively from the classical cAMP-dependent mechanism.

Single-channel properties of L-type calcium channels from failing human ventricle

OBJECTIVE

The aim of our study was to analyse the single-channel properties of L-type calcium channels from failing human heart and to compare them to the respective animal data. Furthermore, we intended to evaluate the feasibility of future single-channel studies on the role of calcium channels in the pathophysiology of heart failure.

METHODS

Single L-type calcium channels were recorded in ventricular myocytes from explanted failing human heart, using the cell-attached configuration of the patch-clamp technique.

RESULTS

One or more successful registrations of calcium channels could be obtained in 11 of 19 cell isolations. Determination of single-channel conductance yielded a mean value of 16.6 +/- 1.2 pS (70 mM Ba2+ as the charge carrier) under control conditions and 23.7 +/- 2.8 pS in presence of the calcium-channel agonist FPL 64176. The rapid gating process could be described by a C<-->C<-->O gating scheme. Slow gating analysis revealed a highly significant clustering of active and non-active sweeps.

CONCLUSION

Single-channel measurements of L-type calcium channels in human failing ventricle are feasible and reproducible despite the varying patient characteristics. Their channel properties are qualitatively comparable to those found in other mammals. Whether there are quantitative differences due to the underlying heart failure can be elucidated in further studies.

C2 region-derived peptides of beta-protein kinase C regulate cardiac Ca2+ channels

We have previously shown that alpha1-adrenergic activation inhibited beta-adrenergic-stimulated L-type Ca2+ current (I(Ca)). To determine the role of protein kinase C (PKC) in this regulation, the inositol trisphosphate pathway was bypassed by direct activation of PKC with 4beta-phorbol 12-myristate 13-acetate (PMA). To minimize Ca2+-induced Ca2+ inactivation, Ba2+ current (I(Ba)) was recorded through Ca2+ channels in adult rat ventricular myocytes. We found that PMA (0.1 micromol/L) consistently inhibited basal I(Ba) by 40.5+/-7.4% and isoproterenol (ISO, 0.1 micromol/L)-stimulated I(Ba) by 48.9+/-7.8%. These inhibitory effects were not observed with the inactive phorbol ester analogue alpha-phorbol 12,13-didecanoate (0.1 micromol/L). To identify the PKC isozymes that mediate these PMA effects, we intracellularly applied peptide inhibitors of a subclass of PKC isozymes, the C2-containing cPKCs. These peptides (betaC2-2 and betaC2-4) specifically inhibit the translocation and function of C2-containing isozymes (alpha-PKC, betaI-PKC, and betaII-PKC), but not the C2-less isozymes (delta-PKC and epsilon-PKC). We first used the pseudosubstrate peptide (0.1 micromol/L in the pipette), which inhibits the catalytic activity of all the PKC isozymes, and found that PMA-induced inhibition of ISO-stimulated I(Ba) was reduced to 16.8+/-7.4% but was not affected by the scrambled pseudosubstrate peptide. The effects of PMA on basal and ISO-stimulated I(Ba) were then determined in the presence of C2-derived peptides or control peptides. When the pipette contained 0.1 micromol/L of betaC2-2 or betaC2-4, PMA-induced inhibition of basal I(Ba) was 26.1+/-4.5% and 23.6+/-2.2%, respectively. Similarly, ISO-stimulated I(Ba) was inhibited by 29.9+/-6.6% and 29.3+/-7.8% in the presence of betaC2-2 and betaC2-4, respectively. In contrast, there was no significant change in the effect of PMA in the presence of control peptides, scrambled betaC2-4, or pentalysine. Finally, PMA-induced inhibition of basal and ISO-stimulated I(Ba) was almost completely abolished in cells dialyzed with both betaC2-2 and betaC2-4. Together, these data suggest a role for C2-containing isozymes in mediating PMA-induced inhibition of L-type Ca2+ channel activity.

Modulation by extracellular ATP of L-type calcium channels in guinea-pig single sinoatrial nodal cell

1. The effects of extracellular adenosine 5'-triphosphate ([ATP]zero) on the L-type Ca2+ channel currents in guinea-pig single sinoatrial nodal (SAN) cells, isolated by enzymatic dissociation, were investigated by use of whole-cell patch-clamp techniques. 2. The application of [ATP]zero (2 microM-1 mM) produced an inhibitory effect on the L-type Ca2+ channel current peak amplitude (10 mM Ba2+ as charge carrier) in a concentration-dependent and reversible manner with an IC50 of 100 microM and a Hill coefficient of 1.83. 3. The presence of the adenosine receptor antagonists, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 0.1 microM) and 8-phenyltheophylline (10 microM) did not affect the [ATP]zero-induced inhibition of the Ca2+ channel currents. Adenosine (100 microM) had little effect on the basal Ca2+ channel currents. Adenosine 500 microM, caused 23% inhibition of the Ca2+ channel current, which was abolished by 0.1 microM DPCPX. 4. The presence of the P2-purinoceptor antagonists, suramin (1, 10 and 100 microM), reactive blue 2 (1 and 10 microM) and pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 50 and 100 microM) failed to affect the inhibitory action of [ATP]zero on Ca2+ channel currents. 5. The relative rank order of potency of different nucleotides and nucleosides, at a concentration of 100 microM, on the inhibition of the Ca2+ channel currents is as follows: adenosine 5'-triphosphate (ATP) = alpha,beta-methylene-ATP (alpha,beta MeATP) > > 2-methylthioATP (2-MeSATP) > or = adenosine 5'-O-(3-thiotriphosphate) (ATP gamma S) > > uridine 5'-triphosphate (UTP) = adenosine 5'-diphosphate (ADP) > adenosine 5'-monophosphate (AMP) > or = adenosine. 6. These results suggest that [ATP]zero may play an important role in the heart beat by inhibiting the L-type Ca2+ channel currents in single SAN cells. This inhibitory effect is not due to the formation of adenosine resulting from the enzymatic degradation of [ATP]zero. Based on the relative order of inhibitory potency of different nucleotides and nucleosides on the L-type Ca2+ channel currents and the ineffectiveness of the purinoceptor antagonists tested, a novel type of purinoceptor may be involved.

Dopamine modulates unitary conductance of single PL-type calcium channels in Roccus chrysops retinal horizontal cells

1. Dopamine modulation of the PL-type calcium channel of white bass retinal horizontal cells was studied in isolated, cultured neurons. Single-channel recordings were made of calcium channels in outside-out patches, under conditions which favoured the expression of calcium channel activity. 2. Analysis of single-channel properties revealed that dopamine potentiated the activity of the sustained calcium channel in three ways. First, it increased unitary conductance through individual channels. Under the influence of dopamine, single-channel conductance doubled. 3. Dopamine also increased the probability of channel opening and increased channel mean open time. The probability of opening increased 4-fold while mean open time doubled. 4. The mean closed time was also affected. The time between individual openings was not affected but the closed time between bursts of openings was shortened by over 50%. 5. The effects of dopamine were mediated via the activation of a D1-type receptor and the resulting activation of a cAMP-mediated second messenger system. 6. The combination of the effects of dopamine significantly increased the net calcium influx into the cell.

Alpha 1-adrenergic activation inhibits beta-adrenergic-stimulated unitary Ca2+ currents in cardiac ventricular myocytes

We have previously shown that whole-cell L-type Ca2+ current that was stimulated through beta-adrenergic receptors was negatively modulated by alpha 1-adrenergic activation. In the present study, we investigated the kinetic basis of this modulation at the single-channel level in adult rat ventricular myocytes using Ba2+ as the charge carrier. Unitary current sweeps were evoked by 300-ms depolarizing pulses to 0 mV, from a holding potential of -50 mV at 0.5 Hz. During control conditions, the ensemble-averaged current amplitude was 0.18 +/- 0.01 pA (n = 7). To achieve beta-adrenergic stimulation (beta effect), cells were superfused with norepinephrine (10 mumol/L) in the presence of prazosin (10 mumol/L), an alpha 1-adrenergic blocker. beta-adrenergic stimulation enhanced ensemble-averaged current (from 0.18 +/- 0.01 to 0.75 +/- 0.04 pA, P < .05, n = 7), increased the open-time constants, and decreased the closed-time constants. To activate alpha 1-receptors while maintaining the beta-adrenergic stimulation, cells were superfused with norepinephrine alone (alpha 1 + beta effects). alpha 1-adrenergic activation reduced ensemble-averaged current (from 0.75 +/- 0.04 to 0.41 +/- 0.03 pA, P < .05, n = 7), decreased open-time constants, and increased closed-time constants. alpha 1-adrenergic activation also inhibited ensemble-averaged currents stimulated by a low concentration (10 mumol/L) of 8-bromo-cAMP but not by (-)Bay K 8644 (1 mumol/L). Calphostin C (1 mumol/L), a specific inhibitor of protein kinase C, attenuated alpha 1-adrenergic inhibition on beta-adrenergic-stimulated unitary currents. We conclude that alpha 1-adrenergic activation exerts an inhibitory effect on beta-adrenergic-stimulated unitary Ba2+ current at the single-channel level. The shortening of the open-time and the lengthening of the closed-time constants and the increase in blank sweeps may explain the inhibition of the Ca(2+)-channel activity and the reduction in whole-cell Ca2+ current previously reported. This inhibition is in part mediated through the protein kinase C pathway.

ATP receptor activation potentiates a voltage-dependent Ca channel in hippocampal neurons

Activation of a purinergic P2 receptor by adenosine 5'-triphosphate (ATP) has previously been shown to open a non-selective cation channel with a reversal potential of approximately 0 mV. We examined the effect of P2 receptor activation on voltage-gated ionic currents in acutely isolated CA3 pyramidal neurons from guinea pig hippocampus using the whole-cell-patch technique. Under conditions designed to isolate current through voltage-dependent Ca channels (ICa), ATP (50 microM) potentiated ICa by 36%. This increase in ICa desensitized back to control levels within 4 min. In contrast to the non-selective cation channel, ICa elicited from a holding potential (HP) of -100 mV showed significant potentiation in response to ATP when depolarized to a test potential (TP) of -10 mV but showed no effect on ICa when the same neuron was alternately depolarized to TP = -70 mV. No change in holding current at HP = -100 mV occurred. Tail currents were unaffected by ATP exposure suggesting that ICa potentiation was not due to modulation of L-type Ca channels. This potentiation was also observed either with ATP-gamma-s, the slowly hydrolyzable ATP analog, or with ATP in the presence of alpha, beta-methylene-ADP, an ectonucleotidase inhibitor, indicating that the effects observed were not due to activation of an adenosine receptor that required ATP hydrolysis. The potentiation of ICa was not observed with the P2X agonist, beta, gamma-methylene-ATP. These results suggest that ATP receptors can modulate voltage- as well as ligand-gated channels permeable to calcium and may play an important role in the dynamics of intracellular Ca2+ in these neurons.

Current state of purinoceptor research

It is hoped that this summary of the history and current status of purinoceptors will convince readers that receptors for purines are now established alongside other well-known extracellular messenger systems. These receptors are primitive, widespread and serve many different systems. Receptors of adenosine (P1-purinoceptors) are clearly different from receptors of ATP (P2-purinoceptors). As for other major transmitters such as acetylcholine, GABA, glutamate and 5-HT, receptors of two major families are activated by ATP, one (the P2X-purinoceptor family) mediates fast responses via ligand-gated ion channels, while the other (the P2Y-purinoceptor family) mediates slower responses via G-proteins (see Table 3). Subclasses of these two families have been suggested on the basis of recent molecular biology studies and the development of new selective agonists and antagonists (Abbracchio and Burnstock, 1994). It would indeed be helpful if the work on purinoceptors could be extended to studies of their chemical structure employing crystallography.

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.