Modulation of basal L-type Ca2+ current by adenosine in ferret isolated right ventricular myocytes

Adenosine A1 Receptor-Mediated Attenuation of Reciprocal Dendro-Dendritic Inhibition in the Mouse Olfactory Bulb

It is well described that A₁ adenosine receptors inhibit synaptic transmission at excitatory synapses in the brain, but the effect of adenosine on reciprocal synapses has not been studied so far. In the olfactory bulb, the majority of synapses are reciprocal dendro-dendritic synapses mediating recurrent inhibition. We studied the effect of A₁ receptor activation on recurrent dendro-dendritic inhibition in mitral cells using whole-cell patch-clamp recordings. Adenosine reduced dendro-dendritic inhibition in wild-type, but not in A₁ receptor knock-out mice. Both NMDA receptor-mediated and AMPA receptor-mediated dendro-dendritic inhibition were attenuated by adenosine, indicating that reciprocal synapses between mitral cells and granule cells as well as parvalbumin interneurons were targeted by A₁ receptors. Adenosine reduced glutamatergic self-excitation and inhibited N-type and P/Q-type calcium currents, but not L-type calcium currents in mitral cells. Attenuated glutamate release, due to A₁ receptor-mediated calcium channel inhibition, resulted in impaired dendro-dendritic inhibition. In behavioral tests we tested the ability of wild-type and A₁ receptor knock-out mice to find a hidden piece of food. Knock-out mice were significantly faster in locating the food. Our results indicate that A₁ adenosine receptors attenuates dendro-dendritic reciprocal inhibition and suggest that they affect odor information processing.

Adenosine does not save the heart of anoxia-tolerant vertebrates during prolonged oxygen deprivation

Despite adenosine being regarded as an important signaling molecule capable of coordinating ATP supply and demand during periods of oxygen deprivation in anoxia-intolerant mammals, the importance of adenosinergic cardiovascular control in anoxia-tolerant vertebrates is poorly understood. Here, we report on adenosinergic cardiovascular control during normoxia and prolonged (hours to days) oxygen deprivation for three vertebrate species tolerant of severe hypoxia/anoxia, the closely related common (Cyprinus carpio) and crucian (Carassius carassius) carp, and the freshwater turtle (Trachemys scripta). Using an intra-arterial injection of the non-specific adenosine receptor antagonist aminophylline while measuring cardiac output (Q), heart rate (f(H)) and arterial blood pressure, we establish that adenosinergic cardiovascular control is unimportant during prolonged anoxia in the freshwater turtle (6 h at 21 degrees C and 14 d at 5 degrees C) and the crucian carp (5 d at 8 degrees C). In contrast, adenosinergic control contributes to the down-regulation of cardiac activity exhibited by 5 degrees C-acclimated common carp during a 12.5 h severe hypoxia (<0.3 mg O2 l(-1)) exposure. Specifically, aminophylline injection resulted in significant increases in f(H) and Q, and a decrease in total peripheral resistance. These species-specific differences in adenosinergic cardiovascular control during prolonged periods of oxygen deprivation may be related to differences displayed by these three species in their anoxia tolerance and survival strategies.

Effects of AMP579 and adenosine on L-type Ca2+ current in isolated rat ventricular myocytes

AIM

To compare the effects of AMP579 and adenosine on L-type Ca2+ current (I(Ca-L)) in rat ventricular myocytes and explore the mechanism by which AMP579 acts on I(Ca-L).

METHODS

I(Ca-L) was recorded by patch-clamp technique in whole-cell configuration.

RESULTS

Adenosine (10 nmol/L to 50 micromol/L) showed no effect on basal I(Ca-L), but it inhibited the I(Ca-L) induced by isoproterenol 10 nmol/L in a concentration-dependent manner with the IC(50) of 13.06 micromol/L. Similar to adenosine, AMP579 also showed an inhibitory effect on the I(Ca-L) induced by isoproterenol. AMP579 and adenosine (both in 10 micromol/L) suppressed isoproterenol-induced ICa-L by 11.1% and 5.2%, respectively. In addition, AMP579 had a direct inhibitory effect on basal I(Ca-L) in a concentration-dependent manner with IC50 (1.17 micromol/L). PD116948 (30 micromol/L), an adenosine A1 receptor blocker, showed no action on the inhibitory effect of AMP579 on basal I(Ca-L). However, GF109203X (0.4 micromol/L), a special protein kinase C (PKC) blocker, could abolish the inhibitory effect of AMP579 on basal I(Ca-L). So the inhibitory effect of AMP579 on basal I(Ca-L) was induced through activating PKC, but not linked to adenosine A1 receptor.

CONCLUSION

AMP579 shows a stronger inhibitory effect than adenosine on the I(Ca-L) induced by isoproterenol. AMP579 also has a strong inhibitory effect on basal I(Ca-L) in rat ventricular myocytes. Activation of PKC is involved in the inhibitory effect of AMP579 on basal I(Ca-L) at downstream-mechanism.

Inhibition of Na+-K+ pump and L-type Ca2+ channel by glibenclamide in Guinea pig ventricular myocytes

Glibenclamide, a potent cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel blocker, is frequently used to study function and regulation of CFTR Cl(-) channels. In this study, the effects of glibenclamide on intracellular Na(+) concentration ([Na(+)](i)), contraction, Ca(2+) transient, and membrane potential were investigated in isolated guinea pig ventricular myocytes. Glibenclamide increased [Na(+)](i) and decreased contraction and Ca(2+) transient. However, glibenclamide did not change membrane potential. To determine whether inhibition of Na(+)-K(+) pumps and L-type Ca(2+) channels is responsible for the increase of [Na(+)](i) and the decrease of contraction, we tested the effects of glibenclamide on Na(+)-K(+) pump current and L-type Ca(2+) current (I(Ca,L)). Glibenclamide decreased Na(+)-K(+) pump current and I(Ca,L) in a concentration-dependent manner. In the presence of Cl(-) channel inhibitors, glibenclamide depolarized diastolic membrane potential and reduced action potential duration. This result suggests that the reason for lack of effect of glibenclamide on membrane potential might be due to its combined inhibitory effects on the Na(+)-K(+) pump, the L-type Ca(2+) channel, and Cl(-) channels, which may have opposing effects on membrane potential. These results indicate that glibenclamide increases [Na(+)(i)] by inhibiting the Na(+)-K(+) pump and decreases contraction and Ca(2+) transient, in addition, by blocking the L-type Ca(2+) channel.

Presynaptic inhibition of spontaneous acetylcholine release induced by adenosine at the mouse neuromuscular junction

1. At the mouse neuromuscular junction, adenosine (AD) and the A(1) agonist 2-chloro-N(6)-cyclopentyl-adenosine (CCPA) induce presynaptic inhibition of spontaneous acetylcholine (ACh) release by activation of A(1) AD receptors through a mechanism that is still unknown. To evaluate whether the inhibition is mediated by modulation of the voltage-dependent calcium channels (VDCCs) associated with tonic secretion (L- and N-type VDCCs), we measured the miniature end-plate potential (mepp) frequency in mouse diaphragm muscles. 2. Blockade of VDCCs by Cd(2+) prevented the effect of the CCPA. Nitrendipine (an L-type VDCC antagonist) but not omega-conotoxin GVIA (an N-type VDCC antagonist) blocked the action of CCPA, suggesting that the decrease in spontaneous mepp frequency by CCPA is associated with an action on L-type VDCCs only. 3. As A(1) receptors are coupled to a G(i/o) protein, we investigated whether the inhibition of PKA or the activation of PKC is involved in the presynaptic inhibition mechanism. Neither N-(2[p-bromocinnamylamino]-ethyl)-5-isoquinolinesulfonamide (H-89, a PKA inhibitor), nor 1-(5-isoquinolinesulfonyl)-2-methyl-piperazine (H-7, a PKC antagonist), nor phorbol 12-myristate 13-acetate (PHA, a PKC activator) modified CCPA-induced presynaptic inhibition, suggesting that these second messenger pathways are not involved. 4. The effect of CCPA was eliminated by the calmodulin antagonist N-(6-aminohexil)-5-chloro-1-naphthalenesulfonamide hydrochloride (W-7) and by ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid-acetoxymethyl ester epsilon6TDelta-BM, which suggests that the action of CCPA to modulate L-type VDCCs may involve Ca(2+)-calmodulin. 5. To investigate the action of CCPA on diverse degrees of nerve terminal depolarization, we studied its effect at different external K(+) concentrations. The effect of CCPA on ACh secretion evoked by 10 mm K(+) was prevented by the P/Q-type VDCC antagonist omega-agatoxin IVA. 6. CCPA failed to inhibit the increases in mepp frequency evoked by 15 and 20 mm K(+). We demonstrated that, at high K(+) concentrations, endogenous AD occupies A1 receptors, impairing the action of CCPA, since incubation with 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, an A(1) receptor antagonist) and adenosine deaminase (ADA), which degrades AD into the inactive metabolite inosine, increased mepp frequency compared with that obtained in 15 and 20 mm K(+) in the absence of the drugs. Moreover, CCPA was able to induce presynaptic inhibition in the presence of ADA. It is concluded that, at high K(+) concentrations, the activation of A(1) receptors by endogenous AD prevents excessive neurotransmitter release.

A model of graded calcium release and L-type Ca2+ channel inactivation in cardiac muscle

We have developed a model of Ca(2+) handling in ferret ventricular myocytes. This model includes a novel L-type Ca(2+) channel, detailed intracellular Ca(2+) movements, and graded Ca(2+)-induced Ca(2+) release (CICR). The model successfully reproduces data from voltage-clamp experiments, including voltage- and time-dependent changes in intracellular Ca(2+) concentration ([Ca(2+)](i)), L-type Ca(2+) channel current (I(CaL)) inactivation and recovery kinetics, and Ca(2+) sparks. The development of graded CICR is critically dependent on spatial heterogeneity and the physical arrangement of calcium channels in opposition to ryanodine-sensitive release channels. The model contains spatially distinct subsystems representing the subsarcolemmal regions where the junctional sarcoplasmic reticulum (SR) abuts the T-tubular membrane and where the L-type Ca(2+) channels and SR ryanodine receptors (RyRs) are localized. There are eight different types of subsystems in our model, with between one and eight L-type Ca(2+) channels distributed binomially. This model exhibits graded CICR and provides a quantitative description of Ca(2+) dynamics not requiring Monte-Carlo simulations. Activation of RyRs and release of Ca(2+) from the SR depend critically on Ca(2+) entry through L-type Ca(2+) channels. In turn, Ca(2+) channel inactivation is critically dependent on the release of stored intracellular Ca(2+). Inactivation of I(CaL) depends on both transmembrane voltage and local [Ca(2+)](i) near the channel, which results in distinctive inactivation properties. The molecular mechanisms underlying many I(CaL) gating properties are unclear, but [Ca(2+)](i) dynamics clearly play a fundamental role.

Cholinergic modulation of the basal L-type calcium current in ferret right ventricular myocytes

The effects of the cholinergic muscarinic agonist carbachol (CCh) on the basal L-type calcium current, I(Ca,L), in ferret right ventricular (RV) myocytes were studied using whole cell patch clamp. CCh produced two major effects : (i) in all myocytes, extracellular application of CCh inhibited I(Ca,L) in a reversible concentration-dependent manner; and (ii) in many (but not all) myocytes, upon washout CCh produced a significant transient stimulation of I(Ca,L) ('rebound stimulation'). Inhibitory effects could be observed at 1 x 10(-10) M CCh. The mean steady-state inhibitory concentration-response relationship was shallow and could be described with a single Hill equation (maximum inhibition = 34.5 %, IC50 = 4 x 10(-8) M, Hill coefficient n = 0.60). Steady-state inhibition (1 or 10 microM CCh) had no significant effect on I(Ca,L) selectivity or macroscopic (i) activation characteristics, (ii) inactivation kinetics, (iii) steady-state inactivation or (iv) kinetics of recovery from inactivation. Maximal inhibition of nitric oxide synthase (NOS) activity (preincubation of myocytes in 1 mM L-NMMA (N(G)-monomethyl-L-arginine) + 1 mM L-NNA (N(G)-nitro-L-arginine) for 2-3 h plus inclusion of 1 mM L-NMMA + 1 mM L-NNA in the patch pipette solution) produced no significant attenuation of the CCh-mediated inhibition of I(Ca,L). Protocols involving (i) the nitric oxide (NO) scavenger PTIO (2-phenyl-4,4,5,5,-tetramethylimidazoline-1-oxyl-3-oxide; 200 microM), (ii) imposition of a 'cGMP clamp' (100 microM 8-Bromo-cGMP), and (iii) inhibition of soluble guanylyl cyclase (ODQ (1H-[1,2,4,]oxadiazolo(4,3,-a)quinoxalin-1-one), 50 microM) all failed to attenuate CCh-mediated inhibition of I(ca,L). While CCh consistently inhibited basal I(Ca,L) in all RV myocytes studied, not all myocytes displayed rebound stimulation upon CCh washout. However, there was no difference between CCh-mediated inhibition of I(Ca,L) between these two RV myocyte types, and in myocytes displaying rebound stimulation neither ODQ nor 8-Bromo-cGMP (8-Br-cGMP) altered the effect. We conclude that NO production, activation of soluble guanylyl cyclase, or changes in intracellular cGMP levels are not obligatorily involved in muscarinic-mediated modulation of basal I(Ca,L) in ferret RV myocytes.

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.

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.

P2 receptor modulation of voltage-gated potassium currents in Brown adipocytes

Using patch voltage-clamp techniques, we find there are two components to the voltage-gated potassium current (IKv) in rat brown adipocytes. The components differ in their gating and responses to purinergic stimulation, but not their pharmacology. IKv-A recovers from inactivation at physiological membrane potentials, while IKv-B inactivation recovers at more negative potentials. Both currents are >90% blocked by similar concentrations of quinine and tetraethylammonium, but not by beta-dendrotoxin, charybdotoxin, or apamin. The two current components are differentially modulated by extracellular ATP. ATP shifts the voltage dependence of IKv-A inactivation negative by 38 +/- 5 mV (n = 35, +/-SEM) and shifts activation by -14 +/- 2 mV in whole-cell experiments. ATP did not affect the steady state inactivation voltage dependence of IKv-B, but did apparently convert IKv-A into IKv-B. The pharmacology of the inactivation shift is consistent with mediation by a P2 purinergic receptor. Purinergic stimulation of perforated-patch clamped cells causes hyperpolarizing shifts in the window current of IKv-A by shifting inactivation -18 +/- 4 mV and activation -7 +/- 2 mV (n = 16). Since perforated-patch recordings will most closely resemble in vivo cell responses, this ATP-induced shift in the window current may facilitate IKv activation when the cell depolarizes. IKv activity is necessary for the proliferation and differentiation of brown adipocytes in culture (Pappone, P.A., and S.I. Ortiz-Miranda. 1993. Am. J. Physiol. 264:C1014-C1019) so purinergic modulation of IKv may be important in altering adipocyte growth and development.

Mediation by nitric oxide of the indirect effects of adenosine on calcium current in rabbit heart pacemaker cells

1. Adenosine (ADO) is a potent negative chronotropic agent in the mammalian myocardium. We have used single myocytes from rabbit sino-atrial node (SAN) to examine whether nitric oxide (NO) is a significant mediator of the effects of ADO on the pacemaker activity, or the underlying Ca2+ and K+ currents. 2. SAN pacemaker cells were isolated from rabbit hearts by enzymatic dispersion, and Ca2+ and K+ currents were recorded by the nystatin-perforated patch voltage clamp method. ADO was applied in the presence of the beta-adrenoceptor agonist, isopremaline (Iso) to mimic the adrenergic tone which the SAN is subjected to in vivo. 3. Control experiments confirmed that isolated SAN cells responded to ADO (10-100 microM) with the expected (i) small increase in background inwardly rectifying K+ current, IK-ADOi and (ii) pronounced decrease in L-type Ca2+ current, ICa-L. These effects were mimicked by a selective A1 purinoceptor agonist, N6-cyclopentyladenosine (CPA, 10 microM); and were inhibited following bath application of the antagonist, DPCPX (10 microM), which selectively blocks A1 purinoceptors. DMPX (10 microM), a blocker of A2 purinoceptor, had no effect on the actions of ADO. 4. A nitric oxide synthase inhibitor, L-NMMA (100 microM), abolished the inhibitory effect of ADO on ICa-L but did not alter activation of IK-ADO. After L-NMMA washoff, it was possible to obtain the normal response (inhibition) of ICa-L to ADO in the same cell. 5. To evaluate whether the observed effect of nitric oxide (NO) on ICa-L was mediated by an increase in guanylyl cyclase (GC) activity and cyclic GMP formation, the guanylyl cyclase inhibitor, LY 83583 (40 microM) was applied prior to ADO. Under these conditions, the inhibitory effect of ADO on ICa-L was abolished, but the activation of IK-ADO was still observed. 6. In combination, these findings strongly suggest that in mammalian primary pacemaker tissue which is under adrenergic tone, the effects of ADO on ICa-L are mediated by NO.

Electrophysiological and functional effects of adenosine on ventricular myocytes of various mammalian species

The goal of this study was to determine the electrophysiological and functional effects of adenosine on ventricular myocytes of guinea pig, rabbit, rat, and ferret hearts. Adenosine (100 microM) shortened the action potential durations of rat and ferret myocytes by 14 +/- 1 and 57 +/- 7%, reduced the amplitudes of cell twitch shortening by 13 +/- 1 and 54 +/- 5%, and increased outward currents by 15 +/- 4 and 55 +/- 5%, respectively, but had no effect on guinea pig and rabbit myocytes. The properties of adenosine-activated outward current in rat and ferret ventricular myocytes indicated that this current is the adenosine-sensitive K+ current [IK(Ado)]. Adenosine had no significant effect on basal Ca2+ current but specifically inhibited isoproterenol-stimulated L-type Ca2+ current in myocytes of all species studied. Binding studies revealed that the density of A1 adenosine receptors (A1AdoR) was highest in ferret and lowest in rabbit myocytes, but the differential effects of adenosine among species could not be solely explained by differences in A1AdoR density. In summary, adenosine shortened the action potential and reduced the twitch shortening of rat and ferret but not of guinea pig and rabbit ventricular myocytes. Shortening of the action potential was associated with the activation of IK(Ado). The anti-beta-adrenergic action of adenosine appeared to be independent of species.

Redox modulation of L-type calcium channels in ferret ventricular myocytes. Dual mechanism regulation by nitric oxide and S-nitrosothiols

The effects of NO-related activity and cellular thiol redox state on basal L-type calcium current, ICa,L, in ferret right ventricular myocytes were studied using the patch clamp technique. SIN-1, which generates both NO. and O2-, either inhibited or stimulated ICa,L. In the presence of superoxide dismutase only inhibition was seen. 8-Br-cGMP also inhibited ICa,L, suggesting that the NO inhibition is cGMP-dependent. On the other hand, S-nitrosothiols (RSNOs), which donate NO+, stimulated ICa,L. RSNO effects were not dependent upon cell permeability, modulation of SR Ca2+ release, activation of kinases, inhibition of phosphatases, or alterations in cGMP levels. Similar activation of ICa,L by thiol oxidants, and reversal by thiol reductants, identifies an allosteric thiol-containing "redox switch" on the L-type calcium channel subunit complex by which NO/O2- and NO+ transfer can exert effects opposite to those produced by NO. In sum, our results suggest that: (a) both indirect (cGMP-dependent) and direct (S-nitrosylation/oxidation) regulation of ventricular ICa,L, and (b) sarcolemma thiol redox state may be an important determinant of ICa,L activity.

Effects of neuropeptide Y on L-type calcium current in guinea-pig ventricular myocytes

1. Neuropeptide Y (NPY) reduces cell shortening at high concentrations in guinea-pig ventricular myocytes. We have studied the effects of the peptide on calcium current in cardiac myocytes. 2. We have recorded L-type calcium current in guinea-pig ventricular myocytes under conditions in which the effects of other overlapping currents have been minimised by using Na(+)-free, K(+)-free external solution and patch-clamp electrodes containing Cs+. 3. Peak inward calcium current is reduced by NPY at concentrations in excess of 1 nM, and maximal inhibition (31%) was found at and above concentrations of 100 nM. The IC50 value for NPY inhibition of peak calcium current was 1.72 nM. 4. NPY had no effect on the voltage-dependence of calcium current amplitude, on the time course of current inactivation, or on the voltage-dependence of the steady-state gating variables. 5. NPY did not reduce the calcium current in the presence of 8-Br-cyclic AMP, and it was also without effect when GTP-gamma-S or GDP-beta-S were included in the patch pipette. 6. We conclude that in guinea-pig ventricular myocytes NPY acts at low concentration to reduce L-type calcium current, via a G-protein-mediated pathway and reduction in intracellular cyclic AMP.

Adenosine inhibits L- and N-type calcium channels in pituitary melanotrophs. Evidence for the involvement of a G protein in calcium channel gating

It has been previously demonstrated that activation of A1 adenosine receptors in frog melanotrophs causes inhibition of spontaneous action potential discharges and alpha-melanocyte-stimulating hormone secretion. In the present study, we have investigated the effect of adenosine on high-voltage-activated (HVA) calcium currents in cultured melanotrophs, using the whole-cell variant of the patch-clamp technique with barium as a charge carrier. Adenosine and the specific A1 adenosine receptor agonist R-PIA (50 microM each) produced a decrease of the amplitude of the barium current, while the selective A2 adenosine receptor agonist CGS 21680 did not affect the current. The inhibitory effect of R-PIA was observed throughout the activation range of the current, with stronger responses at more positive potentials. R-PIA inhibited both the L- and N-type components of the current, the effect on the N-component being two-fold higher than on the L-component. The inhibitory effect of R-PIA was rendered irreversible by addition of GTP gamma S (100 microM) to the intracellular solution. Pre-treatment of the cells with pertussis toxin (1 microgram/ml; 12 h) totally abolished the effect of R-PIA on the HVA calcium channels. Conversely, addition of a high concentration of cAMP (100 microM) together with the phosphodiesterase inhibitor IBMX (100 microM) to the intracellular solution did not modify the effect of R-PIA on the current. It is concluded that, in frog melanotrophs, adenosine induces inhibition of L- and N-calcium currents and that this effect is mediated by a pertussis toxin-sensitive G protein. Our data also indicate that the inhibitory effect of adenosine on the calcium currents is not mediated by inhibition of adenylyl cyclase.

Effects of adenosine on Ca2+ transients and tension in aequorin-injected ferret papillary muscles

The effects of adenosine on Ca2+ transients and tension in ferret papillary muscles were investigated using the aequorin method. Adenosine (0.01-1 mM) reduced the peak of Ca2+ transients and caused a slight concentration-dependent decrease in tension. Adenosine prolonged the decay time of Ca2+ transients but did not alter the time course of tension. In isoproterenol (0.1 microM)-treated preparations, adenosine decreased the peak of Ca2+ transients but did not alter the peak of tension. Adenosine prolonged the isoproterenol-shortened decay time of Ca2+ transients. The effects of adenosine on Ca2+ transients were antagonized by the selective A1 receptor antagonist 8-cyclopentyl-1,3,-dipropylxanthine. In the presence of isoproterenol, adenosine (0.1 mM) shifted the intracellular [Ca2+]/tension relation to the left. These results can be explained by the hypothesis that adenosine inhibits the activity of adenylate cyclase via stimulation of the A1 receptor, other mechanisms however cannot be overlooked. The prolongation of the decay time of Ca2+ transients and the increase in the Ca2+ sensitivity of the contractile elements are the underlying mechanisms of adenosine which maintain developed tension in twitch response, although adenosine decreases the peak of Ca2+ transients.

Mechanisms for vagal modulation of ventricular repolarization and of coronary occlusion-induced lethal arrhythmias in cats

Our goal was to better understand the mechanisms underlying muscarinic receptor actions on the ventricle in vivo. Therefore, we studied the effects of vagal stimulation on ventricular repolarization and of vagal tone on lethal arrhythmias induced by 30 minutes of left anterior descending coronary artery ligation in anesthetized cats. Experimental groups included normal control cats subjected only to coronary ligation and cats pretreated with atropine, pertussis toxin (PTX), or propranolol. All cats received bilateral cervical vagal stimulation (Vstim) at 1, 3, and 5 Hz for 1 minute at 10-minute intervals. Before coronary ligation, Vstim slowed sinus rate, prolonged the PR interval, and lowered blood pressure. Most important from the point of view of electrophysiological function was a vagally induced acceleration of ventricular repolarization in paced and unpaced hearts, which could be explained by the effects of acetylcholine (ie, shortening the subepicardial muscle action potentials). The effect on repolarization was blocked by atropine or PTX but not by propranolol. The extent of sinus slowing and acceleration of repolarization was directly related to the level of functional PTX-sensitive G protein (P < .05). Coronary occlusion was performed during atrial pacing such that the heart rate in all groups was equal. The incidence of ventricular fibrillation (VF) was 10% in the control group and 50% and 54% in atropine and PTX groups, respectively (P < .05). During atrial pacing before coronary occlusion, a vagal index was calculated as percent QTc shortening during Vstim. When the vagal index was 13% to 26%, the incidence of VF during occlusion was zero. When the vagal index was 0% to 12%, VF was 52% (P < .01). Conclusions are as follows: (1) Vstim accelerates ventricular repolarization in cats via a pathway that incorporates a PTX-sensitive G protein and involves an altered gradient between epicardium and endocardium. (2) Removal of vagal tone during ischemia favors VF, as predicted by a vagal index.

Modulation of L-type Ca2+ current by extracellular ATP in ferret isolated right ventricular myocytes

1. The effects of extracellular adenosine triphosphate (ATP) on the basal L-type Ca2+ current (ICa) were investigated in ferret isolated right ventricular myocytes using the gigaohm seal voltage clamp in the whole-cell and cell-attached configurations. 2. Micromolar levels of extracellular ATP reversibly inhibited ICa in a concentration-dependent manner, without any significant changes in the voltage dependence of either the peak ICa I-V relationship or steady-state activation curve. 3. In contrast, micromolar levels of extracellular ATP did significantly alter the inactivation characteristics of ICa. Ten micromolar ATP: (i) increased the degree of steady-state inactivation of ICa; (ii) altered the time constants of ICa inactivation at 0 mV; and (iii) decreased the time constant of ICa recovery from inactivation at -70 mV. 4. The inhibitory effect of ATP on ICa was not blocked by atropine, a muscarinic cholinergic receptor antagonist, or CPDPX (8-cyclopentyl-3,4-dipropylxanthine), an A1 adenosine receptor antagonist. In contrast, the inhibitory effect of 10 microM ATP could be nearly completely antagonized by 100 microM suramin, a purinergic P2 receptor antagonist. 5. The potency order of ATP analogues in inhibiting ICa was 2-methyl-thio-ATP > ATP > alpha,beta-methylene-ATP, indicating involvement of a P2Y-type ATP receptor. 6. Pretreatment of cells with pertussis toxin (PTX) did not prevent the ATP-induced decrease in ICa. However, (i) ATP produced an irreversible decrease of ICa in the presence of intracellular GTP gamma S, and (ii) the inhibitory effect was significantly attenuated in the presence of intracellular GDP beta S, indicating the involvement of a PTX-insensitive G protein in the P2Y receptor-coupling process. 7. Neither (i) replacing extracellular Ca2+ with 1 mM Ba2+, nor (ii) intracellular perfusion of 10 mM BAPTA for at least 30 min attenuated the inhibitory effect of ATP on the current through Ca2+ channels, suggesting that the inhibitory effect was not obligatorily dependent upon influx of Ca2+ or changes in [Ca2+]i. 8. Ensemble-average current behaviour constructed from cell-attached patch recordings of single L-type Ca2+ channels (110 mM BaCl2) demonstrated that when 10 microM ATP was added to the superfusate on the outside of the patch electrode the inhibition of ICa was still observed, providing evidence for the involvement of intracellular diffusible second messenger(s).(ABSTRACT TRUNCATED AT 400 WORDS)