Effects of muscarinic receptor stimulation on Ca2+ transient, cAMP production and pacemaker frequency of rabbit sinoatrial node cells

We investigated the contribution of the intracellular calcium (Cai2+) transient to acetylcholine (ACh)-mediated reduction of pacemaker frequency and cAMP content in rabbit sinoatrial nodal (SAN) cells. Action potentials (whole cell perforated patch clamp) and Cai2+ transients (Indo-1 fluorescence) were recorded from single isolated rabbit SAN cells, whereas intracellular cAMP content was measured in SAN cell suspensions using a cAMP assay (LANCE®). Our data show that the Cai2+ transient, like the hyperpolarization-activated “funny current” (If) and the ACh-sensitive potassium current (IK,ACh), is an important determinant of ACh-mediated pacemaker slowing. When If and IK,ACh were both inhibited, by cesium (2 mM) and tertiapin (100 nM), respectively, 1 μM ACh was still able to reduce pacemaker frequency by 72%. In these If and IK,ACh-inhibited SAN cells, good correlations were found between the ACh-mediated change in interbeat interval and the ACh-mediated change in Cai2+ transient decay (r2 = 0.98) and slow diastolic Cai2+ rise (r2 = 0.73). Inhibition of the Cai2+ transient by ryanodine (3 μM) or BAPTA-AM (5 μM) facilitated ACh-mediated pacemaker slowing. Furthermore, ACh depressed the Cai2+ transient and reduced the sarcoplasmic reticulum (SR) Ca2+ content, all in a concentration-dependent fashion. At 1 μM ACh, the spontaneous activity and Cai2+ transient were abolished, but completely recovered when cAMP production was stimulated by forskolin (10 μM) and IK,ACh was inhibited by tertiapin (100 nM). Also, inhibition of the Cai2+ transient by ryanodine (3 μM) or BAPTA-AM (25 μM) exaggerated the ACh-mediated inhibition of cAMP content, indicating that Cai2+ affects cAMP production in SAN cells. In conclusion, muscarinic receptor stimulation inhibits the Cai2+ transient via a cAMP-dependent signaling pathway. Inhibition of the Cai2+ transient contributes to pacemaker slowing and inhibits Cai2+-stimulated cAMP production. Thus, we provide functional evidence for the contribution of the Cai2+ transient to ACh-induced inhibition of pacemaker activity and cAMP content in rabbit SAN cells

Sodium ion transporters as new therapeutic targets in heart failure

Sodium ion transporters in sarcolemma are involved in numerous vital cell functions, such as excitability, excitation-contraction coupling, energy metabolism, pH and volume regulation, development and growth. In a number of cardiac pathologies, the intracellular sodium concentration ([Na+]i) is elevated. Since [Na+]i and intracellular Ca2+ concentration ([Ca2+]i are coupled through the Na+/Ca(2+)-exchanger, these cardiac pathologies display disturbed calcium handling. For instance, [Na+]i is increased in heart failure (HF) leading to Na+/Ca(2+)-exchanger mediated increase in [Ca2+]i, reduced contractility and increased propensity to arrhythmias. Several studies support the contention that an increase in [Na+]i and [Ca2+]i transduces a signal the nucleus, that triggers development of cardiac remodelling and hypertrophy. Pharmacological intervention, which favourably interferes with [Na+]i and [Ca2+]i homeostasis, might prevent hypertrophy, cardiac remodelling, arrhythmias and HF. The most important sodium transport mechanisms that may underlie increased [Na+]i are: Na+/H(+)-exchanger (NHE-1), Na+-HCO(3)(-) co-transporter (NBC), Na(+)-K(+)-Cl(-) co-transporter (NKCC), Na(+)-channel, Na+/K(+)-ATPase and Na+/Ca(2+)-exchanger (NCX). Preclinical studies showed that pharmacological interventions, targeted against sarcolemmal sodium ion transporters, proved effective in ameliorating heart failure. In this respect: 1) NHE-1 inhibition reduces cardiac remodelling, hypertrophy and HF, although, in the patients following coronary artery bypass graft surgery, it was associated with an increase of stroke. 2) The activity of NBC is up-regulated, during the development of hypertrophy and may be a therapeutic strategy to prevent the development of hypertrophy and HF. 3) NKCC is increased in post-infarction HF, and the inhibition of NKCC attenuated post-infarction remodelling. 4) Inactivation of sodium channels is impaired in HF, which may result, in increased Na+ influx and prolongation of the action potential. 5) Blockade of NCX may be useful as a part of a combined therapeutic approach. Inhibition of reversed mode, or activation of forward mode NCX reduce Ca2+ overload. 6) Inhibition of Na+/K(+)-ATPase (digoxin), is used to increase contractility, however, it enhances progression of HF. Oppositely, new drugs which increase activity of Na+/K(+)-ATPase may prevent the development of cardiac remodelling hypertrophy and H

Calcium transient and sodium-calcium exchange current in human versus rabbit sinoatrial node pacemaker cells

There is an ongoing debate on the mechanism underlying the pacemaker activity of sinoatrial node (SAN) cells, focusing on the relative importance of the "membrane clock" and the "Ca(2+) clock" in the generation of the small net membrane current that depolarizes the cell towards the action potential threshold. Specifically, the debate centers around the question whether the membrane clock-driven hyperpolarization-activated current, I f , which is also known as the "funny current" or "pacemaker current," or the Ca(2+) clock-driven sodium-calcium exchange current, I NaCa, is the main contributor to diastolic depolarization. In our contribution to this journal's "Special Issue on Cardiac Electrophysiology," we present a numerical reconstruction of I f and I NaCa in isolated rabbit and human SAN pacemaker cells based on experimental data on action potentials, I f , and intracellular calcium concentration ([Ca(2+)] i ) that we have acquired from these cells. The human SAN pacemaker cells have a smaller I f , a weaker [Ca(2+)] i transient, and a smaller I NaCa than the rabbit cells. However, when compared to the diastolic net membrane current, I NaCa is of similar size in human and rabbit SAN pacemaker cells, whereas I f is smaller in human than in rabbit cell

Contribution of NHE-1 to cell length shortening of normal and failing rabbit cardiac myocytes

At the same intracellular pH (pHi) Na+/H+ exchange (NHE-1) fluxes of ventricular myocytes of hypertrophied failing hearts (HFH) are increased. We assessed how NHE-1 affected cell length shortening. pHi was measured fluorimetrically in resting and twitching (1-3 Hz) normal and HFH rabbit myocytes. In HEPES-buffered solutions, increased NHE-1 fluxes (P=0.001, n=14) made HFH resting pHi 0.2+/-0.03 units more alkaline than control (n=27). In CO2/HCO3--buffered solutions, HFH resting pHi was not different (7.05+/-0.02, n=30). Twitching myocytes of both groups shortened 15-16% less per 0.1 pH unit acidification. In HEPES-buffered solutions, cariporide depressed cell length shortening of normal myocytes (1-3 Hz) by 16+/-5.4% (n=9, P=0.005). In HFH myocytes cariporide restored the positive force-frequency relationship (n=7, P=0.009), by depressing twitch amplitudes at 1 Hz (16+/-11%, P=0.047) but not at 2 and 3 Hz. The depressions were all caused by pHi acidification. In CO2/HCO3- buffered solutions the cariporide-induced acidification was too small to explain the cell length shortening depression of normal (19+/-5.0%, n=11, P=0.006) and HFH myocytes (14+/-4.7%, n=11, P=0.001). When compared to HEPES-buffered solutions, HFH myocytes in CO2/HCO3--buffered solutions shortened 12+/-6.8% better than expected given the 0.16+/-0.02 units more acidic pHi's at which they twitched. We conclude that in CO2/HCO3--buffered solutions NHE-1 improved cell length shortening of unstretched normal and HFH myocytes via a pHi-independent mechanism. Although NHE-1 was increased in HFH myocytes, the magnitude of the pHi-independent effect of NHE-1 inhibition on cell length shortening was similar in both group

Calcium transient and sodium-calcium exchange current in human versus rabbit sinoatrial node pacemaker cells

There is an ongoing debate on the mechanism underlying the pacemaker activity of sinoatrial node (SAN) cells, focusing on the relative importance of the "membrane clock" and the "Ca 2+ clock" in the generation of the small net membrane current that depolarizes the cell towards the action potential threshold. Specifically, the debate centers around the question whether the membrane clock-driven hyperpolarization-activated current, , which is also known as the "funny current" or "pacemaker current, " or the Ca 2+ clock-driven sodium-calcium exchange current, NaCa , is the main contributor to diastolic depolarization. In our contribution to this journal's "Special Issue on Cardiac Electrophysiology, " we present a numerical reconstruction of and NaCa in isolated rabbit and human SAN pacemaker cells based on experimental data on action potentials, , and intracellular calcium concentration ([Ca 2+ ] ) that we have acquired from these cells. The human SAN pacemaker cells have a smaller , a weaker [Ca 2+ ] transient, and a smaller NaCa than the rabbit cells. However, when compared to the diastolic net membrane current, NaCa is of similar size in human and rabbit SAN pacemaker cells, whereas is smaller in human than in rabbit cells

Ca(2+)-activated Cl(-) current in rabbit sinoatrial node cells

The Ca(2+)-activated Cl(-) current (I(Cl(Ca))) has been identified in atrial, Purkinje and ventricular cells, where it plays a substantial role in phase-1 repolarization and delayed after-depolarizations. In sinoatrial (SA) node cells, however, the presence and functional role of I(Cl(Ca)) is unknown. In the present study we address this issue using perforated patch-clamp methodology and computer simulations. Single SA node cells were enzymatically isolated from rabbit hearts. I(Cl(Ca)) was measured, using the perforated patch-clamp technique, as the current sensitive to the anion blocker 4,4'-diisothiocyanostilbene-2,2'-disulphonic acid (DIDS). Voltage clamp experiments demonstrate the presence of I(Cl(Ca)) in one third of the spontaneously active SA node cells. The current was transient outward with a bell-shaped current-voltage relationship. Adrenoceptor stimulation with 1 microM noradrenaline doubled the I(Cl(Ca)) density. Action potential clamp measurements demonstrate that I(Cl(Ca)) is activate late during the action potential upstroke. Current clamp experiments show, both in the absence and presence of 1 microM noradrenaline, that blockade of I(Cl(Ca)) increases the action potential overshoot and duration, measured at 20 % repolarization. However, intrinsic interbeat interval, upstroke velocity, diastolic depolarization rate and the action potential duration measured at 50 and 90 % repolarization were not affected. Our experimental data are supported by computer simulations, which additionally demonstrate that I(Cl(Ca)) has a limited role in pacemaker synchronization or action potential conduction. In conclusion, I(Cl(Ca)) is present in one third of SA node cells and is activated during the pacemaker cycle. However, I(Cl(Ca)) does not modulate intrinsic interbeat interval, pacemaker synchronization or action potential conductio

Reduced swelling-activated Cl- current densities in hypertrophied ventricular myocytes of rabbits with heart failure

Objective: Hypertrophied myocytes of failing hearts have prolonged action potential durations. It is unknown how the swelling-activated Cl- current affects the abnormal AP configuration. Methods: We studied (I-Cl,I-swell) in ventricular myocytes isolated from failing and age-matched normal rabbit hearts. We applied whole-cell patch-clamp methodology and activated I-Cl,I-swell by lowering tonicity of the superfusate. Results: Neither with ruptured-patch nor with amphotericin B perforated-patch, whole-cell clamp we found I-Cl,I-swell active under isotonic conditions in either the normal or the hypertrophied failing heart (HFH) myocytes. caused AP shortening and resting membrane potential (V-m) depolarization in an osmotic gradient-dependent fashion. However, in the HFH myocytes swelling-induced AP changes were significantly smaller, even though the cells underwent the same relative change in planar cell surface area. Voltage-clamp experiments revealed that in HFH myocytes I-Cl,I-swell current density was similar to50% reduced. Conclusion: Reduced I-Cl,I-swell densities in HFH myocytes cause limited AP shortening and V-m depolarization upon swelling of the cells. (C) 2002 Elsevier Science BY. All rights reserve

Intracellular calcium modulation of voltage-gated sodium channels in ventricular myocytes

AIMS: Cardiac voltage-gated sodium channels control action potential (AP) upstroke and cell excitability. Intracellular calcium (Ca(i)(2+)) regulates AP properties by modulating various ion channels. Whether Ca(i)(2+) modulates sodium channels in ventricular myocytes, is unresolved. We studied whether Ca(i)(2+) modulates sodium channels in ventricular myocytes at Ca(i)(2+) concentrations ([Ca(i)(2+)]) present during the cardiac AP (0-500 nM), and how this modulation affects sodium channel properties in heart failure (HF), a condition in which Ca(i)(2+) homeostasis is disturbed. METHODS: Sodium current (I(Na)) and maximal AP upstroke velocity (dV/dt(max)), a measure of I(Na), were studied at 20 masculineC and 37 masculineC, respectively, in freshly isolated left ventricular myocytes of control and HF rabbits, using whole-cell patch-clamp methodology. [Ca(i)(2+)] was varied using different pipette solutions, the Ca(i)(2+) buffer BAPTA, and caffeine administration. RESULTS: Elevated [Ca(i)(2+)] reduced I(Na) density and dV/dt(max), but caused no I(Na) gating changes. Reductions in I(Na) density occurred simultaneously with increases in [Ca(i)(2+)], suggesting that these effects were due to permeation block. Accordingly, unitary sodium current amplitudes were reduced at higher [Ca(i)(2+)]. While I(Na) density and gating at fixed [Ca(i)(2+)] were not different between HF and control, reductions in dV/dt(max) upon increases in stimulation rate were larger in HF than in control; these differences were abolished by BAPTA. CONCLUSION: Ca(i)(2+) exerts acute modulation of I(Na) density in ventricular myocytes, but does not modify I(Na) gating. These effects, occurring rapidly and in the [Ca(i)(2+)] range observed physiologically, may contribute to beat-to-beat regulation of cardiac excitability in health and diseas

Pacemaker current (I(f)) in the human sinoatrial node

AIMS: Animal studies revealed that the hyperpolarization-activated pacemaker current, I(f), contributes to action potential (AP) generation in sinoatrial node (SAN) and significantly determines heart rate. I(f) is becoming a novel therapy target to modulate heart rate. Yet, no studies have demonstrated that I(f) is functionally present and contributes to pacemaking in human SAN. We aimed to study I(f) properties in human SAN. METHODS AND RESULTS: In a patient undergoing SAN excision, we identified SAN using epicardial activation mapping. From here, we isolated myocytes and recorded APs and I(f) using patch-clamp techniques. Pacemaker cells generated spontaneous APs (cycle length 828 +/- 15 ms) following slow diastolic depolarization, maximal diastolic potential - 61.7 +/- 4.3 mV, and maximal AP upstroke velocity 4.6 +/- 1.2 V/s. They exhibited an hyperpolarization-activated inward current, blocked by external Cs(+) (2 mmol/L), characterizing it as I(f). Fully-activated conductance was 75.2 +/- 3.8 pS/pF, reversal potential - 22.1 +/- 2.4 mV, and half-maximal activation voltage and slope factor of steady-state activation - 96.9 +/- 2.7 and - 8.8 +/- 0.5 mV. Activation time constant ranged from approximately 350 ms (-130 mV) to approximately 1 s (-100 mV), deactivation time constant 156 +/- 45 ms (-40 mV). The role of I(f) in pacemaker activity was demonstrated by slowing of pacemaker cell diastolic depolarization and beating rate by Cs(+). CONCLUSION: I(f) is functionally expressed in human SAN and probably contributes to pacemaking in human SA

Single Cells Isolated from Human Sinoatrial Node: Action Potentials and Numerical Reconstruction of Pacemaker Current

Pacemaker activity of the sinoatrial node has extensively been studied in laboratory animals of various species, but is virtually unexplored in man. Most experimental data have been obtained from rabbit, where the hyperpolarization-activated 'funny' current (If), also known as the 'pacemaker current', plays an important role in diastolic depolarization and thus in setting pacing rate. Recently, we isolated pacemaker cells from excised human sinoatrial node tissue, and recorded action potentials and If using the whole-cell patch-clamp technique in current clamp and voltage clamp mode, respectively. Single sinoatrial node pacemaker cells showed a spontaneous beating rate of 73 +/- 3 beats/min (mean +/- SEM, n = 3) with a remarkably slow diastolic depolarization. If was identified in voltage clamp experiments as the 2 mmol/L Cs+-sensitive inward current activating upon 2-s hyperpolarizing voltage clamp steps. The If reversal potential and (de)activation kinetics were similar to those in rabbit. However, the fully-activated If conductance was 3-4 times smaller than typically found in rabbit. Furthermore, the half-maximal activation voltage was approximately 20 mV more negative than in rabbit. These differences would both act to reduce the functional role of If in human pacemaker cells. To assess this functional role, we carried out a numerical reconstruction of the If time course during an experimentally recorded human sinoatrial node action potential, based on the obtained data on If amplitude and kinetics. This reconstruction revealed that If provides a small but significant inward current in the voltage range of diastolic depolarization. We conclude that human sinoatrial node pacemaker cells functionally express If and that this If contributes to pacemaking in human sinoatrial nod