Mechanism of muscarinic control of the high-threshold calcium current in rabbit sino-atrial node myocytes

Acetylcholine Reduces L-Type Calcium Current without Major Changes in Repolarization of Canine and Human Purkinje and Ventricular Tissue

Vagal nerve stimulation (VNS) holds a strong basis as a potentially effective treatment modality for chronic heart failure, which explains why a multicenter VNS study in heart failure with reduced ejection fraction is ongoing. However, more detailed information is required on the effect of acetylcholine (ACh) on repolarization in Purkinje and ventricular cardiac preparations to identify the advantages, risks, and underlying cellular mechanisms of VNS. Here, we studied the effect of ACh on the action potential (AP) of canine Purkinje fibers (PFs) and several human ventricular preparations. In addition, we characterized the effects of ACh on the L-type Ca²⁺ current (I_CaL) and AP of human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) and performed computer simulations to explain the observed effects. Using microelectrode recordings, we found a small but significant AP prolongation in canine PFs. In the human myocardium, ACh slightly prolonged the AP in the midmyocardium but resulted in minor AP shortening in subepicardial tissue. Perforated patch-clamp experiments on hiPSC-CMs demonstrated that 5 µM ACh caused an ≈15% decrease in I_CaL density without changes in gating properties. Using dynamic clamp, we found that under blocked K⁺ currents, 5 µM ACh resulted in an ≈23% decrease in AP duration at 90% of repolarization in hiPSC-CMs. Computer simulations using the O'Hara-Rudy human ventricular cell model revealed that the overall effect of ACh on AP duration is a tight interplay between the ACh-induced reduction in I_CaL and ACh-induced changes in K⁺ currents. In conclusion, ACh results in minor changes in AP repolarization and duration of canine PFs and human ventricular myocardium due to the concomitant inhibition of inward I_CaL and outward K⁺ currents, which limits changes in net repolarizing current and thus prevents major changes in AP repolarization.

Dual Activation of Phosphodiesterase 3 and 4 Regulates Basal Cardiac Pacemaker Function and Beyond

The sinoatrial (SA) node is the physiological pacemaker of the heart, and resting heart rate in humans is a well-known risk factor for cardiovascular disease and mortality. Consequently, the mechanisms of initiating and regulating the normal spontaneous SA node beating rate are of vital importance. Spontaneous firing of the SA node is generated within sinoatrial nodal cells (SANC), which is regulated by the coupled-clock pacemaker system. Normal spontaneous beating of SANC is driven by a high level of cAMP-mediated PKA-dependent protein phosphorylation, which rely on the balance between high basal cAMP production by adenylyl cyclases and high basal cAMP degradation by cyclic nucleotide phosphodiesterases (PDEs). This diverse class of enzymes includes 11 families and PDE3 and PDE4 families dominate in both the SA node and cardiac myocardium, degrading cAMP and, consequently, regulating basal cardiac pacemaker function and excitation-contraction coupling. In this review, we will demonstrate similarities between expression, distribution, and colocalization of various PDE subtypes in SANC and cardiac myocytes of different species, including humans, focusing on PDE3 and PDE4. Here, we will describe specific targets of the coupled-clock pacemaker system modulated by dual PDE3 + PDE4 activation and provide evidence that concurrent activation of PDE3 + PDE4, operating in a synergistic manner, regulates the basal cardiac pacemaker function and provides control over normal spontaneous beating of SANCs through (PDE3 + PDE4)-dependent modulation of local subsarcolemmal Ca²⁺ releases (LCRs).

Dual Activation of Phosphodiesterase 3 and 4 Regulates Basal Cardiac Pacemaker Function and Beyond

The sinoatrial (SA) node is the physiological pacemaker of the heart, and resting heart rate in humans is a well-known risk factor for cardiovascular disease and mortality. Consequently, the mechanisms of initiating and regulating the normal spontaneous SA node beating rate are of vital importance. Spontaneous firing of the SA node is generated within sinoatrial nodal cells (SANC), which is regulated by the coupled-clock pacemaker system. Normal spontaneous beating of SANC is driven by a high level of cAMP-mediated PKA-dependent protein phosphorylation, which rely on the balance between high basal cAMP production by adenylyl cyclases and high basal cAMP degradation by cyclic nucleotide phosphodiesterases (PDEs). This diverse class of enzymes includes 11 families and PDE3 and PDE4 families dominate in both the SA node and cardiac myocardium, degrading cAMP and, consequently, regulating basal cardiac pacemaker function and excitation-contraction coupling. In this review, we will demonstrate similarities between expression, distribution, and colocalization of various PDE subtypes in SANC and cardiac myocytes of different species, including humans, focusing on PDE3 and PDE4. Here, we will describe specific targets of the coupled-clock pacemaker system modulated by dual PDE3 + PDE4 activation and provide evidence that concurrent activation of PDE3 + PDE4, operating in a synergistic manner, regulates the basal cardiac pacemaker function and provides control over normal spontaneous beating of SANCs through (PDE3 + PDE4)-dependent modulation of local subsarcolemmal Ca²⁺ releases (LCRs).

Genetic Ablation of G Protein-Gated Inwardly Rectifying K+ Channels Prevents Training-Induced Sinus Bradycardia

Background: Endurance athletes are prone to bradyarrhythmias, which in the long-term may underscore the increased incidence of pacemaker implantation reported in this population. Our previous work in rodent models has shown training-induced sinus bradycardia to be due to microRNA (miR)-mediated transcriptional remodeling of the HCN4 channel, leading to a reduction of the "funny" (I _f) current in the sinoatrial node (SAN). Objective: To test if genetic ablation of G-protein-gated inwardly rectifying potassium channel, also known as I _KACh channels prevents sinus bradycardia induced by intensive exercise training in mice. Methods: Control wild-type (WT) and mice lacking GIRK4 (Girk4 ^-/-), an integral subunit of I _KACh were assigned to trained or sedentary groups. Mice in the trained group underwent 1-h exercise swimming twice a day for 28 days, 7 days per week. We performed electrocardiogram recordings and echocardiography in both groups at baseline, during and after the training period. At training cessation, mice were euthanized and SAN tissues were isolated for patch clamp recordings in isolated SAN cells and molecular profiling by quantitative PCR (qPCR) and western blotting. Results: At swimming cessation trained WT mice presented with a significantly lower resting HR that was reversible by acute I _KACh block whereas Girk4 ^-/- mice failed to develop a training-induced sinus bradycardia. In line with HR reduction, action potential rate, density of I _f, as well as of T- and L-type Ca²⁺ currents (I _CaT and I _CaL ) were significantly reduced only in SAN cells obtained from WT-trained mice. I _f reduction in WT mice was concomitant with downregulation of HCN4 transcript and protein, attributable to increased expression of corresponding repressor microRNAs (miRs) whereas reduced I _CaL in WT mice was associated with reduced Ca_v1.3 protein levels. Strikingly, I _KACh ablation suppressed all training-induced molecular remodeling observed in WT mice. Conclusion: Genetic ablation of cardiac I _KACh in mice prevents exercise-induced sinus bradycardia by suppressing training induced remodeling of inward currents I _f, I _CaT and I _CaL due in part to the prevention of miR-mediated transcriptional remodeling of HCN4 and likely post transcriptional remodeling of Ca_v1.3. Strategies targeting cardiac I _KACh may therefore represent an alternative to pacemaker implantation for bradyarrhythmias seen in some veteran athletes.

Dual Activation of Phosphodiesterases 3 and 4 Regulates Basal Spontaneous Beating Rate of Cardiac Pacemaker Cells: Role of Compartmentalization?

Spontaneous firing of sinoatrial (SA) node cells (SANCs) is regulated by cyclic adenosine monophosphate (cAMP)-mediated, protein kinase A (PKA)-dependent (cAMP/PKA) local subsarcolemmal Ca²⁺ releases (LCRs) from ryanodine receptors (RyR). The LCRs occur during diastolic depolarization (DD) and activate an inward Na⁺/Ca²⁺ exchange current that accelerates the DD rate prompting the next action potential (AP). Basal phosphodiesterases (PDEs) activation degrades cAMP, reduces basal cAMP/PKA-dependent phosphorylation, and suppresses normal spontaneous firing of SANCs. The cAMP-degrading PDE1, PDE3, and PDE4 represent major PDE activities in rabbit SANC, and PDE inhibition by 3-isobutyl-1-methylxanthine (IBMX) increases spontaneous firing of SANC by ∼50%. Though inhibition of single PDE1–PDE4 only moderately increases spontaneous SANC firing, dual PDE3 + PDE4 inhibition produces a synergistic effect hastening the spontaneous SANC beating rate by ∼50%. Here, we describe the expression and distribution of different PDE subtypes within rabbit SANCs, several specific targets (L-type Ca²⁺ channels and phospholamban) regulated by basal concurrent PDE3 + PDE4 activation, and critical importance of RyR Ca²⁺ releases for PDE-dependent regulation of spontaneous SANC firing. Colocalization of PDE3 and PDE4 beneath sarcolemma or in striated patterns inside SANCs strongly suggests that PDE-dependent regulation of cAMP/PKA signaling might be executed at the local level; this idea, however, requires further verification.

Unique Ca2+-Cycling Protein Abundance and Regulation Sustains Local Ca2+ Releases and Spontaneous Firing of Rabbit Sinoatrial Node Cells

Spontaneous beating of the heart pacemaker, the sinoatrial node, is generated by sinoatrial node cells (SANC) and caused by gradual change of the membrane potential called diastolic depolarization (DD). Submembrane local Ca²⁺ releases (LCR) from sarcoplasmic reticulum (SR) occur during late DD and activate an inward Na⁺/Ca²⁺ exchange current, which accelerates the DD rate leading to earlier occurrence of an action potential. A comparison of intrinsic SR Ca²⁺ cycling revealed that, at similar physiological Ca²⁺ concentrations, LCRs are large and rhythmic in permeabilized SANC, but small and random in permeabilized ventricular myocytes (VM). Permeabilized SANC spontaneously released more Ca²⁺ from SR than VM, despite comparable SR Ca²⁺ content in both cell types. In this review we discuss specific patterns of expression and distribution of SR Ca²⁺ cycling proteins (SR Ca²⁺ ATPase (SERCA2), phospholamban (PLB) and ryanodine receptors (RyR)) in SANC and ventricular myocytes. We link ability of SANC to generate larger and rhythmic LCRs with increased abundance of SERCA2, reduced abundance of the SERCA inhibitor PLB. In addition, an increase in intracellular [Ca²⁺] increases phosphorylation of both PLB and RyR exclusively in SANC. The differences in SR Ca²⁺ cycling protein expression between SANC and VM provide insights into diverse regulation of intrinsic SR Ca²⁺ cycling that drives automaticity of SANC.

Cardiac neuroanatomy - Imaging nerves to define functional control

The autonomic nervous system regulates normal cardiovascular function and plays a critical role in the pathophysiology of cardiovascular disease. Further understanding of the interplay between the autonomic nervous system and cardiovascular system holds promise for the development of neuroscience-based cardiovascular therapeutics. To this end, techniques to image myocardial innervation will help provide a basis for understanding the fundamental underpinnings of cardiac neural control. In this review, we detail the evolution of gross and microscopic anatomical studies for functional mapping of cardiac neuroanatomy.

Electrophysiological Effects of Dexmedetomidine on Sinoatrial Nodes of Rabbits

BACKGROUND

The purpose of this study was to investigate the electrophysiological effects of dexmedetomidine on pacemaker cells in sinoatrial nodes of rabbits.

METHODS

Healthy rabbits were anesthetized intravenously with sodium pentobarbital, and the hearts were quickly dissected and mounted in a tissue bath. Machine-pulled glass capillary microelectrodes which were connected to a high input impedance amplifier and impaled in dominant pacemaker cells. Thereafter, an intracellular microelectrode technique was used to record action potential.

RESULTS

The amplitude of action potential, velocity of diastolic (phase 4) depolarization, and rate of pacemaker firing in normal pacemaker cells in sinoatrial node were decreased by use of dexmedetomidine (0.5 ng/ml, 5 ng/ml, 50 ng/ml) in a concentration-dependent manner. Pretreatment with yohimbine (1 μM), did not alter the effects of dexmedetomidine (5 ng/ml) on sinoatrial node pacemaker cells. Pretreatment with CsCl (2 mmol/L), dexmedetomidine (5 ng/ml) decreased the amplitude of action potential, but had no significant effect on other parameters of action potential.

CONCLUSIONS

Dexmedetomidine exerts inhibitory electrophysiological effects on pacemaker cells in sinoatrial nodes of rabbits in a concentration-dependent manner, which may not be mediated by alpha 2-adrenoreceptor.

KEY WORDS

Action potential; Cardiology; Dexmedetomidine; Pacemaker activity; Sinoatrial node.

Cell-specific Dynamic Clamp analysis of the role of funny If current in cardiac pacemaking

We used the Dynamic Clamp technique for i) comparative validation of conflicting computational models of the hyperpolarization-activated funny current, If, and ii) quantification of the role of If in mediating autonomic modulation of heart rate. Experimental protocols based on the injection of a real-time recalculated synthetic If current in sinoatrial rabbit cells were developed. Preliminary results of experiments mimicking the autonomic modulation of If demonstrated the need for a customization procedure to compensate for cellular heterogeneity. For this reason, we used a cell-specific approach, scaling the maximal conductance of the injected current based on the cell's spontaneous firing rate. The pacemaking rate, which was significantly reduced after application of Ivabradine, was restored by the injection of synthetic current based on the Severi-DiFrancesco formulation, while the injection of synthetic current based on the Maltsev-Lakatta formulation did not produce any significant variation. A positive virtual shift of the If activation curve, mimicking the Isoprenaline effects, led to a significant increase in pacemaking rate (+17.3 ± 6.7%, p < 0.01), although of lower magnitude than that induced by real Isoprenaline (+45.0 ± 26.1%). Similarly, a negative virtual shift of the activation curve significantly lowered the pacemaking rate (-11.8 ± 1.9%, p < 0.001), as did the application of real Acetylcholine (-20.5 ± 5.1%). The Dynamic Clamp approach, applied to the If study in cardiomyocytes for the first time and rate-adapted to manage intercellular variability, indicated that: i) the quantitative description of the If current in the Severi-DiFrancesco model accurately reproduces the effects of the real current on rabbit sinoatrial cell pacemaking rate and ii) a significant portion (50-60%) of the physiological autonomic rate modulation is due to the shift of the If activation curve.

Effects of tertiapin-Q and ZD7288 on changes in sinoatrial pacemaker rhythm during vagal stimulation

Heart rate slowing produced by cardiac parasympathetic (vagal) stimulation is thought to be the result of modulation of the acetylcholine-activated K(+) current (IK,ACh) and the pacemaker current (If) in sinoatrial (SAN) pacemaker cells. However, the contribution of these and other ion currents to vagal slowing is controversial. Here, we examined the contributions of IK,ACh and If to vagal slowing in 15 isolated, vagal-innervated preparations of guinea-pig atria, using 300 nM tertiapin-Q (TQ) and 2 μM ZD7288 to obtain full and substantial block of these currents, respectively. Blocking IK,ACh alone reduced atrial rate responses to 10-s trains of regular vagal stimulation (supramaximal stimulation, 2-ms duration, 1-10 Hz) by ~50% (P<0.01; N=11); blocking If alone had no effect (N=7). Blocking both IK,ACh and If produced ~90% reduction (P<0.01; N=4). Atrial cycle length response to a single burst of vagal stimuli (3 stimuli at 50 Hz), delivered at the optimum phase of the cycle was strongly suppressed by blocking IK,ACh (reduced by 98%; P<0.01; N=9), and modestly reduced by blocking If alone (by ~43%; P=0.20; N=6). The response was abolished by combined block of IK,ACh and If (P=0.04; N=4). Our data show that modulation of IK,ACh and If is sufficient to account for all the vagal slowing observed in this preparation. The vagally-induced negative shift in activation potential for If will be opposed by hyperpolarisation of SAN through activation of IK,ACh. Thus removal of IK,ACh by TQ may have exaggerated the overall contribution of If to vagal slowing.

The importance of Ca(2+)-dependent mechanisms for the initiation of the heartbeat

Mechanisms underlying pacemaker activity in the sinus node remain controversial, with some ascribing a dominant role to timing events in the surface membrane (“membrane clock”) and others to uptake and release of calcium from the sarcoplasmic reticulum (SR) (“calcium clock”). Here we discuss recent evidence on mechanisms underlying pacemaker activity with a particular emphasis on the many roles of calcium. There are particular areas of controversy concerning the contribution of calcium spark-like events and the importance of I(f) to spontaneous diastolic depolarisation, though it will be suggested that neither of these is essential for pacemaking. Sodium-calcium exchange (NCX) is most often considered in the context of mediating membrane depolarisation after spark-like events. We present evidence for a broader role of this electrogenic exchanger which need not always depend upon these spark-like events. Short (milliseconds or seconds) and long (minutes) term influences of calcium are discussed including direct regulation of ion channels and NCX, and control of the activity of calcium-dependent enzymes (including CaMKII, AC1, and AC8). The balance between the many contributory factors to pacemaker activity may well alter with experimental and clinical conditions, and potentially redundant mechanisms are desirable to ensure the regular spontaneous heart rate that is essential for life. This review presents evidence that calcium is central to the normal control of pacemaking across a range of temporal scales and seeks to broaden the accepted description of the “calcium clock” to cover these important influences.

The G-protein-gated K+ channel, IKACh, is required for regulation of pacemaker activity and recovery of resting heart rate after sympathetic stimulation

Parasympathetic regulation of sinoatrial node (SAN) pacemaker activity modulates multiple ion channels to temper heart rate. The functional role of the G-protein–activated K⁺ current (I_KACh) in the control of SAN pacemaking and heart rate is not completely understood. We have investigated the functional consequences of loss of I_KACh in cholinergic regulation of pacemaker activity of SAN cells and in heart rate control under physiological situations mimicking the fight or flight response. We used knockout mice with loss of function of the Girk4 (Kir3.4) gene (Girk4^−/− mice), which codes for an integral subunit of the cardiac I_KACh channel. SAN pacemaker cells from Girk4^−/− mice completely lacked I_KACh. Loss of I_KACh strongly reduced cholinergic regulation of pacemaker activity of SAN cells and isolated intact hearts. Telemetric recordings of electrocardiograms of freely moving mice showed that heart rate measured over a 24-h recording period was moderately increased (10%) in Girk4^−/− animals. Although the relative extent of heart rate regulation of Girk4^−/− mice was similar to that of wild-type animals, recovery of resting heart rate after stress, physical exercise, or pharmacological β-adrenergic stimulation of SAN pacemaking was significantly delayed in Girk4^−/− animals. We conclude that I_KACh plays a critical role in the kinetics of heart rate recovery to resting levels after sympathetic stimulation or after direct β-adrenergic stimulation of pacemaker activity. Our study thus uncovers a novel role for I_KACh in SAN physiology and heart rate regulation.

Effects of acetylcholine and noradrenalin on action potentials of isolated rabbit sinoatrial and atrial myocytes

The autonomic nervous system controls heart rate and contractility through sympathetic and parasympathetic inputs to the cardiac tissue, with acetylcholine (ACh) and noradrenalin (NA) as the chemical transmitters. In recent years, it has become clear that specific Regulators of G protein Signaling proteins (RGS proteins) suppress muscarinic sensitivity and parasympathetic tone, identifying RGS proteins as intriguing potential therapeutic targets. In the present study, we have identified the effects of 1 μM ACh and 1 μM NA on the intrinsic action potentials of sinoatrial (SA) nodal and atrial myocytes. Single cells were enzymatically isolated from the SA node or from the left atrium of rabbit hearts. Action potentials were recorded using the amphotericin-perforated patch-clamp technique in the absence and presence of ACh, NA, or a combination of both. In SA nodal myocytes, ACh increased cycle length and decreased diastolic depolarization rate, whereas NA decreased cycle length and increased diastolic depolarization rate. Both ACh and NA increased maximum upstroke velocity. Furthermore, ACh hyperpolarized the maximum diastolic potential. In atrial myocytes stimulated at 2 Hz, both ACh and NA hyperpolarized the maximum diastolic potential, increased the action potential amplitude, and increased the maximum upstroke velocity. Action potential duration at 50 and 90% repolarization was decreased by ACh, but increased by NA. The effects of both ACh and NA on action potential duration showed a dose dependence in the range of 1-1000 nM, while a clear-cut frequency dependence in the range of 1-4 Hz was absent. Intermediate results were obtained in the combined presence of ACh and NA in both SA nodal and atrial myocytes. Our data uncover the extent to which SA nodal and atrial action potentials are intrinsically dependent on ACh, NA, or a combination of both and may thus guide further experiments with RGS proteins.

Ca(2+)-stimulated adenylyl cyclases regulate the L-type Ca(2+) current in guinea-pig atrial myocytes

Ca(2+)-stimulated adenylyl cyclases (ACs) have recently been shown to play important roles in pacemaking in the sino-atrial node. Here we present evidence that Ca(2+)-stimulated ACs are functionally active in guinea-pig atrial myocytes. Basal activity of an AC in isolated atrial myocytes was demonstrated by the observations that MDL 12,330A (10 μm), an AC inhibitor, reduced L-type Ca(2+) current (I(CaL)) amplitude, while inhibition of phosphodiesterases with IBMX (100 μm) increased I(CaL) amplitude. Buffering of cytosolic Ca(2+) by exposure of myocytes to BAPTA-AM (5 μm) reduced I(CaL) amplitude, as did inhibition of Ca(2+) release from the sarcoplasmic reticulum with ryanodine (2 μm) and thapsigargin (1 μm). [Ca(2+)]i-activated calmodulin kinase II (CaMKII) inhibition with KN-93 (1 μm) reduced I(CaL), but subsequent application of BAPTA-AM further reduced I(CaL). This effect of BAPTA-AM, in the presence of CaMKII inhibition, demonstrates that there is an additional Ca(2+)-modulated pathway (not dependent on CaMKII) that regulates I(CaL) in atrial myocytes. The effects of BAPTA could be reversed by forskolin (10 μm), a direct stimulator of all AC isoforms, which would restore cAMP levels. In the presence of BAPTA-AM, the actions of IBMX were reduced. In addition, inclusion of cAMP in the patch electrode in the whole-cell configuration prevented the effects of BAPTA. These effects are all consistent with a role for Ca(2+)-stimulated AC in the regulation of atrial myocyte I(CaL).

The muscarinic-activated potassium channel always participates in vagal slowing of the guinea-pig sinoatrial pacemaker

Controversy persists regarding participation of the muscarinic-activated potassium current (c(KACh)) in small and moderate vagal bradycardia. We investigated this by (i) critical examination of earlier experimental data for mechanisms proposed to operate in modest vagal bradycardia (modulation of I(f) and inhibition of a junctional Na(+) current) and (ii) experiments performed on isolated vagally-innervated guinea-pig atria. In 8 superperfused preparations, 10-s trains of vagal stimulation (1 to 20Hz) produced a bradycardia that ranged from 1 to 80%. Hyperpolarisation of sinoatrial cells accompanied bradycardia in 65/67 observations (linear correlation between bradycardia and increase in maximum diastolic potential (mV)=0.076x%; R(2)=0.57; P<0.001). In bath-mounted preparations single supramaximal stimuli to the vagus immediately and briefly increased pacemaker cycle length in 7 of 18 preparations. This response was eliminated by 300nM tertiapin-Q. Trains of 10 single supramaximal vagal stimuli applied at 1-s intervals caused progressive increase in overall cycle length during the train; immediate and brief increases in cycle length occurred following some stimuli. Immediate brief responses and part of the slower response to the stimulus train were removed by 300nM tertiapin-Q.

SUMMARY

experimental data shows that small and modest vagal bradycardia is accompanied by hyperpolarisation of the pacemaker cell which is severely attenuated by tertiapin-Q. These observations support the idea that activation of I(KACh) occurs at all levels of vagal bradycardia. Contradictory conclusions from earlier studies may be attributed to the nature of experimental models and experimental design.

Phosphodiesterases reduce spontaneous sinoatrial beating but not the 'fight or flight' tachycardia elicited by agonists through Gs-protein-coupled receptors

Cyclic AMP (cAMP) steers the generation of basal heart beat in the sinoatrial node. It also induces sinoatrial tachycardia and increased cardiac force, elicited through activation of Gs-protein-coupled receptors (GsPCRs). Phosphodiesterases (PDEs) hydrolyse cAMP. In the heart mainly PDE3 and PDE4 would be expected to limit those functions, and the PDE isoenzymes do indeed reduce basal sinoatrial beating rate and blunt the positive inotropic effects of agonists, mediated by GsPCRs. By contrast, recent evidence shows that GsPCR-mediated sinoatrial tachycardia is not controlled by PDE1-5. A PDE-resistant cAMP pool in sinoatrial cells, generated through activation of GsPCRs, including β(1)- and β(2)-adrenoceptors, appears to guarantee unrestrained tachycardia during fight or flight stress.

Novel oscillatory mechanisms in the cholinergic control of Guinea pig sino-atrial node discharge

Oscillatory Mechanisms in Sinus Node Cholinergic Control. 

INTRODUCTION

The role of the oscillatory after-potential V(os) and pre-potential ThV(os) in cholinergic control of discharge was studied in sino-atrial node (SAN).

METHODS AND RESULTS

A microelectrode technique was used in isolated guinea-pig SAN superfused in vitro in high [K(+) ](o) to visualize V(os) and ThV(os) . The cholinergic agonist carbachol (CCh) decreased the amplitude and slope of V(os) and ThV(os) at a time when there was no increase in maximum diastolic potential. The slowing in SAN rate was due to slower and smaller ThV(os) that missed intermittently the threshold and occurred gradually later in diastole, but not to a decrease in the intrinsic rate of ThV(os) . Eventually, quiescence followed. Larger CCh concentrations quickly induced a hyperpolarization that altogether prevented the occurrence of oscillatory potentials. During CCh washout, ThV(os) reappeared and consistently reinitiated discharge. Lower [Ca(2+) ](o) also decreased slopes and amplitude of V(os) and ThV(os) , thereby slowing and stopping SAN discharge, as CCh did. Overdrive temporarily offset the negative chronotropic effects of CCh and of low [Ca(2+) ](o.) Cesium (a blocker of hyperpolarization-activated current I(f) ) did not abolish CCh inhibitory effects on oscillatory potentials.

CONCLUSIONS

The cholinergic agonist CCh: (1) slows SAN discharge by decreasing the amplitude of V(os) and ThV(os) , but not the rate of ThV(os) ; (2) can cause hyperpolarization that altogether suppresses the oscillatory potentials; (3) is mimicked in its effects by low [Ca(2+) ](o) ; (4) is antagonized by procedures that increase cellular calcium; and (5) modifies the oscillatory potentials independently of I(f) .

Mechanistic links between Na+ channel (SCN5A) mutations and impaired cardiac pacemaking in sick sinus syndrome

RATIONALE

Familial sick sinus syndrome (SSS) has been linked to loss-of-function mutations of the SCN5A gene, which result in decreased inward Na(+) current, I(Na). However, the functional role of I(Na) in cardiac pacemaking is controversial, and mechanistic links between mutations and sinus node dysfunction in SSS are unclear.

OBJECTIVE

To determine mechanisms by which the SCN5A mutations impair cardiac pacemaking.

METHODS AND RESULTS

Action potential (AP) models for rabbit sinoatrial node (SAN) cells were modified to incorporate experimentally reported I(Na) changes induced by 2 groups of SCN5A gene mutations (affecting the activation and inactivation of I(Na), respectively). The cell models were incorporated into an anatomically detailed 2D model of the intact SAN-atrium. Effects of the mutations and vagal nerve activity on cardiac pacemaking at the single-cell and tissue levels were studied. Multielectrode extracellular potential recordings of activation pattern from intact SAN-atrium preparations were performed to test predictions of the models. At the single-cell level, the mutations slowed down pacemaking rates in peripheral, but not in central SAN cells that control the heart rhythm. However, in tissue simulations, the mutations not only slowed down pacemaking, but also compromised AP conduction across the SAN-atrium, leading to a possible SAN exit block or sinus arrest, the major features of SSS. Simulated vagal nerve activity amplified the bradycardiac effects of the mutations. Two groups of SCN5A mutations showed subtle differences in impairing the ability of the SAN to drive the surrounding atrium, primarily attributable to their differential effects on atrial excitability and conduction safety. Experimental data with tetrodotoxin and carbachol confirmed the simulation outcomes.

CONCLUSIONS

Our study substantiates the causative link between SCN5A gene mutations and SSS and illustrates mechanisms by which the mutations impair the driving ability of the SAN.

A novel quantitative explanation for the autonomic modulation of cardiac pacemaker cell automaticity via a dynamic system of sarcolemmal and intracellular proteins

Classical numerical models have attributed the regulation of normal cardiac automaticity in sinoatrial node cells (SANCs) largely to G protein-coupled receptor (GPCR) modulation of sarcolemmal ion currents. More recent experimental evidence, however, has indicated that GPCR modulation of SANCs automaticity involves spontaneous, rhythmic, local Ca(2+) releases (LCRs) from the sarcoplasmic reticulum (SR). We explored the GPCR rate modulation of SANCs using a unique and novel numerical model of SANCs in which Ca(2+)-release characteristics are graded by variations in the SR Ca(2+) pumping capability, mimicking the modulation by phospholamban regulated by cAMP-mediated, PKA-activated signaling. The model faithfully predicted the entire range of physiological chronotropic modulation of SANCs by the activation of beta-adrenergic receptors or cholinergic receptors only when experimentally documented changes of sarcolemmal ion channels are combined with a simultaneous increase/decrease in SR Ca(2+) pumping capability. The novel numerical mechanism of GPCR rate modulation is based on numerous complex synergistic interactions between sarcolemmal and intracellular processes via membrane voltage and Ca(2+). Major interactions include changes of diastolic Na(+)/Ca(2+) exchanger current that couple earlier/later diastolic Ca(2+) releases (predicting the experimentally defined LCR period shift) of increased/decreased amplitude (predicting changes in LCR signal mass, i.e., the product of LCR spatial size, amplitude, and number per cycle) to the diastolic depolarization and ultimately to the spontaneous action potential firing rate. Concomitantly, larger/smaller and more/less frequent activation of L-type Ca(2+) current shifts the cellular Ca(2+) balance to support the respective Ca(2+) cycling changes. In conclusion, our model simulations corroborate recent experimental results in rabbit SANCs pointing to a new paradigm for GPCR heart rate modulation by a complex system of dynamically coupled sarcolemmal and intracellular proteins.

A coupled SYSTEM of intracellular Ca2+ clocks and surface membrane voltage clocks controls the timekeeping mechanism of the heart's pacemaker

Ion channels on the surface membrane of sinoatrial nodal pacemaker cells (SANCs) are the proximal cause of an action potential. Each individual channel type has been thoroughly characterized under voltage clamp, and the ensemble of the ion channel currents reconstructed in silico generates rhythmic action potentials. Thus, this ensemble can be envisioned as a surface "membrane clock" (M clock). Localized subsarcolemmal Ca(2+) releases are generated by the sarcoplasmic reticulum via ryanodine receptors during late diastolic depolarization and are referred to as an intracellular "Ca(2+) clock," because their spontaneous occurrence is periodic during voltage clamp or in detergent-permeabilized SANCs, and in silico as well. In spontaneously firing SANCs, the M and Ca(2+) clocks do not operate in isolation but work together via numerous interactions modulated by membrane voltage, subsarcolemmal Ca(2+), and protein kinase A and CaMKII-dependent protein phosphorylation. Through these interactions, the 2 subsystem clocks become mutually entrained to form a robust, stable, coupled-clock system that drives normal cardiac pacemaker cell automaticity. G protein-coupled receptors signaling creates pacemaker flexibility, ie, effects changes in the rhythmic action potential firing rate, by impacting on these very same factors that regulate robust basal coupled-clock system function. This review examines evidence that forms the basis of this coupled-clock system concept in cardiac SANCs.

Regulation of basal and reserve cardiac pacemaker function by interactions of cAMP-mediated PKA-dependent Ca2+ cycling with surface membrane channels

Decades of intensive research of primary cardiac pacemaker, the sinoatrial node, have established potential roles of specific membrane channels in the generation of the diastolic depolarization, the major mechanism allowing sinoatrial node cells to generate spontaneous beating. During the last three decades, multiple studies made either in the isolated sinoatrial node or sinoatrial node cells have demonstrated a pivotal role of Ca(2+) and, specifically Ca(2+) release from sarcoplasmic reticulum, for spontaneous beating of cardiac pacemaker. Recently, spontaneous, rhythmic local subsarcolemmal Ca(2+) releases from ryanodine receptors during late half of the diastolic depolarization have been implicated as a vital factor in the generation of sinoatrial node cell spontaneous firing. Local Ca(2+) releases are driven by a unique combination of high basal cAMP production by adenylyl cyclases, high basal cAMP degradation by phosphodiesterases and a high level of cAMP-mediated PKA-dependent phosphorylation. These local Ca(2+) releases activate an inward Na(+)-Ca(2+) exchange current which accelerates the terminal diastolic depolarization rate and, thus, controls the spontaneous pacemaker firing. Both the basal primary pacemaker beating rate and its modulation via beta-adrenergic receptor stimulation appear to be critically dependent upon intact RyR function and local subsarcolemmal sarcoplasmic reticulum generated Ca(2+) releases. This review aspires to integrate the traditional viewpoint that has emphasized the supremacy of the ensemble of surface membrane ion channels in spontaneous firing of the primary cardiac pacemaker, and these novel perspectives of cAMP-mediated PKA-dependent Ca(2+) cycling in regulation of the heart pacemaker clock, both in the basal state and during beta-adrenergic receptor stimulation.

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 (Ca_i²⁺) 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 Ca_i²⁺ 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 Ca_i²⁺ transient, like the hyperpolarization-activated “funny current” (I_f) and the ACh-sensitive potassium current (I_K,ACh), is an important determinant of ACh-mediated pacemaker slowing. When I_f and I_K,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 I_f and I_K,ACh-inhibited SAN cells, good correlations were found between the ACh-mediated change in interbeat interval and the ACh-mediated change in Ca_i²⁺ transient decay (r² = 0.98) and slow diastolic Ca_i²⁺ rise (r² = 0.73). Inhibition of the Ca_i²⁺ transient by ryanodine (3 μM) or BAPTA-AM (5 μM) facilitated ACh-mediated pacemaker slowing. Furthermore, ACh depressed the Ca_i²⁺ transient and reduced the sarcoplasmic reticulum (SR) Ca²⁺ content, all in a concentration-dependent fashion. At 1 μM ACh, the spontaneous activity and Ca_i²⁺ transient were abolished, but completely recovered when cAMP production was stimulated by forskolin (10 μM) and I_K,ACh was inhibited by tertiapin (100 nM). Also, inhibition of the Ca_i²⁺ transient by ryanodine (3 μM) or BAPTA-AM (25 μM) exaggerated the ACh-mediated inhibition of cAMP content, indicating that Ca_i²⁺ affects cAMP production in SAN cells. In conclusion, muscarinic receptor stimulation inhibits the Ca_i²⁺ transient via a cAMP-dependent signaling pathway. Inhibition of the Ca_i²⁺ transient contributes to pacemaker slowing and inhibits Ca_i²⁺-stimulated cAMP production. Thus, we provide functional evidence for the contribution of the Ca_i²⁺ transient to ACh-induced inhibition of pacemaker activity and cAMP content in rabbit SAN cells.

Cholinergic receptor signaling modulates spontaneous firing of sinoatrial nodal cells via integrated effects on PKA-dependent Ca(2+) cycling and I(KACh)

Prior studies indicate that cholinergic receptor (ChR) activation is linked to beating rate reduction (BRR) in sinoatrial nodal cells (SANC) via 1) a G(i)-coupled reduction in adenylyl cyclase (AC) activity, leading to a reduction of cAMP or protein kinase A (PKA) modulation of hyperpolarization-activated current (I(f)) or L-type Ca(2+) currents (I(Ca,L)), respectively; and 2) direct G(i)-coupled activation of ACh-activated potassium current (I(KACh)). More recent studies, however, have indicated that Ca(2+) cycling by the sarcoplasmic reticulum within SANC (referred to as a Ca(2+) clock) generates rhythmic, spontaneous local Ca(2+) releases (LCR) that are AC-PKA dependent. LCRs activate Na(+)-Ca(2+) exchange (NCX) current, which ignites the surface membrane ion channels to effect an AP. The purpose of the present study was to determine how ChR signaling initiated by a cholinergic agonist, carbachol (CCh), affects AC, cAMP, and PKA or sarcolemmal ion channels and LCRs and how these effects become integrated to generate the net response to a given intensity of ChR stimulation in single, isolated rabbit SANC. The threshold CCh concentration ([CCh]) for BRR was approximately 10 nM, half maximal inhibition (IC(50)) was achieved at 100 nM, and 1,000 nM stopped spontaneous beating. G(i) inhibition by pertussis toxin blocked all CCh effects on BRR. Using specific ion channel blockers, we established that I(f) blockade did not affect BRR at any [CCh] and that I(KACh) activation, evidenced by hyperpolarization, first became apparent at [CCh] > 30 nM. At IC(50), CCh reduced cAMP and reduced PKA-dependent phospholamban (PLB) phosphorylation by approximately 50%. The dose response of BRR to CCh in the presence of I(KACh) blockade by a specific inhibitor, tertiapin Q, mirrored that of CCh to reduced PLB phosphorylation. At IC(50), CCh caused a time-dependent reduction in the number and size of LCRs and a time dependent increase in LCR period that paralleled coincident BRR. The phosphatase inhibitor calyculin A reversed the effect of IC(50) CCh on SANC LCRs and BRR. Numerical model simulations demonstrated that Ca(2+) cycling is integrated into the cholinergic modulation of BRR via LCR-induced activation of NCX current, providing theoretical support for the experimental findings. Thus ChR stimulation-induced BRR is entirely dependent on G(i) activation and the extent of G(i) coupling to Ca(2+) cycling via PKA signaling or to I(KACh): at low [CCh], I(KACh) activation is not evident and BRR is attributable to a suppression of cAMP-mediated, PKA-dependent Ca(2+) signaling; as [CCh] increases beyond 30 nM, a tight coupling between suppression of PKA-dependent Ca(2+) signaling and I(KACh) activation underlies a more pronounced BRR.

Tertiapin-Q removes a large and rapidly acting component of vagal slowing of the guinea-pig cardiac pacemaker

The participation of acetylcholine-activated potassium current (I(K,ACh)) and hyperpolarization-activated pacemaker current (I(f)) in vagal bradycardia were examined using vagally-innervated preparations of guinea-pig atria. Preparations were maintained in Krebs-Henseleit solution (36 degrees C). Before treatment, trains of vagal stimuli (10 s at 2, 5 and 10 Hz) produced graded bradycardias displaying rapid onset and offset. Tertiapin-Q (300 nM), which blocks I(K,ACh), had no effect on baseline atrial rate. In tertiapin-Q, vagal bradycardia displayed a gradual onset and offset, with a peak response ~50% of that recorded in control conditions. Cumulative addition of 1 mM ZD7288 (blocker of I(f)) caused atrial rate to fall by ~60%, but had no further effect on the amplitude of the vagal bradycardia, while response onset and offset became slightly faster. From these observations, we argue that (i) vagal bradycardia was attributable primarily to activation of I(K,ACh), (ii) vagal modulation of I(f) had a minor influence on the rate of onset and offset of bradycardia, and (iii) removal of the influence of I(K,ACh) unmasked a slow response, of undetermined origin, to vagal stimulation. In a separate set of experiments we compared the effects of 1 mM Ba(2+) and 300 nM tertiapin-Q on vagal bradycardia. Ba(2+) reduced baseline atrial rate and the response to vagal stimulation. Subsequent cumulative addition of tertiapin-Q had no additional effect on baseline atrial rate, but caused further reduction in the amplitude of vagal bradycardia, suggesting that 1 mM Ba(2+) did not achieve a complete block of I(K,ACh) in this preparation.

Synergism of coupled subsarcolemmal Ca2+ clocks and sarcolemmal voltage clocks confers robust and flexible pacemaker function in a novel pacemaker cell model

Recent experimental studies have demonstrated that sinoatrial node cells (SANC) generate spontaneous, rhythmic, local subsarcolemmal Ca(2+) releases (Ca(2+) clock), which occur during late diastolic depolarization (DD) and interact with the classic sarcolemmal voltage oscillator (membrane clock) by activating Na(+)-Ca(2+) exchanger current (I(NCX)). This and other interactions between clocks, however, are not captured by existing essentially membrane-delimited cardiac pacemaker cell numerical models. Using wide-scale parametric analysis of classic formulations of membrane clock and Ca(2+) cycling, we have constructed and initially explored a prototype rabbit SANC model featuring both clocks. Our coupled oscillator system exhibits greater robustness and flexibility than membrane clock operating alone. Rhythmic spontaneous Ca(2+) releases of sarcoplasmic reticulum (SR)-based Ca(2+) clock ignite rhythmic action potentials via late DD I(NCX) over much broader ranges of membrane clock parameters [e.g., L-type Ca(2+) current (I(CaL)) and/or hyperpolarization-activated ("funny") current (I(f)) conductances]. The system Ca(2+) clock includes SR and sarcolemmal Ca(2+) fluxes, which optimize cell Ca(2+) balance to increase amplitudes of both SR Ca(2+) release and late DD I(NCX) as SR Ca(2+) pumping rate increases, resulting in a broad pacemaker rate modulation (1.8-4.6 Hz). In contrast, the rate modulation range via membrane clock parameters is substantially smaller when Ca(2+) clock is unchanged or lacking. When Ca(2+) clock is disabled, the system parametric space for fail-safe SANC operation considerably shrinks: without rhythmic late DD I(NCX) ignition signals membrane clock substantially slows, becomes dysrhythmic, or halts. In conclusion, the Ca(2+) clock is a new critical dimension in SANC function. A synergism of the coupled function of Ca(2+) and membrane clocks confers fail-safe SANC operation at greatly varying rates.

Genesis and regulation of the heart automaticity

The heart automaticity is a fundamental physiological function in higher organisms. The spontaneous activity is initiated by specialized populations of cardiac cells generating periodical electrical oscillations. The exact cascade of steps initiating the pacemaker cycle in automatic cells has not yet been entirely elucidated. Nevertheless, ion channels and intracellular Ca(2+) signaling are necessary for the proper setting of the pacemaker mechanism. Here, we review the current knowledge on the cellular mechanisms underlying the generation and regulation of cardiac automaticity. We discuss evidence on the functional role of different families of ion channels in cardiac pacemaking and review recent results obtained on genetically engineered mouse strains displaying dysfunction in heart automaticity. Beside ion channels, intracellular Ca(2+) release has been indicated as an important mechanism for promoting automaticity at rest as well as for acceleration of the heart rate under sympathetic nerve input. The potential links between the activity of ion channels and Ca(2+) release will be discussed with the aim to propose an integrated framework of the mechanism of automaticity.

Simulated ischemia enhances L-type calcium current in pacemaker cells isolated from the rabbit sinoatrial node

Ischemic-like conditions (a glucose-free, pH 6.6 Tyrode solution bubbled with 100% N2) enhance L-type Ca current ( ICa,L) in single pacemaker cells (PCs) isolated from the rabbit sinoatrial node (SAN). In contrast, studies of ventricular myocytes have shown that acidic extracellular pH, as employed in our “ischemic” Tyrode, reduces ICa,L. Therefore, our goal was to explain why ICa,Lis increased by “ischemia” in SAN PCs. The major findings were the following: 1) blockade of Ca-induced Ca release with ryanodine, exposure of PCs to BAPTA-AM, or replacement of extracellular Ca2+with Ba2+failed to prevent the ischemia-induced enhancement of ICa,L; 2) inhibition of protein kinase A with H-89, or calcium/calmodulin-dependent protein kinase II with KN-93, reduced ICa,Lbut did not prevent its augmentation by ischemia; 3) ischemic Tyrode or pH 6.6 Tyrode shifted the steady-state inactivation curve in the positive direction, thereby reducing inactivation; 4) ischemic Tyrode increased the maximum conductance but did not affect the activation curve; 5) in rabbit atrial myocytes isolated and studied with exactly the same techniques used for SAN PCs, ischemic Tyrode reduced the maximum conductance and shifted the activation curve in the positive direction; pH 6.6 Tyrode also shifted the steady-state inactivation curve in the positive direction. We conclude that the acidic pH of ischemic Tyrode enhances ICa,Lin SAN PCs, because it increases the maximum conductance and reduces inactivation. Furthermore, the opposite results obtained with rabbit atrial myocytes cannot be explained by differences in cell isolation or patch-clamp techniques.

Normal heart rhythm is initiated and regulated by an intracellular calcium clock within pacemaker cells

For almost half a century it has been thought that the heart rhythm originates on the surface membrane of the cardiac pacemaker cells and is driven by voltage-gated ion channels (membrane clocks). Data from several recent studies, however, conclusively show that the rhythm is initiated, sustained, and regulated by oscillatory Ca(2+) releases (Ca(2+) clock) from the sarcoplasmic reticulum, a major Ca(2+) store within sinoatrial node cells, the primary heart's pacemakers. Activation of the local oscillatory Ca(2+) releases is independent of membrane depolarisation and driven by a high level of basal state phosphorylation of Ca(2+) cycling proteins. The releases produce Ca(2+) wavelets under the cell surface membrane during the later phase of diastolic depolarisation and activate the forward mode of Na(+)/Ca(2+) exchanger resulting in inward membrane current, which ignites an action potential. Phosphorylation-dependent gradation of speed at which Ca(2+) clock cycles is the essential regulatory mechanism of normal pacemaker rate and rhythm. The robust regulation of pacemaker function is insured by tight integration of Ca(2+) and membrane clocks: the action potential shape and ion fluxes are tuned by membrane clocks to sustain operation of the Ca(2+) clock which produces timely and powerful ignition of the membrane clocks to effect action potentials.

The effects of tertiapin-Q on responses of the sinoatrial pacemaker of the guinea-pig heart to vagal nerve stimulation and muscarinic agonists

Using Langendorff preparations of the guinea-pig heart, we have examined the participation of the acetylcholine (ACh)-activated potassium channel, IK,ACh, in the bradycardia produced by electrical stimulation of the vagus (parasympathetic) nerve and muscarinic agonists (ACh and bethanecol, bolus i.a.). Hearts from young animals (160-250 g) were perfused with Krebs-Henseleit solution, and pacemaker frequency was determined from the P wave of an ECG. Tertiapin-Q was used to block IK,ACh. Vagal stimulation (10 s trains at 2, 5 and 10 Hz) produced graded reductions in atrial rate that were substantially attenuated, and to a similar extent, by 300 nm and 1 microm tertiapin-Q (to 0.42 +/- 0.12, mean +/- s.d., of the control values; P < 0.001). Acetylcholine (3 nmol) produced brief graded bradycardias that were also attenuated by tertiapin-Q (0.24 +/- 0.24; P = 0.006). Similar results were obtained when experiments were repeated in 2 mm Cs+ (to block the hyperpolarization-activated pacemaker current). Bethanecol (30, 50 and 70 nmol), a muscarinic agonist with no appreciable nicotinic activity, produced sustained bradycardias that were attenuated by 300 nm tertiapin-Q (0.36 +/- 0.21; P < 0.0001). The responses to vagal stimulation and ACh developed more slowly in tertiapin-Q, indicating that a rapidly acting mechanism had been blocked. Responses to vagal stimulation were faster in 2 mm Cs+. Together, these observations show that ACh released from parasympathetic nerve varicosities exerts a considerable part of its effect on the pacemaker by activating IK,ACh and acts in a manner not readily distinguishable from that of directly applied muscarinic agonists.

Ca2+-stimulated adenylyl cyclase isoform AC1 is preferentially expressed in guinea-pig sino-atrial node cells and modulates the I(f) pacemaker current

Ca(2+)-stimulated adenylyl cyclases (AC) are known to play important roles in neurons but have not previously been reported in the heart. Here we present the first evidence for selective expression of Ca(2+)-stimulated AC in the sino-atrial node (SAN) but not in ventricular muscle of the guinea-pig heart. The AC1 isoform of Ca(2+)-stimulated AC was shown to be present in SAN, both as mRNA using RT-PCR and as protein using immuno-blotting with a specific antibody. Confocal immuno-fluorescence studies detected membrane localization of AC1 in SAN cells, but no AC1 in ventricular muscle. Ca(2+)-stimulated AC8 may also be present in SAN. The functional importance of AC activity was investigated by monitoring activation of I(f) (gated by hyperpolarization and regulated by cAMP, which shifts activation to more depolarized voltages). Basal activity of AC in isolated SAN myocytes was demonstrated by the observations that an inhibitor of AC activity (MDL 12330A, 10 microm) shifted activation in the hyperpolarizing direction, while inhibition of phosphodiesterases (IBMX, 100 microm) shifted I(f) activation in the depolarizing direction. Buffering cytosolic Ca(2+) with the Ca(2+) chelator BAPTA (by exposure to BAPTA-AM) shifted activation of I(f) in the hyperpolarizing direction, and under these conditions the AC inhibitor MDL had little or no further effect. The actions of BAPTA were overcome by exposure to forskolin (10 microm), a direct stimulator of all AC isoforms, to restore cAMP levels. These effects are consistent with the functional importance of Ca(2+)-stimulated AC, which is expected to be fundamental to initiation and regulation of the heartbeat.

The Integration of Spontaneous Intracellular Ca2+ Cycling and Surface Membrane Ion Channel Activation Entrains Normal Automaticity in Cells of the Heart's Pacemaker

Although the ensemble of voltage- and time-dependent rhythms of surface membrane ion channels, the membrane "Clock", is the immediate cause of a sinoatrial nodal cell (SANC) action potential (AP), it does not necessarily follow that this ion channel ensemble is the formal cause of spontaneous, rhythmic APs. SANC also generates intracellular oscillatory spontaneous Ca(2+) releases that ignite excitation (SCaRIE) of the surface membrane via Na(+)/Ca(2+) exchanger activation. The idea that a rhythmic intracellular Ca(2+) Clock might keep time for normal automaticity of SANC, however, has not been assimilated into mainstream pacemaker dogma. Recent experimental evidence, derived from simultaneous, confocal imaging of submembrane Ca(2+) and membrane potential of SANC, and supported by numerical modeling, indicates that normal automaticity of SANC is entrained and stabilized by the tight integration of the SR Ca(2+) Clock that generates rhythmic SCaRIE, and the surface membrane Clock that responds to SCaRIE to immediately produce APs of an adequate shape. Thus, tightly controlled, rhythmic SCaRIE does not merely fine tune SANC AP firing, but is the formal cause of the basal and reserve rhythms, insuring pacemaker stability by rhythmically integrating multiple Ca(2+)-dependent functions, and effects normal automaticity by rhythmic ignition of the surface membrane Clock.

The emergence of a general theory of the initiation and strength of the heartbeat

Sarcoplasmic reticulum (SR) Ca(2+) cycling, that is, the Ca(2+) clock, entrained by externally delivered action potentials has been a major focus in ventricular myocyte research for the past 5 decades. In contrast, the focus of pacemaker cell research has largely been limited to membrane-delimited pacemaker mechanisms (membrane clock) driven by ion channels, as the immediate cause for excitation. Recent robust experimental evidence, based on confocal cell imaging, and supported by numerical modeling suggests a novel concept: the normal rhythmic heart beat is governed by the tight integration of both intracellular Ca(2+) and membrane clocks. In pacemaker cells the intracellular Ca(2+) clock is manifested by spontaneous, rhythmic submembrane local Ca(2+) releases from SR, which are tightly controlled by a high degree of basal and reserve PKA-dependent protein phosphorylation. The Ca(2+) releases rhythmically activate Na(+)/Ca(2+) exchange inward currents that ignite action potentials, whose shape and ion fluxes are tuned by the membrane clock which, in turn, sustains operation of the intracellular Ca(2+) clock. The idea that spontaneous SR Ca(2+) releases initiate and regulate normal automaticity provides the key that reunites pacemaker and ventricular cell research, thus evolving a general theory of the initiation and strength of the heartbeat.

Postganglionic nerve stimulation induces temporal inhibition of excitability in rabbit sinoatrial node

Vagal stimulation results in complex changes of pacemaker excitability in the sinoatrial node (SAN). To investigate the vagal effects in the rabbit SAN, we used optical mapping, which is the only technology that allows resolving simultaneous changes in the activation pattern and action potentials morphologies. With the use of immunolabeling, we identified the SAN as a neurofilament 160-positive but connexin 43-negative region ( n = 5). Normal excitation originated in the SAN center with a cycle length (CL) of 405 ± 14 ms ( n = 14), spread anisotropically along the crista terminalis (CT), and failed to conduct toward the septum. Postganglionic nerve stimulation (PNS, 400–800 ms) reduced CL by 74 ± 7% transiently and shifted the leading pacemaker inferiorly (78%) or superiorly (22%) from the SAN center by 2–10 mm. In the intercaval region between the SAN center and the septal block zone, PNS produced an 8 ± 1-mm2 region of transient hyperpolarization and inexcitability. The first spontaneous or paced excitation following PNS could not enter this region for 500–1,500 ms. Immunolabeling revealed that the PNS-induced inexcitable region is located between the SAN center and the block zone and has a 2.5-fold higher density of choline acetyltransferase than CT but is threefold lower than the SAN center. The fact that the inexcitability region does not coincide with the most innervated area indicates that the properties of the myocytes themselves, as well as intercellular coupling, must play a role in the inexcitability induction. Optically mapping revealed that PNS resulted in transient loss of pacemaker cell excitability and unidirectional entrance conduction block in the periphery of SAN.

Responses of the sustained inward current to autonomic agonists in guinea-pig sino-atrial node pacemaker cells

1. The present study was undertaken to examine the responses of the sustained inward current (I(st)) to beta-adrenergic and muscarinic agonists in guinea-pig sino-atrial (SA) node cells using the whole-cell patch-clamp technique. I(st) was detected as the nicardipine (1 microM)-sensitive inward current at potentials between approximately -80 and +20 mV in the presence of low concentration (0.1 mM) of extracellular Ca2+, where the L-type Ca2+ current (I(Ca,L)) was practically abolished. 2. Beta-adrenergic agonist isoprenaline (ISO) in nanomolar concentrations not only increased the amplitude of I(st) but also shifted the membrane potential producing the peak amplitude (Vpeak) to a negative direction by approximately 15 mV without appreciably affecting potential range for the current activation. The stimulatory effect of ISO was concentration-dependent with an EC50 of 2.26 nM and the maximal effect (96.4+/-22.9% increase, n=6) was obtained at 100 nM ISO, when evaluated by the responses at -50 mV. 3. Bath application of acetylcholine (ACh) significantly inhibited I(st), which had been maximally augmented by 100 nM ISO; this inhibitory effect of ACh was concentration-dependent with an IC50 of 133.9 nM. High concentration (1000 nM) of ACh depressed basal I(st) by 10.5+/-2.0% (n=3). 4. In action potential clamp experiments, I(st) was also detected under control conditions and was markedly potentiated by exposure to ISO. 5. These results strongly suggest that I(st) not only contributes to the spontaneous action potentials of mammalian SA node cells but also plays a substantial role in mediating autonomic regulation of SA node pacemaker activity.

Voltage-dependent calcium channels and cardiac pacemaker activity: from ionic currents to genes

The spontaneous activity of pacemaker cells in the sino-atrial node controls the heart rhythm and rate under physiological conditions. Compared to working myocardial cells, pacemaker cells express a specific array of ionic channels. The functional importance of different ionic channels in the generation and regulation of cardiac automaticity is currently subject of an extensive research effort and has long been controversial. Among families of ionic channels, Ca(2+) channels have been proposed to substantially contribute to pacemaking. Indeed, Ca(2+) channels are robustly expressed in pacemaker cells, and influence the cell beating rate. Furthermore, they are regulated by the activity of the autonomic nervous system in both a positive and negative way. In this manuscript, we will first discuss how the concept of the involvement of Ca(2+) channels in cardiac pacemaking has been proposed and then subsequently developed by the recent advent in the domain of cardiac physiology of gene-targeting techniques. Secondly, we will indicate how the specific profile of Ca(2+) channels expression in pacemaker tissue can help design drugs which selectively regulate the heart rhythm in the absence of concomitant negative inotropism. Finally, we will indicate how the new possibility to assign a specific gene activity to a given ionic channel involved in cardiac pacemaking could implement the current postgenomic research effort in the construction of the cardiac Physiome.

Muscarinic regulation of cardiac ion channels

The parasympathetic component of the autonomic nervous system plays an important role in the physiological regulation of cardiac function by exerting significant influence over the initiation as well as propagation of electrical impulses, in addition to being able to regulate contractile force. These effects are mediated in whole or in part through changes in ion channel activity that occur in response to activation of M(2) muscarinic cholinergic receptors following release of the neurotransmitter acetylcholine. The coupling of M(2) receptor activation to most changes in cardiac ion channel function can be explained by one of two general paradigms. The first involves direct G protein-dependent regulation of ion channel activity. The second involves indirect regulation of ion channel activity through modulation of cAMP-dependent responses. This review focuses on recent advances in our understanding of the mechanisms by which M(2) muscarinic receptor activation both inhibits and facilitates cAMP-dependent ion channel responses in the heart.

cGMP-mediated inhibition of cardiac L-type Ca(2+) current by a monoclonal antibody against the M(2) ACh receptor

The effects of a monoclonal antibody (B8E5) directed against the second extracellular loop of the muscarinic M(2) receptor were studied on the L-type Ca(2+) currents (I(Ca,L)) of guinea pig ventricular myocytes using the whole cell patch-clamp technique. Similar to carbachol, B8E5 reduced the isoproterenol (ISO)-stimulated I(Ca,L) but did not significantly affect basal I(Ca,L). Atropine blocked the inhibitory effect of B8E5. The electrophysiological parameters of ISO-stimulated I(Ca,L) were not modified in presence of B8E5. Inhibition of I(Ca,L) by B8E5 was still observed when intracellular cAMP was either enhanced by forskolin or maintained constant by using a hydrolysis-resistant cAMP analog (8-bromoadenosine 3',5'-cyclic monophosphate) or by applying the phosphodiesterase inhibitor IBMX. The effect of B8E5 was mimicked by 8-bromoguanosine 3',5'-cyclic monophosphate, a potent stimulator of cGMP-dependent protein kinase, and prevented by a selective inhibitor of nitric oxide-sensitive guanylyl cyclase [1H-(1,2,4)oxadiazolo[4,3-a]quinoxaline-1-one]. These results indicate that the antibody B8E5 inhibits the beta-adrenergic-stimulated I(Ca,L) through activation of the M(2) muscarinic receptor and further suggest that the antibody acts not via the classical pathway of decreasing intracellular cAMP, but rather by increasing cGMP.

Potentiation by neostigmine of responses to vagal nerve stimulation in the sinus venosus of the toad

The effects of the cholinesterase inhibitor neostigmine on the responses to vagus nerve stimulation of isolated sinus venosus/atrial preparations of the toad, Bufo marinus, were examined. In control solutions, trains of stimuli applied to the vagus nerve led to a decrease in heart rate that was susceptible to muscarinic receptor blockade. Membrane potential recordings made from sinus venosus cells showed that the responses to trains of stimuli, delivered at frequencies of less than 10 Hz, were little changed by the addition of neostigmine. However, the responses to longer trains of stimuli at 10 Hz (30 versus 10 s) were potentiated and the nature of the membrane potential changes was altered. The results suggest that, due to the activity of cholinesterases, acetylcholine (ACh) released from parasympathetic nerves normally has little access to the muscarinic receptors in the pacemaker region, which are linked to potassium channels.

Dynamic nonlinear vago-sympathetic interaction in regulating heart rate

Although the characteristics of the static interactions between the sympathetic and parasympathetic nervous systems in regulating heart rate have been well established, how the dynamic interaction modulates the heart rate response remains unknown. Thus, we investigated the dynamic interaction by estimating the transfer function from nerve stimulation to heart rate, using band-limited Gaussian white noise, in anesthetized rabbits. Concomitant tonic vagal stimulation at 5 and 10 Hz increased the gain of the transfer function relating dynamic sympathetic stimulation to heart rate by 55.0%+/-40.1% and 80.7%+/-50.5%, respectively (P < 0.05). Concomitant tonic sympathetic stimulation at 5 and 10 Hz increased the gain of the transfer function relating dynamic vagal stimulation to heart rate by 18.2%+/-17.9% and 24.1%+/-18.0%, respectively (P < 0.05). Such bidirectional augmentation was also observed during simultaneous dynamic stimulation of the sympathetic and vagal nerves independent of their stimulation patterns. Because of these characteristics, changes in sympathetic or vagal tone alone can alter the dynamic heart rate response to stimulation of the other nerve. We explained this phenomenon by assuming a sigmoidal static relationship between autonomic nerve activity and heart rate. To confirm this assumption, we identified the static and dynamic characteristics of heart rate regulation by a neural network analysis, using large-amplitude Gaussian white noise input. To examine the mechanism involved in the bidirectional augmentation, we increased cytosolic adenosine 3',5'-cyclic monophosphate (cAMP) at the postjunctional effector site by applying pharmacological interventions. The cAMP accumulation increased the gain of the transfer function relating dynamic vagal stimulation to heart rate. Thus, accumulation of cAMP contributes, at least in part, to the sympathetic augmentation of the dynamic vagal control of heart rate.

Parasympathetic modulation of sinoatrial node pacemaker activity in rabbit heart: a unifying model

We have extended our compartmental model [Am. J. Physiol. 266 (Cell Physiol. 35): C832-C852, 1994] of the single rabbit sinoatrial node (SAN) cell so that it can simulate cellular responses to bath applications of ACh and isoprenaline as well as the effects of neuronally released ACh. The model employs three different types of muscarinic receptors to explain the variety of responses observed in mammalian cardiac pacemaking cells subjected to vagal stimulation. The response of greatest interest is the ACh-sensitive change in cycle length that is not accompanied by a change in action potential duration or repolarization or hyperpolarization of the maximum diastolic potential. In this case, an ACh-sensitive K+ current is not involved. Membrane hyperpolarization occurs in response to much higher levels of vagal stimulation, and this response is also mimicked by the model. Here, an ACh-sensitive K+ current is involved. The well-known phase-resetting response of the SAN cell to single and periodically applied vagal bursts of impulses is also simulated in the presence and absence of the beta-agonist isoprenaline. Finally, the responses of the SAN cell to longer continuous trains of periodic vagal stimulation are simulated, and this can result in the complete cessation of pacemaking. Therefore, this model is 1) applicable over the full range of intensity and pattern of vagal input and 2) can offer biophysically based explanations for many of the phenomena associated with the autonomic control of cardiac pacemaking.

Carbachol inhibition of Ca2+ currents in ventricular cells obtained from neonatal and adult rats

We investigated the postnatal developmental changes produced by the muscarinic receptor agonist, carbachol, on the L-type Ca2+ current (ICa(L)) in neonatal (aged 5 to 7 days) and adult (aged 2 to 5 months) rat ventricular cells by using the whole-cell voltage clamp technique. Carbachol inhibited the isoproterenol-stimulated ICa(L). The maximal inhibition was 89.3 +/- 4.8% (n = 5) in neonatal cells and 17.7 +/- 7.7% (n = 9) in adult cells. Carbachol inhibited the forskolin-stimulated ICa(L) to almost same extent as the isoproterenol-stimulated ICa(L). In the cells pretreated with pertussis toxin, carbachol failed to inhibit the isoproterenol-stimulated ICa(L), indicating that carbachol produced its effect via a pertussis toxin-sensitive G-protein pathway. The effects of carbachol in adult cells became more pronounced, increasing from 17.7% to 54.8% (n = 11), with the addition of the synthetic inhibitory G-protein alpha subunit (Gi alpha) (1 microM) to the reaction. Conversely, the alpha subunit of another pertussis toxin-sensitive synthetic G-protein (G(o) alpha, 1 microM) failed to mimic the effect of Gi alpha. These results suggest that, in rat ventricular cells, (1) the action of carbachol on ICa(L) showed a marked decrease during development; (2) the decrease in the effect of carbachol in adult cells is in part due to a decrease in the activity of pertussis toxin-sensitive G protein, especially Gi alpha.

Local cholinergic suppression of pacemaker activity in the rabbit sinoatrial node

The effects of transmural vagal stimulation and acetylcholine (ACh) superfusion on primary and latent pacemaker cells of the rabbit sinoatrial node were studied by using microelectrodes. Both ACh and vagal stimulation lengthened atrial cycle length by 40-60% as compared with control. In the cells from the primary pacemaker area, both ACh superfusion and vagal stimulation suppressed action potential (AP) amplitude and then induced inexcitability. In contrast, cells from subsidiary pacemaker area as well as atrium remained excitable. These effects were completely reversible and also were abolished by atropine, 10(-7) M. Cholinergically induced suppression of AP amplitude is predictable based on the maximal rate of AP upstroke (dV/dt). The probability of amplitude suppression was the highest among pacemaker cells (dV/dt, <3 V/s), in which ACh suppressed amplitude in 27 (93%) of 29 cells, and vagal stimulation did so in 38 (81%) of 47 cells. With increasing upstroke velocity, the probability of amplitude suppression decreased. Inexcitability did not occur in cells whose dV/dt was >15 V/s. The suppression of AP amplitude by ACh occurred in a concentration-dependent manner: the concentration inducing suppression of amplitude in 50% of pacemaker cells was approximately 10 microM. These results indicate that cholinergic effects on typical pacemaker and subsidiary pacemaker cells are different: whereas subsidiary pacemaker cells remain excitable, typical pacemaker cells become quiescent. We hypothesize that quiescent cells create quiescent regions in the center of the sinoatrial node that might functionally be an obstacle for reentrant tachycardias.

Accumulation of cAMP augments dynamic vagal control of heart rate

Recent investigations in our laboratory using a Gaussian white noise perturbation technique have shown that simultaneous sympathetic stimulation augmented the gain of the transfer function from vagal stimulation frequency to heart rate response. However, the mechanism of that augmentation remains to be elucidated. In this study, we examined in anesthetized rabbits how three pharmacological interventions known to cause intracellular accumulation of cAMP affected the transfer function. Isoproterenol (0.3 microg . kg-1 . min-1 iv) increased the dynamic gain of transfer function from 7.12 +/- 0.67 to 12.4 +/- 1.21 beats . min-1 . Hz-1 (P < 0.05) without changing the corner frequency or the lag time. Similar augmentations were observed when forskolin (5 microg . kg-1 . min-1 iv) or theophylline (20 mg/kg iv) was administered under conditions of beta-adrenergic blockade. These results suggest that the accumulation of cAMP at postjunctional effector sites contributes, at least in part, to the sympathetic augmentation of the dynamic vagal control of heart rate.

Cytochrome P450: a novel system modulating Ca2+ channels and contraction in mammalian heart cells

1. Cytochrome P450 (P450) is a ubiquitous enzyme system that catalyses oxidative reactions of numerous endogenous and exogenous compounds. The modulatory effects of P450 on the L-type Ca2+ current (ICa), intracellular free Ca2+ signals and cell shortening were assessed in adult rat single ventricular myocytes. 2. Bath administration of the imidazole antimycotics, clotrimazole, econazole and miconazole, which are potent P450 inhibitors, significantly suppressed cardiac ICa. While the Ca2+ channel antagonist nifedipine blocked ICa within 30 s, clotrimazole-induced suppression of ICa required 5.1 +/- 0.4 min (n = 14) to reach a steady low level. The suppression of ICa was dose dependent and recovered after washout of clotrimazole. Intracellular dialysis with the P450 antibody anti-rat CYP1A2 also significantly reduced cardiac ICa. 3. Additional administration of the beta-adrenergic agonist isoprenaline (1 microM) or the membrane-permeable 8-bromo-cAMP (2 mM) completely reversed the suppressant effects of clotrimazole and NaCN on ICa. In addition, intracellular dialysis with 2 mM cAMP abolished the P450 inhibitor-induced suppression of ICa. Phosphorylation of the channel with hydrolysis-resistant ATPgammaS prevented the suppressant effect of clotrimazole on ICa. Furthermore, dephosphorylation of the Ca2+ channel with intracellular dialysis with phosphatase types I and II reduced ICa by 85 +/- 3 % and abolished clotrimazole-induced suppression of ICa. 4. Extracellular administration of the phospholipase A2 inhibitors mepacrine and 4-bromophenacyl bromide significantly suppressed ICa. 5. Clotrimazole, econazole, miconazole and CN- also significantly inhibited intracellular free Ca2+ signals and cell shortening in rat single ventricular myocytes. 6. Intracellular cAMP content was significantly reduced in isolated ventricular myocytes incubated with clotrimazole or CN-. Extracellular administration of 11, 12-epoxyeicosatrienoic acid, one of the P450-mediated metabolites of arachidonic acid, enhanced ICa and intracellular cAMP content. The epoxyeicosatrienoic acid also restored the amplitude of the reduced ICa in P450 antibody-dialysed myocytes. 7. The present data suggest that cytochrome P450 modulates cardiac ICa and cell contraction, and the modulation may result from changes in intracellular levels of cAMP by P450- mediated metabolites of arachidonic acid.

Negative chronotropic actions of endothelin-1 on rabbit sinoatrial node pacemaker cells

1. The effects of endothelin-1 (ET-1) on sinoatrial (SA) node preparations of the rabbit heart were studied by means of whole-cell clamp techniques. 2. ET-1 at 1 nM slowed the spontaneous beating activity and rendered half of the cells quiescent. At a higher concentration of 10 nM, the slowing and cessation of spontaneous activity were accompanied by hyperpolarization. 3. In voltage-clamp experiments, ET-1 decreased the basal L-type Ca2+ current (Ica(L)) dose-dependently with a half-maximal inhibitory concentration (EC50) of 0.42 nM and maximal inhibitory response (Emax) of 49.5%. The delayed rectifying K+ current (Ik) was also reduced by 33.2 +/- 11.1% at 1 nM. In addition an inwardly rectifying K+ current was activated by ET-1 at higher concentrations (EC50 = 4.8 nM). These ET-1-induced changes in membrane currents were abolished by BQ485 (0.3 microM), a highly selective ETA receptor antagonist. 4. When Ica(L) was inhibited by ET-1 (1 nM), subsequent application of 10 microM ACh showed no additional decrease in Ica(L), suggesting the involvement of cyclic AMP in the effects of ET-1 on Ica(L). In contrast, 1 nM ET-1 further decreased Ica(L) in the presence of 10 microM ACh, suggesting that ET-1 activates some additional mechanism(s) which inhibit Ica(L). The ET-1-induced Ica(L) inhibition was abolished by protein kinase A inhibitory peptide (PKI, 20 microM) or H-89 (5 microM). However, the Ica(L) inhibition was not affected by methylene blue (10 microM), suggesting a minor role for cyclic GMP in the effect of ET-1 under basal conditions. 5. ET-1 failed to inhibit Ica(L) when the pipette contained GDP beta S (200 microM). However, incubation of the 21.5 +/- 9.5%, whereas it abolished the inhibitory effect of ACh on Ica(L). 6. Intracellular perfusion of 8-bromo cyclicAMP (8-Br cyclicAMP, 500 microM) attenuated, but did not abolish the inhibitory effect of ET-1 on Ica(L). This 8-Br cyclicAMP-resistant component (17.5 +/- 14.4%, n = 20) was not affected by combined application of 8-Br cyclicAMP-bromo cyclicGMP (500 microM), ryanodine (1 microM) or phorbol-12-myristate-13-acetate (TPA; 50 nM). 7. In summary, ET-1 exerts negative chronotropic effects on the SA node via ETA-receptors. ET-1 inhibits both ICa(L) and Ik, and increases background K+ current. The inhibition of ICa(L) by ET-1 is mainly due to reduction of the cyclicAMP levels via PTX-sensitive G protein, but some other mechanism(s) also seems to be operative.

Simulations of postvagal tachycardia at the single cell pacemaker level: a new hypothesis

Simulations performed on a single cell model of rabbit sinoatrial node activity after prolonged vagal stimulation have been able to reproduce the known characteristics of cycle length recovery, including the presence of rapid and slow recovery phases and the transient undershoot phenomenon known as postvagal tachycardia (PVT). In the model, the PVT component has been hypothesized to result from the recovery of background levels of the muscarinic K+ current iK,ACh from desensitization due to prolonged exposure to acetylcholine (ACh) neurotransmitter. Other components of the recovery were found to be due to the inactivation of iK,ACh after the hydrolysis of ACh (rapid phase) and the recovery of the hyperpolarizing-activated current i(f) from its ACh-induced inhibition (slow phase). The magnitudes of both the rapid component and the PVT were found to increase linearly with preceding vagally mediated increase in cycle length, whereas the gain of the slow component was found to saturate, reflecting the limited contribution of i(f) inhibition to cycle prolongation.

Muscarinic regulation of the L-type calcium current in isolated cardiac myocytes

Muscarinic agonists regulate the L-type calcium current in isolated cardiac myocytes. The second messengers pathways involved in this regulation are discussed briefly, with particular emphasis on the involvement of cAMP and cGMP pathways.

Regional differences in the response of the isolated sino-atrial node of the rabbit to vagal stimulation

1. The effects of brief postganglionic vagal nerve stimulation on electrical activity in different regions of the rabbit sino-atrial node and surrounding atrial muscle were recorded. 2. At the centre of the node (the leading pacemaker site), the brief stimulation resulted in a large hyperpolarization followed by a depolarization and a shortening of the action potential. All effects were short lasting (time to 90% recovery of membrane potential, 0.8 s). 3. At other sites within the node and in the surrounding atrial muscle, although there was still a substantial action potential shortening, the hyperpolarization was smaller and the depolarization was small or absent. All effects were longer lasting (time to 90% recovery in atrial muscle, 11.4s). 4. The depolarization in the centre of the node was abolished by block of the hyperpolarization-activated current (i(f)) by Cs+ or zatebradine (UL-FS 49). It could, therefore, result from the activation of i(f) during the preceding hyperpolarization. 5. Block of acetylcholinesterase by eserine greatly slowed recovery from vagal stimulation at all sites, demonstrating that recovery is dependent on acetylcholinesterase. The longer lasting effects of vagal stimulation in atrial muscle, therefore, result from lower acetylcholinesterase activity. 6. Vagal stimulation resulted in a short lasting initial slowing of spontaneous action potentials followed by a long-lasting secondary slowing. Whereas the initial slowing coincided with the effects of vagal stimulation on the centre of the node, the secondary slowing coincided with the slower effects of vagal stimulation on the surrounding atrial muscle. The secondary slowing was reduced by 68 +/- 11% (n = 5) by cutting the atrial muscle away from the node. 7. It is concluded that the short-lasting initial slowing of spontaneous action potentials is the direct effect of vagal stimulation on the centre of the sino atrial node, whereas the secondary slowing is the result of the longer lasting effects of vagal stimulation on the surrounding atrial muscle and the electrotonic suppression of the node by the muscle.

Mode of action of bradycardic agent, S 16257, on ionic currents of rabbit sinoatrial node cells

1. The effect of the bradycardic agent S 16257 on the main ionic mechanisms of diastolic depolarization in sinoatrial node cells isolated from rabbit heart, was investigated by the patch-clamp technique in whole-cell and macro-patch recordings. 2. In whole-cell conditions, S 16257 induced a marked exponential use-dependent blockade of the hyperpolarization-activated I(f) current, without shift of the voltage range of its activation curve. The rate of block increased with the drug concentration. The IC50 for the block of I(f) was 2.8 x 10(-6) M. 3. A similar use-dependent decline of I(f) was obtained with 3 microM S 16257, in cell-attached and in inside out macro-patch configurations, suggesting that the bradycardic agent interacts with I(f) channels from the inside of the cell. 4. A high concentration of S 16257 (10 microM) had no detectable effect on T-type calcium current and slightly decreased L-type calcium current (-18.12 +/- 0.66%), without significant use-dependent blockade. 5. S 16257 had no effect on the delayed outward potassium current Ik at 3 microM and slightly decreased it only at high concentrations, -16.3 +/- 1.2% at 10 microM. In contrast, zatebradine, another bradycardic agent, reduced I k by 20.3 +/- 2.5% at 3 microM. 6. In conclusion, S 16257 may lower heart rate without significant negative inotropic action. In comparison with zatebradine, S 16257 had less effect on Ik suggesting less prolongation of repolarization time.