Modulation of rate by autonomic agonists in SAN cells involves changes in diastolic depolarization and the pacemaker current

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.

The cardiac pacemaker current

In mammals cardiac rate is determined by the duration of the diastolic depolarization of sinoatrial node (SAN) cells which is mainly determined by the pacemaker I(f) current. f-channels are encoded by four members of the hyperpolarization-activated cyclic nucleotide-gated gene (HCN1-4) family. HCN4 is the most abundant isoform in the SAN, and its relevance to pacemaking has been further supported by the discovery of four loss-of-function mutations in patients with mild or severe forms of cardiac rate disturbances. Due to its selective contribution to pacemaking, the I(f) current is also the pharmacological target of a selective heart rate-reducing agent (ivabradine) currently used in the clinical practice. Albeit to a minor extent, the I(f) current is also present in other spontaneously active myocytes of the cardiac conduction system (atrioventricular node and Purkinje fibres). In working atrial and ventricular myocytes f-channels are expressed at a very low level and do not play any physiological role; however in certain pathological conditions over-expression of HCN proteins may represent an arrhythmogenic mechanism. In this review some of the most recent findings on f/HCN channels contribution to pacemaking are described.

In vitro characterization of HCN channel kinetics and frequency dependence in myocytes predicts biological pacemaker functionality

The pacemaker current, mediated by hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, contributes to the initiation and regulation of cardiac rhythm. Previous experiments creating HCN-based biological pacemakers in vivo found that an engineered HCN2/HCN1 chimeric channel (HCN212) resulted in significantly faster rates than HCN2, interrupted by 1-5 s pauses. To elucidate the mechanisms underlying the differences in HCN212 and HCN2 in vivo functionality as biological pacemakers, we studied newborn rat ventricular myocytes over-expressing either HCN2 or HCN212 channels. The HCN2- and HCN212-over-expressing myocytes manifest similar voltage dependence, current density and sensitivity to saturating cAMP concentrations, but HCN212 has faster activation/deactivation kinetics. Compared with HCN2, myocytes expressing HCN212 exhibit a faster spontaneous rate and greater incidence of irregular rhythms (i.e. periods of rapid spontaneous rate followed by pauses). To explore these rhythm differences further, we imposed consecutive pacing and found that activation kinetics of the two channels are slower at faster pacing frequencies. As a result, time-dependent HCN current flowing during diastole decreases for both constructs during a train of stimuli at a rapid frequency, with the effect more pronounced for HCN2. In addition, the slower deactivation kinetics of HCN2 contributes to more pronounced instantaneous current at a slower frequency. As a result of the frequency dependence of both instantaneous and time-dependent current, HCN2 exhibits more robust negative feedback than HCN212, contributing to the maintenance of a stable pacing rhythm. These results illustrate the benefit of screening HCN constructs in spontaneously active myocyte cultures and may provide the basis for future optimization of HCN-based biological pacemakers.

Efficacy of the I(f) current inhibitor ivabradine in patients with chronic stable angina receiving beta-blocker therapy: a 4-month, randomized, placebo-controlled trial

AIMS

To evaluate the anti-anginal and anti-ischaemic efficacy of the selective I(f) current inhibitor ivabradine in patients with chronic stable angina pectoris receiving beta-blocker therapy.

METHODS AND RESULTS

In this double-blinded trial, 889 patients with stable angina receiving atenolol 50 mg/day were randomized to receive ivabradine 5 mg b.i.d. for 2 months, increased to 7.5 mg b.i.d. for a further 2 months, or placebo. Patients underwent treadmill exercise tests at the trough of drug activity using the standard Bruce protocol for randomization and at 2 and 4 months. Total exercise duration at 4 months increased by 24.3 +/- 65.3 s in the ivabradine group, compared with 7.7 +/- 63.8 s with placebo (P < 0.001). Ivabradine was superior to placebo for all exercise test criteria at 4 months (P < 0.001 for all) and 2 months (P-values between <0.001 and 0.018). Ivabradine in combination with atenolol was well tolerated. Only 1.1% of patients withdrew owing to sinus bradycardia in the ivabradine group.

CONCLUSION

The combination of ivabradine 7.5 mg b.i.d. and atenolol at the commonly used dosage in clinical practice in patients with chronic stable angina pectoris produced additional efficacy with no untoward effect on safety or tolerability.

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.

The funny current: cellular basis for the control of heart rate

The 'funny' (pacemaker, I(f)) current, first described almost 30 years ago in sinoatrial node (SAN) myocytes, is a mixed sodium/potassium inward current, activated on hyperpolarisation in the diastolic range of voltages. 'Funny' (f) channels are activated by intracellular cyclic adenosine monophosphate (cAMP) concentrations according to a mechanism mediating regulation of heart rate by the autonomic nervous system, as well as by voltage hyperpolarisation. Structural subunits of native f-channels are the hyperpolarisation-activated cyclic nucleotide-gated (HCN) channels; of the four HCN isoforms known, HCN4 is the most highly expressed in SAN tissue. The I(f) current is a natural target in the search for drugs aimed specifically at affecting heart rate, given its function in pacemaking. Increased heart rate has a negative influence on clinical outcome in patients with cardiovascular disease, and indeed is also an established risk factor for cardiovascular and all-cause mortality in the general population. Clearly, therefore, independent reduction of heart rate, through inhibition of the I(f) current, appears to be a suitable therapeutic option for patients with ischaemic heart disease.beta-Adrenoceptor antagonists (beta-blockers) reduce intracellular cAMP levels, and a substantial part of their negative chronotropic effect is therefore attributable to a reduction of the I(f) current. However, neither beta-blockers nor Ca(2+) channel antagonists, both of which have traditionally been used to reduce myocardial ischaemia, are 'pure' heart rate-lowering drugs. These agents may, in fact, have adverse cardiovascular and noncardiovascular effects.Conversely, the novel heart rate-reducing agent ivabradine is a specific blocker of f-channels, hence a selective inhibitor of the pacemaker I(f) current in the SAN. Ivabradine slows heart rate by reducing the I(f) current-regulated steepness of the diastolic depolarisation in SAN myocytes, thereby increasing diastolic duration, without altering action potential duration or causing negative inotropy. As such, ivabradine is particularly useful in patients with chronic stable angina pectoris. Further clinical studies are ongoing to evaluate the efficacy of ivabradine in patients with coronary heart disease, left ventricular dysfunction and heart failure. This short article reviews the current state of knowledge of the properties of the 'funny' current in relation to exploitation of the I(f) function in pacemaking generation and modulation for the pharmacological control of heart rate.