Mechanisms of adrenergic control of sino-atrial node discharge

The funny current If is essential for the fight-or-flight response in cardiac pacemaker cells

The sympathetic nervous system fight-or-flight response is characterized by a rapid increase in heart rate, which is mediated by an increase in the spontaneous action potential (AP) firing rate of pacemaker cells in the sinoatrial node. Sympathetic neurons stimulate sinoatrial myocytes (SAMs) by activating β adrenergic receptors (βARs) and increasing cAMP. The funny current (If) is among the cAMP-sensitive currents in SAMs. If is critical for pacemaker activity, however, its role in the fight-or-flight response remains controversial. In this study, we used AP waveform analysis, machine learning, and dynamic clamp experiments in acutely isolated SAMs from mice to quantitatively define the AP waveform changes and role of If in the fight-or-flight increase in AP firing rate. We found that while βAR stimulation significantly altered nearly all AP waveform parameters, the increase in firing rate was only correlated with changes in a subset of parameters (diastolic duration, late AP duration, and diastolic depolarization rate). Dynamic clamp injection of the βAR-sensitive component of If showed that it accounts for ∼41% of the fight-or-flight increase in AP firing rate and 60% of the decrease in the interval between APs. Thus, If is an essential contributor to the fight-or-flight increase in heart rate.

Animal models of arrhythmia: classic electrophysiology to genetically modified large animals

Arrhythmias are common and contribute substantially to cardiovascular morbidity and mortality. The underlying pathophysiology of arrhythmias is complex and remains incompletely understood, which explains why mostly only symptomatic therapy is available. The evaluation of the complex interplay between various cell types in the heart, including cardiomyocytes from the conduction system and the working myocardium, fibroblasts and cardiac immune cells, remains a major challenge in arrhythmia research because it can be investigated only in vivo. Various animal species have been used, and several disease models have been developed to study arrhythmias. Although every species is useful and might be ideal to study a specific hypothesis, we suggest a practical trio of animal models for future use: mice for genetic investigations, mechanistic evaluations or early studies to identify potential drug targets; rabbits for studies on ion channel function, repolarization or re-entrant arrhythmias; and pigs for preclinical translational studies to validate previous findings. In this Review, we provide a comprehensive overview of different models and currently used species for arrhythmia research, discuss their advantages and disadvantages and provide guidance for researchers who are considering performing in vivo studies.

Essential role of diastolic oscillatory potentials in adrenergic control of guinea pig sino-atrial node discharge

Background

The diastolic oscillatory after-potential V_os and pre-potential ThV_os play an essential role in the pacemaker mechanism of sino-atrial node (SAN). The aim of this study was to investigate whether these oscillatory potentials are also involved in adrenergic control of SAN discharge.

Methods

V_os and ThV_os were visualized by superfusing guinea pig SAN in high [K⁺]_o. The actions of adrenergic agonists on oscillatory potentials were studied by means of a microelectrode technique. Statistical significance was determined by means of Student's paired t-test.

Results

In non-spontaneous SAN, norepinephrine (NE) decreased the resting potential into a voltage range ("oscillatory zone") where increasingly larger ThV_os appeared and initiated spontaneous discharge. In slowly discharging SAN, NE gradually increased the rate by increasing the amplitude and slope of earlier-occurring ThV_os and of V_os until these oscillations fused with initial diastolic depolarization (DD₁). In the presence of NE, sudden fast rhythms were initiated by large V_os that entered a more negative oscillatory zone and initiated a large ThV_os. Recovery from NE exposure involved the converse changes. The β-adrenergic agonist isoproterenol had similar actions. Increasing calcium load by decreasing high [K⁺]_o, by fast drive or by recovery in Tyrode solution led to growth of V_os and ThV_os which abruptly fused when a fast sudden rhythm was induced. Low [Ca²⁺]_o antagonized the adrenergic actions. Cesium (a blocker of I_f) induced spontaneous discharge in quiescent SAN through ThV_os. In spontaneous SAN, Cs⁺increased V_os and ThV_os, thereby increasing the rate. Cs⁺ did not hinder the positive chronotropic action of NE. Barium increased the rate, as Cs⁺ did.

Conclusion

Adrenergic agonists: (i) initiate SAN discharge by decreasing the resting potential and inducing ThV_os; (ii) gradually accelerate SAN rate by predominantly increasing size and slope of earlier and more negative ThV_os; (iii) can induce sudden fast rhythms through the abrupt fusion of large V_os with large ThV_os; (iv) increase V_os and ThV_osby increasing cellular calcium; and (v) do not modify the oscillatory potentials by means of the hyperpolarization-activated current I_f. The results provide evidence for novel mechanisms by which the SAN dominant pacemaker activity is initiated and enhanced by adrenergic agonists.

The vicissitudes of the pacemaker current I Kdd of cardiac purkinje fibers

The mechanisms underlying the pacemaker current in cardiac tissues is not agreed upon. The pacemaker potential in Purkinje fibers has been attributed to the decay of the potassium current I (Kdd). An alternative proposal is that the hyperpolarization-activated current I (f) underlies the pacemaker potential in all cardiac pacemakers. The aim of this review is to retrace the experimental development related to the pacemaker mechanism in Purkinje fibers with reference to findings about the pacemaker mechanism in the SAN as warranted. Experimental data and their interpretation are critically reviewed. Major findings were attributed to K(+) depletion in narrow extracellular spaces which would result in a time dependent decay of the inward rectifier current I (K1). In turn, this decay would be responsible for a "fake" reversal of the pacemaker current. In order to avoid such a postulated depletion, Ba(2+) was used to block the decay of I (K1). In the presence of Ba(2+) the time-dependent current no longer reversed and instead increased with time and more so at potentials as negative as -120 mV. In this regard, the distinct possibility needs to be considered that Ba(2+) had blocked I (Kdd) (and not only I (K1)). That indeed this was the case was demonstrated by studying single Purkinje cells in the absence and in the presence of Ba(2+). In the absence of Ba(2+), I (Kdd) was present in the pacemaker potential range and reversed at E (K). In the presence of Ba(2+), I (Kdd) was blocked and I (f) appeared at potentials negative to the pacemaker range. The pacemaker potential behaves in a manner consistent with the underlying I (Kdd) but not with I (f). The fact that I (f) is activated on hyperpolarization at potential negative to the pacemaker range makes it suitable as a safety factor to prevent the inhibitory action of more negative potentials on pacemaker discharge. It is concluded that the large body of evidence reviewed proves the pacemaker role of I (Kdd) (but not of I (f)) in Purkinje fibers.

Mechanisms underlying overdrive suppression and overdrive excitation in guinea pig sino-atrial node

The hypothesis that the pause that follows overdrive of the sino-atrial node (SAN) might be the net result of overdrive excitation and overdrive suppression was tested by studying rate and force patterns induced by overdrive in isolated guinea pig SAN superfused in vitro. In Tyrode solution, the pause is short and changes but little with longer or faster drives. In high [K(+)](o) solution, longer overdrives increase force percent-wise more than in Tyrode solution, shorten the pause and are followed by greater rate and force. When the SAN (quiescent in high [K(+)](o)) is driven at 6/min, faster overdrives are followed by stronger, slowly decreasing contractions. Alternating 10 s drives with 10 s pauses have little effect on force and rate in Tyrode solution, but progressively increase force and rate in high [K(+)](o). Cesium has effects similar to high [K(+)](o). High [Ca(2+)](o) increases force and in high [K(+)](o) increases the rate as well as it shortens the pause, whereas Ni(2+) decreases force as well as rate and lengthens the pause. Barium dissociates the effects on force and rate. Lidocaine and tetrodotoxin decrease rate and force, and increase the pause duration. In overdrive excitation, the increase in rate is associated with an enhancement of diastolic voltage oscillations. It is concluded that in SAN the prevalence of Ca(2+) load leads to overdrive excitation whereas the prevalence of Na(+) load leads to overdrive suppression. In Tyrode solution, the pause after drive appears to be the net result of these two different mechanisms.