Obligatory role of diastolic voltage oscillations in sino-atrial node discharge

Ca2+-regulated-cAMP/PKA signaling in cardiac pacemaker cells links ATP supply to demand

RATIONALE

In sinoatrial node cells (SANC), Ca(2+) activates adenylate cyclase (AC) to generate a high basal level of cAMP-mediated/protein kinase A (PKA)-dependent phosphorylation of Ca(2+) cycling proteins. These result in spontaneous sarcoplasmic-reticulum (SR) generated rhythmic Ca(2+) oscillations during diastolic depolarization, that not only trigger the surface membrane to generate rhythmic action potentials (APs), but, in a feed-forward manner, also activate AC/PKA signaling. ATP is consumed to pump Ca(2+) to the SR, to produce cAMP, to support contraction and to maintain cell ionic homeostasis.

OBJECTIVE

Since feedback mechanisms link ATP-demand to ATP production, we hypothesized that (1) both basal ATP supply and demand in SANC would be Ca(2+)-cAMP/PKA dependent; and (2) due to its feed-forward nature, a decrease in flux through the Ca(2+)-cAMP/PKA signaling axis will reduce the basal ATP production rate.

METHODS AND RESULTS

O(2) consumption in spontaneous beating SANC was comparable to ventricular myocytes (VM) stimulated at 3 Hz. Graded reduction of basal Ca(2+)-cAMP/PKA signaling to reduce ATP demand in rabbit SANC produced graded ATP depletion (r(2)=0.96), and reduced O(2) consumption and flavoprotein fluorescence. Neither inhibition of glycolysis, selectively blocking contraction nor specific inhibition of mitochondrial Ca(2+) flux reduced the ATP level.

CONCLUSIONS

Feed-forward basal Ca(2+)-cAMP/PKA signaling both consumes ATP to drive spontaneous APs in SANC and is tightly linked to mitochondrial ATP production. Interfering with Ca(2+)-cAMP/PKA signaling not only slows the firing rate and reduces ATP consumption, but also appears to reduce ATP production so that ATP levels fall. This distinctly differs from VM, which lack this feed-forward basal cAMP/PKA signaling, and in which ATP level remains constant when the demand changes.

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) .

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.

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.

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.

On the mechanisms underlying diastolic voltage oscillations in the sinoatrial node

AIM

The study of the mechanisms underlying the oscillatory afterpotential (V(os)) and prepotential (ThV(os)).

BACKGROUND

It has been recently shown that V(os) and ThV(os) play an obligatory role in the dominant sinoatrial node (SAN) discharge.

METHODS

Guinea pig isolated SAN was studied in vitro by means of a microelectrode technique.

RESULTS

High [K(+)](o) and premature stimuli unmask V(os) superimposed on early diastolic depolarization and ThV(os) within a less negative voltage range ("oscillatory zone"). Subthreshold stimuli elicit ThV(os) in the oscillatory zone, but not at more negative values. Drive and caffeine shift the oscillatory zone in a negative direction. Low caffeine concentrations increase the size of V(os) and of ThV(os), rate, and force. High caffeine concentrations suppress V(os) but increase the size of ThV(os) and shift them to more negative values until they eventually miss the threshold. In quiescent SAN in high caffeine, a fast drive enhances ThV(os) size, thereby initiating a transient spontaneous rhythm ("overdrive excitation"). Adrenergic agonists potentiate caffeine-induced overdrive excitation through an increase in ThV(os). In high caffeine, the first twitch after quiescence is not larger, twitch relaxation is slower, V(os) is abolished, and the prolonged nonoscillatory afterdepolarization V(ex) is induced, consistent with an impairment of Ca2+ handling by the sarcoplasmic reticulum. The effects of caffeine in Tyrode's solution are accounted for by the caffeine-induced changes in the oscillatory potentials. Tetrodotoxin decreases force and size of both V(os) and ThV(os).

CONCLUSIONS

The mechanism underlying V(os) is related to a diastolic release of Ca2+ from a Ca2+-overloaded sarcoplasmic reticulum, whereas that of ThV(os) appears to be related to ionic currents in the resting potential range that can initiate and sustain spontaneous discharge.

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.