The Relative Contributions of Various Time-Dependent Membrane Currents to Pacemaker Activity in the Sino Atrial Node. In: Bouman LN, Jongsma HJ, editors. Cardiac Rate and Rhythm. Dordrecht: Springer Netherlands; 1982. pp. 53-68. [DOI: 10.1007/978-94-009-7535-4_5]

The missing link in the mystery of normal automaticity of cardiac pacemaker cells

Earlier studies of the initiating event of normal automaticity of the heart's pacemaker cells, inspired by classical quantitative membrane theory, focused upon ion currents (IK, I f) that determine the maximum diastolic potential and the early phase of the spontaneous diastolic depolarization (DD). These early DD events are caused by the prior action potential (AP) and essentially reflect a membrane recovery process. Events following the recovery process that ignite APs have not been recognized and remained a mystery until recently. These critical events are linked to rhythmic intracellular signals initiated by Ca2+ clock (i.e., sarcoplasmic reticulum [SR] cycling Ca2+). Sinoatrial cells, regardless of size, exhibit intense ryanodine receptor (RyR), Na+/Ca2+ exchange (NCX)-1, and SR Ca2+ ATPase-2 immunolabeling and dense submembrane NCX/RyR colocalization; Ca2+ clocks generate spontaneous stochastic but roughly periodic local subsarcolemmal Ca2+ releases (LCR). LCRs generate inward currents via NCX that exponentially accelerate the late DD. The timing and amplitude of LCR/I NCX-coupled events control the timing and amplitude of the nonlinear terminal DD and therefore ultimately control the chronotropic state by determining the timing of the I CaL activation that initiates the next AP. LCR period is precisely controlled by the kinetics of SR Ca2+ cycling, which, in turn, are regulated by 1) the status of protein kinase A-dependent phosphorylation of SR Ca2+ cycling proteins; and 2) membrane ion channels ensuring the Ca2+ homeostasis and therefore the Ca2+ available to Ca2+ clock. Thus, the link between early DD and next AP, missed in earlier studies, is ensured by a precisely physiologically regulated Ca2+ clock within pacemaker cells that integrates multiple Ca2+-dependent functions and rhythmically ignites APs during late DD via LCRs-I NCX coupling.

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.

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.

The mammalian sinoatrial node

The sinoatrial node (SAN) was discovered in 1906 by Keith and Flack. The relation between its ultrastructure and function was first studied by Trautwein and Uchizono in 1963, whereas this relation was definitely established by Taylor and coworkers in 1978. The impulse originates from cells with a relatively low percentage of myofilaments. Earliest discharge is restricted to one site only in rabbit, guinea pig, cat, and pig and presumably also in larger animals. From this primary pacemaker area, the impulse is preferentially conducted towards the crista terminalis. The amount of cells in the primary pacemaker area may vary from a few hundred to a few thousand. In rabbit, guinea pig, cat, and pig, the amount of collagen is considerable. Normal SAN function was observed in the cat although the SAN volume occupied by myocytes was less than 5%. Changes in ionic composition of the perfusion fluid and the addition of autonomic substances may cause pacemaker shifts and altered activation patterns.

Acetylcholine and the mammalian 'slow inward' current: a computer investigation

Although it is accepted that acetylcholine has two possible actions on cardiac muscle, one being the opening of time-independent potassium channels and the other a block of time-dependent calcium current, there is considerable doubt about which mechanism, if either, is chiefly responsible for modulating vagal control in any given region of the heart. It seems of particular importance to resolve this problem in the case of the mammalian sino-atrial node because this tissue not only receives the densest vagal innervation, but it is also the primary pacemaker. Recent studies on intact rabbit node (Shibata et al. 1985) have produced results which can be largely reproduced by using a computer model based on experimental studies of the sinus node current mechanisms (Noble & Noble 1984). We studied perturbations of node activity by modifying a number of these mechanisms, including an acetylcholine-activated K+ channel, iK,ACh, introduced into the model for the first time. Our findings lead us to the conclusion that the most important factor in modulating moderate chronotropic changes is a block of calcium current permeability. Complete vagal inhibition may also depend as much upon this effect as upon activation of iK,ACh current. Because the experimental studies under consideration here used beta-blockers to abolish the action of noradrenaline released by vagal stimulation, we suggest how a muscarinic block of isi might occur in vivo in the absence of beta-agonists.

Threshold effects of acetylcholine on primary pacemaker cells of the rabbit sino-atrial node

Leading or primary pacemaker cells located within the rabbit sino-atrial node have been identified by using electrophysiological and pharmacological techniques. Stable intracellular recordings lasting 20-30 min from cells within the s.a. node reveal three distinct patterns of spontaneous intracellular responses: (i) leading or primary pacing; (ii) follower or subsidiary pacing; and (iii) 'anomalous' pacemaker discharge. Our main objective was to measure the first detectable effect, or effects, of acetylcholine on the spontaneous intracellular electrical activity in mammalian primary pacemaker cells. Trains of brief 'field' stimuli were applied to evoke transmitter release from endogenous nerve varicosities. Systematic variations in the amplitude and duration of each stimulus, and in the train length; in conjunction with application of beta blockers (l-pindolol (10(-6) M); l-propranolol, (2 X 10(-7) M)) yielded small and transient, but very consistent negative chronotropic effects. These electrophysiological changes were blocked by atropine (1 X 10(-7) M) and were mimicked by bath application of low doses of acetylcholine (10(-7)-10(-6) M) or muscarine chloride (10(-8)-10(-7) M). In primary cells the first, or threshold effect of vagal excitation is a decrease in the slope of the pacemaker potential, without a detectable (less than 2 m V) hyperpolarization or change in action potential duration. A reduction in the dV/dtmax of the initial depolarization is also quite consistently observed. Application of longer stimulus trains yield the classical hyperpolarizing response, which is often assumed to be the major electrophysiological correlate of the negative chronotropic effect. These data provide a detailed electrophysiological description of the 'physiological' effects of the vagus nerve excitation on primary or leading pacemaker cells of the mammalian s.-a. node. A plausible explanation for the absence of hyperpolarization is suggested; and a working hypothesis is presented for the changes in ionic current or currents, that underlie this negative chronotropic effect.

A model of sino-atrial node electrical activity based on a modification of the DiFrancesco-Noble (1984) equations

DiFrancesco & Noble's (1984) equations (Phil. Trans. R. Soc. Lond. B (in the press.] have been modified to apply to the mammalian sino-atrial node. The modifications are based on recent experimental work. The modified equations successfully reproduce action potential and pacemaker activity in the node. Slightly different versions have been developed for peripheral regions that show a maximum diastolic potential near --75 mV and for central regions that do not hyperpolarize beyond --60 to --65 mV. Variations in extracellular potassium influence the frequency of pacemaker activity in the s.a. node model very much less than they do in the Purkinje fibre model. This corresponds well to the experimental observation that the node is less sensitive to external [K] than are Purkinje fibres. Activation of the Na-K exchange pump in the model by increasing intracellular sodium can suppress pacemaker activity. This phenomenon may contribute to the mechanism of overdrive suppression.

The ionic currents underlying pacemaker activity in rabbit sino-atrial node: experimental results and computer simulations

The membrane currents underlying the pacemaker depolarization have been investigated in rabbit s.a. node preparations using the two-microelectrode voltage clamp technique. Many of the experimental results have been simulated using a computer model of s.a. node electrical activity. Changes of three time-dependent membrane currents which could contribute to pacemaker depolarization are found to occur in the relevant potential range: decay of the potassium current, iK, and activation of the inward current, if, and of the slow inward current, isi. The contribution of if activation to the pacemaker depolarization ranges from nil to an appreciable part depending on the preparation; when Cs (1 mM) blocks if, it nevertheless does not prevent pacemaking. In the model, holding the if activation variable at zero slows but does not stop pacemaking; doubling if conductance and shifting its activation curve by 15 mV in the positive direction causes a 15% faster rate of pacemaking. The slow time course of re-availability of isi must be allowed for when determining the isi threshold. A voltage clamp protocol designed to mimic as closely as possible an action potential followed by a pacemaker depolarization gives an estimate of isi threshold at the potential level of the last third of the pacemaker depolarization. This has been confirmed in experiments in which the voltage clamp was switched on at different points in the pacemaker depolarization. In the computer simulation, 'blocking' isi depolarizes the membrane to the zero current level (close to the potential reached at the end of a pacemaker depolarization) and stops the generation of action potentials. The decay of iK contributes to the pacemaker depolarization; with both our own model and that of K. Yanagihara, A. Noma and H. Irisawa, Jap. J. Physiol. 30, 841-857 (1980) 'blocking' iK decay abolishes pacemaker activity. Computations of extracellular K+ concentration changes compared with iK decay in a cylindrical model allow re-assessment of the interpretation of K+ concentration measurements during pacemaking made by J. Maylie, M. Morad and J. Weiss, J. Physiol., Lond. 311, 167-178 (1981).(ABSTRACT TRUNCATED AT 400 WORDS)

Propagation through electrically coupled cells. Effects of regional changes in membrane properties

The normal process of excitation of the heart involves propagation of action potentials through cardiac regions of different anatomy and different intrinsic membrane properties. Although our understanding of these properties is still incomplete, it is well accepted that the parameters measured from a single cell penetration in an electrical syncytium (e.g., action potential duration, rate of rise, and velocity) reflect not only the properties of that cell but also the electrotonic interactions with other cells to which the recorded cell is electrically coupled. We have used simulation techniques to predict the spatial distribution of action potential parameters resulting from discretely localized alterations in the intrinsic membrane properties of some of the cells of an electrical syncytium. We have shown that the resulting spatial distribution is markedly different for alterations in plateau and pacemaker currents vs. rising phase currents, and that other factors, such as the site of stimulation and the underlying spatial pattern of cell-cell coupling resistance, also modify the spatial distribution of action potential properties resulting from a discrete regional change in intrinsic membrane properties.