Potassium current during the pacemaker depolarization in rabbit sinoatrial node cell

Roles and mechanisms of natural drugs on sinus node dysfunction

Sinus node dysfunction is a common arrhythmia disorder with a high incidence and significant social and economic burden. Currently, there are no effective drugs for treating chronic sinus node dysfunction. The disease is associated with ion channel disturbances caused by aging, fibrosis, inflammation, oxidative stress, and autonomic dysfunction. Natural active substances and Chinese herbal medicines have been widely used and extensively studied in the medical community for the treatment of arrhythmias. Multiple studies have demonstrated that various active ingredients and Chinese herbal medicines, such as astragaloside IV, quercetin, and ginsenosides, exhibit antioxidant effects, reduce fibrosis, and maintain ion channel stability, providing promising drugs for treating sinus node dysfunction. This article summarizes the research progress on natural active ingredients and Chinese herbal formulas that regulate sick sinoatrial node function, providing valuable references for the treatment of sinus node dysfunction.

Evolution of mathematical models of cardiomyocyte electrophysiology

Advanced computational techniques and mathematical modeling have become more and more important to the study of cardiac electrophysiology. In this review, we provide a brief history of the evolution of cardiomyocyte electrophysiology models and highlight some of the most important ones that had a major impact on our understanding of the electrical activity of the myocardium and associated transmembrane ion fluxes in normal and pathological states. We also present the use of these models in the study of various arrhythmogenesis mechanisms, particularly the integration of experimental pharmacology data into advanced humanized models for in silico proarrhythmogenic risk prediction as an essential component of the Comprehensive in vitro Proarrhythmia Assay (CiPA) drug safety paradigm.

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.

Structural and functional determinants in the S5-P region of HCN-encoded pacemaker channels revealed by cysteine-scanning substitutions

Hyperpolarization-activated cyclic nucleotide-modulated (HCN) channels are responsible for the membrane pacemaker current that underlies the spontaneous generation of bioelectrical rhythms. However, their structure-function relationship is poorly understood. Previously, we identified several pore residues that influence HCN gating properties and proposed a pore-to-gate mechanism. Here, we systematically introduced cysteine-scanning substitutions into the descending portion of the P loop (residues 339-345) of HCN1-R (where R is resistance to sulfhydryl-reactive agents) channels, in which all endogenous cysteines except C303 have been removed or replaced. F339C, K340C, A341C, M342C, S343C, and M345C did not produce functional currents. Interestingly, the loss of function phenotype of F339C could be rescued by the reducing agent dithiothreitol (DTT). H344C but not HCN1-R and DTT-treated F339C channels were sensitive to blockade by divalent Cd(2+) (current with 100 microM Cd(2+)/control current at -140 mV = 67.6 +/- 2.9%, 109.3 +/- 3.1%, and 103.8 +/- 1.7%, respectively). Externally applied methanethiosulfate ethylammonium, a covalent sulfhydryl-reactive compound, irreversibly modified H344C by reducing the current at -140 mV (to 43.7 +/- 6.5%), causing a hyperpolarizing steady-state activation shift (change in half-activation voltage: approximately 6 mV) and decelerated gating kinetics (by up to 3-fold). Based on these results, we conclude that pore residues 339-345 are important determinants of the structure-function properties of HCN channels and that the side chain of H344 is externally accessible.

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.

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.

Pore-to-gate coupling of HCN channels revealed by a pore variant that contributes to gating but not permeation

Although ample evidence suggests the presence of an intracellular activation gate in HCN (pacemaker) channels, mutations in the outer pore can alter gating properties. Here we investigated the role of the outer pore residue A354 in HCN1 gating by systematically converting it to the equivalent residues (T, Y, and F) found in K(+)-channels. A354T negatively shifted steady-state activation (DeltaV(1/2) approximately -25 mV), decelerated gating kinetics (by up to 8-fold), and abolished the effects of external ions on gating. A354Y and A354F did not yield functional currents when expressed alone, although immunofluorescence microscopy indicated the presence of these channel proteins on the membrane surface. Currents recorded after co-expressing A354Y with WT HCN1 were reduced in amplitude (relative to WT alone) and had changes in gating similar to those of A354T. We conclude that the pore variant at position 354 contributes to gating but not permeation, and that the HCN outer pore may be involved in gating via a pore-to-gate coupling mechanism.

Critical intra-linker interactions of HCN1-encoded pacemaker channels revealed by interchange of S3-S4 determinants

Hyperpolarization-activated cyclic nucleotide-modulated (HCN) channels contribute to the spontaneous rhythmic activities in cardiac and neuronal cells. Recently, we reported that the S3-S4 linker of HCN1 channels influences activation, and that part of the linker is helical with the determinants G231, M232, and E235 clustered on one side. Here we explored the undefined role of the G(231)E(235)M(232) triplet by systematic substitutions. Replacing G231 or M232 next to the "neighboring" E235 in the S3-S4 helix with an anionic residue (i.e., G231E, M232E) rendered channels non-functional although they were localized on the membrane surface. Interestingly, this loss of function could be readily rescued either by introducing a countercharge at position 235 (G231E/E235R, M232E/E235R) or by interchanging residues 231 or 232 and 235 (G231E/E235G, M232E/E235M). We conclude that residues 231, 232, and 235 are in close spatial proximity to each other, and uniquely interact with one another to shape the phenotypes of HCN channels.

The multiple personalities of h-channels

Concepts regarding the function of the hyperpolarization-activated current (Ih) in shaping the excitability of single cells and neuronal ensembles have been evolving rapidly following the recent cloning of genes that encode the underlying 'h-channels' - the HCN genes. This article reviews new information about the transcriptional regulation of these channels, highlighting novel studies that demonstrate short- and long-term modulation of HCN expression, and linking this modulation to mechanisms of neurological diseases.

Roles of L-type Ca2+ and delayed-rectifier K+ currents in sinoatrial node pacemaking: insights from stability and bifurcation analyses of a mathematical model

To elucidate the dynamical mechanisms of the sinoatrial (SA) node pacemaker activity, we investigated the roles of L-type Ca2+ (ICa,L) and delayed-rectifier K+ (IKr) currents in pacemaking by stability and bifurcation analyses of our rabbit SA node model (Kurata Y, Hisatome I, Imanishi S, and Shibamoto T. Am J Physiol Heart Circ Physiol 283: H2074-H2101, 2002). Equilibrium points (EPs), periodic orbits, stability of EPs, and Hopf bifurcation points were calculated as functions of conductance or gating time constants of the currents for constructing bifurcation diagrams. Structural stability (robustness) of the system was also evaluated by computing stability and dynamics during applications of constant bias currents (Ibias). Blocking ICa,L or IKr caused stabilization of an EP and cessation of pacemaking via a Hopf bifurcation. The unstable zero-current potential region determined with Ibias applications, where spontaneous oscillations appear, shrunk and finally disappeared as ICa,L diminished, but shrunk little when IKr was eliminated. The reduced system, including no time-dependent current except ICa,L, exhibited pacemaker activity. These results suggest that ICa,L is responsible for EP instability and pacemaker generation, whereas IKr is not necessarily required for constructing a pacemaker cell system. We further explored the effects of various K+ currents with different kinetics on stability and dynamics of the model cell. The original IKr of delayed activation and inward rectification appeared to be most favorable for generating large-amplitude oscillations with stable frequency, suggesting that IKr acts as an oscillation amplifier and frequency stabilizer. IKr may also play an important role in preventing bifurcation to quiescence of the system.

Role of the hyperpolarization-activated cation current (Ih) in pacemaker activity in area postrema neurons of rat brain slices

To clarify the functional properties of the hyperpolarization-activated cation current (Ih) as a pacemaker current in area postrema neurons, whole-cell recordings were made in visually identified cells in rat brain slices. The activation of Ih was identified in approximately 62 % of area postrema neurons tested. The cells displaying Ih showed a depolarizing "sag" in responses to hyperpolarizing current injection in current-clamp mode. The reversal potential for the Ih was -36 mV, and this was shown to depend on the external concentration of Na+ and K+ ions. Extracellular Cs+ ions (2 mM) and ZD7288 (100 microM), a potent selective Ih channel antagonist, blocked Ih and induced a membrane potential hyperpolarization, suggesting the sustained activation of Ih near the resting potential and a contribution from Ih to membrane potential maintenance at more depolarized levels. In contrast, extracellular Ba2+ ions caused a depolarization of the membrane potential, suggesting the blockade of inward rectifier K+ currents. ZD7288 decreased the spontaneous discharge rate by prolonging the slow depolarization between two spikes, with minimal effect on the amplitude of the afterhyperpolarization or action potential waveforms. Ih stabilized the latency of rebound action potentials. Ih was weakly activated by external 8-bromoadenosine 3',5' cyclic monophosphate (1 mM) or forskolin (50-100 microM), indicating that the Ih channel subtypes in area postrema cells could be modulated by intracellular cAMP. Our findings indicate that Ih contributes to the subthreshold membrane and firing properties of rat area postrema neurons and may regulate their resting membrane potential and firing patterns.

KCNE2 modulates current amplitudes and activation kinetics of HCN4: influence of KCNE family members on HCN4 currents

The HCN4 gene encodes a hyperpolarization-activated cation current contributing to the slow components of the pacemaking currents I(f) in the sinoatrial node and I(h) or I(q) in the thalamus. Heterologous expression studies of individual HCN channels have, however, failed to reproduce fully the diversity of native I(f/h/q) currents, suggesting the presence of modulating auxiliary subunits. Consistent with this is the recent description of KCNE2, which is highly expressed in the sinoatrial node, as a beta-subunit of rapidly activating HCN1 and HCN2 channels. To determine whether KCNE2 can also modulate the slow component of native I(f/h/q) currents, we co-expressed KCNE2 with HCN4 in Xenopus oocytes and in Chinese hamster ovary (CHO) cells and analysed the resulting currents using two-electrode voltage-clamp and patch-clamp techniques, respectively. In both cell types, co-expressed KCNE2 enhanced HCN4-generated current amplitudes, slowed the activation kinetics and shifted the voltage for half-maximal activation of currents to more negative voltages. In contrast, the related family members KCNE1, KCNE3 and KCNE4 did not change current characteristics of HCN4. Consistent with these electrophysiological results, the carboxy-terminal tail of KCNE2, but not of other KCNE subunits, interacted with the carboxy-terminal tail of HCN4 in yeast two-hybrid assays. KCNE2, by modulating I(f) or I(h) currents, might thus contribute to the electrophysiological diversity of known pacemaking currents in the heart and brain.

Helical secondary structure of the external S3-S4 linker of pacemaker (HCN) channels revealed by site-dependent perturbations of activation phenotype

If, encoded by the hyperpolarization-activated cyclic nucleotide-modulated channel family (HCN1-4), contributes significantly to neuronal and cardiac pacing. Recently, we reported that the S3-S4 residue Glu-235 of HCN1 influences activation by acting as a surface charge. However, it is uncertain whether other residues of the external S3-S4 linker are also involved in gating. Furthermore, the secondary conformation of the linker is not known. Here we probed the structural and functional role of the HCN1 S3-S4 linker by introducing systematic mutations into the entire linker (defined as 229-237) and studying their effects. We found that the mutations K230A (-62.2 +/- 3.4 mV versus -72.2 +/- 1.7 mV of wild type (WT)), G231A (-64.4 +/- 1.3 mV), M232A (V(1/2) = -63.1 +/- 1.1 mV), and E235G (-65.4 +/- 1.5 mV) produced depolarizing activation shifts. Although E229A and M232A decelerated gating kinetics (<13- and 3-fold, respectively), K230A and G231A accelerated both activation and deactivation (< approximately 2-3-fold). D233A, S234A, V236A, and Y237A channels exhibited WT properties (p > 0.05). Shortening the linker (EVY235-237deltadeltadelta) caused depolarizing activation shift and slowed kinetics that could not be explained by removing the charge at position 235 alone. Secondary structural predictions by the modeling algorithms SSpro2 and PROF, along with refinements by our experimental data, suggest that part of the S3-S4 linker conforms a helical structure with the functionally important residues Met-232, Glu-235, and Gly-231 (|deltadeltaG|>1 kcal/mol) clustered on one side.

Identification of a surface charged residue in the S3-S4 linker of the pacemaker (HCN) channel that influences activation gating

I(f), encoded by the hyperpolarization-activated cyclic nucleotide-modulated (HCN) channel family, is a key player in cardiac and neuronal pacing. Although HCN channels structurally resemble voltage-gated K(+) (Kv) channels, their structure-function correlation is much less clear. Here we probed the functional importance of the HCN1 S3-S4 linker by multiple substitutions of its residues. Neutralizing Glu(235), an acidic S3-S4 linker residue conserved in all hyperpolarization-activated channels, by Ala substitution produced a depolarizing activation shift (V(12) = -65.0 +/- 0.7 versus -70.6 +/- 0.7 mV for wild-type HCN1); the charge-reversed mutation E235R shifted activation even more positively (-56.2 +/- 0.5 mV). Increasing external Mg(2+) mimicked the progressive rightward shifts of E235A and E235R by gradually shifting activation (V(12) = 1 < 3 < 10 < 30 mm); Delta V(12) induced by 30 mm Mg(2+) was significantly attenuated for E235A (+7.9 +/- 1.2 versus +11.3 +/- 0.9 mV for wild-type HCN1) and E235R (+3.3 +/- 1.4 mV) channels, as if surface charges were already shielded. Consistent with an electrostatic role, the energetic changes associated with Delta V(12) resulting from various Glu(235) substitutions (i.e. Asp, Ala, Pro, His, Lys, and Arg) displayed a strong correlation with their charges (Delta Delta G = -2.1 +/- 0.3 kcal/mol/charge; r = 0.94). In contrast, D233E, D233A, D233G, and D233R did not alter activation gating. D233C (in C318S background) was also not externally accessible when probed with methanethiosulfonate ethylammonium (MTSEA). We conclude that the S3-S4 linker residue Glu(235) influences activation gating, probably by acting as a surface charge.

Dynamical description of sinoatrial node pacemaking: improved mathematical model for primary pacemaker cell

We developed an improved mathematical model for a single primary pacemaker cell of the rabbit sinoatrial node. Original features of our model include 1) incorporation of the sustained inward current (I(st)) recently identified in primary pacemaker cells, 2) reformulation of voltage- and Ca(2+)-dependent inactivation of the L-type Ca(2+) channel current (I(Ca,L)), 3) new expressions for activation kinetics of the rapidly activating delayed rectifier K(+) channel current (I(Kr)), and 4) incorporation of the subsarcolemmal space as a diffusion barrier for Ca(2+). We compared the simulated dynamics of our model with those of previous models, as well as with experimental data, and examined whether the models could accurately simulate the effects of modulating sarcolemmal ionic currents or intracellular Ca(2+) dynamics on pacemaker activity. Our model represents significant improvements over the previous models, because it can 1) simulate whole cell voltage-clamp data for I(Ca,L), I(Kr), and I(st); 2) reproduce the waveshapes of spontaneous action potentials and ionic currents during action potential clamp recordings; and 3) mimic the effects of channel blockers or Ca(2+) buffers on pacemaker activity more accurately than the previous models.

Role of IK and If in the pacemaker mechanisms of sino-atrial node myocytes

The role of IK (delayed rectifier current) and If (hyperpolarization-activated current) in dominant and subsidiary pacemaker ranges was studied in single myocytes isolated from the guinea pig sino-atrial node by means of a perforated patch-clamp technique. In the dominant pacemaker range (approx. –55 to –40 mV), IK tails are present whereas If is not activated. In the subsidiary pacemaker range (approx. –80 to –70 mV), If is large whereas IK is minimal and reversing. The threshold for If activation is more negative at short time intervals. Larger or longer depolarizations to –40 mV and +20 mV deactivate If more and are followed by faster reactivation of If. Steps of 200–300 ms duration to +20 mV completely deactivate If. The slope conductance decreases during depolarizations at –40 and +20 mV and quickly re-increases after the steps. The If deactivation range is between –70 and +10 mV, with a V1/2 of –35 mV. Depolarizations from –80 to +20 mV at a rate of 120/min limit the subsequent If reactivation owing to the short diastole. We conclude that IK plays a predominant role in the dominant pacemaker range and If does so in the subsidiary pacemaker range. Either pacemaker mechanism is used by sino-atrial node cells depending on the diastolic potential range. A previous depolarization markedly increases the amplitude and rate of If reactivation.Key words: dominant and subsidiary pacemaker ranges, sinus discharge and dual pacemaker mechanisms, delayed rectifier current IK, hyperpolarization-activated current If, factors regulating If, pacemaker shifts.

On the mechanisms of cholinergic control of the sinoatrial node discharge

It has been proposed that cholinergic agonists inhibit the sinoatrial node discharge by shifting the activation range of the hyperpolarization-activated inward current If to more negative values or by increasing potassium conductance. In the former instance, cesium will potentiate the cholinergic inhibition by blocking any residual If; in the latter instance, Cs+ and Ba2+ will antagonize the inhibitory action by blocking K+ channels. The changes in discharge induced by high and low concentrations of carbachol were studied using an electrophysiologic technique in isolated guinea pig sinoatrial node perfused in vitro in the absence and presence of different concentrations of Cs+ and Ba2+. In Tyrode solution, high carbachol concentrations (0.5-2 microM) slowed the sinoatrial node by hyperpolarizing the membrane and by reducing the amplitude of diastolic depolarization; and stopped the sinoatrial node by preventing the attainment of threshold potential. Adding Cs+ (10 mM) to carbachol increased the rate in slowly discharging sinoatrial node and induced spontaneous discharge in quiescent sinoatrial node. In high [K+]o (approximately 12 mM), carbachol slowed or stopped the slow responses and adding Cs+ accelerated or induced discharge. Both in Tyode and in high [K+]o, in the presence of Cs+, carbachol did stop the sinoatrial node. In the presence of carbachol, Ba2+ (0.1 mM) accelerated or induced discharge, as Cs+ did. Atropine (1 microM) prevented both the slowing or suppression by carbachol and the acceleration of sinoatrial node by Cs+ in the presence of carbachol. Low carbachol concentrations (0.05-0.1 microM) decreased the rate to a similar extent in the absence and the presence of a low concentration of Cs+ (2 mM, which blocks If but not K+ channels), but markedly less in the presence of 0.5-0.75 mM Ba2+ (which block K+ channels but not If). We conclude that cholinergic agonists slow or stop the sinoatrial node by a shifting the membrane potential toward the more negative subsidiary pacemaker range and away from the threshold. The results with Cs+ and Ba2+ indicate that both high and low concentrations of carbachol decrease sinoatrial node discharge by activating the I(K,ACh) channels rather than by decreasing If.

Distribution and prevalence of hyperpolarization-activated cation channel (HCN) mRNA expression in cardiac tissues

HCN cation channel mRNA expression was determined in the rabbit heart and neonatal and adult rat ventricle using RNase protection assays. In the rabbit SA node, the dominant HCN transcript is HCN4, representing >81% of the total HCN message. HCN1 is also expressed, representing >18% of the total HCN mRNA. Rabbit Purkinje fibers contained almost equal amounts of HCN1 and HCN4 transcripts with low levels of HCN2, whereas rabbit ventricle contained predominantly HCN2. The SA node contained 25 times the total HCN message of Purkinje fibers and 140 times the total HCN message of ventricle. No reports of hyperpolarization-activated current (If) exist in rabbit Purkinje fibers, and we could not record If in rabbit ventricular myocytes. To investigate the possible role of isoform switching in determining the voltage dependence of If, we determined the prevalence of HCN isoforms in neonatal and adult rat ventricle. We had previously determined the threshold for activation of If to be approximately -70 mV in neonatal rat ventricle and -113 mV in adult rat ventricle. In both neonatal and adult rat ventricle, only HCN2 and HCN4 transcripts are present. The ratio of HCN2 to HCN4 is approximately 5:1 in the neonate and 13:1 in the adult. Taken together, these results suggest that different cardiac regions express different isoforms of the HCN family. The HCN1 and HCN4 isoforms are most closely associated with a depolarized threshold for If activation, whereas the HCN2 isoform is associated with a more negative activation curve.

Molecular characterization of the hyperpolarization-activated cation channel in rabbit heart sinoatrial node

We cloned a cDNA (HAC4) that encodes the hyperpolarization-activated cation channel (If or Ih) by screening a rabbit sinoatrial (SA) node cDNA library using a fragment of rat brain If cDNA. HAC4 is composed of 1150 amino acid residues, and its cytoplasmic N- and C-terminal regions are longer than those of HAC1-3. The transmembrane region of HAC4 was most homologous to partially cloned mouse If BCNG-3 (96%), whereas the C-terminal region of HAC4 showed low homology to all HAC family members so far cloned. Northern blotting revealed that HAC4 mRNA was the most highly expressed in the SA node among the rabbit cardiac tissues examined. The electrophysiological properties of HAC4 were examined using the whole cell patch-clamp technique. In COS-7 cells transfected with HAC4 cDNA, hyperpolarizing voltage steps activated slowly developing inward currents. The half-maximal activation was obtained at -87.2 +/- 2.8 mV under control conditions and at -64.4 +/- 2.6 mV in the presence of intracellular 0.3 mM cAMP. The reversal potential was -34.2 +/- 0.9 mV in 140 mM Na+o and 5 mM K+o versus 10 mM Na+i and 145 mM K+i. These results indicate that HAC4 forms If in rabbit heart SA node.

Contribution of L-type Ca2+ current to electrical activity in sinoatrial nodal myocytes of rabbits

The role of L-type calcium current (ICa,L) in impulse generation was studied in single sinoatrial nodal myocytes of the rabbit, with the use of the amphotericin-perforated patch-clamp technique. Nifedipine, at a concentration of 5 microM, was used to block ICa,L. At this concentration, nifedipine selectively blocked ICa,L for 81% without affecting the T-type calcium current (ICa,T), the fast sodium current, the delayed rectifier current (IK), and the hyperpolarization-activated inward current. Furthermore, we did not observe the sustained inward current. The selective action of nifedipine on ICa,L enabled us to determine the activation threshold of ICa,L, which was around -60 mV. As nifedipine (5 microM) abolished spontaneous activity, we used a combined voltage- and current-clamp protocol to study the effects of ICa,L blockade on repolarization and diastolic depolarization. This protocol mimics the action potential such that the repolarization and subsequent diastolic depolarization are studied in current-clamp conditions. Nifedipine significantly decreased action potential duration at 50% repolarization and reduced diastolic depolarization rate over the entire diastole. Evidence was found that recovery from inactivation of ICa,L occurs during repolarization, which makes ICa,L available already early in diastole. We conclude that ICa,L contributes significantly to the net inward current during diastole and can modulate the entire diastolic depolarization.

Cesium effects on i(f) and i(K) in rabbit sinoatrial node myocytes: implications for SA node automaticity

Cesium blocks the hyperpolarization-activated current i(f) but blocks neither the delayed-rectifier current i(K) nor the sinoatrial (SA) node discharge. It has been proposed that the failure of Cs+ to block SA discharge is either an incomplete block or a negative shift of i(f). However, an alternative possibility is that i(K) (rather than i(f)) has a predominant role in the SA-pacemaker potential. To investigate this point, the effects of Cs+ on both i(f) and i(K) in the pacemaker range of potentials were studied in the same single SA node cell at the same time by means of the perforated patch-clamp technique. Hyperpolarizing steps from a holding potential (Vh) of -35 mV into and past the pacemaker-potential range resulted in a progressively larger i(f) associated with an increasing slope conductance. Cs+ (2 mM) reversibly blocked both i(f) and the slope conductance increase, suggesting that the current activated was indeed predominantly i(f). Subsequently, hyperpolarizing steps to -50, -60, and -70 mV were applied in the absence (to activate only i(f)) and in the presence of a prior depolarizing step to +10 mV (to activate i(K) as well, as the action potential normally does). Cs+ almost abolished i(f) but only slightly decreased i(K). It is concluded that the failure of Cs+ to block the SA- node spontaneous discharge is not due to a shift of i(f) out of the pacemaker range (due to run-down) or an incomplete block of i(f). Instead, the resistance of i(K) to block by Cs+ is consistent with a predominant role of i(K) for the discharge of the SA node, although i(f) can contribute under normal or special circumstances. The reduction of i(K) by Cs+ raises the question whether the Cs+ slows the SA-node discharge not only by suppressing I(f), but also by reducing i(K).

Cellular electrophysiologic mechanisms of cardiac arrhythmias

Cardiac arrhythmias are caused by alterations in the electrophysiologic properties of the cardiac cells, which affect the characteristics of the transmembrane potentials. The electrophysiologic properties that cause arrhythmias are automaticity, triggered activity, and reentrant excitation. Each of these mechanisms is described in terms of the characteristics of the transmembrane potentials and how these influence the appearance of the arrhythmia on the electrocardiogram.

IK independent class III actions of MS-551 compared with sematilide and dofetilide during reperfusion in anaesthetized rats

1. The antiarrhythmic and haemodynamic effects of three class III antiarrhythmic drugs, MS-551, sematilide and dofetilide, were examined in the coronary artery, ligation-reperfusion model of pentobarbitone-anaesthetized rats, a species deficient in functional cardiac IK. MS-551 is a non-selective potassium channel blocker, while both sematilide and dofetilide are selective delayed rectifier potassium (K) channel (IK) blockers. 2. Before coronary ligation, 3 and 10 mg kg-1 MS-551 decreased the heart rate by 6% (P < 0.01) and 12% (P < 0.01), and increased mean arterial pressure (MAP) by 14% (P < 0.05) and 33% (P < 0.01), respectively. Sematilide at 10 and 30 mg kg-1 also decreased the heart rate by 4% (P < 0.01) and 9% (P < 0.01), respectively, and the higher dose of 30 mg kg-1 decreased MAP by 29% (P < 0.01). Dofetilide, 1 mg kg-1, decreased the heart rate (P < 0.01), but had no significant effect on MAP. 3. The QT interval was increased by 10% (P < 0.01) and 31% (P < 0.01), when 3 and 10 mg kg-1 MS-551 were given. Sematilide and dofetilide had no effect on the QT interval. 4. Immediately after reperfusion, lethal ventricular fibrillation (VF) was induced in 80% of the saline group. MS-551 at 3 and 10 mg kg-1, reduced the incidence of lethal VF to 50% and 20% (P < 0.05). Neither dofetilide 1 mg kg-1 nor sematilide (10 and 30 mg kg-1) decreased the incidence of lethal VF (70%, 80% and 50%, respectively). None of the three drugs had any effect on the occurrence of reperfusion-induced VT or the total incidence of VF. However, 10 mg kg-1 MS-551 delayed the onset of reperfusion-induced VF (27 +/- 5 s compared with 12 +/- 2 s of the control group, P < 0.05). 5. In conclusion, in rats which are deficient in cardiac IK MS-551 prolonged the QT interval and reduced the incidence of sustained VF after reperfusion. Blockade of channels other than IK might participate in the defibrillatory effect of MS-551. Sematilide and dofetilide, which are selective IK blockers, did not increase the QT interval nor did they show antiarrhythmic effects Mechanisms other than K channel block may be involved in the different effects of the three drugs on blood pressure.

Use-dependent block of the pacemaker current I(f) in rabbit sinoatrial node cells by zatebradine (UL-FS 49). On the mode of action of sinus node inhibitors

BACKGROUND

Zatebradine (UL-FS 49) is a drug with a specific bradycardiac electrophysiological profile. It reduces heart rate by lengthening the duration of diastolic depolarization in the sinoatrial (SA) node. The ionic basis of this action, however, is not clarified.

METHODS AND RESULTS

We used the whole-cell patch-clamp technique to study the effects of zatebradine on ionic currents underlying diastolic depolarization of isolated rabbit SA node cells. Low concentrations of zatebradine simultaneously reduced diastolic depolarization rate and the pacemaker current I(f). The drug blocked the pacemaker current, I(f), in a use-dependent manner without causing a shift of its activation curve. At hyperpolarized potentials, unblock of I(f) occurred. Clinically relevant concentrations of the drug have little effect on the L-type calcium current or delayed rectifier potassium current.

CONCLUSIONS

This use-dependent block of the If channel can account for most of the pharmacological characteristics of zatebradine and is probably the mechanism of heart rate reduction caused by this agent. Thus, the sinus node inhibitor zatebradine belongs to a new class of "I(f) blockers" with possible advantages over currently available drugs for the treatment of ischemic heart disease.

Negative chronotropic effect of a novel class III antiarrhythmic drug, UK-68,798, devoid of beta-blocking action on isolated guinea-pig atria

1. The chronotropic effects of a novel class III antiarrhythmic drug, UK-68,798, and the beta-adrenoceptor blocker, propranolol, for comparison, were studied on spontaneously beating right atria isolated from guinea-pigs in the absence and presence of increasing concentrations of isoprenaline (10(-10)-10(-4) M). 2. UK-68,798 (10(-9)-10(-5) M) decreased spontaneous atrial rate by 6-21%. Propranolol (10(-8) -10(-6) M) also had a negative but significantly smaller chronotropic effect. 3. UK-68,798 dose-dependently reduced the maximal positive chronotropic effect induced by isoprenaline, but without significantly shifting the concentration-response curve for isoprenaline in a parallel fashion. A pD'2 value of 5.88 was obtained. As expected, propranolol displayed a competitive inhibition with a pA2 value of 8.21. 4. The results demonstrate a negative chronotropic effect of UK-68,798, which is not associated with a beta-adrenoceptor blocking action. We suggest that the negative chronotropic effect is linked with potassium channel blockade and thereby the class III antiarrhythmic action of UK-68,798.

Electrophysiology of single heart cells from the rabbit tricuspid valve

1. The electrophysiology of single myocytes isolated from the rabbit tricuspid valve was studied using the patch-clamp method (whole-cell configuration). Cell dispersion was achieved by collagenase treatment, using the Langendorff retrograde perfusion procedure. 2. After isolation, and while incubating in the recovery (Kraftbrühe) solution, cells had clear striations and were mostly spindle-shaped, or rod-like (less than 10%), with length varying from 35 microns to over 150 microns, and diameter from 3 to 10 microns. 3. Upon exposure to Tyrode solution, the calcium-tolerant cells were mostly rounded with smooth surfaces and well-defined borders. The mean diameter of these cells was 15 +/- 5 microns (S.D., n = 9). A smaller percentage (about 30%) retained the original elongated shape. 4. Patch pipette recordings showed the presence of spontaneous activity in about 30% of round cells, and less frequently in elongated cells. Maximum diastolic potentials (MDPs) in the round cells averaged -82 +/- 6 mV, with a take-off potential of -56 +/- 3 mV (n = 9), and an average maximum upstroke velocity (Vmax) value of 6.3 +/- 0.6 V/s (n = 4). In quiescent cells, the mean resting potential was 69 +/- 12 mV (n = 43). 5. Voltage clamp ramps revealed a steady-state I-V relation with a negative slope region. The mean input resistance value was 25 +/- 9 M omega (n = 16) for the elongated, and 883 +/- 481 M omega (n = 8) for the round cells. 6. Hyperpolarizing 5 s pulses (holding potential = -50 mV) occasionally revealed a slow, time-dependent inward current whose peak increased progressively as a function of clamp potential. The slowly activating current was sensitive to caesium 2 mM), indicating its similarity to the so-called 'pacemaker current' (iF). In alternate voltage- and current-clamp experiments, blocking of iF did not stop pacemaker activity, but there was up to a fourfold increase in pacemaker cycle length. 7. In some cells, 5 s hyperpolarizing steps from a holding potential of -40 or -50 mV produced large, inwardly directed and voltage-dependent current surges that decayed rapidly with time, similar to the inactivation described for the inward rectifier current, iK1. The current was very prominent at voltages more negative than -100 mV, and its decay process was best fitted by two time constants, one fast and one slow. For example, at -150 mV the time constants were 61 and 634 ms. The inward current was blocked by barium (1 mM).(ABSTRACT TRUNCATED AT 400 WORDS)

Cardiac electrophysiologic effects of AHR 5360C

We used microelectrode and blood superfusion techniques to study the cardiac electrophysiologic effects of a new drug, AHR 5360C, which has antihypertensive and calcium channel blocking properties in several experimental models. AHR 5360C, 10(-7) M significantly depressed the amplitude of the slow response action potential in canine Purkinje fibers. The fast response action potential was also depressed in a dose-dependent fashion, but with a threshold concentration of 5 X 10(-6) M. AHR 5360C decreased normal automaticity and barium-induced abnormal automaticity at concentrations of 5 X 10(-6) and 10(-5) M respectively, as well as ouabain-induced delayed afterdepolarizations at a threshold concentration of 10(-6) M. In blood superfusion studies; i.v. administration of AHR 5360C, 0.3 mg/kg, significantly reduced blood pressure. Doses of 1.0 mg/kg induced a high degree of A-V block, further reduction of blood pressure, and no physiological changes in heart rate and in the blood superfused fibers. In conclusion, AHR 5360C has calcium blocking properties that depress A-V conduction at concentrations that do not affect the sodium-dependent fast response action potential.

Contribution of two types of calcium currents to the pacemaker potentials of rabbit sino-atrial node cells

1. Two types of calcium currents, the transient type and long-lasting type, were examined by both whole-cell and cell-attached patch-clamp modes in single isolated sino-atrial node cells of the rabbit. 2. In the whole-cell clamp mode, in response to a depolarizing pulse to -40 mV from a holding potential of -80 mV, a transient type calcium current with an amplitude of 2.1 +/- 0.7 pA/pF (mean +/- S.D.; n = 15) was recorded. The threshold potential was approximately -50 mV. 3. Nickel (40 microM) and tetramethrin (0.1 microM) blocked the transient type calcium current without appreciable effects on the long-lasting type. Nifedipine and D600 blocked the long-lasting type, but did not affect the transient type. Cadmium (20 microM) and cobalt (2 mM) inhibited both types of calcium currents equally. 4. Both types of calcium currents showed an increased amplitude with increasing extracellular calcium concentration. The values of the Michaelis constant, Km, were 0.95 mM for the transient type and 3.92 mM for the long-lasting type, indicating that these types represent two different classes of channels. 5. In the cell-attached patch-clamp mode, the single-channel conductance of the transient type calcium current was 8.5 pS, by using 100 mM-BaCl2 in the pipette, whereas that of the long-lasting type was 16.0 pS, under the same conditions. Each of these values was similar to those found in other cells, respectively. 6. In the whole-cell clamp mode, the transient type current began to inactivate at -70 mV and was fully inactivated at -40 mV. The steady-state inactivation curve of the transient type current was approximately 50 mV negative to that of the long-lasting type. The overlap of the membrane potential between the activation and inactivation curves was small. The time constant of the inactivation shortened from 20 to 5 ms as the potential became progressively positive over the range from -80 to +30 mV. 7. Isoprenaline (1 microM) increased the amplitude of the long-lasting type Ca2+ current, but was not effective on the transient type, suggesting that the long-lasting type calcium current may be responsible for the positive chronotropic effect of isoprenaline. 8. While recording spontaneous electrical activity of the cell, application of 40 microM-nickel induced bradycardia and this effect was enhanced when the membrane was constantly hyperpolarized.(ABSTRACT TRUNCATED AT 400 WORDS)

Barium-induced automatic activity in isolated ventricular myocytes from guinea-pig hearts

1. A suction-pipette whole-cell clamp technique was applied to single ventricular myocytes isolated from guinea-pig hearts, in order to investigate the ionic mechanism underlying Ba2+-induced automatic activity. 2. The application of 0.1 mM or less Ba2+ to the myocytes caused a depolarization of the resting membrane potential without inducing spontaneous activity. The stimulated action potential showed a prolonged repolarization phase followed by an after-hyperpolarization. 3. Concentrations of Ba2+ of 0.2 mM or greater produced further depolarization of the resting membrane potential and induced spontaneous activity. Spontaneous activity developed from the slow diastolic depolarization preceded by after-hyperpolarizations of spontaneous or stimulated action potentials. 4. Under voltage-clamp conditions, a decaying outward or inward current in response to hyperpolarizing clamp steps from depolarized potentials appeared in the presence of Ba2+. The Ba2+-induced current decay showed a faster time course with increasing hyperpolarizing clamp pulses and reversed its polarity at around -90 mV, the presumed equilibrium potential for K+ (EK). In the late current-voltage (I-V) relation, Ba2+ almost eliminated the inward-rectifying property. These effects on the cardiac membrane are consistent with a time- and voltage-dependent blocking action of Ba2+ on inward-rectifying K+ currents as reported for other excitable tissues. 5. The concentration- and voltage-dependence of the steady-state block of the inward rectifying K+ current (IK1) was fitted by a simple model assuming 1:1 binding of Ba2+ to a site within the membrane. The apparent dissociation constant at the holding potential of 0 mV (K(0] was 0.3 mM, and the parameter for the membrane potential dependence of Ba2+ blockade (mu) was approximately 0.5. 6. A computer model of the ventricular action potential proposed by Beeler & Reuter (1977) was modified, based on the recent experiments using single cardiac myocytes. The modifications include (1) the current-voltage relationship of IK1, (2) time courses of activation and inactivation of the Ca2+ current (ICa), (3) the activation voltage range for the delayed outward K+ current (IK). 7. The time- and voltage-dependent blocking action of Ba2+ on IK1, including the experimentally determined values for K(0) and mu, were incorporated into the modified version of the action potential model. The computer model reproduced an after-hyperpolarization at doses of Ba2+ lower than 0.1 mM and automatic activity at doses higher than 0.15 mM.(ABSTRACT TRUNCATED AT 400 WORDS)

Membrane currents in cardiac pacemaker tissue

The present work is a brief survey of the mechanism of the cardiac pacemaker in sinoatrial node cells. Information on the pacemaker mechanism in cardiac tissue has been greatly enhanced by the development of the single cell isolation technique and the patch clamp technique. These methods circumvent to a large extent the difficulties involved in voltage clamping multicellular preparations. The calcium current (ICa), delayed rectifier potassium current (IK), transient outward current (Ito;IA), and the hyperpolarization activated inward current (Ih or If) were found both in whole cell preparations and in single channel analysis. The physiological significance of these currents, together with the exchange current systems for the pacemaker depolarization are discussed.

Conductance and kinetics of delayed rectifier potassium channels in nodal cells of the rabbit heart

1. The delayed rectifier K+ current (IK) of single pace-maker cells from the sino-atrial node and the atrioventricular node of the rabbit heart was investigated using the whole-cell and cell-attached configurations of the patch-clamp technique. 2. The activation kinetics of the macroscopic IK were not altered by varying the extracellular K+ concentration ([K+]o) between 5.4 and 150 mM. The amplitude of the tail current of IK, however, was about 10-fold larger at a [K+]o of 150 mM than that at a [K+]o of 5.4 mM. 3. By using a high-[K+]o solution, inward single-channel currents were observed on repolarization from potentials positive to -40 mV. The current-voltage (I-V) relation was linear over the negative potential range and the reversal potential estimated by extrapolating the I-V curve was shifted by about 60 mV for a 10-fold increase in [K+]o, indicating that the channel was highly selective for K+. 4. The single-channel conductance was 11.1 pS at a [K+]o of 150 mM and varied in proportion to the square root of [K+]o. The total number of channels was estimated as approximately 1000 per cell (0.7/micron 2). On repolarization, the averaged single-channel current disappeared with a time constant similar to that of the macroscopic tail current of IK. 5. At potentials between -50 and -100 mV, the open and closed times of the single channel fitted well with single-exponential and biexponential distributions, respectively. As the membrane was progressively depolarized, the open time was shortened while the closed time was prolonged, suggesting a decrease of open probability. These changes were in the opposite direction to those expected from the delayed rectifier K+ current which progressively increases in magnitude at more positive potentials. 6. At the beginning of the macroscopic tail current, a transient increase of the inward current was found to precede the time-dependent decrease. This rapid initial change can be attributed to a quick removal of inactivation of IK which had occurred during the depolarizing pulse. This inactivation gate of the channel has very fast kinetics and could be responsible for the inward-going rectification observed in the 'fully activated' IK.

Electrophysiological effects of diclofurime on rabbit and frog atrial heart muscle

The effects of diclofurime on the electrical activity of the rabbit sinus node, rabbit atria and frog atrial fibres were studied using microelectrode and the double sucrose gap voltage-clamp techniques respectively. In rabbit sinus node, diclofurime (10(-7) M to 10(-6) M) decreased the action potential (AP) amplitude and maximum rate of depolarization (Vmax), increased the AP duration and slowed the sinus rate. In rabbit atria, the drug reduced the amplitude of the depolarizing phase and Vmax, lengthened the AP duration and decreased the resting membrane potential. In frog atrial fibres, the drug (10(-5) M) depolarized the resting membrane potential, decreased Vmax as well as the plateau amplitude. It inhibited the sodium current (INa) with a dissociation constant of 3.7 X 10(-6) M and a one to one relationship between the drug molecule and the Na channel. Diclofurime did not alter the apparent reversal potential for the fast Na current (ENa) but it inhibited the sodium conductance (GNa) in a frequency-dependent manner. Diclofurime also blocked the slow inward current (Islow) without alteration of Eslow. The block of Islow occurred with a dissociation constant of 2 X 10(-5) M and unity stoichiometry. The data suggest that diclofurime might be effective in the control of cardiac arrythmias since it exhibited both local anaesthetic-like and calcium antagonistic properties.

Effects of calcium antagonistic drugs on the electrical activity of rabbit sino-atrial node

The effects of several calcium antagonistic drugs (slow channel blockers), mesudipine, FR-34235 (nilvadipine), nifedipine, and verapamil, were compared on the naturally-occurring slow action potentials (APs) of the isolated spontaneously-contracting sinoatrial (SA)-node of the rabbit. Mesudipine, at 1 X 10(-8) M, had little or no effect on the AP parameters and beating frequency. At 1 X 10(-7) M, mesudipine depressed the amplitude, + Vmax and frequency of the APs. A higher concentration of mesudipine (3 X 10(-7) M) completely blocked the slow APs. The concentration of FR-34235 required for 50% depression of Vmax was 3 X 10(-8) M, and complete block required 1 X 10(-7) M. The verapamil and nifedipine concentrations required for complete block were 1 X 10(-6) M and 1 X 10(-7) M, respectively. The cells blocked by these drugs were depolarized to about -40 mV. Cells blocked and depolarized by the drugs responded to intensive field stimulation with a transient after-hyperpolarization, followed by damped oscillations. When [Ca]o was elevated from 1.8 mM to 5.4 mM, full block required a higher concentration of mesudipine (1 X 10(-6)M) and FR-34235 (1 X 10(-6)M). The order of potency of the drugs tested was: nifedipine = FR-34235 greater than mesudipine greater than verapamil. The effects of mesudipine, FR-34235, and nifedipine were easily reversible upon washout for 10 min, whereas those of verapamil required longer periods. The results indicate that the slow channel blockers, mesudipine, FR-34235, nifedipine, and verapamil depress the APs and automaticity of the SA-node.

Barium-induced blockade of the inward rectifier in calf Purkinje fibres

The Ba-induced blockade of iK1 in calf Purkinje fibres was studied in the low concentration range (3-1,000 microM). The results showed that the blockade induced by hyperpolarizations was time-dependent, and that the rate of blockade increased with the Ba concentration and the negative pulse amplitude. At Ba concentrations higher than 1 mM the time course of iK1 blockade was not separable from the capacity transient. At these concentrations, and in a limited voltage range, the Ba-sensitive instantaneous current displayed a marked inward rectification and behaved with external K as expected for a pure K+ current, suggesting that under these conditions the only current affected by Ba was iK1. The Ba-sensitive current deceased with time during hyperpolarizations even when high external K was present, indicating that a voltage-dependent inactivation process could be at least in part responsible for the observed current decline.

Action potential and membrane currents of single pacemaker cells of the rabbit heart

Single, viable pacemaker cells were isolated from sinoatrial (S-A) and atrioventricular (A-V) nodes by treating with collagenase. In normal Tyrode solution containing 1.8 mM Ca2+, these pacemaker cells had a round configuration and contracted rhythmically at a frequency of about 150-260/min. The amplitude, duration, and maximum rate of rise of the spontaneous action potentials recorded using patch clamp electrodes were similar to those obtained from multicellular preparations. Amplitudes of the recorded membrane current were normalized with reference to the surface area of the cell by assuming the cell shape as a plane oblate spheroid. The membrane resistance of the isolated nodal cells was 14.9 +/- 4.0 k omega . cm2 (n = 12) at about -35 mV and the membrane capacitance was 1.30 +/- 0.24 microF/cm2 (n = 18). The inactivation time course of the slow inward current, isi, was fitted with a sum of two exponentials with time constants of 6.7 +/- 0.6 ms and 46.6 +/- 15.3 ms (n = 4) at +10 mV. The amplitude of isi peaked at 0 approximately +10 mV in the current-voltage relationship and was 18.2 +/- 8.4 microA/cm2. The potassium current, iK, was activated in the voltage range positive to -50 mV and was saturated at about +20 mV. The amplitude of the fully-activated iK at -40 mV was 3.3 +/- 1.4 microA/cm2 (n = 10) and showed an inward-going rectification. The activation of the hyperpolarization-activated current was observed at potentials negative to -70 mV in seven of 14 experiments. The current density and membrane capacitance calculated could be overestimated and the membrane resistance underestimated, because of the presence of caveolae on the cell surface. However, these data give the nearest possible estimates of the electrical constants in the nodal cells, which cannot be measured accurately in the conventional multicellular preparations.

Ionic currents responsible for the generation of pace-maker current in the rabbit sino-atrial node

The ionic nature of the pace-maker current (delta Ip, If, Ih) was investigated in rabbit sino-atrial node using a single sucrose-gap voltage-clamp technique. The pace-maker current was activated by hyperpolarizing clamp steps negative to -50 mV and the pace-maker potential was activated by an action potential or a depolarizing clamp step. Neither pace-maker current nor pace-maker potential were altered by addition of tetrodotoxin, but a tetrodotoxin-sensitive channel could be activated in sino-atrial nodal strips following hyperpolarizing clamp steps. Ca2+-channel blockers did not affect the voltage dependence of delta Ip or the maximum diastolic potential (m.d.p.) significantly. Removal of Ca2+ did not affect the pace-maker current at clamp potentials near the pace-maker potential range (-60 to -80 mV), but it did reduce the potential dependence of the m.d.p. Removal of Na+ suppressed completely the pace-maker current and hyperpolarized the membrane. Removal of Na+ also increased membrane conductance, most likely through an increase in resting K+ permeability. Low concentration of Cs+ (less than 5 mM), but not Ba2+ or tetraethylammonium, markedly suppressed activation delta Ip and reduced the rate of pacing slightly. Cs+ also decreased the membrane conductance and hyperpolarized the membrane. In 50% of experiments designed to determine contribution of IK to pace-maker current, a double-pulse procedure revealed a time-dependent component of delta Ip which reversed near the K+ equilibrium potential, EK. Release of depolarizing or hyperpolarizing test clamps was followed by pace-maker potentials, the magnitudes of which were dependent on the test-clamp potential. The m.d.p. approached values near EK following depolarizing clamps and near -45 mV following hyperpolarizing clamps. The results suggest that delta Ip is carried primarily by Na+ and is blocked by Cs+. It is likely, however, that Ca2+ alters the rate of pacing not only through its contribution to the Isi system, but also through activation of a K+ conductance.

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)

The surprising heart: a review of recent progress in cardiac electrophysiology

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Adenosine-5'-triphosphate-sensitive single potassium channel in the atrioventricular node cell of the rabbit heart

The patch-clamp method was applied to single atrioventricular (a.v.) node cells of the rabbit heart to study the characteristics of the K+ channel. When the electrode contained 5.4 mM-K+, depolarizations of the cell-attached patch membrane induced outward single channel currents characterized by burst-like openings; the open-state probability increased from 0.005-0.01 at -40 mV to 0.07-0.1 at +20 mV of membrane potential. The reversal potentials of the current at K+ concentrations of 5.4, 20 and 130 mM in the electrode agreed with those given by the Nernst equation, indicating that this channel is selective for K+ ions. The slope conductance of the channel decreased beyond 60-90 mV positive to the reversal potential (inward-going rectification). The conductance near the reversal potential increased with increasing K+ concentrations on either side of the membrane: from 31-32 pS at 5.4 mM-K+ to 41-42 pS at 20 mM-K+ on the outside, and from 19 pS at 90 mM-K+ to 29.3 pS at 130 mM-K+ on the inside. Superfusion of the cell with 5.4 mM-CN-, glucose-free Tyrode markedly increased the number of channel openings in the cell-attached patch. In the inside-out patch, application of 1 mM-adenosine-5'-triphosphate (ATP) at the inner surface of the patch membrane blocked reversibly the channel activity, while 1 mM-adenosine-5'-diphosphate (ADP) failed to block it. The conductance and kinetics of the channel were not modified by increasing the Ca2+ concentration from 10(-8) M to 5 X 10(-6) M on the inner side of the membrane, while a further increase in Ca2+ to 10(-4) M decreased the open-state probability. The probability density for the open time fitted well with an exponential distribution (time constant of 5.4 ms at 60 mV positive to the resting potential), while that for the closed time was separated into a fast and a slow component (time constants of 4.0 and 132.0 ms, respectively). The time constant of the slow component decreased significantly with depolarization in some preparations. However, neither the time constant of the fast component of the closed-time histogram nor that of the open-time histogram was voltage-dependent.

Electrophysiologic effects of flecainide acetate on sinus node function, anomalous atrioventricular connections, and pacemaker thresholds

The acute electrophysiologic effects of i.v. flecainide acetate (2 mg/kg body weight) were assessed in 71 patients undergoing electrophysiologic study. Ten patients underwent investigation for sinus node dysfunction. Sinus cycle length shortened slightly, from 980 +/- 292 to 931 +/- 276 ms (p less than 0.01). Uncorrected or corrected sinus node recovery times or sinoatrial conduction time (according to the methods of Strauss and Narula) did not change in 6 patients with normal sinus node function and in 3 of 4 patients with abnormal sinus node function at rest. In the remaining patient maximal sinus node recovery time increased from a value at rest of 5,185 ms to 23,460 ms after flecainide. In the same patient sinoatrial conduction times at rest increased from 159 ms (Strauss method) and 143 ms (Narula method) to 1,398 and 1,455 ms, respectively, after flecainide. Thirty-three patients underwent electrophysiologic evaluation of anomalous atrioventricular (AV) pathways and reentrant tachycardias. Flecainide significantly prolonged accessory AV pathway anterograde and retrograde refractoriness. Anterograde accessory pathway block occurred in 33% of patients and retrograde accessory pathway block in 44%. Flecainide was successful in the acute termination of 86% of orthodromic atrioventricular reentrant tachycardias. In 15 patients with dual AV nodal pathways, only retrograde "fast" AH pathway refractoriness was significantly increased by flecainide, which was successful in the acute termination of 88% of intra-AV nodal reentrant tachycardias. In 28 patients who underwent endocardial pacing threshold assessment before and after i.v. flecainide, the acute threshold rose by a maximum of 117%, whereas the chronic threshold rose by a maximum of 83%.(ABSTRACT TRUNCATED AT 250 WORDS)

Pathophysiologic mechanisms of cardiac arrhythmias

Arrhythmias result from abnormalities of impulse initiation or impulse conduction or a combination of both. Abnormal impulse initiation results from either automaticity or triggered activity. Automaticity can further be subdivided into (1) automaticity caused by the normal automatic mechanism (a normal property of cardiac cells in the sinus node, in some parts of the atria, in the atrioventricular junctional region, and in the His-Purkinje system) and (2) automaticity caused by an abnormal mechanism (resulting from a decrease in membrane potential of cardiac fibers, which normally have a high level of membrane potential). Triggered activity is caused by afterdepolarizations, which are second depolarizations that occur either during repolarization (referred to as early afterdepolarizations) or after repolarization is complete or nearly complete (referred to as delayed afterdepolarizations). Abnormal impulse conduction results in reentrant excitation. Usually a combination of slowed conduction and unidirectional conduction block provides the conditions necessary for reentry to occur. Slow conduction and block may result from a decrease in the resting potential and velocity of depolarization of the action potential or may be a consequence of the anisotropic structure of cardiac muscle, in which case resting potential and action potential upstroke velocity may be normal.

Optical evidence for calcium-action potentials in early embryonic precontractile chick heart using a potential-sensitive dye

Using an optical method for monitoring membrane potential, spontaneous action potentials in the 7- to 9-somite embryonic precontractile chick hearts were measured. The optical action potential in the 7- to 9-somite embryonic heart was lacking 'phase 0' and 'phase 1' attributable to the fast Na+ current. The embryonic precontractile heart continued to generate spontaneous action potentials in a Na+-free solution or in the presence of tetrodotoxin. Such an action potential was blocked by adding Co2+, Mn2+, Ni2+, La3+, D-600 or GEDTA, and the frequency, the amplitude, and the rate of rise of the spontaneous action potentials depended closely upon the external Ca2+ concentration; reducing the external Ca2+ concentration resulted in suppression of the spontaneous excitability. From the above results, we concluded that the spontaneous action potential in the early phases of cardiogenesis is characterized as a Ca2+-dependent action potential.

Does the "pacemaker current" generate the diastolic depolarization in the rabbit SA node cells?

Small preparations of spontaneously beating rabbit sino-atrial node (SA node) were voltage clamped with the two-microelectrode technique. The effects of 0.25-5 mM Cs+ on the spontaneous pacing rate and the time-dependent inward "pacemaker" current, ih, were studied. In the presence of 2 mM Cs+, the spontaneous pacing rate decreased only slightly even though ih was strongly depressed at potentials negative to -60 mV Cs+ had little or no effect on other time-dependent currents observed with clamp pulses less negative than -50 mV. Since no voltage-dependence to the Cs+ effect on ih could be measured (between -90 mV and -20 mV), it was considered unlikely that the lack of Cs+ effect on the rate of diastolic depolarization results from a voltage-dependent effect of Cs+ on the ih channel. Adrenaline produced a marked positive chronotropic effect in Cs+-treated SA node cells. This effect was accompanied by marked enhancement of the slow inward current (isi) with no change in the Cs+-blocked ih current. These results are consistent with the idea that ih plays a minor role in generation of pacemaker depolarization, and suggest a more prominent role of isi in the generation of diastolic depolarization in SA nodal cells.

Effects of barium on the membrane currents in the rabbit S-A node

In small preparations of rabbit sinoatrial node voltage clamp experiments with the two microelectrode technique were carried out. The effects of extracellular barium ions on the slow inward current and outward currents were studied and the following results were obtained: 1. Ba increased the amplitude of the slow inward current without a change in the time course of inactivation. In 10 mM Ba the steady-state inactivation curve (f infinity) shifted in the positive direction (3-4 mV), suggesting a neutralization of negative surface charges. A similar shift of the steady state activation curve (d infinity) could not be detected. 2. Ba reduced the amplitude of the time-dependent (IK, Ix) and time-independent (Ibg) potassium currents in a concentration-dependent manner. 3. The block of the time-dependent potassium current, IK, depended on the membrane potential. The block was stronger at negative than at positive potentials. The block could be relieved by depolarizing pulses, the degree of unblock increased with longer duration of the depolarizing pulse. 4. Ba blocked the slow outward current Ix in a voltage- and time-dependent manner. Like for IK, the block of Ix was stronger at negative than at positive potentials. A given concentration of Ba produced stronger block of Ix than of IK and the removal of block of Ix by depolarizing pulses was slower than the removal of IK block. 5. The effects of Ba on Ix suggest that this current is a potassium current.