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

The trunk-tail junctional region in Ciona larvae autonomously expresses tail-beating bursts at ∼20 second intervals

Swimming locomotion in aquatic vertebrates, such as fish and tadpoles, is expressed through neuron networks in the spinal cord. These networks are arranged in parallel, ubiquitously distributed and mutually coupled along the spinal cord to express undulation patterns accommodated to various inputs into the networks. While these systems have been widely studied in vertebrate swimmers, their evolutionary origin along the chordate phylogeny remains unclear. Ascidians, representing a sister group of vertebrates, give rise to tadpole larvae that swim freely in seawater. In the present study, we examined the locomotor ability of the anterior and posterior body fragments of larvae of the ascidian Ciona that had been cut at an arbitrary position. Examination of more than 200 fragments revealed a necessary and sufficient body region that spanned only ∼10% of the body length and included the trunk-tail junction. 'Mid-piece' body fragments, which included the trunk-tail junctional region, but excluded most of the anterior trunk and posterior tail, autonomously expressed periodic tail-beating bursts at ∼20 s intervals. We compared the durations and intervals of tail-beating bursts expressed by mid-piece fragments, and also by whole larvae under different sensory conditions. The results suggest that body parts outside the mid-piece effect shortening of swimming intervals, particularly in the dark, and vary the burst duration. We propose that Ciona larvae express swimming behaviors by modifying autonomous and periodic locomotor drives that operate locally in the trunk-tail junctional region.

Cardiac transmembrane ion channels and action potentials: cellular physiology and arrhythmogenic behavior

Cardiac arrhythmias are among the leading causes of mortality. They often arise from alterations in the electrophysiological properties of cardiac cells and their underlying ionic mechanisms. It is therefore critical to further unravel the pathophysiology of the ionic basis of human cardiac electrophysiology in health and disease. In the first part of this review, current knowledge on the differences in ion channel expression and properties of the ionic processes that determine the morphology and properties of cardiac action potentials and calcium dynamics from cardiomyocytes in different regions of the heart are described. Then the cellular mechanisms promoting arrhythmias in congenital or acquired conditions of ion channel function (electrical remodeling) are discussed. The focus is on human-relevant findings obtained with clinical, experimental, and computational studies, given that interspecies differences make the extrapolation from animal experiments to human clinical settings difficult. Deepening the understanding of the diverse pathophysiology of human cellular electrophysiology will help in developing novel and effective antiarrhythmic strategies for specific subpopulations and disease conditions.

Mechanisms of spontaneous pacing: sinoatrial nodal cells, neonatal cardiomyocytes, and human stem cell derived cardiomyocytes

The sinoatrial (SA) node is the primary site from which the mammalian heart is paced, but the mechanisms underlying the pacemaking still remain clouded. It is generally believed that the hyperpolarization-activated current If, encoded by hyperpolarization-activated cyclic nucleotide–gated (HCN) genes, contributes significantly to pacing, which in tandem with inward current generated by efflux of Ca2+ via the Na+–Ca2+ exchanger (NCX), resulting from the released Ca2+, mediates the diastolic depolarization. Here, we review the data that implicate If as the “pacemaker current” and conclude that there is not only a significant discrepancy between the range of diastolic depolarization potential (–60 to –40 mV) and the activation potential of If (negative to –70 mV), but that also the kinetics of If and its pharmacology are incompatible with the frequency of a heartbeat in rodents and humans. We propose that If serves as a functional insulator, which protects the SA-nodal cells against the large negative electrical sink of atrial tissue connected to it with connexins. We also evaluate the role of If and calcium signaling in mediating the diastolic depolarization in rat neonatal cardiomyocytes (rN-CM), and human induced pluripotent stem-cell derived cardiomyocytes (hiPSC-CM), and provide evidence for a possible involvement of mitochondrial Ca2+ in initiating the oscillatory events required for the spontaneous pacing.

Small functional If current in sinoatrial pacemaker cells of the brown trout (Salmo trutta fario) heart despite strong expression of HCN channel transcripts

Funny current (I_f), formed by hyperpolarization-activated cyclic nucleotide-gated channels (HCN channels), is supposed to be crucial for the membrane clock regulating the cardiac pacemaker mechanism. We examined the presence and activity of HCN channels in the brown trout (Salmo trutta fario) sinoatrial (SA) pacemaker cells and their putative role in heart rate (f_H) regulation. Six HCN transcripts (HCN1, HCN2a, HCN2ba, HCN2bb, HCN3, and HCN4) were expressed in the brown trout heart. The total HCN transcript abundance was 4.0 and 4.9 times higher in SA pacemaker tissue than in atrium and ventricle, respectively. In the SA pacemaker, HCN3 and HCN4 were the main isoforms representing 35.8 ± 2.7 and 25.0 ± 1.5%, respectively, of the total HCN transcripts. Only a small I_f with a mean current density of -1.2 ± 0.37 pA/pF at -140 mV was found in 4 pacemaker cells out of 16 spontaneously beating cells examined, despite the optimization of recording conditions for I_f activity. I_f was not found in any of the 24 atrial myocytes and 21 ventricular myocytes examined. HCN4 coexpressed with the MinK-related peptide 1 (MiRP1) β-subunit in CHO cells generated large I_f currents. In contrast, HCN3 (+MiRP1) failed to produce I_f in the same expression system. Cs⁺ (2 mM), which blocked 84 ± 12% of the native I_f, reversibly reduced f_H 19.2 ± 3.6% of the excised multicellular pacemaker tissue from 53 ± 5 to 44 ± 5 beats/min (P < 0.05). However, this effect was probably due to the reduction of I_Kr, which was also inhibited (63.5 ± 4.6%) by Cs⁺ These results strongly suggest that f_H regulation in the brown trout heart is largely independent on I_f.

Regionally diverse mitochondrial calcium signaling regulates spontaneous pacing in developing cardiomyocytes

The quintessential property of developing cardiomyocytes is their ability to beat spontaneously. The mechanisms underlying spontaneous beating in developing cardiomyocytes are thought to resemble those of adult heart, but have not been directly tested. Contributions of sarcoplasmic and mitochondrial Ca(2+)-signaling vs. If-channel in initiating spontaneous beating were tested in human induced Pluripotent Stem cell-derived cardiomyocytes (hiPS-CM) and rat Neonatal cardiomyocytes (rN-CM). Whole-cell and perforated-patch voltage-clamping and 2-D confocal imaging showed: (1) both cell types beat spontaneously (60-140/min, at 24°C); (2) holding potentials between -70 and 0mV had no significant effects on spontaneous pacing, but suppressed action potential formation; (3) spontaneous pacing at -50mV activated cytosolic Ca(2+)-transients, accompanied by in-phase inward current oscillations that were suppressed by Na(+)-Ca(2+)-exchanger (NCX)- and ryanodine receptor (RyR2)-blockers, but not by Ca(2+)- and If-channels blockers; (4) spreading fluorescence images of cytosolic Ca(2+)-transients emanated repeatedly from preferred central cellular locations during spontaneous beating; (5) mitochondrial un-coupler, FCCP at non-depolarizing concentrations (∼50nM), reversibly suppressed spontaneous pacing; (6) genetically encoded mitochondrial Ca(2+)-biosensor (mitycam-E31Q) detected regionally diverse, and FCCP-sensitive mitochondrial Ca(2+)-uptake and release signals activating during INCX oscillations; (7) If-channel was absent in rN-CM, but activated only negative to -80mV in hiPS-CM; nevertheless blockers of If-channel failed to alter spontaneous pacing.

Kinetic relationship between the voltage sensor and the activation gate in spHCN channels

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are activated by membrane hyperpolarizations that cause an inward movement of the positive charges in the fourth transmembrane domain (S4), which triggers channel opening. The mechanism of how the motion of S4 charges triggers channel opening is unknown. Here, we used voltage clamp fluorometry (VCF) to detect S4 conformational changes and to correlate these to the different activation steps in spHCN channels. We show that S4 undergoes two distinct conformational changes during voltage activation. Analysis of the fluorescence signals suggests that the N-terminal region of S4 undergoes conformational changes during a previously characterized mode shift in HCN channel voltage dependence, while a more C-terminal region undergoes an additional conformational change during gating charge movements. We fit our fluorescence and ionic current data to a previously proposed 10-state allosteric model for HCN channels. Our results are not compatible with a fast S4 motion and rate-limiting channel opening. Instead, our data and modeling suggest that spHCN channels open after only two S4s have moved and that S4 motion is rate limiting during voltage activation of spHCN channels.

An unexpected requirement for brain-type sodium channels for control of heart rate in the mouse sinoatrial node

Voltage-gated Na(+) channels are composed of pore-forming alpha and auxiliary beta subunits. The majority of Na(+) channels in the heart contain tetrodotoxin (TTX)-insensitive Na(v)1.5 alpha subunits, but TTX-sensitive brain-type Na(+) channel alpha subunits are present and functionally important in the transverse tubules of ventricular myocytes. Sinoatrial (SA) nodal cells were identified in cardiac tissue sections by staining for connexin 43 (which is expressed in atrial tissue but not in SA node), and Na(+) channel localization was analyzed by immunocytochemical staining with subtype-specific antibodies and confocal microscopy. Brain-type TTX-sensitive Na(v)1.1 and Na(v)1.3 alpha subunits and all four beta subunits were present in mouse SA node, but Na(v)1.5 alpha subunits were not. Na(v)1.1 alpha subunits were also present in rat SA node. Isolated mouse hearts were retrogradely perfused in a Langendorff preparation, and electrocardiograms were recorded. Spontaneous heart rate and cycle length were constant, and heart rate variability was small under control conditions. In contrast, in the presence of 100 nM TTX to block TTX-sensitive Na(+) channels specifically, we observed a significant reduction in spontaneous heart rate and markedly greater heart rate variability, similar to sick-sinus syndrome in man. We hypothesize that brain-type Na(+) channels are required because their more positive voltage dependence of inactivation allows them to function at the depolarized membrane potential of SA nodal cells. Our results demonstrate an important contribution of TTX-sensitive brain-type Na(+) channels to SA nodal automaticity in mouse heart and suggest that they may also contribute to SA nodal function and dysfunction in human heart.

Effects of K(+)-channel blockers on A1-adenosine receptor-mediated negative inotropy and chronotropy of guinea-pig isolated left and right atria

Adenosine has previously been shown to stimulate K(+)-efflux and to block L-type calcium channels in atrial myocytes. The aim of the present study was to evaluate the contribution of K(+)-channels in the development of the negative inotropic and chronotropic responses to adenosine agonists in guinea-pig left and right atria, respectively. Tetraethylammonium (TEA) potentiated the negative inotropic and chronotropic responses to R-(-)-N6-(2-phenyl-isopropyl)-adenosine (R-PIA), seen as leftward shifts of the concentration-response curves. Glibenclamide had no effect on the negative inotropic response to R-PIA but increased the rate of onset of the negative chronotropic response in right atria. 4-Aminopyridine (4-AP, 10 mM), potentiated the left atrial inotropic responses to R-PIA, seen as a leftward shift of the concentration-response curve, but slowed the speed of onset of the response to a single concentration (10(-6) M) of R-PIA. This reduction in speed of onset of the response can explain the differences in effects of 4-AP on concentration-response curves reported here and previously. In the right atria, 4-AP (10 mM) inhibited the negative chronotropic responses to R-PIA, seen as a rightward shift of the concentration-response curve and reduction of the maximum response. 4-AP also slowed the onset of the right atrial rate response to R-PIA. These results support the theory that K(+)-efflux plays only a minor role in the negative inotropic responses of guinea-pig left atria to R-PIA. This apparently controls the speed of onset of the response. The negative chronotropic response of guinea-pig right atria to R-PIA appears to be much more dependent upon K(+)-efflux than for the negative inotropic response of the left atria.

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).

Overdrive Excitation in the Guinea Pig Sinoatrial Node Superfused in High [K(+)](o)

The aim of the present experiments was to study the characteristics and mechanisms of the rhythm induced by overdrive ('overdrive excitation', ODE) in the sinoatrial node (SAN) superfused in high [K(+)](o) (8-14 mM). It was found that: (1) overdrive may induce excitation in quiescent SAN and during a slow drive; (2) in spontaneously active SAN, overdrive may accelerate the spontaneous discharge; (3) immediately after the end of overdrive, a pause generally precedes the onset of the induced rhythm; (4) during the pause, an oscillatory potential (V(os)) may be superimposed on the early diastolic depolarization (DD); (5) during the subsequent late DD, a different kind of oscillatory potential appears near the threshold for the upstroke (ThV(os)) which is responsible for the initiation of spontaneous activity; (6) once started, the induced rhythm is fastest soon after overdrive; (7) faster drives induce longer and faster spontaneous rhythms; (8) the induced action potentials are slow responses followed by DD with a superimposed V(os), but ThV(os) is responsible for ODE; (9) the induced rhythm subsides when ThV(os) miss the threshold and gradually decay; (10) low [Ca(2+)](o) abolishes ODE; (11) in quiescent SAN, high [Ca(2+)](o) induces spontaneous discharge through ThV(os) and increases its rate by enhancing V(os) and shifting the threshold to more negative values, and (12) tetrodotoxin abolishes ODE as welll as the spontaneous discharge induced by high [Ca(2+)](o). In conclusion, in K(+)-depolarized SAN, ODE may be present in the apparent absence of calcium overload, is Ca(2+)- and Na(+)-dependent and is mediated by ThV(os) and not by V(os). Copyright 1997 S. Karger AG, Basel

Sodium dependency of the inward potassium rectifier in horizontal cells isolated from the white bass retina

The ionic properties underlying the inwardly rectifying potassium current in cultured voltage-clamped white bass horizontal cells were studied. Anomalous rectification was apparent upon membrane hyperpolarization with a reversal potential depolarized from the predicted value of EK. In raised extracellular potassium, the current increased and the reversal potential shifted toward a more depolarized membrane potential. Solutions containing decreased sodium caused a rapid decrease in the inward rectifier current but only slightly affected the reversal potential. Extracellular cesium or barium caused a reversible voltage-dependent reduction of the inward current. We interpret these results to mean that the inward rectifying channel in white bass horizontal cells is mainly permeable to potassium ions, but is sodium dependent. It may shape the photoresponses of the horizontal cells and may contribute to a hyperpolarization activated conductance increase measured in situ.

Responses to sympathetic nerve stimulation of the sinus venosus of the toad

1. The changes in membrane potential produced by sympathetic nerve stimulation were recorded from sinus venosus preparations of the toad, Bufo marinus, in which beating had been prevented by the dihydropyridine calcium antagonist, nifedipine. 2. Supramaximal sympathetic stimuli initiated long-lasting excitatory junction potentials which started with the same latencies, some 1 to 2 s, as did sympathetic tachycardias recorded from beating preparations. 3. Brief trains of stimuli increased the amplitude of excitatory junction potentials and shortened their latency of onset. Similarly when excitatory junction potentials were facilitated their latency of onset was shortened. 4. The time courses of excitatory junction potentials were prolonged by cooling the preparation but unchanged when the neuronal uptake of catecholamines was inhibited. 5. In arrested preparations, beta-adrenoceptor activation causes a hyperpolarization, as did the inhibition of phosphodiesterases or the activation of adenylate cyclase. This contrasts with the depolarization produced by sympathetic nerve stimulation which could be mimicked by the rapid application of either adrenaline or noradrenaline but not by beta-adrenoceptor activation, phosphodiesterase inhibition or by adenylate cyclase activation. 6. The results are discussed in relation to the idea that neuronally released adrenaline activates a set of adrenoceptors which are linked to a set of channels by a pathway that does not involve cyclic AMP.

Ionic currents activated during hyperpolarization of single right atrial myocytes from cat heart

Whole-cell recording techniques were used on single right atrial myocytes to study the ionic currents that may be responsible for the diverse diastolic voltage characteristics of atrial tissue. Ionic currents were activated by hyperpolarizing voltage pulses negative to -30 mV. In general, four different types of cells were identified based primarily on the ionic currents elicited during hyperpolarization. The first cell type exhibited an inward current that decayed with time at more negative voltages, reversed near the potassium equilibrium potential, inwardly rectified at more positive voltages, increased in elevated extracellular potassium, and was blocked by 3 mM barium or 10 mM cesium. This current was identified as the potassium current iK1. A second cell type exhibited a time-dependent inward current that increased at more negative voltages, had an activation range between -50 and -110 mV, had a reversal potential of -26 mV, and was blocked by 3 mM cesium. This current was identified as an if current. A third cell type exhibited an inward current that initially decayed and then became more inward with time. Barium (3 mM) abolished the initial inward current and revealed a time-dependent increasing inward current that was blocked by 3 mM cesium. This current was composed of both the iK1 and if currents. A fourth cell type exhibited only small time-independent leak currents in response to hyperpolarization. These results indicate that individual cells within the right atrium are electrophysiologically heterogeneous with respect to the types of ionic channels present in their sarcolemmal membranes. This specialization in ionic currents partially explains the diverse diastolic voltage characteristics and functional properties of atrial tissue.

Voltage clamp measurements of the hyperpolarization-activated inward current I(f) in single cells from rabbit sino-atrial node

1. The kinetics and ion transfer characteristics of the hyperpolarization-activated inward current, I(f), have been studied in single cells obtained by enzymatic dispersion from the rabbit sino-atrial (S-A) node. These experiments were done to assess the role of I(f) in the generation of the pacemaker depolarization in the S-A node. 2. The activation and the deactivation of I(f) in these single cells are accompanied by significant conductance increases and decreases respectively, confirming earlier findings from multicellular man-made strips of rabbit S-A node, and from mammalian Purkinje fibres. 3. The steady-state activation of I(f) lies between -40 and -120 mV, and its voltage dependence can be described by a Boltzmann relation with the half-activation point at approximately -70 mV. 4. The delay or sigmoidicity in both the onset of I(f) and the deactivation of the tail currents can be accounted for semi-quantitatively by using a second-order Hodgkin-Huxley kinetic scheme. 5. The reversal potential for I(f) is -24 +/- 2 mV (mean +/- S.E.M., n = 6). It does not change significantly as a function of the amount of I(f) which is activated, indicating that ion accumulation or depletion phenomena are not important variables controlling the time course of I(f), or its selectivity. 6. The fully-activated current-voltage relationship for I(f) is approximately linear with a slope conductance of 12.0 +/- 0.88 nS per cell (mean +/- S.E.M., n = 6). 7. A simple mathematical model based on the measured values of maximum conductance, reversal potential, and kinetics of I(f) has been developed to simulate the size and time course of I(f) during typical spontaneous pacemaker activity in rabbit sino-atrial node cells. The calculations show that I(f) can change significantly during pacing and suggest that this current change is, at least in part, responsible for the pacemaker depolarization.

Effects of vagal stimulation and applied acetylcholine on the arrested sinus venosus of the toad

1. The effects of vagal stimulation and applied acetylcholine were compared on sinus venosus preparations of the toad, Bufo marinus, in which beating had been inhibited by adding the organic calcium antagonist nifedipine. 2. Bath-applied acetylcholine and vagal stimulation each caused membrane hyperpolarizations which were abolished by hyoscine. 3. Whereas the hyperpolarization that accompanied vagal stimulation was largely unaffected by barium ions, that produced by bath-applied acetylcholine was almost abolished. 4. Caesium ions also prevented the hyperpolarization produced by bath-applied acetylcholine but potentiated the responses to vagal stimulation. 5. The membrane resistance of arrested sinus venosus cells was found to be higher during vagal stimulation than in the absence of stimulation. In contrast when a similar hyperpolarization was produced by adding acetylcholine, the membrane resistance was found to be lower than in control solution. 6. The results are discussed in relation to the idea that neuronally released acetylcholine causes membrane hyperpolarization by suppressing inward current flow and applied acetylcholine acts to increase outward current flow.

Isolated cells of the frog sinus venosus: properties of the inward current activated during hyperpolarization

Single sinus venosus cells from frog, Rana esculenta, were isolated using an enzymic dispersion procedure, involving applications of collagenase and protease. About 40%-60% of the cells showed spontaneous contractions. Isolated cells were studied in the whole-cell configuration. Regenerative action potentials were tetrodotoxin-insensitive and similar to those recorded in multicellular preparations. Hyperpolarizing pulses in the voltage range negative to -50 mV induced the activation of a time-dependent inward current, which was blocked by 4 mM caesium but less affected by barium ions. A lower concentration of caesium (1 mM) exerted a voltage-dependent reduction of the current and decreased the spontaneous pacing rate. The activation range of the hyperpolarization-activated current approximately extended from -50 mV to -110 mV, but varied from cell to cell. A high variability was observed in the behaviour of the activation kinetics. The current had a reversal potential near -20 mV that was shifted positively by increasing the external potassium concentration (from 3 mM to 30 mM) and negatively by reducing the external sodium concentration (from 115 mM to 30 mM). The hyperpolarization-activated inward current of the frog sinus venosus cell appears to be carried by both sodium and potassium ions. It shows electrophysiological properties similar to those of the If current of the mammalian heart. The role of the current in the spontaneous activity is discussed.

Sialic acid and the surface charge associated with hyperpolarization-activated, inward rectifying channels

The whole-cell configuration of the patch-clamp technique was used with cultured pacemaker cells from the rabbit sinoatrial node to test the hypothesis that sialic acid residues (NANA) constitute much of the negative surface charge associated with hyperpolarization-activated, inward rectifying channels. Activation-voltage relationships (between -70 and -140 mV) were determined for hyperpolarization-activated (inward rectifying) current (i(f)). Addition of 10 mM Ca2+ shifted the half-activation potential (V 1/2) from -89.5 +/- 0.9 mV to -77.9 +/- 2.6 mV (P less than 0.01), confirming the presence of negative fixed charges on the myocytes after 3 to 5 days in culture. Addition of 20 mM dimethonium, an organic divalent cation that "screens" but does not bind to negative surface charge, shifted V 1/2 from -86.8 +/- 1.4 mV to -75.0 +/- 1.7 mV (P less than 0.001) without affecting the amplitude of the current. In contrast, 10 mM Ca2+ reduced the amplitude of i(f) significantly. Incubation of cells with a highly purified preparation of neuraminidase (0.1-2.0 U/ml, 1 hr, 37 degrees C), an enzyme that selectively removes NANA from glycoproteins and glycolipids, failed to alter V 1/2 or the amplitude of i(f) significantly. Pretreatment of cells with neuraminidase (1.0 U/ml, 1 hr, 37 degrees C) failed to alter the positive shift of V 1/2 produced by dimethonium. The results suggest that NANA does not constitute the negative surface charge associated with hyperpolarization-activated, inward rectifying channels.

Mechanisms of automaticity in subsidiary pacemakers from cat right atrium

Intracellular recordings were made from eustachian ridge of cat right atrium to determine mechanisms responsible for subsidiary pacemaker automaticity. Pacemaker action potentials exhibited two phases of diastolic depolarization: an initial steeper slope (D1) followed by a more gradual slope (D2). Cesium (1 mM) decreased D1 (-45.6%) to a significantly greater extent than D2 (-33.6%) and increased spontaneous cycle length (SCL) (+37.7%). Tetrodotoxin (10(-6) M) had no effect on maximum rate of rise of upstroke, although it increased SLC (+23.9%). Verapamil (0.4-1.0 microM) progressively increased SCL by decreasing late diastolic slope, resulting in oscillatory potentials and eventual quiescence. Both norepinephrine (2 x 10(-9) M) and Bay K 8644 (10(-7) M) elicited a significantly greater increase in D2 than in D1, resulting in a decrease in SCL. Ryanodine (10(-6) M) caused a small but significant initial decrease (-3.7%) followed by a progressive increase in SCL (+172%). Ryanodine decreased D2 without changing D1, increased maximum rate of rise and overshoot potential, and abolished tension. In the presence of ryanodine, Bay K 8644 progressively increased D1 amplitude, resulting in a cyclic pattern of dysrhythmic activity. In the presence of ryanodine, cesium significantly decreased D1 (-39.3%), shifted the late diastolic potential more negative, and increased SCL (+25.7%). These results indicated that multiple mechanisms participate in subsidiary pacemaker automaticity. They include 1) a cesium-sensitive component that contributes to a greater extent during the initial phase of diastolic depolarization, 2) a component mediated via calcium released from the sarcoplasmic reticulum that contributes primarily during the latter half of diastolic depolarization, and 3) possibly a direct contribution by the slow inward calcium current.

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.

Properties of the hyperpolarizing-activated current (if) in cells isolated from the rabbit sino-atrial node

Individual cells were isolated from the sino-atrial node area of the rabbit heart using an enzyme medium containing collagenase and elastase. After enzymatic treatment the cells were placed in normal Tyrode solution, where beating resumed in a fraction of them. Isolated cells were studied in the whole cell configuration. Action potentials as well as membrane currents under voltage-clamp conditions were similar to those in multicellular preparations. Pulses to voltages more negative than about -50 mV caused activation of the hyperpolarizing-activated current, if. Investigation of the properties of this current was carried out under conditions that limited the influence of other current systems during voltage clamp. The if current activation range usually extended approximately from -50 to -100 mV, but varied from cell to cell. In several cases, pulsing to the region of -40 mV elicited a sizeable if. Both current activation and deactivation during voltage steps had S-shaped time courses. A high variability was however observed in the sigmoidal behaviour of if kinetics. Plots of the fully-activated current-voltage (I-V) relation in different extracellular Na and K concentrations showed that both ions carry the current if. While changes in the external Na concentration caused the current I-V relation to undergo simple shifts along the voltage axis, changes in extracellular K concentration were also associated with changes in its slope. Again, a large variability was observed in the increase of I-V slope on raising the external K concentration. The current if was strongly depressed by Cs, and the block induced by 5 mM-Cs was markedly voltage dependent. Adrenaline (1-5 microM) and noradrenaline (1 microM) increased the current if around the half-activation voltage range and accelerated its activation at more negative voltages. Often, however, drug application failed to elicit any modification of if. Current run-down was observed in nearly all cells, although at a highly variable rate. It was accelerated by raising the extracellular K concentration but did not show a marked use dependence. Both the if activation curve and the fully activated I-V relation were affected by run-down, the former being shifted to more negative values along the voltage axis and the latter being depressed with no apparent change of the if reversal potential.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of potassium conductance inhibitors on spontaneous diastolic depolarization and abnormal automaticity in human atrial fibers

The capability of generating spontaneous diastolic depolarization and automaticity was investigated in vitro by means of standard microelectrode techniques in 50 human atrial preparations. Samples were classified within two groups: group 1 was composed of 12 well-polarized preparations exhibiting action potentials that were fast responses (mean maximum diastolic potential: -75.5 mV and Vmax greater than 100 V/s); group 2 was composed of 38 partially-depolarized samples (mean maximum diastolic potential: -50.3 mV and Vmax less than 10 V/s) and was further divided into two subgroups. Subgroup 2A consisted of 20 spontaneously beating preparations and subgroup 2B consisted of 18 non-automatic partially-depolarized specimens. Highly-polarized fibers from group 1, although exhibiting a slight diastolic depolarization which was almost entirely suppressed by 2 mM caesium, never presented spontaneous activity under our experimental conditions. 90% of automatic fibers from subgroup 2A were sampled from dilated atria. In automatic preparations, diastolic depolarization was usually separated into two phases: an initial phase, also present in non-automatic fibers, and a late phase. Changes in the initial phase were not accompanied by concomitant changes in the spontaneous rate. Abnormal automaticity was clearly related to the late diastolic phase (absent in non-automatic fibers), the generation of which appeared to be a specific property of automatic fibers. The use of K conductance inhibitors (caesium, 4-aminopyridine, barium, low K solutions) provided indirect evidence that neither delayed outward ix current nor if type inward current are principally responsible for abnormal automaticity.

Negative chronotropic effects of thyroxine on the isolated bullfrog heart

An acute application of L-thyroxine (T4) to everted sinus-atrium preparations of bullfrog showed negative chronotropic effects consisting of two processes: an initial peak at 2-5 min and a delayed plateau at 40-60 min. The initial effect was blocked by Cd2+ and tetraethyl-ammonium (TEA). The delayed effect was suppressed by Cd2+ and NaCN.

Voltage clamp of bull-frog cardiac pace-maker cells: a quantitative analysis of potassium currents

Spontaneously active single cells have been obtained from the sinus venosus region of the bull-frog, Rana catesbeiana, using an enzymic dispersion procedure involving serial applications of trypsin, collagenase and elastase in nominally 0 Ca2+ Ringer solution. These cells have normal action potentials and fire spontaneously at a rate very similar to the intact sinus venosus. A single suction micro-electrode technique (Hamill, Marty, Neher, Sakmann & Sigworth, 1981; Hume & Giles, 1983) has been used to record the spontaneous diastolic depolarizations or pace-maker activity as well as the regenerative action potentials in these cells. This electrophysiological activity is completely insensitive to tetrodotoxin (TTX; 3 X 10(-6) M) and is very similar to that recorded from an in vitro sinus venosus preparation. The present experiments were aimed at identifying the transmembrane potassium currents, and analysing their role(s) in the development of the pace-maker potential and the repolarization of the action potential. Depolarizing voltage-clamp steps from the normal maximum diastolic potential (-75 mV) elicit a time- and voltage-dependent activation of an outward current. The reversal potential of this current in normal Ringer solution [( K+]0 2.5 mM) is near -95 mV; and it shifts by 51 mV per tenfold increase in [K+]0, which strongly suggests that this current is carried by K+. We therefore labelled it IK. The reversal potential of IK did not shift in the positive direction following very long (20 s) depolarizing clamp steps to +20 mV, indicating that 'extracellular' accumulation of [K+]0 does not produce any significant artifacts. The fully activated instantaneous current-voltage (I-V) relationship for IK is approximately linear over the range of potentials -130 to -30 mV. Thus, the ion transfer mechanism of IK may be described as a simple ohmic conductance in this range of potentials. Positive relative to -30 mV, however, the I-V exhibits significant inward rectification. A Hodgkin-Huxley analysis of the kinetics of IK, including a demonstration that the envelope of tails quantitatively matches the time course of the onset of IK during a prolonged depolarizing clamp step has been completed. The steady-state activation variable (n infinity) of IK spans the voltage range approximately -40 to +10 mV. It is well-fitted by a Boltzmann distribution function with half-activation at -20 mV. The time course of decay of IK is a single exponential. However, the activation or onset of IK shows clear sigmoidicity in the range of potentials from the activation threshold (-40 mV) to 0 mV.(ABSTRACT TRUNCATED AT 400 WORDS)