Cardiac Pacemaker Tissue Research Articles

Canonical Wnt signaling promotes pacemaker cell specification of cardiac mesodermal cells derived from mouse and human embryonic stem cells.

Cardiac differentiation of embryonic stem cells (ESCs) can give rise to de novo chamber cardiomyocytes and nodal pacemaker cells. Compared with our understanding of direct differentiation toward atrial and ventricular myocytes, the mechanisms for nodal pacemaker cell commitment are not well understood. Taking a cue from the prominence of canonical Wnt signaling during cardiac pacemaker tissue development in chick embryos, we asked if modulations of Wnt signaling influence cardiac progenitors to bifurcate to either chamber cardiomyocytes or pacemaker cells. Omitting an exogenous Wnt inhibitor, which is routinely added to maximize cardiac myocyte yield during differentiation of mouse and human ESCs, led to increased yield of spontaneously beating cardiomyocytes with action potential properties similar to those of native sinoatrial node pacemaker cells. The pacemaker phenotype was accompanied by enhanced expression of genes and gene products that mark nodal pacemaker cells such as Hcn4, Tbx18, Tbx3, and Shox2. Addition of exogenous Wnt3a ligand, which activates canonical Wnt/β-catenin signaling, increased the yield of pacemaker-like myocytes while reducing cTNT-positive pan-cardiac differentiation. Conversely, addition of inhibitors of Wnt/β-catenin signaling led to increased chamber myocyte lineage development at the expense of pacemaker cell specification. The positive impact of canonical Wnt signaling on nodal pacemaker cell differentiation was evidenced in direct differentiation of two human ESC lines and human induced pluripotent stem cells. Our data identify the Wnt/β-catenin pathway as a critical determinant of cardiac myocyte subtype commitment during ESC differentiation: endogenous Wnt signaling favors the pacemaker lineage, whereas its suppression promotes the chamber cardiomyocyte lineage.

Pacemaker cell characteristics of differentiated and HCN4-transduced human mesenchymal stem cells

AimsCell-based biological pacemakers aim to overcome limitations and side effects of electronic pacemaker devices. We here developed and tested different approaches to achieve nodal-type differentiation using human adipose- and bone marrow-derived mesenchymal stem cells (haMSC, hbMSC). Main methodshaMSC and hbMSC were differentiated using customized protocols. Quantitative RT-PCR was applied for transcriptional pacemaker-gene profiling. Protein membrane expression was analyzed by immunocytochemistry. Pacemaker current (If) was studied in haMSC with and without lentiviral HCN4-transduction using patch clamp recordings. Functional characteristics were evaluated by co-culturing with neonatal rat ventricular myocytes (NRVM). Key findingsCulture media-based differentiation for two weeks generated cells with abundant transcription of ion channel genes (Cav1.2, NCX1), transcription factors (TBX3, TBX18, SHOX2) and connexins (Cx31.9 and Cx45) characteristic for cardiac pacemaker tissue, but lack adequate HCN transcription. haMSC-derived cells revealed transcript levels, which were closer related to sinoatrial nodal cells than hbMSC-derived cells. To substitute for the lack of If, we performed lentiviral HCN4-transduction of haMSC resulting in stable If. Co-culturing with NRVM demonstrated that differentiated haMSC expressing HCN4 showed earlier onset of spontaneous contractions and higher beating regularity, synchrony and rate compared to co-cultures with non-HCN4-transduced haMSC or HCN4-transduced, non-differentiated haMSC. Confocal imaging indicated increased membrane expression of cardiac gap junctional proteins in differentiated haMSC. SignificanceBy differentiation haMSC, rather than hbMSC attain properties favorable for cardiac pacemaking. In combination with lentiviral HCN4-transduction, a cellular phenotype was generated that sustainably controls and stabilizes rate in co-culture with NRVM.

Analysis of heterogeneous cardiac pacemaker tissue models and traveling wave dynamics

The sinoatrial-node (SAN) is a complex heterogeneous tissue that generates a stable rhythm in healthy hearts, yet a general mechanistic explanation for when and how this tissue remains stable is lacking. Although computational and theoretical analyses could elucidate these phenomena, such methods have rarely been used in realistic (large-dimensional) gap-junction coupled heterogeneous pacemaker tissue models. In this study, we adapt a recent model of pacemaker cells (Severi et al., 2012), incorporating biophysical representations of ion channel and intracellular calcium dynamics, to capture physiological features of a heterogeneous population of pacemaker cells, in particular “center” and “peripheral” cells with distinct intrinsic frequencies and action potential morphology. Large-scale simulations of the SAN tissue, represented by a heterogeneous tissue structure of pacemaker cells, exhibit a rich repertoire of behaviors, including complete synchrony, traveling waves of activity originating from periphery to center, and transient traveling waves originating from the center. We use phase reduction methods that do not require fully simulating the large-scale model to capture these observations. Moreover, the phase reduced models accurately predict key properties of the tissue electrical dynamics, including wave frequencies when synchronization occurs, and wave propagation direction in a variety of tissue models. With the reduced phase models, we analyze the relationship between cell distributions and coupling strengths and the resulting transient dynamics. Further, the reduced phase model predicts parameter regimes of irregular electrical dynamics. Thus, we demonstrate that phase reduced oscillator models applied to realistic pacemaker tissue is a useful tool for investigating the spatial-temporal dynamics of cardiac pacemaker activity.

Store-operated calcium entry and the localization of STIM1 and Orai1 proteins in isolated mouse sinoatrial node cells.

In many non-excitable and excitable cells, store-operated calcium entry (SOCE) represents an additional pathway for calcium entry upon Ca2+ store depletion. In a previous study, we demonstrated SOCE activity in intact mouse cardiac pacemaker tissue, specifically from sinoatrial node (SAN) tissue. However, store content as a key determinant of SOCE activity is difficult to measure in intact SAN tissue. Therefore, to investigate the interaction between SOCE and store content and its role in cardiac pacemaking, it is necessary to investigate SOCE activity in single cardiac pacemaker cells. Furthermore, recent studies in other tissues have identified two new proteins involved in SOCE, stromal interacting molecule (STIM), which is an ER Ca2+ sensor, and the surface membrane channel Orai, a prototypic gene encoding for SOCE. However, whether STIM and Orai are expressed in native pacemaker cells is still unknown. In this current study, we examined SOCE activity in single firing pacemaker cells isolated from mouse sinoatrial node tissue. We found a significant rise in Ca2+ entry in response to Ca2+ store depletion. SOCE blockers reduced the amplitude and frequency of spontaneous Ca2+ transients and reduced Ca2+ store content. We demonstrated for the first time that STIM and Orai are expressed in pacemaker cells. After store depletion, STIM1 redistributed to the cell periphery and showed increased co-localization with surface membrane located Orai1, indicating a possible involvement of these proteins in SOCE activity in native cardiac pacemaker cells. These results suggest the novel concept that SOCE plays a functional role in regulating intracellular Ca2+ of cardiac pacemaker cells.

The Large Scale Transcriptome Analysis of Mouse Sinoatrial Node (SAN)

Novel functional analyses and numerical models have led to a paradigm shift in sinoatrial nodal pacemaker cell biology. Recent reports of transcriptional profiles of SAN, however, have focused on a limited number of pre-selected transcripts, while the large-scale transcriptome profile of SAN pacemaker cells remains unknown. Using Illumina Ref8v2 bead chips we performed large-scale transcriptome analysis of sinoatrial node (SAN) and compared it to those of left and right atrium (LA and RA), left and right ventricle (LV and RV) and spontaneously beating cultured HL-1 cells.Principal component analysis of transcriptomic relationship among samples revealed a distinct supraventricular grouping (LA, RA, SAN and HL-1), a ventricular grouping (LV, RV), a native heart tissue grouping, (LA, RA, SAN, LV and RV) and the cultured cell line. Overexpressed genes specific to each native heart tissue (greater than 75th percentile for that group) were 3946 transcripts evenly expressed among its component tissues. The supraventricular group expression of 87 of these genes was specifically enriched in SAN; 95, in RA and 205 in LA; ventricular grouping embraced 2773 common transcripts, with 901 transcripts being enriched in LV and 142 in RV. Some findings in SAN compare to other heart tissues, for example, PDE1A, Tbx3, RGS4, ISL1, were highly overexpressed. The co-expression gene networks by the weighted gene co-expression analysis reveal that neuron projection, fatty acid metabolic process, neuron differentiation and response to endogenous stimulus pathways were highly represented in SAN.In summary, this is the first large-scale transcriptomic analysis of cardiac pacemaker tissue and its comparison to other cardiac tissue. Novel gene markers for SAN were observed. The results provide valuable clues with respect to regeneration of SAN-like beating cells from pluripotent cell lines and to the design of biological pacemakers.

Liénard-type models for the simulation of the action potential of cardiac nodal cells

Existing models of cardiac cells which include multi-variable cardiac transmembrane current are too complex to simulate the long time dynamical properties of the heart rhythm. The large number of parameters that need to be defined and set for such models make them not only cumbersome to use but also require a large computing power. Consequently, the application of such models for the bedside analysis of heart rate of a specific patient may be difficult. Other ways of modelling need to be investigated.We consider the general problem of developing a model of cardiac pacemaker tissue that allows to combine the investigation of phenomena at a time scale of thousands of heart beats with the ability to reproduce realistic tissue-level characteristics of cell dynamics. We propose a modified van der Pol–Duffing equation–a Liénard-type oscillator–as a phenomenological model for cardiac nodal tissue, with certain important physiological similarities to ion-channel models of cardiac pacemaker cells.The model presented here is specifically designed to qualitatively reproduce mesoscopic characteristics of cell dynamics, including action potential duration (APD) restitution properties, phase response characteristics, and phase space structure.We show that these characteristics agree qualitatively with the extensive ionic models and experimental results in the literature [Anumonwo et al., 1991, [33], Cao et al., 1999, [49], Coster and Celler, 2003, [31], Qu, 2004, [45], Tsalikakis et al., 2007, [32], Inada et al., 2009, [14], Qu et al., 2010, [50]]. Furthermore, we discuss reasons for the memory effect [Cherry and Fenton, 2007, [42]] obtained for the pacemaker cells.The usefulness of the general concepts presented here is illustrated by a simulation of the atrio-ventricular re-entry tachycardia (AVNRT) in a pseudo one dimensional strip of tissue. We compare the results with other kinds of simulation in the literature.

HCN Pacemaker Channels and Pain: A Drug Discovery Perspective

This article reviews evidence that hyperpolarization-activated, cation nonselective (HCN) channels, the molecular basis of the Ih current, potentially represent valid targets for novel analgesic agents. Ih is a prominent current in many peripheral sensory nerves, with highest current density typically found in large diameter neurons. Recent data suggest that Ih may represent a valid target for the treatment of spontaneous pain and allodynia associated with nerve injury. The majority of available electrophysiological and molecular evidence suggests that fast activating, weakly cyclic adenosine monophosphate (cAMP) sensitive HCN1-based channels may make a significant contribution to Ih, especially in large diameter, mechanosensitive fibers, where the Ih current appears to support abnormal spontaneous firing after nerve injury. In contrast, HCN4 channels seem to play the predominant role in cardiac pacemaker tissue. These observations raise the possibility that HCN1 selective blockers may inhibit pain associated with nerve injury without dramatic effects on heart rate. Development of novel HCN blocking analgesics presents a number of significant technical challenges. Although a number of HCN blockers are available, such as ZD-7288, ivabradine, and others, these drugs inhibit all HCN isoforms with the same potency. As a result, these compounds have powerful effects on heart rate, severely limiting their utility for non-cardiac indications such as pain. Selectivity challenges, mechanisms of compound interaction with the channel, and assay methods are described in detail.

Characteristics of single cells isolated from the atrioventricular node of the adult guinea-pig heart.

To date, data regarding the cellular electrophysiology of the atrioventricular node (AVN) have derived from AVN cells isolated from the rabbit heart. The aim of this study was to characterise for the first time the electrophysiological properties of single cells isolated from the AVN of the guinea-pig heart. Cells were isolated from the AVN region by a combination of enzymatic and mechanical dispersion. Mean (+/-SEM) cell dimensions were 110.8+/-2.3 microm in length by 11.4+/-1.3 micro m in width (n=76 cells). Electrophysiological recordings were made using the whole-cell patch-clamp technique at 37 degrees C. Mean cell capacitance was 25.1+/-0.9 pF (n=43) and mean cell input resistance was 1,377+/-178 Muomega (n=21). Spontaneously active cells exhibited action potentials characteristic of cardiac pacemaker tissue. Under whole-cell voltage clamp, the mean 'zero current' potential was -39.7+/-4.1 mV (n=21). Voltage commands of 500 ms duration to a range of test potentials from a holding potential of -40 mV revealed a number of distinct current components. At potentials positive to -40 mV an early inward current was observed that exhibited a bell-shaped current voltage (I-V) relation, typical of L-type calcium current. A delayed outward current that increased progressively with test potential magnitude was also observed. Analysis of the outward current 'tails' on repolarisation to -40 mV showed this to be comprised of two components with half-maximal activation voltages of -17.2+/-1.8 mV, and +27.1+/-3.6 mV (n=13). 'Transient outward' current appeared absent from guinea-pig AVN cells. Hyperpolarising test pulses activated net inward current, for which three components could be observed: a barium-sensitive (100 microM Ba(2+)) inwardly rectifying current evident at the start of the voltage command and prominent at potentials negative to the diastolic potential range; a time-dependent, hyperpolarisation-activated current, and a residual background current. On repolarisation to -40 mV, a large inward current spike typical of cardiac Na current was observed in some cells. Notably, the following electrophysiological characteristics of guinea-pig AVN cells are distinct from those characteristics previously reported for the rabbit AVN: (1) an absence of transient outward current, (2) the presence of two components of delayed outward current, and (3) the presence of a Ba(2+)-sensitive inwardly rectifying current at negative voltages. The guinea-pig AVN isolated cell preparation may be valuable in providing additional insights into the cellular electrophysiology of this important region of the heart.

T Lymphocyte Memory in Non-mammary Silicone Encounters

Purpose: To study the effect of injected or implanted silicones other than mammary devices on siloxane T H cell immune memory by lymphoblast transformation. Design: Study of 43 individuals with silicone implants compared with controls. Materials and Methods: Twenty-two men and 21 women, injected or implanted with silicones other than mammary devices, were tested for siloxane T H cell immune memory by lymphoblast transformation. They received an arm muscle implant, block silicone as bone grafts, penile and testicular devices, silicone-containing temporomandibular joint prostheses, silicone-sheathed cardiac pacemaker wires, chin implants, toe replacements, tissue injections, or heavy industrial exposure. The control subjects were drawn from the medical center staff. Concanavalin A (Con A) was the mitogenic control. Results: The mean stimulation indexes for the Con A control were 269.78 &#45 30.24 (subjects) and 284.17 &#45 39.18 (controls), p = 0.3857, not significant. The mean stimulation index for siloxan...

An obligatory role for nitric oxide in autonomic control of mammalian heart rate.

Cholinergic modulation of heart rate in isolated spontaneously beating single cells from the rabbit sino-atrial node was investigated by measuring transmembrane ionic currents using the nystatin-perforated patch whole-cell voltage-clamp technique. Carbamylcholine (CCh), a stable analogue of acetylcholine (ACh), significantly inhibited L-type calcium currents (Ica(L) which had been augmented by beta-adrenergic stimulation. In addition, CCh activated a potassium outward current (IK(ACh)). Both effects were blocked by atropine. The possible involvement of nitric oxide (NO) in these responses was evaluated by inhibiting NO synthesis. In the presence of NG-monomethyl-L-arginine (L-NMMA, 100 microM) or nitro-L-arginine methyl ester (L-NAME, 1 mM), two specific inhibitors of nitric oxide synthase (NOS), CCh no longer inhibited ICa(L). IK(ACh) could still be activated. Co-incubation of cells in L-NAME or in L-NMMA with arginine (the endogenous substrate of NOS) restored the CCh-induced attenuation of ICa(L), indicating that L-NAME or L-NMMA did not interfere directly with the muscarinic action of CCh on ICa(L). Effects of the NO-releasing agent molsidomine (SIN-1) on CCh-induced changes in ICa(L) were also investigated. After ICa(L) had been augmented by beta-adrenergic stimulation, SIN-1 (0.1 mM) inhibited ICa(L); however, SIN-1 had no further inhibitory effect after a maximal CCh concentration had been applied. These findings suggest that NO generation is an obligatory process in cholinergic inhibition of ICa(L) in mammalian cardiac pacemaker tissue.

Analysis of the cardiovascular changes induced in the rat by graded levels of systemic hypoxia.

1. In rats anaesthetized with Saffan, we have further analysed the respiratory, cardiac and regional vascular responses induced by 3 min periods of graded hypoxia (breathing 15, 12, 8 or 6% O2 in N2). 2. Frequently, hypoxia evoked an episode, lasting 1-1.5 min, of tachycardia, renal and mesenteric vasoconstriction and skeletal muscle vasodilatation. The tachycardia and muscle vasodilatation persisted after vagotomy indicating they were not initiated by pulmonary stretch receptors secondary to hyperventilation. We propose that such episodes represented the cardiovascular components of the alerting-defence response initiated by activation of the brain stem defence areas by peripheral chemoreceptors. 3. Each of these episodes was superimposed upon gradual hyperventilation, tachycardia, fall in arterial pressure and vasodilatation in renal, mesenteric and muscle circulation the magnitudes of which at 2 min were generally graded with the level of hypoxia. In the 3rd minute, respiration and heart rate tended to wane below control levels. 4. Vagotomy had little effect on the heart rate changes and only slightly reduced the peripheral vasodilatation allowing the conclusion that the gradual tachycardia and peripheral vasodilatation was not a reflex initiated by pulmonary stretch receptors. 5. Guanethidine given after vagotomy abolished the tachycardia indicating it was sympathetically mediated; possible initiating factors are discussed. But the secondary bradycardia persisted indicating it reflected the direct effect of hypoxia on cardiac pacemaker tissue. 6. The peripheral vasodilatation persisted after guanethidine or phentolamine indicating it was mainly attributable to the local vasodilator effects of tissue hypoxia. 7. It is proposed that the components of the alerting response are an integral part of the response to systemic hypoxia. Further, that in the rat this response is superimposed upon, but may be overcome by the direct effects of hypoxia on peripheral vasculature, heart and central nervous system.

Ionic mechanisms of adenosine actions in pacemaker cells from rabbit heart.

1. Whole-cell and patch clamp techniques have been applied to cells isolated from the rabbit sino-atrial (S-A) node to study the ionic mechanism(s) of adenosine-induced slowing of cardiac pacemaker activity. 2. Viable spontaneously active cells were isolated from the central region of the S-A node of the rabbit heart by an enzymatic dispersion procedure similar to that reported by Giles & van Ginneken (1985) and van Ginneken & Giles (1988). In these spontaneously beating cells application of adenosine caused a dose-dependent slowing accompanied by a small hyperpolarization of the maximum diastolic potential. Relatively high doses of adenosine (greater than 20 microM) caused complete arrest, associated with a hyperpolarization of 12-15 mV. 3. In corresponding whole-cell voltage clamp experiments adenosine activated a time-independent potassium current, IK(ADO), which at -50 mV is approximately 30 pA in normal Tyrode solution and 50 pA in high [K+]o (20 mM) Tyrode solution. This current is similar to the one identified previously in guinea-pig atrium (Belardinelli & Isenberg, 1983a; Kurachi, Nakajima & Sugimoto, 1986). 4. Patch clamp recordings of the single-channel events underlying IK(ADO) showed that they have a conductance of approximately 25.0 +/- 1.9 pS. The whole-cell or macroscopic current, IK(ADO), and the adenosine-induced single-channel events exhibit strong inward-going rectification. 5. Adenosine in doses (10 microM) which significantly activate IK(ADO) failed to produce any measurable effect on the calcium current, ICa, in these isolated cardiac pacemaker cells. However, after ICa has been enhanced by the addition of isoprenaline, adenosine (1-10 microM) caused a significant inhibition: it reduced ICa back to approximately the control levels. 6. A similar 'indirect' effect of adenosine was observed on If, the slow time- and voltage-dependent inward current which is activated by hyperpolarizing these S-A node cells. Adenosine (10(-5) M) failed to influence the control or basal If; however, after If was enhanced by isoprenaline, adenosine markedly inhibited it. 7. These results provide explanations for both the direct and the indirect effects of adenosine in mammalian cardiac pacemaker tissue: activation of IK(ADO), and of a time-independent background potassium current and inhibition of ICa and If, respectively. Since it is known that there is significant adrenergic tone in the mammalian S-A node both the indirect and the direct effects of adenosine may be of physiological importance.

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.

Ionic currents that generate the spontaneous diastolic depolarization in individual cardiac pacemaker cells.

An enzymatic dispersion procedure has been developed to obtain viable, spontaneously active single myocytes from cardiac pacemaker tissue: the bullfrog (Rana catesbeiana) sinus venosus. Recordings of time- and voltage-dependent Ca2+ and K+ currents have been made by using a single suction-microelectrode technique. The results show that two time- and voltage-dependent currents interact to modulate the slope of the pacemaker potential. These are: (i) the decay of a delayed rectifier K+ current and (ii) the activation of a Ca2+ current. In addition, the data strongly suggest that cardiac pacemaker tissue does not have an inwardly rectifying background K+ current.

Intracellular potassium activity in rabbit sinoatrial node. Evaluation during spontaneous activity and arrest.

HomeCirculation ResearchVol. 51, No. 3Intracellular potassium activity in rabbit sinoatrial node. Evaluation during spontaneous activity and arrest. Free AccessAbstractPDF/EPUBAboutView PDFSections ToolsAdd to favoritesDownload citationsTrack citationsPermissions ShareShare onFacebookTwitterLinked InMendeleyReddit Jump toFree AccessAbstractPDF/EPUBIntracellular potassium activity in rabbit sinoatrial node. Evaluation during spontaneous activity and arrest. A O Grant and H C Strauss A O GrantA O Grant Search for more papers by this author and H C StraussH C Strauss Search for more papers by this author Originally published1 Sep 1982https://doi.org/10.1161/01.RES.51.3.271Circulation Research. 1982;51:271–279 Previous Back to top Next FiguresReferencesRelatedDetailsCited By Shi W and Zhou M Optimal Single-Pulse for Pacemakers Based on a Sinoatrial Model, IEEE/ASME Transactions on Mechatronics, 10.1109/TMECH.2011.2176745, 18:1, (348-354) Opthof T (1988) The mammalian sinoatrial node, Cardiovascular Drugs and Therapy, 10.1007/BF02125744, 1:6, (573-597), Online publication date: 1-Mar-1988. Irisawa H (1987) Membrane currents in cardiac pacemaker tissue, Experientia, 10.1007/BF01945510, 43:11-12, (1131-1135), Online publication date: 1-Dec-1987. Campbell T (2012) Differing electrophysiological effects of class IA, IB and IC antiarrhythmic drugs on guinea-pig sinoatrial node, British Journal of Pharmacology, 10.1111/j.1476-5381.1987.tb10294.x, 91:2, (395-401), Online publication date: 1-Jun-1987. Molyvdas P and Sperelakis N (2012) Effects of calcium antagonistic drugs on the electrical activity of rabbit sino-atrial node, British Journal of Pharmacology, 10.1111/j.1476-5381.1986.tb09493.x, 88:1, (249-258), Online publication date: 1-May-1986. (1997) Threshold effects of acetylcholine on primary pacemaker cells of the rabbit sino-atrial node, Proceedings of the Royal Society of London. Series B. Biological Sciences, 10.1098/rspb.1985.0006, 223:1232, (355-378), Online publication date: 22-Jan-1985. Nakayama T and Irisawa H (1985) Transient outward current carried by potassium and sodium in quiescent atrioventricular node cells of rabbits., Circulation Research, 57:1, (65-73), Online publication date: 1-Jul-1985. Molyvdas P and Sperelakis N (1984) Effects of Calcium Antagonistic Drugs on Various Heart Tissues, Including Blockade of the Slow Channels and Depression of Postdrive Hyperpolarization. Calcium Antagonists, 10.1007/978-1-4613-3810-9_12, (153-178), . Sperelakis N (1984) Effects of Calcium Slow Channel Blockers on the Slow Action Potentials of Cardiac Muscle and Vascular Smooth Muscle Calcium Antagonists, 10.1007/978-1-4613-3810-9_3, (21-45), . Baumgarten C, Singer D and Fozzard H (1984) Intra- and extracellular potassium activities, acetylcholine and resting potential in guinea pig atria., Circulation Research, 54:1, (65-73), Online publication date: 1-Jan-1984. September 1, 1982Vol 51, Issue 3 Advertisement Article InformationMetrics Copyright © 1982 by American Heart Associationhttps://doi.org/10.1161/01.RES.51.3.271 Originally publishedSeptember 1, 1982 PDF download Advertisement

A mathematical model of primary pacemaking cell in SA node of the heart.

Application of voltage-clamp techniques to cardiac primary pacemaker tissue is currently a subject of considerable interest in cardiac electrophysiology. Information regarding the electrical behavior of this type of membrane is by no means complete, yet sufficiently detailed information is available in the literature that would allow the formulation of a reasonably quantitative model to represent the electrical activity of primary pacemaker tissue. In this study the well-known McAllister-Noble-Tsien (MNT) model of the cardiac Purkinje fiber is modified to account for the electrical activity of the primary pacemaking cell (P-cell) of the sinoatrial (SA) node in the heart. Modification is accomplished by appropriately deleting (or inactivating) selected inward- or outward-current channels in the MNT model and adjusting the range of voltage dependence of the various channel-gating variables. Wherever possible, this is accomplished by taking into account voltage-clamp and other electrophysiological data from experiments on the SA node. The resultant model mimics published data quite well and is capable of characterizing the free-running behavior of the primary pacemaker as well as its response to injected currents.

Are acetylcholine-induced increases in 42K efflux mediated by intracellular cyclic GMP in turtle cardiac pace-maker tissue?

1. 42K efflux has been measured from small strips of turtle sinus venosus tissue in order: (a) to characterize further the pharmacology of the acetylcholine response and (b) to test whether cyclic guanosine 3':5'-monophosphoric acid (cyclic GMP) is the intracellular mediator of this response. 2. The 42K wash-out curves show that the fractional escape rate (FER) of 42K efflux is nearly constant after 60-80 min, indicating that after this time period 42K FER is controlled by barrier-limited diffusion from a single intracellular compartment. 3. The threshold of the dose-response relationship for the acetylcholine-induced increase in 42K FER is about 10(-8) M and the Km is 2.75 x 10(-7) in non-eserinized preparations. 4. This acetylcholine response is completely blocked by atropine; but nicotinic blockers produce no detectable reduction of it. 5. Exogenous application of lipid-soluble analogues of cyclic GMP (dibutyryl or 8-bromo-cyclic GMP applied at 2-3 mM for 30 min) failed to mimic the acetylcholine-induced augmentation of 42K FER. 6. Experiments in which sodium nitroprusside (5 x 10(-4) M for 30 min) was applied in order to stimulate the guanylate cyclase and hence produce a large, maintained increase in intracellular cyclic GMP failed to show a significant increase in 42K FER. 7. When acetylcholine (10(-6)M) was applied in the presence of O[Ca2+]0 (in an attempt to inhibit the guanylate cyclase) there was no significant reduction in the acetylcholine-induced increases in 42K FER. 8. Hence, these three indirect tests indicate that the muscarinic acetylcholine-induced increase in 42K FER in cardiac pace-maker tissue is unlikely to be mediated entirely by changes in the levels of intracellular cyclic GMP.

Quantitative changes in beta-adrenergic responses of isolated atria from hyper- and hypothyroid rats.

Concentration-response curves for the chronotropic and inotropic effects of isoprenaline, in the absence and presence of propranolol, were obtained on heart atria isolated from normo- or dysthyroid rats. Hyperthyroidism increased the chronotropic potency and efficacy of the beta-adrenergic agonist. The results are compatible with the view that thyroid hormone increases the density of functional beta-adrenoceptors in cardiac pacemaker tissue.