Electrophysiology of the sinoatrial node

Getting to the heart of rhythm: A century of progress

The human heart beats over eighty thousand times a day, and the average person's heart may have beaten up to 3 billion times by the age of 80. During the early stages of pregnancy, the heart beat provides the first visual and auditory sign of life of the foetus. Conversely, the first audible sound that the foetus is likely to hear is the heart beat of the mother. How fitting then, that at the "birth" Physiological Reviews the very first article published in 1921 written by Eyster and Meek addressed "The origin and conduction of the heart beat".¹ In their insightful review, the authors discussed the landmark discoveries made from the mid-19^th century on the electrical function of the heart. Now, a hundred years later, at the start of the next century of Physiological Reviews, an update on the huge progress made in the "exciting" field of cardiac electrophysiology is warranted. Guided by a number of excellent reviews published in Physiological Reviews since 1921 as well as a large body of literature, an overview of the important advancements made on the topic is provided here.

Pacemaking in cardiac tissue. From IK2 to a coupled-clock system

Initially, diastolic depolarization in Purkinje fibers was explained by deactivation of gK2 in the presence of inward current. Weakness of the hypothesis was a too negative reversal potential, sensitivity to external Na+ ions, existence of K+ depletion, and fake current during hyperpolarizing clamps. The development of a sinus node preparation of almost microscopic dimensions allowing uniform voltage clamps created new possibilities. Three different groups discovered in this improved node preparation an hyperpolarization induced time-dependent inward current, with a reversal potential positive to the resting potential, carried by a mixture of Na+ and K+ ions. A new current, If, or funny current was born. It is not the only pacemaker current. The following sequence of currents (membrane clock) has been proposed: diastole starts as a consequence of IK deactivation and If activation; followed by activation of the T-type Ca2+ current, Ca2+ -induced Ca2+ release from the SR, and activation of sodium-calcium exchange current with further depolarization of the membrane till threshold of the L-type Ca2+ current is reached. The release of Ca2+ can also occur spontaneously independently from a T-type Ca2+ current. The system acts then as a primary intracellular clock. The review is completed by description of an evolution in the direction of biological pacing using induced pluripotent stem cells or transcription factors. See also: https://doi.org/10.14814/phy2.13860 & https://doi.org/10.14814/phy2.13861.

SK4 K+ channels are therapeutic targets for the treatment of cardiac arrhythmias

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a stress‐provoked ventricular arrhythmia, which also manifests sinoatrial node (SAN) dysfunction. We recently showed that SK4 calcium‐activated potassium channels are important for automaticity of cardiomyocytes derived from human embryonic stem cells. Here SK4 channels were identified in human induced pluripotent stem cell‐derived cardiomyocytes (hiPSC‐CMs) from healthy and CPVT2 patients bearing a mutation in calsequestrin 2 (CASQ2‐D307H) and in SAN cells from WT and CASQ2‐D307H knock‐in (KI) mice. TRAM‐34, a selective blocker of SK4 channels, prominently reduced delayed afterdepolarizations and arrhythmic Ca²⁺ transients observed following application of the β‐adrenergic agonist isoproterenol in CPVT2‐derived hiPSC‐CMs and in SAN cells from KI mice. Strikingly, in vivo ECG recording showed that intraperitoneal injection of the SK4 channel blockers, TRAM‐34 or clotrimazole, greatly reduced the arrhythmic features of CASQ2‐D307H KI and CASQ2 knockout mice at rest and following exercise. This work demonstrates the critical role of SK4 Ca²⁺‐activated K⁺ channels in adult pacemaker function, making them promising therapeutic targets for the treatment of cardiac ventricular arrhythmias such as CPVT.

SK4 Ca2+ activated K+ channel is a critical player in cardiac pacemaker derived from human embryonic stem cells

Proper expression and function of the cardiac pacemaker is a critical feature of heart physiology. Two main mechanisms have been proposed: (i) the "voltage-clock," where the hyperpolarization-activated funny current If causes diastolic depolarization that triggers action potential cycling; and (ii) the "Ca(2+) clock," where cyclical release of Ca(2+) from Ca(2+) stores depolarizes the membrane during diastole via activation of the Na(+)-Ca(2+) exchanger. Nonetheless, these mechanisms remain controversial. Here, we used human embryonic stem cell-derived cardiomyocytes (hESC-CMs) to study their autonomous beating mechanisms. Combined current- and voltage-clamp recordings from the same cell showed the so-called "voltage and Ca(2+) clock" pacemaker mechanisms to operate in a mutually exclusive fashion in different cell populations, but also to coexist in other cells. Blocking the "voltage or Ca(2+) clock" produced a similar depolarization of the maximal diastolic potential (MDP) that culminated by cessation of action potentials, suggesting that they converge to a common pacemaker component. Using patch-clamp recording, real-time PCR, Western blotting, and immunocytochemistry, we identified a previously unrecognized Ca(2+)-activated intermediate K(+) conductance (IK(Ca), KCa3.1, or SK4) in young and old stage-derived hESC-CMs. IK(Ca) inhibition produced MDP depolarization and pacemaker suppression. By shaping the MDP driving force and exquisitely balancing inward currents during diastolic depolarization, IK(Ca) appears to play a crucial role in human embryonic cardiac automaticity.

Aging is a primary risk factor for cardiac arrhythmias: disruption of intracellular Ca2+ regulation as a key suspect

Aging is an inevitable time-dependent progression associated with a functional decline of the cardiovascular system even in 'healthy' individuals. Age positively correlates with an increasing risk of cardiac problems including arrhythmias. Not only the prevalence but also the severity of arrhythmias escalates with age. The reasons for this are multifactorial but dysregulation of intracellular calcium within the heart is likely to play a key role in initiating and perpetuating these life-threatening events. We now know that several aspects of cardiac calcium regulation significantly change with advancing age - changes that could produce electrical instability. Further development of knowledge of the mechanisms underlying these changes will allow us to reduce what currently is an inevitable increase in the incidence of arrhythmias in the elderly.

Calcium oscillations and pacemaking

Calcium plays important role in biological systems where it is involved in diverse mechanisms such as signaling, muscle contraction and neuromodulation. Action potentials are generated by dynamic interaction of ionic channels located on the plasma-membrane and these drive the rhythmic activity of biological systems such as the smooth muscle and the heart. However, ionic channels are not the only pacemakers; an intimate interaction between intracellular Ca(2+) stores and ionic channels underlie rhythmic activity. In this review we will focus on the role of Ca(2+) stores in regulation of rhythmical behavior.

Bifurcation analysis and effects of changing ionic conductances on pacemaker rhythm in a sinoatrial node cell model

The electrical excitation (action potential generation) of sinoatrial node (cardiac pacemaker) cells is directly related to various ion channels (pore-forming proteins) in cell membranes. In order to analyze the relation between action potential generation and ion channels, we use the Yanagihara-Noma-Irisawa (YNI) model of sinoatrial node cells, which is described by the Hodgkin-Huxley-type equations with seven variables. In this paper, we analyze the global bifurcation structure of the YNI model by varying various conductances of ion channels, and examine the effects of these conductance changes on pacemaker rhythm (frequency of action potential generation). The coupling effect on pacemaker rhythm is also examined approximately by applying external current to the YNI model.

Genesis and regulation of the heart automaticity

The heart automaticity is a fundamental physiological function in higher organisms. The spontaneous activity is initiated by specialized populations of cardiac cells generating periodical electrical oscillations. The exact cascade of steps initiating the pacemaker cycle in automatic cells has not yet been entirely elucidated. Nevertheless, ion channels and intracellular Ca(2+) signaling are necessary for the proper setting of the pacemaker mechanism. Here, we review the current knowledge on the cellular mechanisms underlying the generation and regulation of cardiac automaticity. We discuss evidence on the functional role of different families of ion channels in cardiac pacemaking and review recent results obtained on genetically engineered mouse strains displaying dysfunction in heart automaticity. Beside ion channels, intracellular Ca(2+) release has been indicated as an important mechanism for promoting automaticity at rest as well as for acceleration of the heart rate under sympathetic nerve input. The potential links between the activity of ion channels and Ca(2+) release will be discussed with the aim to propose an integrated framework of the mechanism of automaticity.

Epoxygenases and peroxisome proliferator-activated receptors in mammalian vascular biology

Epoxygenases, particularly of the CYP2C and CYP2J families, are important lipid-metabolizing enzymes. Epoxygenases are found throughout the cardiovascular system where their lipid products, particularly the epoxyeicosatrienoic acids (EETs), which are arachidonic acid metabolites, have the potential to regulate vascular tone, cellular proliferation, migration, inflammation and cardiac function. The receptors for EETs are, however, poorly understood. The peroxisome proliferator-activated receptors (PPARs) are a family of three (alpha, beta/delta and gamma) nuclear receptors that are activated by lipid metabolites. Activation of PPAR alpha and PPAR gamma, similar to the longer term effects of EETs, causes the inhibition of vascular cell proliferation, migration and inflammation. Interestingly, EETs and their metabolites have recently been found to active both PPAR alpha and PPAR gamma. The epoxygenase-EET-PPAR pathway may therefore represent a novel endogenous protective pathway by which short-lived lipid mediators control vascular cell activation.

[Sinus rhythm: mechanisms and function]

The normal cardiac rhythm originates in a specialized region of the heart, the sinus node that is part of the nodal tissue. The rhythmic, impulse initiation of sinus node pacemaker cells results from a spontaneous diastolic depolarization that is initiated immediately after repolarization of the preceding actions potential. This slow diastolic depolarisation is typical of automatic cells and essential to their function. Several currents are involved in this diastolic depolarisation: a hyperpolarization activated inward current, termed "pacemaker" I(f) current, two Ca2+ currents (a L type and a T type), a delayed K+ current and a Na/Ca exchange current. The frequency of the automatic discharge is the main determinant of heart rate. However the sinus node activity is regulated by adrenergic and cholinergic neurotransmitters. Acetylcholine provokes the hyperpolarization of pacemaker cells and decreases the speed of the spontaneous diastolic depolarisation, thus slowing the sinus rate. Catecholamines lead to sinus tachycardia by increasing the diastolic depolarisation speed. In normal conditions, the observed resting heart rate is lower than the intrinsic frequency of the sinus node due to a "predominance" of the vagal tone. Neural regulation of the heart rate aims at meeting the metabolic needs of the tissues through a varying blood flow. Differences between diurnal and nocturnal mean heart rates are accounted for by neural influences. During the night, the increased vagal tone results in decreased heart rate. The exercise-induced tachycardia results from the sympathetic stimulation. It allows more blood to reach skeletal muscles, and as a consequence an increased supply of oxygen and nutrients. Compared to the variety of clinical arrhythmias, sinus rhythm is the basis for optimal exercise capacity and quality of life.

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.

Declining into failure: the age-dependent loss of the L-type calcium channel within the sinoatrial node

BACKGROUND

The spontaneous activity of pacemaker cells in the sinoatrial (SA) node controls heart rate under normal physiological conditions. Clinical studies have shown the incidence of SA node dysfunction increases with age and occurs with peak prevalence in the elderly population. The present study investigated whether aging affected the expression of Ca(v)1.2 channels and whether these changes could affect pacemaker activity, in turn leading to age-related SA node degeneration.

METHODS AND RESULTS

The SA node region was isolated from the right atrium of guinea pigs between birth and 38 months of age. Immunofluorescence studies showed Ca(v)1.2 protein was present as punctate labeling around the outer membrane of atrial cells but was absent from the center of the SA node. The area lacking Ca(v)1.2-labeled protein progressively increased from 2.06+/-0.1 (mean+/-SEM) mm2 at 1 month to 18.72+/-2.2 mm2 at 38 months (P<0.001). Western blot provided verification that Ca(v)1.2 protein expression within the SA node declined during aging. Functional measurements showed an increased sensitivity to the L-type calcium blocker nifedipine; SA node preparations stopped beating in 100 micromol/L nifedipine at 1 day old, compared with 30 micromol/L at 1 month and 10 micromol/L at 38 months of age. Furthermore, the amplitude of extracellular potentials declined within the center and periphery of the SA node during aging.

CONCLUSIONS

The present data show Ca(v)1.2 channel protein decreases concurrently with reduced spontaneous activity of the SA node with increased age, which provides further evidence of mechanisms underlying the age-related deterioration of the cardiac pacemaker.

Enrichment of cardiac pacemaker-like cells: neuregulin-1 and cyclic AMP increase I(f)-current density and connexin 40 mRNA levels in fetal cardiomyocytes

Generation of a large number of cells belonging to the cardiac pacemaker system would constitute an important step towards their utilization as a biological cardiac pacemaker system. The aim of the present study was to identify factors, which might induce transformation of a heterogenous population of fetal cardiomyocytes into cells with a pacemaker-like phenotype. Neuregulin-1 (alpha- and beta-isoform) or the cAMP was added to fresh cell cultures of murine embryonic cardiomyocytes. Quantitative northern blot analysis and flowcytometry were performed to detect the expression of connexins 40, 43 and 45. Patch clamp recordings in the whole cell configuration were performed to determine current density of I (f), a characteristic ion current of pacemaker cells. Fetal cardiomyocytes without supplement of neuregulin or cAMP served as control group. Neuregulin and cAMP significantly increased mRNA levels of connexin 40 (Cx-40), a marker of the early differentiating conduction system in mice. On the protein level, flowcytometry revealed no significant differences between treated and untreated groups with regard to the expression of connexins 40, 43 and 45. Treatment with cAMP (11.2 +/- 2.24 pA/pF; P < 0.001) and neuregulin-1-beta (6.23 +/- 1.07 pA/pF; P < 0.001) significantly increased the pacemaker current density compared to control cardiomyocytes (1.76 +/- 0.49 pA/pF). Our results indicate that neuregulin-1 and cAMP possess the capacity to cause significant transformation of a mixed population of fetal cardiomyocytes into cardiac pacemaker-like cells as shown by electrophysiology and increase of Cx-40 mRNA. This method may allow the development of a biological cardiac pacemaker system when applied to adult or embryonic stem cells.

Fundamental importance of Na+-Ca2+ exchange for the pacemaking mechanism in guinea-pig sino-atrial node

Na+-Ca2+ exchange (NCX) current has been suggested to play a role in cardiac pacemaking, particularly in association with Ca2+ release from the sarcoplasmic reticulum (SR) that occurs just before the action potential upstroke. The present experiments explore in more detail the contribution of NCX to pacemaking. Na+-Ca2+ exchange current was inhibited by rapid switch to low-Na+ solution (with Li+ replacing Na+) within the time course of a single cardiac cycle to avoid slow secondary effects. Rapid switch to low-Na+ solution caused immediate cessation of spontaneous action potentials. ZD7288 (3 microM), to block I(f) (funny current) channels, slowed but did not stop the spontaneous activity, and tetrodotoxin (10 microM), to block Na+ channels, had little effect, but in the presence of either of these agents, rapid switch to low-Na+ solution again caused immediate cessation of spontaneous action potentials. Spontaneous electrical activity was also stopped following loading of the cells with the Ca2+ chelators BAPTA and EGTA, and by exposure to the NCX inhibitor KB-R7943 (5 microM). When rapid switch to low-Na+ solution caused cessation of spontaneous activity, this was found (using confocal microscopy, with fluo-4 as the Ca2+ probe) to be accompanied by an initial fall in cytosolic [Ca2+], with subsequent appearance of Ca2+ waves. Inhibition of SR Ca2+ uptake with cyclopiazonic acid (CPA, 30 microM) slowed but did not stop spontaneous activity. Rapid switch to low-Na+ solution in the presence of CPA caused abolition of spontaneous Ca2+ transients and a progressive rise in cytosolic [Ca2+]. With ratiometric fluorescence methods (indo-5F as the Ca2+ probe), the minimum level of [Ca2+] between beats was found to be approximately 225 nM, and abolition of beating with nifedipine, acetylcholine or adenosine caused a fall in cytosolic [Ca2+] below this level. These observations support the hypothesis that NCX current is essential for normal pacemaker activity under the conditions of our experiments. A continuous depolarizing influence of current through the NCX protein might result from maintained electrogenic NCX (with 3:1 stoichiometry, supported by a cytosolic [Ca2+] that normally does not fall below 225 nM between beats) and/or from a novel, recently suggested role of the NCX protein to allow a Na+ leak pathway.

Requirement of neuronal- and cardiac-type sodium channels for murine sinoatrial node pacemaking

The majority of Na+ channels in the heart are composed of the tetrodotoxin (TTX)-resistant (KD, 2-6 microm) Nav1.5 isoform; however, recently it has been shown that TTX-sensitive (KD, 1-10 nm) neuronal Na+ channel isoforms (Nav1.1, Nav1.3 and Nav1.6) are also present and functionally important in the myocytes of the ventricles and the sinoatrial (SA) node. In the present study, in mouse SA node pacemaker cells, we investigated Na+ currents under physiological conditions and the expression of cardiac and neuronal Na+ channel isoforms. We identified two distinct Na+ current components, TTX resistant and TTX sensitive. At 37 degrees C, TTX-resistant iNa and TTX-sensitive iNa started to activate at approximately -70 and approximately -60 mV, and peaked at -30 and -10 mV, with a current density of 22 +/- 3 and 18 +/- 1 pA pF(-1), respectively. TTX-sensitive iNa inactivated at more positive potentials as compared to TTX-resistant iNa. Using action potential clamp, TTX-sensitive iNa was observed to activate late during the pacemaker potential. Using immunocytochemistry and confocal microscopy, different distributions of the TTX-resistant cardiac isoform, Nav1.5, and the TTX-sensitive neuronal isoform, Nav1.1, were observed: Nav1.5 was absent from the centre of the SA node, but present in the periphery of the SA node, whereas Nav1.1 was present throughout the SA node. Nanomolar concentrations (10 or 100 nm) of TTX, which block TTX-sensitive iNa, slowed pacemaking in both intact SA node preparations and isolated SA node cells without a significant effect on SA node conduction. In contrast, micromolar concentrations (1-30 microm) of TTX, which block TTX-resistant iNa as well as TTX-sensitive iNa, slowed both pacemaking and SA node conduction. It is concluded that two Na+ channel isoforms are important for the functioning of the SA node: neuronal (putative Nav1.1) and cardiac Nav1.5 isoforms are involved in pacemaking, although the cardiac Nav1.5 isoform alone is involved in the propagation of the action potential from the SA node to the surrounding atrial muscle.

Mechanisms of adrenergic control of sino-atrial node discharge

Among the mechanisms proposed for the increase in discharge of sino-atrial node (SAN) by norepinephrine (NE) are an increase in the hyperpolarization-activated current I(f) and in the slow inward current I(Ca,L). If I(f) is the primary mechanism, cesium (a blocker of I(f)) should eliminate the positive chronotropic effect of NE. If I(Ca,L), is involved, [Ca(2+)](o) should condition NE effects. We studied the electrophysiological changes induced by NE in isolated guinea pig SAN superfused in vitro with Tyrode solution (both SAN dominant and subsidiary pacemaker mechanisms are present) as well as with high [K(+)](o), higher Cs(+) or Ba(2+) (only the dominant pacemaker mechanism is present). In Tyrode solution, NE (0.5-1microM) increased the SAN rate and adding Cs(+) (approximately 12 mM) caused a decaying voltage tail during diastole in subsidiary pacemakers. NE enhanced the Cs(+)-induced tail, and increased the rate but less than in Tyrode solution. In higher [Cs(+)](o) (15- 18 mM), Ba(2+) (1 mM) or Ba(2+) plus Cs(+) (10 mM) dominant action potentials (not followed by a tail) were present and NE accelerated them as in Tyrode solution. In high [K(+)](o), NE increased the rate in the absence and presence of Cs(+), Ba(2+) or Ba(2+) plus Cs(+). In these solutions, NE increased the overshoot and maximum diastolic potential of dominant action potentials (APs) and increased the rate by steepening diastolic depolarization and shifting the threshold for upstroke to more negative values. High [Ca(2+)](o) alone increased the rate and NE enhanced this action, whereas low [Ca(2+)](o) reduced or abolished the increase in rate by NE. In SAN quiescent in high [K(+)](o) plus indapamide, NE induced spontaneous discharge by decreasing the resting potential and initiating progressively larger voltage oscillations. Thus, NE increases the SAN rate by acting primarily on dominant APs in a manner consistent with an increase of I(Ca,L) and I(K) and under conditions where I(f) is either blocked or not activated. NE INITIATES spontaneous discharge by inducing voltage oscillations unrelated to I(f).

Potentiation by neostigmine of responses to vagal nerve stimulation in the sinus venosus of the toad

The effects of the cholinesterase inhibitor neostigmine on the responses to vagus nerve stimulation of isolated sinus venosus/atrial preparations of the toad, Bufo marinus, were examined. In control solutions, trains of stimuli applied to the vagus nerve led to a decrease in heart rate that was susceptible to muscarinic receptor blockade. Membrane potential recordings made from sinus venosus cells showed that the responses to trains of stimuli, delivered at frequencies of less than 10 Hz, were little changed by the addition of neostigmine. However, the responses to longer trains of stimuli at 10 Hz (30 versus 10 s) were potentiated and the nature of the membrane potential changes was altered. The results suggest that, due to the activity of cholinesterases, acetylcholine (ACh) released from parasympathetic nerves normally has little access to the muscarinic receptors in the pacemaker region, which are linked to potassium channels.

Permanent pacemakers in older persons

OBJECTIVE

To review (1) the physiologic changes of aging that may lead to the need for a permanent pacemaker; (2) the current standard indications for pacemaker implantation as reported in expert guidelines; (3) newer investigational uses of pacemakers; (4) advances in pacemaker technology; and (5) cost-effectiveness of permanent pacing.

DATA SOURCES

Computer-assisted search of the English language literature (MEDLINE database), manual search of articles bibliographies, and review of data provided by a major pacemaker manufacturer.

DESIGN

Pertinent articles were reviewed and data extracted. Studies and data involving older persons were emphasized, and these data were extracted and presented.

RESULTS

Abnormalities in impulse generation and conduction are common in older people and form the substrate for the need of pacemaker implantation. Pacemaker use is high in older people, with an estimated 70 to 80% of all permanent pacemakers implanted in individuals aged 65 years and older. The hemodynamic changes of aging include a reduction of ventricular compliance and increased contribution of atrial contraction to ventricular filling. Pacemakers that maintain synchrony between atria and ventricles may, therefore, be particularly advantageous in older adults. Recent studies have validated this theoretical reasoning. Chronotropic incompetence is common in older people, and rate responsive ventricular pacing has been shown to improve quality of life compared with fixed rate devices in older patients. Sequential, dual chamber pacemakers reduce the symptoms of pacemaker syndrome and recurrences of atrial fibrillation in certain groups of patients. Potential utility of permanent pacing is being investigated in patients with severe left ventricular dysfunction, markedly prolonged atrioventricular conduction time, hypertrophic and dilated cardiomyopathy, and after cardiac transplantation. Biventricular pacing as therapy for severe heart failure is in the very early phases of investigation. Newer implantable pacemakers provide a host of technological options but are somewhat more expensive and require more frequent follow-up. Controversies still exist regarding the need for pacemakers in certain clinical conditions but are decreasing as new high quality studies are completed.

CONCLUSIONS

Permanent pacing is highly cost-effective, safe, and simple to perform. Pacemakers are implanted in patients with sinus node dysfunction, acquired (both native and postsurgical) atrioventricular block, some forms of neurally mediated syndromes, fascicular blocks, and, occasionally, for the prevention of supraventricular or ventricular tachyarrhythmias. Although pacemakers are implanted in individuals of all ages, they are most often utilized in older adults; it is estimated that 70 to 80% of all pacemakers are implanted in patients 65 years of age or older. This is attributable to an increase in abnormalities of impulse generation and conduction with advancing age. Dual chamber pacemakers that maintain synchrony between atria and ventricles are preferable in older patients because of the increased contribution of atrial contraction to ventricular filling in this age group. This theoretical advantage has been confirmed by prospective studies in limited patient subgroups.

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.

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.

Inhibition of cardiac L-type calcium channels by epoxyeicosatrienoic acids

Epoxyeicosatrienoic acids (EETs), products of the cytochrome P-450 monooxygenase metabolism of arachidonic acid, can regulate the activity of ion channels. We examined the effects of EETs on cardiac L-type Ca2+ channels that play important roles in regulating cardiac contractility, controlling heart rate, and mediating slow conduction in normal nodal cells and ischemic myocardium. Our experimental approach was to reconstitute porcine L-type Ca2+ channels into planar lipid bilayers where we could control the aqueous and lipid environments of the channels and the regulatory pathways that change channel properties. We found that 20 to 125 nM EETs inhibited the open probability of reconstituted L-type Ca2+ channels, accelerated the inactivation of the channels, and reduced the unitary current amplitude of open channels. There was no selectivity among different EET regioisomers or stereoisomers. When 11,12-EET was esterified to the sn-2 position of phosphatidylcholine, restricting it to the hydrophobic phase of the planar lipid bilayer, the reconstituted channels were similarly inhibited, suggesting that the EET interacts directly with Ca2+ channels through the lipid phase. The inhibitory effects of EET persisted in the presence of microcystin, an inhibitor of protein phosphatases 1 and 2A, suggesting that dephosphorylation was not the mechanism through which these eicosanoids down-regulate channel activity. This inhibition may be an important protective mechanism in the setting of cardiac ischemia where arachidonic acid levels are dramatically increased and EETs have been shown to manifest preconditioning-like effects.

Role of nitric oxide production in carbachol-induced negative chronotropy in cultured rat ventricular myocytes

It has been reported that nitric oxide (NO) plays a physiological role in mediating the effect of vagal stimulation in the autonomic regulation of the heart. In this study, the changes in NO production induced by carbachol were investigated by measuring the NO metabolites, nitrite (NO2-) and nitrate (NO3-), with a high-performance liquid chromatography-Griess reaction system, and the carbachol-induced chronotropic response was simultaneously investigated. Cultured rat ventricular myocytes exhibited a dose-dependent negative chronotropic response and NO metabolite production in response to carbachol. The negative chronotropy and the enhancement of NO metabolite production induced by 10(-4) M carbachol were completely abolished by 10(-6) M atropine. Both of these effects of carbachol were completely abolished by NO synthase inhibitors such as 3 X 10(-4) M NG-monomethyl-L-arginine acetate and 10(-5) M methylene blue. Furthermore, the negative chronotropic effect induced by 10(-4) M carbachol was also abolished by 10(-6) M 1 H-[1,2,4]oxadiazolo[4,3-alpha]quanoxalin-1-one, a selective guanylyl cyclase inhibitor. In addition, 10(-4) M 8-bromoguanosine 3':5'-cyclic monophosphate, a cell-permeable analogue of guanosine 3':5'-cyclic monophosphate, caused a negative chronotropic effect. These results suggest that the NO-signaling pathway may play an important role in the muscarinic cholinergic regulation of myocardial function.

Inhibition of myocardial L- and T-type Ca2+ currents by efonidipine: possible mechanism for its chronotropic effect

Effects of efonidipine, a dihydropyridine phosphonate Ca2+ channel antagonist, on the guinea-pig heart were compared with those of nifedipine. In the sino-atrial node, 1 microM efonidipine produced increase in cycle length accompanied by prolongation of the phase 4 depolarization which was not prominent with 0.1 microM nifedipine. In ventricular myocytes, both efonidipine and nifedipine produced inhibition of the L-type Ca2+ current, nifedipine being tenfold more potent than efonidipine. Efonidipine also inhibited the T-type Ca2+ current at higher concentrations but nifedipine did not. Both Ca2+ channel antagonists had no or only a weak effect on K+ currents. In addition, 40 microM Ni2+, which selectively inhibited the T-type Ca2+ current, had no effect on myocardial Ca2+ transients and contractile force. In conclusion, efonidipine was shown to have inhibitory effects on both L- and T-type Ca2+ currents, which may contribute to its high negative chronotropic potency.

Distribution of atrial and nodal cells within the rabbit sinoatrial node: models of sinoatrial transition

BACKGROUND

In the sinoatrial node (SAN) the course of the action potential gradually changes from the primary pacemaker region toward the atrium. It is not known whether this gradient results from different intrinsic characteristics of the nodal cells, from an increasing electrotonic interaction with the atrium, or from both. Therefore we have characterized the immunohistochemical, morphological, and electrophysiological correlates of this functional gradient.

METHODS AND RESULTS

The distribution of rabbit nodal myocytes in the SAN has been studied by immunohistochemistry. After cell isolation, the electrophysiological characteristics of different nodal cell types were measured. (1) The staining pattern of a neurofilament protein coincides with the electrophysiologically mapped pacemaker region in the SAN. (2) Enzymatic digestion of the SAN reveals three morphologically different nodal cell types and one atrial type. Of each nodal cell type, neurofilament-positive as well as neurofilament-negative myocytes are found. Atrial cells are all neurofilament-negative. (3) In contrast to previous findings, we observed atrial cells in the very center of the SAN. The relative number of atrial cells gradually increases from the central pacemaker area toward the atrium. (4) Differences in electrophysiological characteristics between individual nodal cells are not associated with differences in cell type.

CONCLUSIONS

(1) The expression of neurofilaments can be used to delineate the nodal area in the intact SAN but is not sufficiently sensitive for characterizing all individual isolated nodal cells. (2) A fundamentally different organization of the SAN is presented: The gradual increase in density of atrial cells from the dominant area toward the crista terminalis in the SAN causes a gradual increase of atrial electrotonic influence that may be an important cause of the gradual transition of the nodal to the atrial type of action potential.

Effects of Ca2+ channel antagonists on sinus node: prolongation of late phase 4 depolarization by efonidipine

Effects of various Ca2+ channel antagonists on the action potential configuration of rabbit sino-atrial node tissue were examined with standard microelectrode techniques. All Ca2+ channel antagonists decreased the maximum rate of phase 0 depolarization (Vmax) and increased the cycle length. The potency order to increase the cycle length was nisoldipine = verapamil > nifedipine = clentiazem > efonidipine > diltiazem. The potency order to decrease Vmax and to shift the threshold potential to a positive direction was the same as that to increase the cycle length, indicating that the major mechanism of negative chronotropism was inhibition of the L-type Ca2+ current. All Ca2+ channel antagonists except efonidipine shifted the maximum diastolic potential to the positive direction, decreased the action potential amplitude and prolonged the action potential duration. The effects of nifedipine were slightly weaker than those of other drugs when compared at equally bradycardiac concentrations. These differences may reflect differences in drug effects on currents other than the L-type Ca2+ current. A characteristic feature of efonidipine was selective suppression of the later phase of pacemaker depolarization with no effect on action potential amplitude and duration. Similar suppression of the later phase was observed with 50 microM Ni2+, which is reported to inhibit the T-type, but not L-type, Ca2+ current. Thus, efonidipine appears to suppress selectively the later phase of pacemaker depolarization through inhibition of both L- and T-type Ca2+ currents, which may be the underlying mechanism for its reported potent negative chronotropic but weak inotropic activity.

Atrioventricular junctional rhythm induced by sympathetic stimulation in E-4031-treated dog hearts

To investigate the role of delayed rectifier potassium current (IK) on the sympathetic control of the heart, we studied the effects of E-4031, a blocker of the rapidly activating type of IK (IKr), on the chronotropic, dromotropic, and inotropic responses to sympathetic nerve stimulation in the autonomically decentralized hearts of open-chest anesthetized dogs, E-4031 (0.01-3 mumol/kg intravenously, i.v.) decreased the heart rate (HR) dose dependently without affecting other cardiac functions. After E-4031 treatment, cardiac sympathetic nerve stimulation changed the sinus rhythm to the atrioventricular (AV) junctional rhythm in 6 of 11 anesthetized dogs (55%). In three of six junctional rhythm hearts, sinus rhythm supervened during sympathetic stimulation for 2 min. The number of pacemaker shifts to junctional rhythm increased as the dose of E-4031 was increased. However, E-4031 attenuated neither the positive chronotropic, dromotropic, nor right atrial and ventricular inotropic responses to sympathetic nerve stimulation. These results suggest that IKr inhibition may induce the AV junctional rhythm due to the combination of the different participation of IKr, the different resting potentials, and the different sensitivity to sympathetic activation among cardiac pacemaker cells.

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.

Role of T-type Ca2+ channel inhibitors in the pacemaker depolarization in rabbit sino-atrial nodal cells

1. Effects of T-type Ca2+ channel inhibitors, Ni2+ and tetramethrin, on the spontaneous action potentials in rabbit sino-atrial nodal cells were examined. 2. The firing rate of spontaneous activity was 201 +/- 11 beats/min (n = 18). Experiments were performed at 36 degrees C. 3. Ni2+ (10(-5) to 10(-4) M) and tetramethrin (10(-7) to 5 x 10(-5) M) caused a negative chronotropic effect. Both inhibitors did not affect the maximum diastolic potential, and slowed the rate of depolarization during the diastole. 4. In the presence of TTX (10(-7) M), both inhibitors caused a stronger negative chronotropic effect, and hyperpolarized the maximum diastolic potential. The maximum rate of depolarization was enhanced, and the action potential duration (at 50% repolarization) was prolonged. The action potential amplitude was unaffected. Ni2+ had more potent actions than tetramethrin. 5. T-type and other Ca2+ channel inhibitors affected only the late phase of pacemaker potential, resulting in a negative chronotropic effect. 6. These results indicate that T-type Ca2+ channel inhibitors (Ni2+ and tetramethrin) slow the pacemaker depolarization at the late phase (but not at the initial phase), resulting in a negative chronotropic effect in the sino-atrial nodal cells.

Effects of delayed rectifier current blockade by E-4031 on impulse generation in single sinoatrial nodal myocytes of the rabbit

The role of the delayed rectifier current (IK) in impulse generation was studied in single sinoatrial nodal myocytes of the rabbit. We used the class III antiarrhythmic drug E-4031, which blocks IK in rabbit ventricular myocytes. In single sinoatrial nodal cells, E-4031 (0.1 mumol/L) significantly prolonged cycle length and action potential duration, depolarized maximum diastolic potential, and reduced both the upstroke velocity of the action potential and the diastolic depolarization rate. Half of the cells were arrested completely. At higher concentrations (1 and 10 mumol/L), spontaneous activity ceased in all cells. Three ionic currents fundamental for pacemaking, ie, IK, the long-lasting inward calcium current (ICa,L), and the hyperpolarization-activated current (I(f)), were studied by using the whole-cell and amphotericin-perforated patch technique. E-4031 blocked part of the outward current during depolarizing steps as well as the tail current upon subsequent repolarization (ITD) in a dose-dependent manner. E-4031 (10 mumol/L) depressed ITD (88 +/- 4%) (n = 6), reduced peak ICa,L at 0 mV (29 +/- 15%) (n = 4), but did not affect I(f). Lower concentrations did not affect ICa,L. Additional use of 5 mumol/L nifedipine demonstrated that ITD is carried in part by a calcium-sensitive current. Interestingly, complete blockade of IK and ICa,L unmasked the presence of a background current component with a reversal potential of -32 +/- 5.4 mV (n = 8) and a conductance of 39.5 +/- 5.6 pS/pF, which therefore can contribute both to the initial part of repolarization and to full diastolic depolarization.(ABSTRACT TRUNCATED AT 250 WORDS)

Depression of the spontaneous activity by phorbol esters in young embryonic chick cardiomyocytes

Effects of phorbol esters, 12-O-tetradecanoyl-phorbol-13-acetate (TPA) and 4-beta-phorbol-12,13-dibutyrate (PDB), that stimulate protein kinase C (PK-C) on the spontaneous action potential and the ionic currents in cultured embryonic chick ventricular cardiomyocytes were examined using whole-cell voltage-clamp and current-clamp modes. Experiments were performed at room temperature (22 degrees C). The firing rate of spontaneous activity was 61.7 +/- 1.6 beats/min (n = 12). Phorbol esters (100 nM and 1 microM) caused a negative chronotropic effect, and they inhibited the maximum rate of depolarization and the action potential amplitude. The action potential duration (at 50% repolarization) tended to decrease, and the maximum diastolic potential was unaffected. In whole-cell voltage-clamp experiments, both TPA and PDB inhibited the L-type Ca2+ current and the delayed rectifier K+ current. The fast time constant of the inactivation phase for the Ca2+ current was decreased, but the slow component was unaffected. In addition, PDB (100 nM) enhanced the T-type Ca2+ current, accompanied with an increase in its time constant. In contrast, 4-alpha-phorbol-12,13-didecanoate (PDD), an inactive analogue on PK-C, failed to produce significant changes. These results suggest that the PK-C stimulation induced by phorbol esters might affect the ionic currents and modulate the [Ca]i, resulting in regulation of the spontaneous activity of embryonic chick heart cells.

Electrophysiological actions of taurine on spontaneously beating rabbit sino-atrial nodal cells

Effects of taurine on the spontaneous action potentials in rabbit sino-atrial nodal cells were examined at different extracellular Ca2+ concentrations ([Ca]o). Experiments were performed at 36 degrees C. The firing rate of spontaneous activity was 132.5 +/- 12.1 beats/min (n = 18) in normal Tyrode's solution ([Ca]o = 1.8 mM). Increasing [Ca]o level from 0.9 to 10.8 mM significantly changed the maximum rate of depolarization. Other parameters of the action potentials were unaffected. When [Ca]o was 0.9 mM, application of taurine (1 to 20 mM) tended to cause a positive chronotropic effect and hyperpolarized the maximum diastolic potential. In the normal solution (at 1.8 mM [Ca]o), taurine significantly enhanced only the maximum rate of depolarization. In contrast, under high [Ca]o (5.4 and 10.8 mM), taurine at 1 and 5 mM had a negative chronotropic effect, but 20 mM taurine had a positive chronotropic effect. Also, taurine shortened the action potential duration and hyperpolarized the maximum diastolic potential. The maximum rate of depolarization was inhibited. In 10.8 mM [Ca]o solution, irregular spontaneous activity (dysrhythmias) occurred in 4 of 6 preparations, and addition of taurine (1 to 20 mM) abolished it. These results indicate that taurine modulates the action potential configuration in the sino-atrial nodal cells dependent on [Ca]o.

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.

Permanent cardiac pacemakers in the elderly

OBJECTIVE

To review (1) Changes in cardiac impulse generation, conduction, and ventricular filling in normal aging and disease; (2) Pacemaker technology and nomenclature; (3) Expert guidelines about pacemaker use; (4) Studies of pacemaker effectiveness and utilization.

DESIGN

Articles were identified through a Medline search, review of articles' bibliographies, and contact with pacemaker manufacturer representatives for information on device features and costs. These articles were reviewed, and the relevant data are presented.

RESULTS

Abnormalities in impulse generation and conduction are common in the elderly. Pacemaker use is higher in the elderly than in other population groups. Hemodynamic changes associated with aging include an increased contribution of atrial contraction to ventricular filling. Pacemakers, which maintain the synchrony between the atria and ventricles, may be particularly advantageous in the elderly for this reason. Rate-responsive ventricular pacemakers improve the quality of life compared with fixed rate devices in some patients over the age of 75. Dual-chamber, sequential pacemakers are more likely to reduce symptoms of pacemaker syndrome than ventricular pacemakers and probably also prolong survival and reduce risk of atrial fibrillation in certain groups of patients. However, dual chamber devices are more expensive and require more frequent follow-up. Pacemaker utilization can vary widely by region. Decisions about pacemakers require explicit tradeoffs between risk and quality of life on one hand and cost on the other. In many clinical situations, there is controversy as to whether pacemakers should be used.

CONCLUSIONS

Pacemakers provide definite benefits to some patients, whereas in others, the likelihood of benefit is uncertain. More sophisticated devices may provide some additional benefit, but they are more costly. Further data is still required to define precisely which groups of patients substantially benefit from complex and expensive pacing modalities compared with simpler ones.

Cloning and functional expression of a cardiac inward rectifier K+ channel

We have isolated a cDNA coding for an inward rectifier K+ channel (RBHIK1) from rabbit heart. The cloned cDNA encodes a protein of 427 amino acids with two putative transmembrane segments. The primary structure of RBHIK1 is highly homologous to that of IRK1 which is an inward rectifier K+ channel recently cloned from mouse macrophage by expression cloning. When expressed in Xenopus oocytes, RBHIK1 current showed strong inward rectification and was inhibited by extracellular Ba2+ and Cs+. RNA blot analysis revealed the expression of RBHIK1 mRNA in various rabbit tissues, especially high level in the ventricular muscle.

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.

Effects of sympathetic nerve stimulation on the sino-atrial node of the guinea-pig

1. The effects of sympathetic nerve stimulation on the generation of pacemaker action potentials, recorded from the sino-atrial node of the guinea-pig, were determined by using intracellular recording techniques. 2. Trains of stimuli applied to the right stellate ganglion led to an increase in heart rate after a delay of a few seconds. During the initial phase of the tachycardia the rate of discharge of pacemaker action potentials increased and the rate of diastolic depolarization increased, but both the peak diastolic potential and the maximum rate of rise of the action potentials were reduced. During the later phase of the tachycardia the peak diastolic potential, the amplitude of the action potentials, the maximum rate of rise and the rate of repolarization of the action potentials were increased. 3. When membrane potential recordings were made from sino-atrial node cells, in which beating had been abolished by adding the organic calcium antagonist nifedipine, sympathetic nerve stimulation initiated excitatory junction potentials (EJPs) which had time courses similar to those of the tachycardias recorded from beating preparations. 4. Although both the tachycardias produced by either sympathetic nerve stimulation or added noradrenaline were largely abolished by beta-adrenoceptor antagonists, the membrane potential changes recorded during the responses to sympathetic nerve stimulation or added noradrenaline were different. Bath-applied noradrenaline caused a tachycardia which was associated with an increase in the amplitudes of pacemaker action potentials, an increase in the peak diastolic potential and a shortening in the duration of pacemaker action potentials. 5. The addition of agents which cause the accumulation of cyclic AMP in the cytoplasm of the cells produced a tachycardia which was associated with a similar sequence of changes in the membrane potentials to those produced by added noradrenaline; again the membrane potential changes produced by these agents differed from those produced by sympathetic nerve stimulation. 6. The results are discussed in relation to the idea that neurally released noradrenaline activates a set of receptors which cause tachycardia by increasing inward current flow during diastole, whereas added noradrenaline activates a set of receptors that are linked to a cyclic AMP-dependent pathway which modifies the properties of some of the voltage-dependent channels involved in pacemaking activity.

Propranolol and lidocaine inhibit neural norepinephrine release in hearts with increased extracellular potassium and ischemia

BACKGROUND

Propranolol and lidocaine are effective antiarrhythmic drugs in myocardial ischemia and infarction. As sympathetic nerve activation and norepinephrine release in ischemic hearts are arrhythmogenic, we tested the possibility that both agents inhibit neural norepinephrine release following sympathetic activation in the ischemic environment.

METHODS AND RESULTS

The model used was an in situ perfused innervated rat heart. Norepinephrine release was induced by electrical stimulation of the left cervicothoracic stellate ganglion and analyzed using radioenzymatic assay or high-performance liquid chromatography. In normoxically perfused hearts, evoked norepinephrine release was not affected by either of the two agents at doses of 1 to 10 mumol/L when extracellular K+ concentration was 4 mmol/L but dose-dependently reduced at 10 mmol/L K+ (D,L-propranolol: -53 +/- 4% at 1 mumol/L and -64 +/- 6% at 10 mumol/L K+, lidocaine: -37 +/- 11% at 0.1 mumol/L, -67 +/- 5% at 1 mumol/L, and -75 +/- 6% at 10 mumol/L). At 10 mmol/L K+, norepinephrine release was not affected by timolol or atenolol (both 10 mumol/L but was equally inhibited by D- or L-propranolol at 10 mumol/L (-56 +/- 5% and -53 +/- 9%, respectively), indicating a beta-blocking-independent mechanism. In hearts with metabolic acidosis (pH 6.85) at K+ of 4 mmol/L, neural norepinephrine release was also reduced by propranolol at 10 mumol/L (-37%). Finally, in hearts perfused with 4 mmol/L K+ and subjected to 6-minute periods of ischemia, neural norepinephrine release was similarly suppressed by D,L-propranolol (-38 +/- 6% at 0.1 mumol/L, -44 +/- 5% at 1 mumol/L, and -78 +/- 3% at 10 mumol/L) or lidocaine (-39 +/- 7% at 0.1 mumol/L, -58 +/- 9% at 1 mumol/L, and -91 +/- 3% at 10 mumol/L).

CONCLUSIONS

These data indicate that propranolol and lidocaine inhibit neural norepinephrine release via a Na+ channel-blocking mechanism that is synergistic with changes induced by ischemia, primarily raised extracellular K+. This mechanism may contribute to the anti-ischemic and antiarrhythmic properties of both agents in acute myocardial ischemia, which induces increased extracellular K+ and sympathetic activation.

Hyperpolarization-activated inward current in embryonic chick cardiac myocytes: developmental changes and modulation by isoproterenol and carbachol

Modulation of the hyperpolarization-activated inward current (If) in embryonic chick ventricular myocytes was examined using whole-cell voltage-clamp. Long (3 s) hyperpolarizing pulses were applied from a holding potential of -30 mV to steps of -40 to -120 mV. If was marked in 3-day-old cells, diminished at 10 days, and was almost completely gone at 17 days. If current density (at -120 mV) was -6.7 +/- 1.3 (3 days), -3.3 +/- 1.0 (10 days), and -2.0 +/- 0.5 pA/pF (17 days). If reduction paralleled the decrease in spontaneous activity. In 3-day cells, the threshold potential was -50 to -60 mV, and the reversal potential was -13.4 +/- 1.3 mV. The time course of activation was fitted by a single exponential and was temperature dependent: tau was 1.3 +/- 0.4 s at 20 degrees C and 0.7 +/- 0.4 s at 30 degrees C (at -120 mV). If amplitude was enhanced by 12.1 +/- 1.8% at 30 degrees C compared with 20 degrees C. Cs+ (3 mM) blocked If and had a negative chronotropic effect (rate decreased by 61%). Isoproterenol (1 microM) caused a positive chronotropic effect (17.1 +/- 2.9%) and increased If by 65.2 +/- 5.6%. Carbachol (0.1 microM) had a negative chronotropic effect (26.3 +/- 3.4%), and decreased If by 41.2 +/- 1.3%; it also reversed the enhancement produced by isoproterenol. Intracellular application of 100 microM GTP-gamma S decreased basal If by 35.2 +/- 5.0%, but potentiated the stimulant effect of isoproterenol (by 37.8 +/- 4.7%) and the inhibitory effect of carbachol (21.2 +/- 4.3%).(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of the effects of vagal stimulation on the sinus venous of the toad

In amphibians and mammals, vagal stimulation leads to the release of acetylcholine, ACh, which causes bradycardia. However, the responses to nerve stimulation are not well mimicked by exogenously applied ACh. These observations have led to the suggestion that there are subpopulations of muscarinic receptors on pacemaker cells and that during vagal stimulation neuronally released ACh caused slowing by suppressing inward current flow during diastole. After the generation of action potentials has been prevented by applying an organic calcium antagonist, vagal stimulation causes a hyperpolarization and an increase in membrane resistance: this observation suggests that the hyperpolarization results from a suppression of inward, presumably Na+, current flow. In this study we describe the effects of vagal stimulation on membrane potentials recorded from arrested and beating hearts by using a computer model. The model of Noble & Noble (Proc. R. Soc. Lond. B 222, 295 (1984)) was modified to describe the shape of amphibian pacemaker action potentials. A voltage-dependent Na conductance was included as well as two voltage-independent conductances, a background Na conductance and a background K conductance. Subsequently the hypothesis that the changes in membrane potential recorded during vagal stimulation from arrested preparations resulted from a reduction in Na conductance and this represented the sole action of vagally released. ACh, was tested. If this were so, the changes in membrane conductance that occur during vagal inhibitory junction potentials recorded from arrested preparations should produce changes in pacemaker action potentials similar to those recorded experimentally from beating preparations. This was found to be the case. Thus the analyses are consistent with the idea that vagal inhibition of pacemaker cells results solely from a suppression of the two pacemaker sodium currents.

Muscarinic receptors--characterization, coupling and function

At least five muscarinic receptor genes have been cloned and expressed. Muscarinic receptors act via activation of G proteins: m1, m3 and m5 muscarinic receptors couple to stimulate phospholipase C, while m2 and m4 muscarinic receptors inhibit adenylyl cyclase. This review describes the localization, pharmacology and function of the five muscarinic receptor subtypes. The actions of muscarinic receptors on the heart, smooth muscle, glands and on neurons (both presynaptic and postsynaptic) in the autonomic nervous system and the central nervous system are analyzed in terms of subtypes, biochemical mechanisms and effects on ion channels, including K+ channels and Ca2+ channels.

Membrane properties of rat suprachiasmatic nucleus neurons receiving optic nerve input

1. The electrophysiological properties of suprachiasmatic nucleus (SCN) neurons (n = 33) receiving optic nerve input were studied with intracellular recordings in rat hypothalamic slices maintained in vitro. Our major goal was to provide baseline data concerning the intrinsic membrane properties of these neurons and to test the hypothesis that the neurons are homogeneous electrophysiologically. 2. Action potentials were short in duration and followed by a pronounced hyperpolarizing after-potential. Spike amplitude (58.2 +/- 1.1 mV, mean +/- S.E.M.; measured from threshold), spike duration (0.83 +/- 0.03 ms; measured at half amplitude) and hyperpolarizing after-potential amplitude (23.9 +/- 1.0 mV; measured from threshold) appeared unimodally distributed and did not co-vary. 3. Intracellular injection of depolarizing current pulses evoked spike trains, and spike inactivation, spike broadening and frequency accommodation were always present. An after-hyperpolarization followed the spike train in all but one neuron. 4. Membrane time constant ranged from 7.5 to 21.1 ms (11.4 +/- 0.7 ms, n = 27), and its distribution appeared to be unimodal with the peak at approximately 10 ms. Input resistance ranged from 105 to 626 M omega (301 +/- 23 M omega, n = 33); the distribution also appeared unimodal with its peak at approximately 250 M omega. 5. A subpopulation (16 of 33, 48%) of the neurons exhibited slight (6-29%) time-dependent inward rectification in their voltage responses to hyperpolarizing current injection. Of the neurons lacking the time-dependent rectification, some (n = 5) exhibited time-independent inward rectification of 6-20% and others showed no (or < 3%) such rectification. The degree of inward rectification was correlated with neuronal excitability (r = 0.60, P < 0.002; assessed by measuring the steepness of the primary slope of the frequency-current plot) and with the spontaneous firing rate (r = 0.49, P < 0.007). Furthermore, the neurons with > 6% inward rectification (neurons with time-dependent and -independent rectification were combined) were more excitable (362 +/- 43 Hz/nA (n = 15) vs. 221 +/- 37 Hz/nA (n = 9), P < 0.05) and had a higher spontaneous firing rate (11.1 +/- 1.9 Hz (n = 19) vs. 3.9 +/- 1.5 Hz (n = 11), P < 0.02) than the neurons with no or negligible (i.e. < 3%) inward rectification. The two groups, however, were not significantly different in membrane time constant and input resistance. 6. When adequately hyperpolarized, twelve of seventeen (71%) neurons generated small low-threshold spike (LTS) potentials in response to depolarizing current pulses.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Intercellular communication in smooth muscle

The functioning of a group of cells as a tissue depends on intercellular communication; an example is the spread of action potentials through intestinal tissue resulting in synchronized contraction. Recent evidence for cell heterogeneity within smooth muscle tissues has renewed research into cell coupling. Electrical coupling is essential for propagation of action potentials in gastrointestinal smooth muscle. Metabolic coupling may be involved in generation of pacemaker activity. This review deals with the role of cell coupling in tissue function and some of the issues discussed are the relationship between electrical synchronization and gap junctions, metabolic coupling, and the role of interstitial cells of Cajal in coupling.

Structure of autonomic neuromuscular junctions in the sinus venosus of the toad

The structure of cholinergic and adrenergic neuromuscular junctions in the sinus venosus of the toad, Bufo marinus, was determined by electron microscopy. From random sections of sinus venosus tissue it appeared that there were variable separations between cholinergic or adrenergic varicosities and the nearest sinus venosus muscle cell. However, when the structure of complete cholinergic and adrenergic varicosities was determined by examining serial electron micrographs, virtually all varicosities that lost their covering of Schwann cell were found to form an area of close apposition with an adjacent muscle cell. At the region of close apposition, the neuromuscular cleft was filled with a single layer of basal lamina to give a neuromuscular separation of about 70 nm. Synaptic vesicles within a varicosity were usually found to be concentrated towards the region of close apposition. These observations are discussed in relationship to the idea that when transmission occurs at these neuromuscular junctions the transmitters act on discrete pools of specialized subsynaptic receptors.

The Sicilian gambit. A new approach to the classification of antiarrhythmic drugs based on their actions on arrhythmogenic mechanisms. Task Force of the Working Group on Arrhythmias of the European Society of Cardiology

The Queen's Gambit is an opening move in chess that provides a variety of aggressive options to the player electing it. This report represents a similar gambit (the Sicilian Gambit) on the part of a group of basic and clinical investigators who met in Taormina, Sicily to consider the classification of antiarrhythmic drugs. Paramount to their considerations were 1) dissatisfaction with the options offered by existing classification systems for inspiring and directing research, development, and therapy, 2) the disarray in the field of antiarrhythmic drug development and testing in this post-Cardiac Arrhythmia Suppression Trial (CAST) era, and 3) the desire to provide an operational framework for consideration of antiarrhythmic drugs that will both encourage advancement and have the plasticity to grow as a result of the advances that occur. The multifaceted approach suggested is, like the title of the article, a gambit. It is an opening rather than a compendium and is intended to challenge thought and investigation rather than to resolve issues. The article incorporates first, a discussion of the shortcomings of the present system for drug classification; second, a review of the molecular targets on which drugs act (including channels and receptors); third, a consideration of the mechanisms responsible for arrhythmias, including the identification of "vulnerable parameter" that might be most accessible to drug effect; and finally, clinical considerations with respect to antiarrhythmic drugs. Information relating to the various levels of information is correlated across categories (i.e., clinical arrhythmias, cellular mechanisms, and molecular targets), and a "spread sheet" approach to antiarrhythmic action is presented that considers each drug as a unit, with similarities to and dissimilarities from other drugs being highlighted. A complete reference list for this work would require as many pages as the text itself. For this reason, referencing is selective and incomplete. It is designed, in fact, to provide sufficient background information to give the interested reader a starting frame of reference rather than to recognize the complete body of literature that is the basis for this article.