Effects of high potassium and the bradycardic agents ZD7288 and cesium on heart rate of rabbits and guinea pigs

Drivers of Sinoatrial Node Automaticity in Zebrafish: Comparison With Mechanisms of Mammalian Pacemaker Function

Cardiac excitation originates in the sinoatrial node (SAN), due to the automaticity of this distinct region of the heart. SAN automaticity is the result of a gradual depolarisation of the membrane potential in diastole, driven by a coupled system of transarcolemmal ion currents and intracellular Ca²⁺ cycling. The frequency of SAN excitation determines heart rate and is under the control of extra- and intracardiac (extrinsic and intrinsic) factors, including neural inputs and responses to tissue stretch. While the structure, function, and control of the SAN have been extensively studied in mammals, and some critical aspects have been shown to be similar in zebrafish, the specific drivers of zebrafish SAN automaticity and the response of its excitation to vagal nerve stimulation and mechanical preload remain incompletely understood. As the zebrafish represents an important alternative experimental model for the study of cardiac (patho-) physiology, we sought to determine its drivers of SAN automaticity and the response to nerve stimulation and baseline stretch. Using a pharmacological approach mirroring classic mammalian experiments, along with electrical stimulation of intact cardiac vagal nerves and the application of mechanical preload to the SAN, we demonstrate that the principal components of the coupled membrane- Ca²⁺ pacemaker system that drives automaticity in mammals are also active in the zebrafish, and that the effects of extra- and intracardiac control of heart rate seen in mammals are also present. Overall, these results, combined with previously published work, support the utility of the zebrafish as a novel experimental model for studies of SAN (patho-) physiological function.

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

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

A model of cellular cardiac-neural coupling that captures the sympathetic control of sinoatrial node excitability in normotensive and hypertensive rats

Hypertension is associated with sympathetic hyperactivity. To represent this neural-myocyte coupling, and to elucidate the mechanisms underlying sympathetic control of the cardiac pacemaker, we developed a new (to our knowledge) cellular mathematical model that incorporates signaling information from cell-to-cell communications between the sympathetic varicosity and sinoatrial node (SAN) in both normotensive (WKY) and hypertensive (SHR) rats. Features of the model include 1), a description of pacemaker activity with specific ion-channel functions and Ca(2+) handling elements; 2), dynamic β-adrenergic modulation of the excitation of the SAN; 3), representation of ionic activity of sympathetic varicosity with NE release dynamics; and 4), coupling of the varicosity model to the SAN model to simulate presynaptic transmitter release driving postsynaptic excitability. This framework captures neural-myocyte coupling and the modulation of pacemaking by nitric oxide and cyclic GMP. It also reproduces the chronotropic response to brief sympathetic stimulations. Finally, the SHR model quantitatively suggests that the impairment of cyclic GMP regulation at both sides of the sympathetic cleft is crucial for development of the autonomic phenotype observed in hypertension.

Mapping cardiac pacemaker circuits: methodological puzzles of the sinoatrial node optical mapping

Historically, milestones in science are usually associated with methodological breakthroughs. Likewise, the advent of electrocardiography, microelectrode recordings and more recently optical mapping have ushered in new periods of significance of advancement in elucidating basic mechanisms in cardiac electrophysiology. As with any novel technique, however, data interpretation is challenging and should be approached with caution, as it cannot be simply extrapolated from previously used methodologies and with experience and time eventually becomes validated. A good example of this is the use of optical mapping in the sinoatrial node (SAN): when microelectrode and optical recordings are obtained from the same site in myocardium, significantly different results may be noted with respect to signal morphology and as a result have to be interpreted by a different set of principles. Given the rapid spread of the use of optical mapping, careful evaluation must be made in terms of methodology with respect to interpretation of data gathered by optical sensors from fluorescent potential-sensitive dyes. Different interpretations of experimental data may lead to different mechanistic conclusions. This review attempts to address the origin and interpretation of the "double component" morphology in the optical action potentials obtained from the SAN region. One view is that these 2 components represent distinctive signals from the SAN and atrial cells and can be fully separated with signal processing. A second view is that the first component preceding the phase 0 activation represents the membrane currents and intracellular calcium transients induced diastolic depolarization from the SAN. Although the consensus from both groups is that ionic mechanisms, namely the joint action of the membrane and calcium automaticity, are important in the SAN function, it is unresolved whether the double-component originates from the recording methodology or represents the underlying physiology. This overview aims to advance a common understanding of the basic principles of optical mapping in complex 3D anatomic structures.

Regulation of basal and reserve cardiac pacemaker function by interactions of cAMP-mediated PKA-dependent Ca2+ cycling with surface membrane channels

Decades of intensive research of primary cardiac pacemaker, the sinoatrial node, have established potential roles of specific membrane channels in the generation of the diastolic depolarization, the major mechanism allowing sinoatrial node cells to generate spontaneous beating. During the last three decades, multiple studies made either in the isolated sinoatrial node or sinoatrial node cells have demonstrated a pivotal role of Ca(2+) and, specifically Ca(2+) release from sarcoplasmic reticulum, for spontaneous beating of cardiac pacemaker. Recently, spontaneous, rhythmic local subsarcolemmal Ca(2+) releases from ryanodine receptors during late half of the diastolic depolarization have been implicated as a vital factor in the generation of sinoatrial node cell spontaneous firing. Local Ca(2+) releases are driven by a unique combination of high basal cAMP production by adenylyl cyclases, high basal cAMP degradation by phosphodiesterases and a high level of cAMP-mediated PKA-dependent phosphorylation. These local Ca(2+) releases activate an inward Na(+)-Ca(2+) exchange current which accelerates the terminal diastolic depolarization rate and, thus, controls the spontaneous pacemaker firing. Both the basal primary pacemaker beating rate and its modulation via beta-adrenergic receptor stimulation appear to be critically dependent upon intact RyR function and local subsarcolemmal sarcoplasmic reticulum generated Ca(2+) releases. This review aspires to integrate the traditional viewpoint that has emphasized the supremacy of the ensemble of surface membrane ion channels in spontaneous firing of the primary cardiac pacemaker, and these novel perspectives of cAMP-mediated PKA-dependent Ca(2+) cycling in regulation of the heart pacemaker clock, both in the basal state and during beta-adrenergic receptor stimulation.

Cholinergic receptor signaling modulates spontaneous firing of sinoatrial nodal cells via integrated effects on PKA-dependent Ca(2+) cycling and I(KACh)

Prior studies indicate that cholinergic receptor (ChR) activation is linked to beating rate reduction (BRR) in sinoatrial nodal cells (SANC) via 1) a G(i)-coupled reduction in adenylyl cyclase (AC) activity, leading to a reduction of cAMP or protein kinase A (PKA) modulation of hyperpolarization-activated current (I(f)) or L-type Ca(2+) currents (I(Ca,L)), respectively; and 2) direct G(i)-coupled activation of ACh-activated potassium current (I(KACh)). More recent studies, however, have indicated that Ca(2+) cycling by the sarcoplasmic reticulum within SANC (referred to as a Ca(2+) clock) generates rhythmic, spontaneous local Ca(2+) releases (LCR) that are AC-PKA dependent. LCRs activate Na(+)-Ca(2+) exchange (NCX) current, which ignites the surface membrane ion channels to effect an AP. The purpose of the present study was to determine how ChR signaling initiated by a cholinergic agonist, carbachol (CCh), affects AC, cAMP, and PKA or sarcolemmal ion channels and LCRs and how these effects become integrated to generate the net response to a given intensity of ChR stimulation in single, isolated rabbit SANC. The threshold CCh concentration ([CCh]) for BRR was approximately 10 nM, half maximal inhibition (IC(50)) was achieved at 100 nM, and 1,000 nM stopped spontaneous beating. G(i) inhibition by pertussis toxin blocked all CCh effects on BRR. Using specific ion channel blockers, we established that I(f) blockade did not affect BRR at any [CCh] and that I(KACh) activation, evidenced by hyperpolarization, first became apparent at [CCh] > 30 nM. At IC(50), CCh reduced cAMP and reduced PKA-dependent phospholamban (PLB) phosphorylation by approximately 50%. The dose response of BRR to CCh in the presence of I(KACh) blockade by a specific inhibitor, tertiapin Q, mirrored that of CCh to reduced PLB phosphorylation. At IC(50), CCh caused a time-dependent reduction in the number and size of LCRs and a time dependent increase in LCR period that paralleled coincident BRR. The phosphatase inhibitor calyculin A reversed the effect of IC(50) CCh on SANC LCRs and BRR. Numerical model simulations demonstrated that Ca(2+) cycling is integrated into the cholinergic modulation of BRR via LCR-induced activation of NCX current, providing theoretical support for the experimental findings. Thus ChR stimulation-induced BRR is entirely dependent on G(i) activation and the extent of G(i) coupling to Ca(2+) cycling via PKA signaling or to I(KACh): at low [CCh], I(KACh) activation is not evident and BRR is attributable to a suppression of cAMP-mediated, PKA-dependent Ca(2+) signaling; as [CCh] increases beyond 30 nM, a tight coupling between suppression of PKA-dependent Ca(2+) signaling and I(KACh) activation underlies a more pronounced BRR.

Role of pacemaking current in cardiac nodes: Insights from a comparative study of sinoatrial node and atrioventricular node

Cardiac pacemaking in the sinoatrial (SA) node and atrioventricular (AV) node is generated by an interplay of many ionic currents, one of which is the funny pacemaker current (If). To understand the functional role of If in two different pacemakers, comparative studies of spontaneous activity and expression of the HCN channel in mouse SA node and AV node were performed. The intrinsic cycle length (CL) is 179+/-2.7 ms (n=5) in SA node and 258+/-18.7 ms (n=5) in AV node. Blocking of If current by 1 micromol/L ZD7288 increased the CL to 258+/-18.7 ms (n=5) and 447+/-92.4 ms (n=5) in SA node and AV node, respectively. However, the major HCN channel, HCN4 expressed at low level in the AV node compared to the SA node. To clarify the discrepancy between the functional importance of If and expression level of HCN4 channel, a SA node cell model was used. Increasing the If conductance resulted in decreasing in the CL in the model, which explains the high pacemaking rate and high expression of HCN channel in the SA node. Resistance to the blocking of If in the SA node might result from compensating effects from other currents (especially voltage sensitive currents) involved in pacemaking. The computer simulation shows that the difference in the intrinsic CL could explain the difference in response to If blocking in these two cardiac nodes.

Characterization of hyperpolarization-activated current (Ih) in dorsal root ganglion neurons innervating rat urinary bladder

Afferent pathways innervating the urinary bladder consist of myelinated Adelta-fibers and unmyelinated C-fibers. Normal voiding is dependent on mechanoceptive Adelta-fiber bladder afferents that respond to bladder distention. However, the mechanisms for controlling the excitability of Adelta-fiber bladder afferents are not fully understood. We therefore used whole cell patch-clamp techniques to investigate the properties of hyperpolarization-activated, cyclic nucleotide-gated (HCN) currents (I(h)) in dorsal root ganglion (DRG) neurons innervating the urinary bladder of rats. The neurons were identified by axonal tracing with a fluorescent dye, Fast Blue, injected into the bladder wall. Hyperpolarizing voltage step pulses from -40 to -130 mV produced voltage- and time-dependent inward I(h) currents in bladder afferent neurons. The amplitude and current density of I(h) at a holding potential of -130 mV was significantly larger in medium-sized bladder afferent neurons (diameter: 37.8 +/- 0.3 microm), a small portion (19%) of which were sensitive to capsaicin (1 microM), than in uniformly capsaicin-sensitive small-sized (27.6 +/- 0.5 microm) bladder neurons. In medium-sized bladder neurons, a selective HCN channel inhibitor, ZD7288, dose-dependently inhibited I(h) currents. ZD7288 (10 microM) also increased the time constant of the slow depolarization phase of spike after-hyperpolarization from 91.8 to 233.0 ms. These results indicate that I(h) currents are predominantly expressed in medium-sized bladder afferent neurons innervating the bladder and that inhibition of I(h) currents delayed recovery from the spike after-hyperpolarization. Thus, it is assumed that I(h) currents could control excitability of mechanoceptive Adelta-fiber bladder afferent neurons, which are usually capsaicin-insensitive and larger in size than capsaicin-sensitive C-fiber bladder afferent neurons.

Hyperpolarization-activated cation currents: from molecules to physiological function

Hyperpolarization-activated cation currents, termed If, Ih, or Iq, were initially discovered in heart and nerve cells over 20 years ago. These currents contribute to a wide range of physiological functions, including cardiac and neuronal pacemaker activity, the setting of resting potentials, input conductance and length constants, and dendritic integration. The hyperpolarization-activated, cation nonselective (HCN) gene family encodes the channels that underlie Ih. Here we review the relation between the biophysical properties of recombinant HCN channels and the pattern of HCN mRNA expression with the properties of native Ih in neurons and cardiac muscle. Moreover, we consider selected examples of the expanding physiological functions of Ih with a view toward understanding how the properties of HCN channels contribute to these diverse functional roles.

Natriuretic peptides like NO facilitate cardiac vagal neurotransmission and bradycardia via a cGMP pathway

We tested the hypothesis that natriuretic peptide receptors (NPRs) that are coupled to cGMP production act in a similar way to nitric oxide (NO) by enhancing acetylcholine release and vagal-induced bradycardia. The effects of enzyme inhibitors and channel blockers on the action of atrial natriuretic peptide (ANP), brain-derived natriuretic peptide (BNP), and C-type natriuretic peptide (CNP) were evaluated in isolated guinea pig atrial-right vagal nerve preparations. RT-PCR confirmed the presence NPR B and A receptor mRNA in guinea pig sinoatrial node tissue. BNP and CNP significantly (P < 0.05) enhanced the heart rate (HR) response to vagal nerve stimulation. CNP had no effect on the HR response to carbamylcholine and facilitated the release of [(3)H]acetylcholine during atrial field stimulation. The particulate guanylyl cyclase-coupled receptor antagonist HS-142-1, the phosphodiesterase 3 inhibitor milrinone, the protein kinase A inhibitor H89, and the N-type calcium channel blocker omega-conotoxin all blocked the effect of CNP on vagal-induced bradycardia. Like NO, BNP and CNP facilitate vagal neurotransmission and bradycardia. This may occur via a cGMP-PDE3-dependent pathway increasing cAMP-PKA-dependent phosphorylation of presynaptic N-type calcium channels.

Electrolyte and pH dependence of heart rate during hemodialysis: a computer model analysis

The influence of hemodialysis-induced modifications in extracellular fluid characteristics on heart rate was investigated by using a detailed computer model of sinus-node electrical activity. Changes similar to those occurring in the course of hemodialysis in extracellular concentrations of sodium (from 138 to 140 mM), potassium (from 6 to 3.3 mM), and calcium (from 1.2 to 1.5 mM) ions as well as in pH (from 7.31 to 7.4) and intracellular volume were simulated. The model predicted that such changes may largely influence the rhythm of the sinoatrial node pacemaker, causing the heart rate to range from 69 to 86 bpm. Heart rate increases after removing potassium (up to 7 bpm) and also after calcium perfusion (up to 11 bpm) whereas restoring pH slows heart beat (up to 6 bpm). Extracellular sodium has no significant influence, but the heart rate strictly depends on intracellular sodium concentration (5 bpm/mM). A complex dependence of heart rate on electrolytes and pH was also recognized. Providing extracellular potassium concentration is maintained above 5 mM, heart rate exhibits low sensitivity to changes in calcium and potassium. When potassium concentration is reduced below 4.5 mM, heart rate sensitivity to calcium and potassium increases significantly to 10 and 30 bpm/mM, respectively. A sustained increase in heart rate always corresponds to an increase in intracellular sodium concentration.

Nitric oxide donors can increase heart rate independent of autonomic activation

Administration of nitric oxide (NO) donors in vivo is accompanied by a baroreflex-mediated increase in heart rate (HR). In vitro, however, NO donors can increase HR directly by stimulating a pathway that involves NO, cGMP, and the hyperpolarization-activated current (I(f)). The aim of this study was to assess the functional significance of this pathway in vivo by testing whether NO donors can increase HR in the anesthetized rabbit independent of the autonomic nervous system. New Zealand White rabbits were vagotomized, cardiac sympathectomized, and treated with propranolol (0.3 mg/kg iv). The NO donor molsidomine (0.2 mg/kg iv) caused a progressive increase (Delta) in HR (DeltaHR, 14 +/- 3 beats/min; P < 0.01). This effect was significantly reduced by the I(f) blocker ZD-7288 (0.2 mg/kg iv; DeltaHR, 2 +/- 3 beats/min; P = not significant). Similar results were seen with sodium nitroprusside. The positive chronotropic effect of sodium nitroprusside (50 microM) was confirmed in the isolated working rabbit heart preparation (DeltaHR, 17 +/- 3 beats/min; P < 0.01). In conclusion, NO donors exert a small, but significant, positive chronotropic effect in vivo that is independent of the autonomic nervous system. These results are also consistent with data in sinoatrial node cells that show that NO donors increase HR by stimulating I(f).

Nitric oxide can increase heart rate by stimulating the hyperpolarization-activated inward current, I(f)

We investigated the chronotropic effect of increasing concentrations of sodium nitroprusside (SNP, n = 8) or 3-morpholinosydnonimine (SIN-1, n = 6) in isolated guinea pig spontaneously beating sinoatrial node/atrial preparations. Low concentrations of NO donors (nanomolar to micromolar) gradually increased the beating rate, whereas high (millimolar) concentrations decreased it. The increase in rate was (1) enhanced by superoxide dismutase (50 to 100 U/mL, n = 6), (2) prevented by the guanylyl cyclase inhibitors 6-anilino-5,8-quinolinedione (5 mumol/L, n = 6) or 1H-(1,2,4)oxadiazolo(4,3-a)quinoxalin-1-one (10 mumol/L, n = 6), and (3) mimicked by 8-bromo-cGMP (n = 6) with no additional positive chronotropic effect of SIN-1 (n = 5). The response to 10 mumol/L SNP (n = 28) or 50 mumol/L SIN-1 (n = 16) was unaffected by IcaL antagonism with nifedipine (0.2 mumol/L) but was abolished after blockade of the hyperpolarization-activated inward current (I(f)) by Cs+ (2 mmol/L) or 4-(N-ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino)pyrimidinium chloride (1 mumol/L). The effect on I(f) was further evaluated in rabbit isolated patch-clamped sinoatrial node cells (n = 21), where we found that 5 mumol/L SNP or SIN-1 caused a reversible Cs(+)-sensitive increase in this current (+130% at -70 mV and +250% at -100 mV). In conclusion, NO donors can affect pacemaker activity in a concentration-dependent biphasic fashion. Our results indicate that the increase in beating rate is due to stimulation of I(f) via the NO-cGMP pathway. This may contribute to the sinus tachycardia in pathological conditions associated with an increase in myocardial production of NO.