Rabbit Sinoatrial Node Research Articles

Downward gradient in action potential duration along conduction path in and around the sinoatrial node.

Regional differences in electrical activity in rabbit sinoatrial node have been investigated by recording action potentials throughout the intact node or from small balls of tissue from different regions. In the intact node, action potential duration was greatest at or close to the leading pacemaker and declined markedly in all directions from it, e.g., by 74 +/- 4% (mean +/- SE, n = 4) to the crista terminalis. Similar data were obtained from the small balls. The gradient is down the conduction pathway and will help prevent reentry. In the intact node, a zone of inexcitable tissue with small depolarizations of <25 mV or stable resting potentials was discovered in the inferior part of the node, and this will again help prevent reentry. The intrinsic pacemaker activity of the small balls was slower in tissue from more inferior (as well as more central) parts of the node [e.g., cycle length increased from 339 +/- 13 ms (n = 6) to 483 +/- 13 ms (n = 6) in transitional tissue from more superior and inferior sites], and this may help explain pacemaker shift.

Effect of isoprenaline, carbachol, and Cs+ on Na+ activity and pacemaker potential in rabbit SA node cells.

Effects of isoprenaline, carbachol, and Cs+ on intracellular Na+ activity (a(i)Na) and spontaneous action potentials were studied in multicellular and single cell preparations isolated from rabbit sinoatrial (SA) nodes. a(i)Na was measured with double-barreled Na+-selective microelectrodes and the fluorescent Na+-indicator sodium-binding benzofuran isophthalate (SBFI). In spontaneously beating cells, aiNa measured with Na+-selective microelectrodes and SBFI were 4.5 +/- 1.2 mM (means +/- SD, n = 21) in multicellular preparations and 4.0 +/- 1.1 mM (n = 16) in single cells, respectively. Measurements of a(i)Na with microelectrodes showed that isoprenaline increased a(i)Na from 4.7 +/- 1.2 to 5.5 +/- 1.6 mM (n = 16, P < 0.01) and shortened the action potential cycle length (ACL) from 338 +/- 46 to 269 +/- 35 ms (n = 16, P < 0.01). However, increasing the action potential rate by pacing produced a much smaller increase in a(i)Na. Changes in a(i)Na and ACL produced by isoprenaline were blocked by Cs+. The selective hyperpolarization-activated inward current (If) blocker ZD-7288 decreased a(i)Na from 5.2 +/- 1.0 to 4.6 +/- 1.3 mM (n = 4, P < 0.01) and prolonged ACL from 394 +/- 20 to 553 +/- 68 ms (n = 4, P < 0.01). The If blocker substantially inhibited the increase in a(i)Na produced by isoprenaline. Carbachol and Cs+ decreased aiNa from 4.6 +/- 1.4 to 3.9 +/- 1.2 mM (n = 15, P < 0.01) and from 4.9 +/- 1.0 to 3.9 +/- 1.3 mM (n = 18, P < 0.01), respectively. In addition, carbachol and Cs+ prolonged ACL from 345 +/- 44 to 587 +/- 100 ms (n = 15, P < 0.01) and from 353 +/- 30 to 464 +/- 87 ms (n = 18, P < 0.01), respectively. However, carbachol and Cs+ almost did not change a(i)Na when SA node cells became quiescent in a 25.4 mM extracellular K+ concentration. The results suggest that isoprenaline, ZD-7288, carbachol, or Cs+ might have changed a(i)Na and action potential rate by possibly stimulating or inhibiting If carried by Na+. Measurements of a(i)Na with SBFI showed that isoprenaline, carbachol, and Cs+ produced a(i)Na changes that were similar to those measured with the microelectrodes.

Dual Effect of Nitric Oxide on the Hyperpolarization-activated Inward Current (If) in Sino-atrial Node Cells of the Rabbit

Using the whole-cell voltage-clamp technique, we have investigated the effect of nitric oxide (NO) donor (sodium nitroprusside, SNP) on hyperpolarization-activated inward current,If, in isolated rabbit sinoatrial node (SAN) cells.Ifin the basal state increased when NO was applied but decreased whenIfwas pre-stimulated by isoproterenol (ISO) or by adding cAMP to the pipette solution. Both the stimulatory and the inhibitory effects of NO were abolished by guanylyl cyclase inhibitor, methylene blue (MB), suggesting that the effect of NO is mediated by cGMP. The inhibitory effect of NO was abolished whenIfwas pre-stimulated by 3-isobutyl-1-methylxanthine (IBMX), which is a phosphodiesterase (PDE) inhibitor, or by adding 8Br-cAMP (which is resistant to PDE) to the pipette solution. An analogue of cGMP, 8Br-cGMP, which is a potent stimulator of cGMP-dependent protein kinase (PKG) but has little effect on PDE, did not inhibitIfwhenIfwas pre-stimulated by ISO. In its basal state,Ifwas still increased by 8Br-cGMP, and this effect was not prevented by the pretreatment with H-7, PKG inhibitor. The effect of acetylcholine (ACh) was not identical to that of NOIfdecreased when pre-stimulated not only by ISO, but also by IBMX. The above results suggest that via cGMP, NO exerts a dual effect onIf: the inhibitory effect is mediated by cGMP-stimulated PDE, and the stimulatory effect may be attributable to direct binding of cGMP toIfchannels.

Two distinct pathways of muscarinic current responses in rabbit sino-atrial node myocytes.

Acetycholine (ACh) slows the heart rate by acting on sino-atrial node currents. Low ACh concentrations act on muscarinic receptors to inhibit the hyperpolarization-activated current (if) by a adenosine 3',5'-cyclic monophosphate (cAMP)-dependent cytoplasmic pathway. ACh also activates a muscarinic potassium conductance (iK,ACh) via a pertussis toxin-sensitive guanine nucleotide-binding protein (G-protein) that gates the channel directly. This pathway has been called membrane-delimited or "fast" because cytoplasmic components are not required and hence activation is relatively rapid. Such a pathway has also been proposed for the muscarinic inhibition of if. Here we show that, under steady-state current conditions, 0.1-1 microM ACh activates iK,ACh with a time constant of 1 s or less that is inversely proportional to ACh concentration, consistent with a fast, membrane-delimited pathway. ACh also causes a significantly slower inhibition of if which is not proportional to ACh binding. The changes in if are consistent with muscarinic effects mediated exclusively through the cAMP pathway.

Regional differences in effects of 4-aminopyridine within the sinoatrial node.

4-Aminopyridine (4-AP)-sensitive transient outward current (Ito) has been observed in the sinoatrial node, but its role is unknown. The effect of block of Ito by 5 mM 4-AP on small ball-like tissue preparations (diameter approximately 0.3-0.4 mm) from different regions of the rabbit sinoatrial node has been investigated. 4-AP elevated the plateau, prolonged the action potential, and decreased the maximum diastolic potential. Effects were greater in tissue from the periphery of the node than from the center. In peripheral tissue, 4-AP abolished the action potential notch, if present. 4-AP slowed pacemaker activity of peripheral tissue but accelerated that of central tissue. Differences in the response to 4-AP were also observed between tissue from more superior and inferior regions of the node. In the intact sinoatrial node, 4-AP resulted in a shift of the leading pacemaker site consistent with the regional differences in the response to 4-AP. It is concluded that 4-AP-sensitive outward current plays a major role in action potential repolarization and pacemaker activity in the sinoatrial node and that its role varies regionally.

Selective cardiodepressant activity of fluodipine, a fluorenone-1,4-dihydropyridine derivative

The effect of the dihydropyridine derivative, 1,4-dihydro-2,6-dimethyl-4-(fluorenon-4-yl)pyridine-3,5-dicarboxylic acid diallyl ester (fluodipine) was studied in vitro in different rabbit, rat and guinea pig preparations and in vivo in the rabbit in order to characterize its pharmacological profile at cardiac and at vascular sites. Compared to nifedipine, fluodipine showed a similar cardiodepressant activity, and a much lower inhibitory activity on vascular contraction. The highest tissue selectivity was observed in guinea pig preparations: fluodipine was about 2–3 times more effective than nifedipine on chronotropism and inotropism in isolated atria, and about 150 times less effective on aortic strip contraction. Accordingly, fluodipine (i) showed high-affinity binding to guinea pig ventricular L-type cardiac Ca2+ channels (Ki=2.57 nM), (ii) was about 80 times less effective than nifedipine to inhibit Ca2+ influx in vascular smooth muscle cells and (iii) induced a significant reduction of heart rate in the anesthetized rabbit (ID25=8.5 mg kg−1, i.v.) without affecting the blood pressure up to 20 mg kg−1, whereas nifedipine showed a significant hypotensive effect at very low doses (ID25=0.18 mg kg−1, i.v.). The pacemaker current If of rabbit sino-atrial node myocytes was not affected by fluodipine. These findings demonstrate that fluodipine exerts selective cardiodepressant activity, likely due to a higher affinity for cardiac than for vascular Ca2+ channels.

Local cholinergic suppression of pacemaker activity in the rabbit sinoatrial node.

The effects of transmural vagal stimulation and acetylcholine (ACh) superfusion on primary and latent pacemaker cells of the rabbit sinoatrial node were studied by using microelectrodes. Both ACh and vagal stimulation lengthened atrial cycle length by 40-60% as compared with control. In the cells from the primary pacemaker area, both ACh superfusion and vagal stimulation suppressed action potential (AP) amplitude and then induced inexcitability. In contrast, cells from subsidiary pacemaker area as well as atrium remained excitable. These effects were completely reversible and also were abolished by atropine, 10(-7) M. Cholinergically induced suppression of AP amplitude is predictable based on the maximal rate of AP upstroke (dV/dt). The probability of amplitude suppression was the highest among pacemaker cells (dV/dt, <3 V/s), in which ACh suppressed amplitude in 27 (93%) of 29 cells, and vagal stimulation did so in 38 (81%) of 47 cells. With increasing upstroke velocity, the probability of amplitude suppression decreased. Inexcitability did not occur in cells whose dV/dt was >15 V/s. The suppression of AP amplitude by ACh occurred in a concentration-dependent manner: the concentration inducing suppression of amplitude in 50% of pacemaker cells was approximately 10 microM. These results indicate that cholinergic effects on typical pacemaker and subsidiary pacemaker cells are different: whereas subsidiary pacemaker cells remain excitable, typical pacemaker cells become quiescent. We hypothesize that quiescent cells create quiescent regions in the center of the sinoatrial node that might functionally be an obstacle for reentrant tachycardias.

Electrical interactions between a rabbit atrial cell and a nodal cell model.

Atrial activation involves interactions between cells with automaticity and slow-response action potentials with cells that are intrinsically quiescent with fast-response action potentials. Understanding normal and abnormal atrial activity requires an understanding of this process. We studied interactions of a cell with spontaneous activity, represented by a "real-time" simulation of a model of the rabbit sinoatrial (SA) node cell, simultaneously being electrically coupled via our "coupling clamp" circuit to a real, isolated atrial myocyte with variations in coupling conductance (Gc) or stimulus frequency. The atrial cells were able to be driven at a regular rate by a single SA node model (SAN model) cell. Critical Gc for entrainment of the SAN model cell to a nonstimulated atrial cell was 0.55 +/- 0.05 nS (n = 7), and the critical Gc that allowed entrainment when the atrial cell was directly paced at a basic cycle length of 300 ms was 0.32 +/- 0.01 nS (n = 7). For each atrial cell we found periodic phenomena of synchronization other than 1:1 entrainment when Gc was between 0.1 and 0.3 nS, below the value required for frequency entrainment, when the atrial cell was directly driven at a basic cycle length of either 300 or 600 ms. In conclusion, the high input resistance of the atrial cells allows successful entrainment of nodal and atrial cells at low values of Gc, but further uncoupling produces arrhythmic interactions.

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

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. 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. (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.

Differences Between the Delayed Rectifier K+ Currents in Myocytes From Rabbit Sinoatrial and Atrioventricular Nodes

Differences Between the Delayed Rectifier K+ Currents in Myocytes From Rabbit Sinoatrial and Atrioventricular Nodes

Differences between the delayed rectifier K+currents in myocytes from rabbit sinoatrial and atrioventricular nodes

Differences between the delayed rectifier K+currents in myocytes from rabbit sinoatrial and atrioventricular nodes

Endothelin-1 Inhibits Pacemaker Currents in Rabbit SA Node Cells

Our recent study demonstrated that endothelin-1 (ET-1) inhibits the pacemaker activity of sinoatrial (SA) node cells via changes in the L-type Ca2+, delayed K+, and background K+ currents. Using the whole-cell patch-clamp technique in the same preparation, we found that ET-1 reduces other pacemaker currents, the T-type Ca2+ current (ICa,T) and the hyperpolarization-activated inward current (I(f)). The inhibitory actions of ET-1 on these currents were concentration-dependent, i.e., EC50 of 0.9 nM for ICa,T and 2.3 nM for I(f), with little reversal after washout of the peptide. In the presence of BQ485, both currents were not affected by ET-1. These results indicate additional mechanisms underlying negative chronotropic actions of ET-1 on the rabbit SA node.

Pacemaker synchronization of electrically coupled rabbit sinoatrial node cells.

The effects of intercellular coupling conductance on the activity of two electrically coupled isolated rabbit sinoatrial nodal cells were investigated. A computer-controlled version of the "coupling clamp" technique was used in which isolated sinoatrial nodal cells, not physically in contact with each other, were electrically coupled at various values of ohmic coupling conductance, mimicking the effects of mutual interaction by electrical coupling through gap junctional channels. We demonstrate the existence of four types of electrical behavior of coupled spontaneously active cells. As the coupling conductance is progressively increased, the cells exhibit: (a) independent pacemaking at low coupling conductances, (b) complex dynamics of activity with mutual interactions, (c) entrainment of action potential frequency at a 1:1 ratio with different action potential waveforms, and (d) entrainment of action potentials at the same frequency of activation and virtually identical action potential waveforms. The critical value of coupling conductance required for 1:1 frequency entrainment was <0.5 nS in each of the five cell pairs studied. The common interbeat interval at a relatively high coupling conductance (10 nS), which is sufficient to produce entrainment of frequency and also identical action potential waveforms, is determined most by the intrinsically faster pacemaker cell and it can be predicted from the diastolic depolarization times of both cells. Evidence is provided that, at low coupling conductances, mutual pacemaker synchronization results mainly from the phase-resetting effects of the action potential of one cell on the depolarization phase of the other. At high coupling conductances, the tonic, diastolic interactions become more important.

Ionic basis of ryanodine's negative chronotropic effect on pacemaker cells isolated from the sinoatrial node.

Spontaneous electrical activity and indo 1 fluorescence ratios were recorded simultaneously in cultured pacemaker cells isolated from the rabbit sinoatrial node. Ryanodine (10 microM) reduced the amplitude of action potential-induced intracellular Ca2+ (Ca2+i) transients by 19 +/- 3%, increased the time constant for their decay by 51 +/- 5%, and slowed spontaneous firing by 32 +/- 3%. 1,2-Bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA)-acetoxymethyl ester (AM; 25 microM) inhibited the Ca2+i transients and slowed spontaneous firing by 28 +/- 4%. Ryanodine did not alter hyperpolarization-activated or time-independent inward current, but it reduced the sum of L- and T-type Ca2+ currents (ICa,L and ICa,T) in both the presence and absence of BAPTA-AM. In contrast, ICa,L was unchanged by ryanodine. Slow inward current tails, presumed to be Na/Ca exchange current (INa/Ca), were abolished by BAPTA or ryanodine. The results suggest that a decrement of ICa,T, due to reduction of the intracellular Ca2+ concentration or a direct effect of ryanodine on T-type Ca2+ channels, contributes to the negative chronotropic effect. Another possibility, based primarily on theory and results in other preparations, is that a reduction of INa/Ca, as a consequence of the smaller action potential-induced Ca2+i transients, contributes to the effect of ryanodine.

Stereospecific in vitro and in vivo effects of the new sinus node inhibitor (+)-S 16257.

The effects of the two isomers, (+)-S 16257 and (-)-S 16260, of a new bradycardic agent, (+/-)-S 15544 (7,8-dimethoxy 3-[3-[[(4.5-dimethoxybenzocyclobutan-1-yl)methyl] methylamino]propyl]1,3,4,5-tetrahydro-2H-3-benzazepin-2-one), were compared in vitro and in vivo on cardiac spontaneous rate and repolarization time. In the isolated rabbit sino-atrial node, the three compounds (3 microM) were equi-effective to reduce the action potential firing rate. In anesthetized pigs, both isomers (0.03, 0.1, 0.3 and 1 mg kg(-1) i.v.) were equipotent to reduce heart rate. For all compounds, the negative chronotropic effect resulted from a reduction in the slope of diastolic depolarization of pacemaker cells. In sino-atrial node cells, (-)-S 16260 (3 microM) increased action potential duration while (+)-S 16257 had a smaller effect. In driven guinea-pig papillary muscles exposed to increasing concentrations of compounds (0.1 to 10 microM) a small prolongation of action potential duration was observed. This prolongation was more marked in rabbit Purkinje fibers stimulated at a low rate. In all cardiac preparations the highest prolongation was observed with (-)-S 16260. In vivo, (-)-S 16260 prolonged QTc at the two highest doses tested while (+)-S 16257 had no effect. In conclusion, resolution of (+/-)-S 15544 into its two enantiomers yielded compounds with the same bradycardic effects. Of the isomers, (+)-S 16257 has an increased specificity with minimal direct effect on action potential repolarization.

Simulations of postvagal tachycardia at the single cell pacemaker level: a new hypothesis.

Simulations performed on a single cell model of rabbit sinoatrial node activity after prolonged vagal stimulation have been able to reproduce the known characteristics of cycle length recovery, including the presence of rapid and slow recovery phases and the transient undershoot phenomenon known as postvagal tachycardia (PVT). In the model, the PVT component has been hypothesized to result from the recovery of background levels of the muscarinic K+ current iK,ACh from desensitization due to prolonged exposure to acetylcholine (ACh) neurotransmitter. Other components of the recovery were found to be due to the inactivation of iK,ACh after the hydrolysis of ACh (rapid phase) and the recovery of the hyperpolarizing-activated current i(f) from its ACh-induced inhibition (slow phase). The magnitudes of both the rapid component and the PVT were found to increase linearly with preceding vagally mediated increase in cycle length, whereas the gain of the slow component was found to saturate, reflecting the limited contribution of i(f) inhibition to cycle prolongation.

Tyrosine kinase inhibition reduces i(f) in rabbit sinoatrial node myocytes.

We studied pacemaker current (i(f)), the inward current activated by hyperpolarization in rabbit sinoatrial (SA) node myocytes, with the permeabilized-patch-clamp technique. The tyrosine kinase inhibitors genistein (50 microM) or herbimycin A (35 microM) reduced the amplitude of i(f) in response to step hyperpolarizations in the diastolic range of potentials. A two-step voltage-clamp protocol revealed that the reduction in i(f) is due to a decrease in maximal i(f) conductance. The observed effects are due to tyrosine kinase inhibition since an inactive analog of genistein did not reduce i(f). To further examine the mechanism of action, we added 2 mM chlorophenylthio cAMP (CPTcAMP, a membrane-permeant cAMP analog) to the bathing Tyrode, which increased i(f). Genistein still reduced i(f) in the presence of CPTcAMP. This suggests that the pathway mediating the actions of tyrosine kinase inhibition on i(f) is independent of cAMP- or protein-kinase-A-mediated phosphorylation.

Antiarrhythmic and electrophysiological effects of the novel K ATP channel opener, rilmakalim, in rabbit cardiac cells

1. 1. The effects of rilmakalim, a potassium channel opener, were studied on rabbit cardiac Purkinje, ventricular muscle and atrial fibers, with the use of conventional microelectrode techniques. 2. 2. Rilmakalim (0.24-7.2 μM) shortened, in a concentration-dependent manner, the action potential duration (APD) in Purkinje and ventricular muscle without affecting other parameters of the action potential. Pinacidil (30–300 μM) also decreased the APD of Purkinje fibers. 3. 3. Rilmakalim (2.4 μM) and cromakalim (100 μM) hyperpolarized and abolished abnormal automaticity of cardiac Purkinje fibers pretreated with barium (0.2–0.3 mM). Glibenclamide (5 μM) blocked the hyperpolarizing effect. 4. 4. Stable early afterdepolarizations induced in Purkinje fibers by berberine (100 μM) were reversibly blocked by rilmakalim (2.4 μM), which also suppressed late afterdepolarizations induced in Purkinje fibers treated with ouabain (0.3–0.5 μM). 5. 5. The rate of spontaneous discharges of the rabbit sinoatrial node was not affected by rilmakalim (7.2 μM) or by pinacidil (100 μM). Both agents were also unable to affect the APD of atrial muscle fibers. 6. 6. In cardiac Purkinje fibers, tetraethylammonium (TEA; 20 mM) significantly reduced the effects of rilmakalim (2.4 μM) on the APD. However, neither TEA nor glibenclamide (100 μM) reduced the shortening of the APD induced by dinitrophenol (30 μM) or by salicylate (1 mM).

Activation of f-channels by cAMP analogues in macropatches from rabbit sino-atrial node myocytes.

1. The action of the two diastereometric phosphorothioate derivatives of cAMP, Rp-cAMPs and Sp-cAMPs, was investigated on hyperpolarization-activated 'pacemaker' current (i(f)) recorded in inside-out macropatches from rabbit sino-atrial (SA) node myocytes. 2. When superfused on the intracellular side of f-channels at the concentration of 10 microM, both cAMP derivatives accelerated i(f) activation; their action was moderately less pronounced than that due to the same concentration of cAMP. 3. The measurement of the i(f) conductance-voltage relation by voltage ramp protocols indicated that both cAMP analogues shift the activation curve of i(f) to more positive voltages with no change in maximal (fully activated) conductance. 4. Dose-response relationships of the shift of the i(f) activation curve showed that both Rp-cAMPs and Sp-cAMPs act as agonists in the cAMP-dependent direct f-channel activation. Fitting data to the Hill equation resulted in maximal shifts of 9.6 and 9.5 mV, apparent dissociation constants of 0.82 and 5.4 microM, and Hill coefficients of 0.82 and 1.12 for Sp-cAMPs and Rp-cAMPs, respectively. 5. The activating action of Rp-cAMPs, a known antagonist of cAMP in the activation of cAMP-dependent protein kinase, confirms previously established evidence that f-channel activation does not involve phosphorylation. These results also suggest that the cAMP binding site of f-channels may be structurally similar to the cyclic nucleotide binding site of olfactory receptor channels.

Variation in effects of Cs+, UL-FS-49, and ZD-7288 within sinoatrial node.

The effects of Cs+, UL-FS-49, and ZD-7288 on action potentials recorded from central, transitional, and peripheral sites in the rabbit sinoatrial (SA) node have been investigated. In isolated right atrial preparations, including the whole SA node, Cs+ (2 mM), UL-FS-49 (1 microM), and ZD-7288 (3 microM), decreased the spontaneous rate by 12 +/- 2% (n = 21), 16 +/- 3% (n = 10), and 13 +/- 3% (n = 10). They decreased the slope of the pacemaker potential without affecting action potential characteristics. The decrease of pacemaker slope was maximal (69-120%) in the periphery and minimal (22-25%) in the center. In experiments on small ball-like tissue specimens (approximately 0.3 mm diam), a decrease in spontaneous rate and pacemaker slope by Cs+ (2 mM) was largest (approximately 19 and approximately 47%) in tissue from the periphery and least (approximately 7 and approximately 17%) in tissue from the center. Because the hyperpolarization-activated inward current (If) is reduced by each of these agents, the results suggest that this current might play a relatively minor role in the center but a more important role in the periphery of the SA node.