Anisotropy of electrotonus in the sinoatrial node of the rabbit heart

In fibers of the sinoatrial node of isolated right atria of rabbits the decay of the electrotonic potential caused by intracellular current injection was measured in two directions: parallel to the crista terminalis and perpendicular to it. A K+-perfused extracellular suction electrode was used to apply current pulses (10(-5) A, 60 ms) to fibers located in the primary center of the SA node every fourth cardiac cycle at a fixed moment during diastole. The decay of the electrotonic spread was measured in a series of impalements on a straight line from the current source. Space constants were calculated by fitting single exponential curves to the data. Considerable regional differences in space constant values were found in either direction. Parallel to the crista terminalis the mean value was 529 +/- 446 microns (S.D., n = 7), perpendicular to it 306 +/- 295 microns (n = 12); the difference was not significant (P less than 0.2). However, a significant anisotropy (P less than 0.05) of the electrotonic spread was found when measurements were taken from small areas of the node. Large abrupt changes in the electrotonic potential within 200 microns were observed in the center of the node. These data indicate a non-uniformity of electrotonic spread in this part of the SA node.     

Effect of transmural vagal stimulation on electrotonic current spread in the rabbit sinoatrial node.

OBJECTIVE: The effect of vagal stimulation on the decay of electrotonic potential caused by intracellular current injection and on input resistance was measured in the sinoatrial node of isolated rabbit right atria. METHODS: Studies were performed on New Zealand White rabbits weighing approximately 2-3 kg. Vagal stimulation was achieved by transmural stimulation of intramural nerve fibres in the presence of propranolol. A K+ perfused suction electrode was used to inject hyperpolarising current pulses; input resistance was measured by means of a double barrel microelectrode. RESULTS: Vagal stimulation which caused a 14-20% increase of cycle length diminished electronic potential significantly by a decrease of membrane resistance. The input resistance of the sinoatrial node was not affected. Space constant values calculated by using either a one or a two dimensional model of electrotonic current spread were decreased on average by 13% and 14% respectively. CONCLUSIONS: The results from this study show that vagal stimulation which gave rise to a moderate negative chronotropic effect and marked changes in action potential configuration of nodal fibres affects the electrotonic interaction within the sinoatrial node. This may have consequences for the electrical activity and synchronisation of the sinoatrial nodal fibres.     

Microelectrode study of Wenckebach periodicity in canine Purkinje fibers

Electrophysiologic mechanisms responsible for Wenckebach periodicity produced experimentally in a segment of blocked canine Purkinje fibers were investigated with multiple microelectrode recordings. Transmission through the zone of block was electrotonic. The Wenckebach mechanism was related to a progressive decrease in the efficacy of the transmitted electrotonic potential as a stimulus for the regenerative response at the distal block boundary with successive impulses of the cycle. This was manifested in the transmembrane potentials as a progressive decrease in upstroke velocity and a voltage change that caused the electrotonic potential to become progressively removed from the threshold for stimulation. The immediate cause of this phenomenon was a progressive increase in the voltage level (more negative) from which the electrotonic potential originated and a resulting change in voltage-dependent membrane resistance that further attenuated the signal with successive impulses of the cycle.Two main mechanisms appeared to be responsible for these phenomena: (1) a progressive increase in maximal repolarization voltage (more negative) of the transmembrane potential, and (2) a progressive decrease in the diastolic interval that, in the presence of enhanced phase 4 diastolic depolarization, caused the voltage level from which the electrotonic potential originated to become further removed from the stimulation threshold with successive impulses of the cycle.     

Increased spread of electrotonic potentials during diastolic depolarization in cardiac muscle

The spread of electrotonic potentials during diastolic depolarization was measured in canine Purkinje fibres immersed in normal Tyrode solution. The results indicate that the space constant is gradually enhanced during the pacemaker potential reaching an optimum before the firing of the action potential. The increase in space constant is mainly due to a rise in rm but a decline in ri was also found. Epinephrine (1 microgram/ml) increased the spread of electrotonic potentials during diastolic depolarization. Assuming that the myoplasmic resistivity is not changing during the pacemaker potential the results obtained might indicate an increase in junctional conductance during this phase of cardiac cycle.     

Microelectrode study of high grade block in canine Purkinje fibers

Previous reports from this laboratory described an electrotonic mechanism for simple impulse transmission through blocked segments of canine Purkinje tissue with slow diastolic depolarization assuming a vital role in second degree block. Utilizing an electrical blocking current, a blocked segment of canine Purkinje tissue was produced. Transmembrane events were recorded from blocked segments during higher grades of block (3:1 to complete) to delineate further mechanisms responsible for a periodic distal boundary response. Our results confirm that slow diastolic depolarization is an important determinant in sustaining periodic impulse conduction. Its importance is related to (1) progressive decrease of the resting membrane potential toward threshold at the distal block boundary, and (2) augmentation of the transmitted electrotonic potential in accordance with voltage dependent changes in membrane resistance. These data further lend definition to the distinction between electrotonic, partially active, and active transmembrane potentials. Impulse transmission through a segment of inactivated tissue is electrotonic and slow diastolic depolarization plays an important role in the maintenance of periodic impulse transmission.     

Sustained inward current during pacemaker depolarization in mammalian sinoatrial node cells

Several time- and voltage-dependent ionic currents have been identified in cardiac pacemaker cells, including Na(+) current, L- and T-type Ca(2+) currents, hyperpolarization-activated cation current, and various types of delayed rectifier K(+) currents. Mathematical models have demonstrated that spontaneous action potentials can be reconstructed by incorporating these currents, but relative contributions of individual currents vary widely between different models. In 1995, the presence of a novel inward current that was activated by depolarization to the potential range of the slow diastolic depolarization in rabbit sinoatrial (SA) node cells was reported. Because the current showed little inactivation during depolarizing pulses, it was called the sustained inward current (I(st)). A similar current is also found in SA node cells of the guinea pig and rat and in subsidiary pacemaker atrioventricular node cells. Recently, single-channel analysis has revealed a nicardipine-sensitive, 13-pS Na(+) current, which is activated by depolarization to the diastolic potential range in guinea pig SA node cells. This channel differs from rapid voltage-gated Na(+) or L-type Ca(2+) channels both in unitary conductance and gating kinetics. Because I(st) was observed only in spontaneously beating SA node cells, ie, it was absent in quiescent cells dissociated from the same SA or atrioventricular node, an important role of I(st) for generation of intrinsic cardiac automaticity was suggested.     

Effects of bepridil on the electrophysiologic properties of isolated canine and rabbit myocardial fibers

Bepridil hydrochloride is a relatively new calcium antagonist which appears to have a complex pharmacologic profile, but its concentration-response characteristics with respect to its electrophysiologic properties of varying concentrations (0.1 to 10.0 micrograms/ml) of the drug were therefore determined in rabbit and canine myocardial fiber preparations in vitro by standard microelectrode techniques. The following were measured: sinus cycle length (SCL), action potential amplitude (APA), maximum diastolic potential (MDP), threshold potential (TP), slope of phase 4 depolarization, action potential duration (APD), and dV/dtmax of phase O depolarization (Vmax) in rabbit sinoatrial (SA) node. Also measured were APA, membrane resting potential (MRP), Vmax, APD at 50% and 90% repolarization (APD50 and APD90), and effective refractory period (ERP) in rabbit atria and canine Purkinje fibers and ventricular muscle. At the lowest concentrations bepridil selectively prolonged SCL by reducing the slope of phase 4 and decreased APA and MDP in a concentration-dependent manner in the sinus node. At higher concentrations, bepridil exerted additional effects in producing concentration-dependent decreases in APA and Vmax in rabbit atria and in canine Purkinje fibers and ventricular muscle. During superfusion with 1.0 micrograms/ml bepridil, Vmax fell by 22.2% (p less than 0.05) in Purkinje fibers and by 11.8% (NS) in ventricular muscle; at 10.0 micrograms/ml, Vmax fell by 46.5% (p less than 0.01), respectively. The depression of Vmax was frequency dependent. There was a differential effect of bepridil on repolarization in Purkinje fibers as compared to that in ventricular muscle.(ABSTRACT TRUNCATED AT 250 WORDS)     

Evidence for the presence of electrotonic depression of pacemakers in the rabbit atrioventricular node. The effects of uncoupling from the surrounding myocardium

Summary In the isolated AV junctional preparation of the rabbit heart, the presence of electrotonic influences on impulse formation was investigated. After disconnection of the sinus node, impulse formation started in the junctional area with a mean frequency of 72 beats/min (n=17), which is about 40% of the sinus rate. Intracellular recordings were obtained to determine pacemaker location and activation sequence in the junctional area. The pacemaker was always located in the area of the lower nodal fibers of the AV node (thus distally from the site of maximal conduction delay) and these fibers had the highest rate of diastolic depolarization. Since it is known that pacemaker fibers are electrotonically influenced by their neighbouring cells, we investigated whether AV nodal automaticity was influenced by its surrounding tissue. Therefore the AV node was isolated from the atrial tissue and His bundle. This caused an enormous increase in diastolic depolarization rate, especially in the lower nodal fibers, accompanied by a rhythm acceleration to a mean of 137 beats/min. From the findings of the present study it was concluded that under normal conditions impulse formation in the lower nodal fibers of the rabbit AV node is electrotonically depressed by the connecting myocardium.     

Non-uniform electrophysiological properties and electrotonic interaction in hypertrophied rat myocardium

We studied the distribution and nature of the electrical changes associated with myocardial hypertrophy induced by renal hypertension in rats. Standard microelectrode techniques were used to study transmembrane action potentials recorded from endocardial, papillary muscle, and epicardial stimulation from hypertrophied (HBP) and normal (SHAM) hearts. We also determined the effects of stimulation frequency on the action potentials recorded from these preparations. To assess whether altered intercellular electrical connections contribute to the electrophysiological changes associated with hypertrophy, we analyzed the spatial steady state voltage decrement produced by passing intracellular constant current pulses and determined the effective input resistance (Rin) of endocardial HBP and SHAM preparations. Our results show that the action potential prolongation that accompanies hypertrophy is not uniform. Thus, the entire course of repolarization is prolonged in epicardial and papillary muscle fibers, but only the latter half of repolarization is prolonged in epicardial fibers. Endocardial action potentials is general, and HBP action potentials in particular, have a distinctive steep relation between duration and stimulation frequency which may be due to a difference in the rate dependence of a membrane conductance(s), although relatively greater accumulation of extracellular potassium or altered activity of the Na+-K+ pump cannot be excluded as contributing factors. In addition, the similarity in the profile of spatial voltage decrement and the values for Rin in HBP and SHAM preparations indicates that alterations in electrotonic coupling between cells are unlikely to account for the prolonged action potentials of hypertrophied myocardium.     

Effect of hypoxia on the sinoatrial node, atrium, and atrioventricular node in the rabbit heart.

We used intracellular microelectrodes to study the effects of hypoxia on the isolated, superfused sinoatrial (SA) node, atrium, and atrioventricular (AV) node of the rabbit heart. Hypoxia decreased the rate of spontaneous impulse initiation in SA nodal fibers by decreasing the slope of diastolic depolarization. With gradually decreasing Po2, the sinus rate was reduced; concomitantly, the corrected sinus node recovery time after rapid atrial stimulation was much less affected demonstrating marked prolongation only under severe anoxic conditions. Hypoxia decreased the amplitude of action potentials of the SA node and of the AV node but not of the atrium. SA and AV nodal conduction were slowed by hypoxia; intraatrial conduction was not significantly affected. AV nodal conduction block occurred at lower atrial rates, and the effective refractory period of the AV node was prolonged. Inhomogeneity of SA and AV nodal impulse propagation often was observed in the presence of hypoxia. This was associated with concealed reentry within both nodal areas. The extracellular K+ concentration of the atrial tissue was measured with ion-sensitive microelectrodes. [K+]o remained unchanged even after prolonged periods of severe hypoxia. These results are consistent with the hypothesis that acute hypoxia predominantly inhibits slow response activity but has only little effect on the fast inward sodium current.     

Action potentials that mimic fibrillation activate sodium current.

Ventricular fibrillation (VF) has brief action potentials (50-70 ms) with short diastolic intervals (10-30 ms). Under these conditions ion channel activity may be grossly different to normal sinus rhythm (NSR). In particular, sodium channel activation may not contribute to the generation and propagation of action potentials during VF. This study determined if sodium channels can be activated when action potentials mimic VF. Isolated chick ventricular myocytes (n=7) were voltage-clamped to quantitate fast inward sodium current. The voltage clamp protocol simulated VF with a 10 pulse train at 10 Hz (100 ms cycle length (CL)) and depolarization interval (action potential duration) ranging from 90 to 20 ms. After each train a test pulse was delivered from holding (-80 mV) in 10-ms steps. The train preceded each step pulse. Peak sodium current for control and each VF protocol occurred at a membrane potential (V(m)) of -10 mV. Sodium current was evident during brief resting intervals as short as 20 ms, albeit 10-20% of baseline. Resting intervals less than 60 ms shifted the sodium conductance activation curve from Vm(0.5)-30 mV to -22 mV membrane potential. Similar findings occurred when resting potential was at -65 mV, although there was less sodium current with all tested protocols. There was significantly less inactivation of sodium current when the prepulse was shorter (100 v 1000 ms). There was approximately 20% greater sodium current when the test pulse followed a short v long depolarized (>-80 mV) prepulse. Although the longer depolarization pulses produce approximately 20% greater sodium current at membrane potentials more negative than -80 mV. Lastly the time for half recovery of sodium current from activation was significantly less when the inactivating prepulse was short v long (45.9+/-9 v 118+/-20 ms, P<0.05). In conclusion, sodium current is evident when the diastolic rest interval is as brief as 10-20 ms. Rest interval, length of membrane depolarization and membrane potential interact to affect sodium channel activation, inactivation and recovery from inactivation. These data demonstrate that the brief action potentials at more depolarized membrane potentials seen during VF allow for inward sodium current upon depolarization, less sodium channel inactivation, and a faster recovery from inactivation, thereby compensating for a short diastolic rest interval. Therefore, it is likely that the inward sodium channel contributes to wave front propagation during ventricular fibrillation.     

Synaptic background activity influences spatiotemporal integration in single pyramidal cells.

The standard one-dimensional Rall cable model assumes that the electrotonic structure of neurons does not change in response to synaptic input. This model is used in a great number of both theoretical and anatomical-physiological structure-function studies. In particular, the membrane time constant, tau m, the somatic input resistance, Rin, and the electrotonic length are used to characterize single cells. However, these studies do not take into account that neurons are embedded in a network of spontaneously active cells. Synapses from these cells will contribute significantly to the membrane conductance, especially if recent evidence of very high specific membrane resistance, Rm = 100 k omega.cm2, is taken into account. We numerically simulated the electrical behavior of an anatomically reconstructed layer V cortical pyramidal cell receiving input from 4000 excitatory and 1000 inhibitory cells firing spontaneously at 0-7 Hz. We found that, over this range of synaptic background activity, tau m and Rin change by a factor of 10 (80-7 msec, 110-14 M omega) and the electrotonic length of the cell changes by a factor of 3. We show that this significantly changes the response of the cell to temporal desynchronized versus temporal synchronized synaptic input distributed throughout the neuron. Thus, the global activity of the network can control how individual cells perform spatial and temporal integration.     

Automatism: an intrinsic property of cardiac tissues

Automaticity is an intrinsic property of some types of cardiac tissues, like the nodes and the conducting fibers. Automatic cells are characterized by the appearance, during diastole, of a slow depolarization which is capable of reaching threshold and generate an action potential. This depolarization is the so-called slow diastolic depolarization, or pacemaker potential. Tissues with automaticity do not have the same intrinsic firing frequency, which determines the existence of a true pacemaker; normally this role is assumed by the sinus node and several latent, or subsidiary pacemakers. The latter do not give any manifestation, because, since they have a slower firing frequency than that of the true pacemaker, the impulses generated by the latter will activate them before they can reach threshold. The ionic mechanisms that originate the pacemaker potential are just some of the processes that constitute the electrophysiological properties of the cardiac cell membranes. The mechanism includes the slow inactivation of an outward current combined with a constant inward current. Recently published evidence suggests that the pacemaker potential could be the resultant of an inward current that is slowly activated during the final part of the repolarization, but these data are still controversial and difficult to interpret. The factors that regulate heart rate modify the electrophysiological characteristics of the membrane by means of three major mechanisms: the slope of the pacemaker potential, the voltage of the threshold potential and, the maximum diastolic potential. Some of the most important factors that modulate heart rate include the neurotransmitters of the automatic nervous system, the plasmatic levels of some ions, like potassium and calcium, and changes in the intrinsic properties of the membrane, like the influence of heart rate on the activity of the sodium pump.     

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

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

Sinus node automaticity during atrial fibrillation in isolated rabbit hearts.

BACKGROUND. It is still unclear what role the sinus node may play in the genesis or perpetuation of atrial fibrillation. Therefore, we studied the electrical activity in different regions of the sinus node during atrial fibrillation. METHODS AND RESULTS. In Langendorff-perfused rabbit hearts, paroxysms of atrial fibrillation were induced by burst pacing. Standard microelectrode techniques were used to record transmembrane potentials from different regions of the sinus node. We found that during atrial fibrillation, a high degree (5:1) of sinoatrial entrance block was present that protected the pacemaker fibers in the center of the sinus node against the high rate of fibrillatory impulses. As a result, the true pacemaker fibers in the center of the node were activated with only a slightly higher average rate than during sinus rhythm. Spontaneous diastolic depolarization was still present but was modulated by electrotonic depolarizations due to intranodal conduction block of atrial fibrillatory impulses. Incidentally, phase 4 depolarization resulted in the generation of spontaneous action potentials in the sinus node. However, the high activation rate in the sinoatrial border during atrial fibrillation prevented these spontaneous impulses to exit from the sinus node. Because of the minimal degree of sinus node overdrive suppression (9%) and the presence of concealed automaticity during atrial fibrillation, spontaneous termination of atrial fibrillation was promptly followed by resumption of normal sinus rhythm. CONCLUSIONS. During atrial fibrillation, sinus automaticity still is present in the center of the sinus node and hardly overdrive suppressed due to a high degree of sinoatrial entrance block.     

The Cardiac Pacemaker Current if

Pacemaker Current if. Since the hyperpolarization-activated current, if, was originally associated with the diastolic depolarization phase of action potential in the sinoatrial (SA) node in 1979, its central role in the generation and control of pacemaker activity has become increasingly clear through a series of experimental findings, some of which have substantially modified the pre-existing theories of cardiac pacemaking and its modulation by the autonomic transmitters. Thus, the pacemaker current of Purkinje fibers, formerly described as a deactivating pure potassium (K) current, was found to be in fact, like the nodal if, inward and activating on hyperpolarization. Furthermore, in SA node cells, as well as mediating rhythm acceleration induced by catecholamines, if was found to underlie the slowing effect of low acetylcholine (ACh) concentrations, in contrast with the generally accepted hypothesis that activation of a K conductance is the main process responsible for cardiac slowing. A final, atypical property of if recently demonstrated concerns the activating action exerted on if by intracellular cAMP. Unlike that on other voltage-gated, cAMP-modulated cardiac channels, this action is independent of phosphorylation and involves a direct binding of cAMP to if channels. (J Cardiovasc Electrophysiol, Vol. 3, pp. 334–344, August 1992)     

Mechanisms by which discharge modulates diastolic depolarization in sheep and dog Purkinje fibers

For reasons unknown, a fast drive is prone to induce overdrive excitation in sheep Purkinje fibers under conditions that still induce overdrive suppression in dog Purkinje fibers. Our aim was to study by means of a microelectrode technique diastolic depolarization (DD) and its changes with overdrive in sheep and dog Purkinje fibers perfused in vitro under identical conditions. The major results are: (a) At a slow rate, diastolic depolarization is much faster and larger in sheep than in dog Purkinje fibers. (b) Faster rates increase DD slope and amplitude in sheep and decrease them in dog Purkinje fibers. (c) DD slope and amplitude increase in sheep and decrease in dog if the same number of action potentials are separated by a shorter diastole. (d) The change in DD slope and amplitude induced by a fast drive persists after a subsequent slow drive of approximately 20 s. (e) The fastest drives can induce an oscillatory potential superimposed on early DD in sheep. (f) In both species, high [Ca(2+)](o) increases and low [Ca(2+)](o) decreases DD slope and amplitude. (g) Neither high nor low [Ca(2+)](o) change the DD rate-dependence patterns peculiar to either species. (h) DD amplitude in dog in high [Ca(2+)](o) is still smaller than that in sheep in Tyrode solution. (i) Caffeine prevents the steepening of early DD by drive, but not the subsequent increase which can lead to overdrive excitation in both species. (j) TTX decreases DD slope and amplitude in both species. (k) Cs(+) markedly reduces DD slope and amplitude and more so at faster rates, especially in the sheep. We conclude that the differences in diastolic depolarization and the different behavior of DD with overdrive in the two species account for the propensity of sheep Purkinje fibers to develop overdrive excitation and for that of dog Purkinje fibers to develop overdrive suppression.     

Contribution of sodium channel mutations to bradycardia and sinus node dysfunction in LQT3 families

One variant of the long-QT syndrome (LQT3) is caused by mutations in the human cardiac sodium channel gene. In addition to the characteristic QT prolongation, LQT3 carriers regularly present with bradycardia and sinus pauses. Therefore, we studied the effect of the 1795insD Na+ channel mutation on sinoatrial (SA) pacemaking. The 1795insD channel was previously characterized by the presence of a persistent inward current (Ipst) at -20 mV and a negative shift in voltage dependence of inactivation. In the present study, we first additionally characterized Ipst over the complete voltage range of the SA node action potential (AP) by measuring whole-cell Na+ currents (INa) in HEK-293 cells expressing either wild-type or 1795insD channels. Ipst for 1795insD channels varied between 0.8+/-0.2% and 1.9+/-0.8% of peak INa. Activity of 1795insD channels during SA node pacemaking was confirmed by AP clamp experiments. Next, Ipst and the negative shift were implemented into SA node AP models. The -10-mV shift decreased sinus rate by decreasing diastolic depolarization rate, whereas Ipst decreased sinus rate by AP prolongation, despite a concomitant increase in diastolic depolarization rate. In combination, moderate Ipst (1% to 2%) and the shift reduced sinus rate by approximately 10%. An additional increase in Ipst could result in plateau oscillations and failure to repolarize completely. Thus, Na+ channel mutations displaying an Ipst or a negative shift in inactivation may account for the bradycardia seen in LQT3 patients, whereas SA node pauses or arrest may result from failure of SA node cells to repolarize under conditions of extra net inward current.     

The role of Ca2+ release from sarcoplasmic reticulum in the regulation of sinoatrial node automaticity

Summary The role of Ca²⁺ release channels in the sarcoplasmic reticulum in modulating physiological automaticity of the sinoatrial (SA) node was studied by recording transmembrane action potentials and membrane ionic currents in small preparations of the rabbit SA node. Ryanodine, which modifies the conductance and gating behavior of the Ca²⁺ release channels, was used to block Ca²⁺ release from the sarcoplasmic reticulum. Superfusion of 1-mM ryanodine decreased the spontaneous firing frequency as well as the maximal rate of depolarization of the SA, and these reductions reached a steady state within approximately 5min. The action potential recordings revealed that the latter part of diastolic depolarization was depressed and that the take-off potential became less negative. This suggested that the negative chronotropic effect of ryanodine resulted from the blockade of physiological Ca²⁺ release from the sarcoplasmic reticulum. In voltage clamp experiments, using double-microelectrode techniques, ryanodine did not markedly reduce the Ca²⁺ current (I_Ca) but decreased the delayed rectifying K+ current (I_K), the steady-state inward current (I_ss), and the hyperpolarization-activated inward current (I_h). These observations suggest that, even when the function of Ca²⁺ channels in the cell membrane is normally maintained, depression of Ca²⁺ release channels in the sarcoplasmic reticulum would prevent sufficient elevation of the Ca²⁺ concentration in SA node cells for the activation of various ionic currents, and, thus adversely affect the physiological automaticity of this primary cardiac pacemaker.     

Electrophysiology of the ageing rabbit and cat sinoatrial node -- a comparative study

Intrinsic properties of the human sinoatrial (SA) node havebeen shown to decline with age. In the present study we aimedat investigating the underlying mechanisms of age-dependentchanges in intrinsic cycle length and sinoatrial conductiontime. To this end, the cycle length and transmembrane potentialsof the SA nodes of rabbits (2 days–5.6 years) and cats(6 weeks–1.8 years) were recorded and nodal conductionwas reconstructed. The size of the SA nodes was measured inSirius Red stained sections. Cycle length increases with age in both the rabbit and cat SAnode, and in both species cycle length is dependent on diastolicdepolarization rate and action potential duration. Nodal actionpotential duration increases with age in both rabbit and cat,whereas diastolic depolarization rate decreases in the cat only. The location of the primary pacemaker is not related to age.With age, sinoatrial conduction time increases in both speciesas a result of an enlargment of the area with low phase 0 upstrokevelocities. The size of the SA node of adult animals does notincrease with age.