Rabbit Sinus Node Research Articles

Effects of niludipine (Bay-a-7168) on transmembrane action potentials of rabbit sinus node cells and atrial muscle fibers.

Effects of niludipine (bis(2-propoxyethyl)-1,4-dihydro-2,6-dimethyl-4-(3-nitrophenyl)-3,5-pyridinedic arboxylate; Bay-a-7168) on isolated rabbit sinus node and right atrial preparations were studied using intracellular microelectrode techniques. Niludipine in concentrations above 1 X 10(-8) M caused dose-dependent changes in the membrane action potentials of the preparations. In spontaneously firing sinus node cells, a marked decrease in the amplitude (AMP) and maximum rate of depolarization (MRD) of action potentials as well as a slight decrease in their maximum diastolic potential (MDP) were observed after drug administration. In constantly driven atrial muscle fibers, the early repolarizing phase was accelerated (shortening of action potential duration) and AMP was decreased by several millivolts. However, little change occurred in the MDP and MRD of atrial muscle fibers even at the highest concentration (1 X 10(-6) M). Niludipine prolonged the effective refractory period of atrial muscle fibers by 4--8 msec. At the same time, it caused a marked decrease in the amplitude of the premature action potentials and prolonged their propagation time. Most of these effects of niludipine, which may be attributed to the reduction of the slow inward current through the myocardial cell membrane, were antagonized by the concomitant administration of isoproterenol (1 X 10(-6) M).

Functional and morphological organization of the rabbit sinus node.

In isolated right atria of the rabbit heart, we studied the activation pattern within the sinus node, using the microelectrode technique. After the electrophysiological experiments, the preparations were subjected to a correlative morphological investigation, using light or electron microscopy. Different criteria for defining the dominant pacemaker were compared. A group of at least 5000 cells, located within the central part of the node where the most characteristic tissue architecture was found, was considered to be responsible for generation of the impulse. At the ultrastructural level, this leading cell group appeared to be part of a larger uniform cell group. The number of gap junctions observed suggests that all nodal cells are coupled by these structures. Toward the periphery, the excitation wave was propagated preferentially in an oblique cranial direction toward the crista terminalis. Neither morphologically nor electrophysiologically specific pathways were found for the conduction, but the preferential direction could be explained by the tissue architecture.

Post-Overdrive Suppression of the Rabbit Sinus Node Pacemaker Cells : Symposium on Sinus Node Function and Its Abnormality

Effects of rapid stimulation (overdrive) on the isolated rabbit sinus node pacemaker cells were examined in order to elucidate the mechanism of post-overdrive suppression. Small specimens of sinus node tissue having a size of approximately half or less of the length constant of sinus nodes were driven at a fast rate through single sucrose gap methods for 10 to 120 seconds. Under the control perfusion (35°C oxygenated Tyrode's solution), overdrive was followed by a slight suppression of spontaneous activity of the preparations. The post-overdrive suppression was accompanied by the decrease in maximum diastolic potential (MDP), overshoot (OS) and maximum rate of depolarization ((dV/dt)max) as well as a slowing of the rate of diastolic depolarization. The values of MDP, OS, (dV/dt)max and the spontaneous activity returned to the pre-overdrive level within a few seconds. Atropine (2 × 10-6g/ml) did not influence the effects of overdrive. After ouabain administration (2 × 10-7g/ml) or in the low temperature perfusate (25°C), the effects of overdrive were potentiated and a marked post-overdrive suppression characterized by a long pause of over several seconds was obtained. The decrease in MDP, OS and (dV/dt)max following overdrive was also accentuated in both experimental conditions. These results would suggest that the post-overdrive suppression of sinus node is, at least partly, due to some ionic shifts following overdrive and may be potentiated by the metabolic dysfunction of pacemaker cells.

Effects of rapid stimulation on the transmembrane action potentials of rabbit sinus node pacemaker cells.

We studied the mechanism of post-overdrive suppression in superfused rabbit sinus node pacemaker cells. Small specimens of sinus node tissue isolated from rabbit hearts were driven at a fast rate (overdrive) for 10-120 seconds using single sucrose gap methods. During the control perfusion (35 degrees C Tyrode's solution), overdrive caused a progressive decrease in maximum diastolic potential (MDP), overshoot (OS), and maximum rate of depolarization at phase 0 [dV/dt)max]. After cessation of the overdrive, the rate of diastolic depolarization decreased, and the spontaneous activity was suppressed temporarily (post-overdrive suppression). MDP, OS, (dV/dt)max, and the spontaneous activity returned within a few seconds to the level observed before overdrive. Atropine (2 x 10(-6) g/ml) did not influence the effects of overdrive. After ouabain administration (3 x 10(-7) g/ml) or in low temperature perfusate (25 degrees C), the effects of overdrive were accentuated, and a marked suppression of spontaneous activity with a long pause of over several seconds was seen following the overdrive. These results suggest that the post-overdrive suppression of sinus node is attributable, at least in part, to ionic shifts following overdrive, and may be potentiated by metabolic dysfunction of pacemaker cells.

The plasma membrane of leading pacemaker cells in the rabbit sinus node. A qualitative and quantitative ultrastructural analysis.

We studied the fine structure of the plasma membrane of electrophysiologically identified leading pacemaker cells from the rabbit sinus node, using both ultrathin sections of fixed tissue and replicas of freeze-cleaved material. We found that differences exist between sinus node and working myocardial membranes, but these are only quantitative. The caveolae or sarcolemmal invaginations are present in very large numbers; they increase the surface area of the plasma membrane by about 100%. The small macular nexuses that are present represent 0.2% of the membrane surface area. Nexuses are therefore about 10 times less numerous in leading sinus node cells than in working myocardium cells. A single equivalent electrical representation of the sinus node shows, nevertheless, that an appreciable electrical coupling may be expected.

Electrophysiological actions of mexiletine on isolated rabbit atria and canine ventricular muscle and Purkinje fibres

The electrophysiological effects of varying concentrations of mexiletine on isolated rabbit sinus node and atrial preparations. Purkinje fibres, Purkinje-fibre-ventricular muscle junction and distal conduction system ('gate') were studied in oxygenated Tyrode solution at 35 degrees C using standard microelectrode techniques. 'Therapeutic' concentrations of mexiletine (0.5 to 2.0 micrograms.cm-3) had little effect on the atrial action potentials or on sinus node automaticity but sino-atrial conduction was delayed. In Purkinje fibres, these concentrations of the drug depressed the dV/dtmax of phase 0, conduction velocity and membrane responsiveness accompanied by significant shortening of the action potential duration and the effective refractory period, the change in the former always being greater than that in the latter. Mexiletine decreased antegrade as well as retrograde conduction at the Purkinje-fibre-ventricular muscle junction, while producing a uniform shortening of the action potential duration in the distal conduction system without a preferential effect at the 'gate'. The mechanisms of antiarrhythmic actions of mexiletine are thus complex but resemble those of lignocaine in superfused isolated cardiac muscle.

An analysis of the effects of acetylcholine on conduction and refractoriness in the rabbit sinus node.

The effects of acetylcholine (ACh) on sinus node automaticity, atrio-sinus conduction, and refractoriness were studied in 41 isolated rabbit right atrial preparations. Average control rate was 126 beats/min, and ACh 5 x 10(-8) M, 5 x 10(-7) M, and 5 x 10(-6) M significantly decreased heart rate by 7, 15, and 43%, respectively (P less than 0.01, 0.001, and 0.001). Atrio-sinus conduction time at a pacing cycle length of 400 msec did not significantly change during exposure to ACh 5 x 10(-8) and 5 x 10(-7) M. However, the mean effective refractory period (ERP) of the sinus node, at a pacing cycle length of 400 msec, increased from 183 +/- 16 msec to 210 +/- 24 msec during exposure to ACh 5 x 10(-7) M (P less than 0.025). The change in ERP followed the change in action potential duration. In contrast to the lack of effect of ACh 5 x 10(-7) M on atrio-sinus conduction time, ACh 5 x 10(-6) M caused 2:1 atrio-sinus block in 8 of 10 experiments. The site of block was identified using multiple microelectrode impalements, and occurred between the perinodal fibers bordering on the edge of the sinus node and the pacemaker area in the sinus node proper. When the pacing cycle length was increased and 1:1 atrio-sinus conduciton was present, conduction time did not significantly differ from control. At this longer pacing cycle length the mean ERP of the sinus node was 380 msec greater than control and lasted well after repolarization was completed. Thus, atrio-sinus block during exposure to ACh 5 x 10(-6) M resulted from a marked prolongation of refractoriness.

The effect of brief vagal stimulation on the isolated rabbit sinus node.

We developed an isolated rabbit atrial preparation which responds consistently and reproducibly to brief, submaximal stimulation of the autonomic nerves contained in it. In 6 of 11 preparations in the presence of propranolol (1 mg/liter), the time course of changes in the atrial rate following 120 msec vagal stimulation was bimodal. The maximal slowing occurred at 0.64 +/- 0.16 second, and the peak secondary slowing occurred at 2.3 +/- 1.0 seconds. An acceleratory component occurred between the first and second peaks between 0.8 and 1.6 seconds. The total time course of vagal effect lasted for 5.0 +/- 2.0 seconds. These changes in rate could not be explained by shifts in the location of the primary pacemaker. The acceleratory component was due to a 4.7 +/- 2.0 (SD) mV depolarization of the maximum diastolic membrane potential of the primary pacemaker of the sinus node which lasted for 1.8 +/- 0.3 seconds. Following vagal stimulation, there was an increase of 0.2 mM in the activity of potassium in the extracellular space recorded with a potassium-sensitive electrode; this peaked between 1.4 and 2.5 seconds and cleared with an exponential time course. The halftimes for recovery ranged between 2.8 and 4.6 seconds. The initial peak slowing of the bimodal time course and the acceleratory component therefore appear to be direct effects of acetylcholine. The secondary slowing occurs after acetylcholine presumably has been inactivated and occurs coincidently with the accumulation of potassium in the extracellular space.

Sinus node response to premature atrial stimulation in the rabbit studied with multiple microelectrode impalements.

In this study we investigated the response of the isolated rabbit sinus node to ectopic atrial premature beats elicited late in the atrial cycle. In three experiments the response of at least 45 different fibers of the sinus node was recorded, whereas, in other experiments, investigation was less extensive. In this way the spread of activation of the total pacemaker area could be mapped accurately both during spontaneous beating and the induction of ectopic atrial beats of different degrees of prematurity. We found that: (1) The conduction of an impulse from the dominant pacemaker area to the atrium during spontaneous rhythm (antegrade conduction) is slower than the conduction of an ectopic atrial impulse towards the center of the sinus node (retrograde conduction). (2) The action potential of the dominant pacemaker fibers in the sinus node is shortened because of premature activation caused by an ectopic impulse from the atrium. Late premature beats had no effect on diastolic depolarization. If the retrograde activation wave did not reach the area of the dominant pacemaker before the spontaneous discharge of these fibers, there was electrotonic influence demonstrable over a distance of about 0.5 mm. (3) Comparison of the true sinoatrial conduction time with the estimated sinoatrial conduction calculated indirectly from the length of the postextrasystolic atrial cycle revealed that, in the isolated rabbit heart, the calculated value is a serious underestimation of the true antegrade sinoatrial conduction time.

Mechanism of rhythm determination among pacemaker cells of the mammalian sinus node.

The isolated rabbit sinus node was partly divided into two parts by a cut in the middle portion. Microelectrode recording near the bridge connecting the two parts revealed an interference between action potentials from the two parts. Comparison of microelectrode recordings from the two parts taken near the bridge suggested that an induced peculiar rhythm change in one part was probably induced by the electrotonic effects of the action potentials of the other part. To prove this, a subthreshold depolarizing square-wave pulse was applied extracellularly to the isolated uncut sinus node. When the pulse was applied in the early portion of slow diastolic depolarization, diastole was prolonged, and when it was applied in the later portion, diastole was shortened. These findings can explain the observed peculiar thythm and suggest that in the mammalian sinus node, pacemaker cells accelerate or decelerate mutually by the electrotonic effects of their action potentials, depending on the phase of application of the effects. In particular, for some time the faster pacemaker cells could be influenced by dragging effects from the neighboring slower pacemaker cells and the slower pacemaker cells by pulling effects from the neighboring faster pacemaker cells.

Anatomy of the cardiac conduction system in the rabbit.

Anatomy of the cardiac conduction system was studied in 15 rabbits with serial histologic sectioning of 4. The rabbit's sinus node is a simple and distinct structure located at the junction of the crista terminalis with the sinus intercavarum nearly midway between the two cavae; it contains an unusually large percentage of P cells but is not organized about a large central artery. The AV node and His bundle are relatively small and are displaced anteriorly by the large coronary sinus normally present in the rabbit because of its persisting left superior vena cava. The AV node is organized with input system and bypass tracts similar to those of man and the dog. Doth the AV node and His bundle contain many conspicuously large nerve trunks. The blood supply of the sinus node is from terminal small branches of both the right and left coronary arteries and that to the AV node and His bundle is from similar terminal branches of the septal artery only. The possible functional significance of some of these anatomic features is discussed.

Effect of temperature on pacemaker activity of rabbit sinus node.

ARTICLESEffect of temperature on pacemaker activity of rabbit sinus nodeS Yamagishi, and T SanoS Yamagishi, and T SanoPublished Online:01 Apr 1967https://doi.org/10.1152/ajplegacy.1967.212.4.829MoreSectionsPDF (1 MB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations ShareShare onFacebookTwitterLinkedInWeChat Previous Back to Top Next Download PDF FiguresReferencesRelatedInformation Cited ByMaximum heart rate in brown trout (Salmo trutta fario) is not limited by firing rate of pacemaker cellsJaakko Haverinen, Denis V. Abramochkin, Andre Kamkin, and Matti Vornanen1 February 2017 | American Journal of Physiology-Regulatory, Integrative and Comparative Physiology, Vol. 312, No. 2Effects of Low Temperature on the Chronotropic and Inotropic Responses to Zatebradine, E-4031 and Verapamil in Isolated Perfused Dog Atria.The Japanese Journal of Pharmacology, Vol. 78, No. 4 More from this issue > Volume 212Issue 4April 1967Pages 829-834 Copyright & PermissionsCopyright © 1967 by American Physiological Societyhttps://doi.org/10.1152/ajplegacy.1967.212.4.829PubMed6024446History Published online 1 April 1967 Published in print 1 April 1967 Metrics