Effects of calcium channel antagonists on the vagally mediated cardiac chronotropic response in dogs

A brief electrical stimulation of the vagus nerve may elicit a triphasic response comprising (i) an initial prolongation of the same or the next cardiac cycle, (ii) a return of the subsequent cardiac cycle to about the level prior to vagal stimulation, and (iii) a secondary prolongation of cardiac cycle length that lasts several beats. We compared the effects of two calcium channel antagonists, verapamil and nifedipine, on this triphasic response to vagal stimulation in chloralose-anesthetized, open-chest dogs. In the absence of vagal stimulation, nifedipine (doses of 10, 40, and 50 micrograms/kg for a total dose of 100 micrograms/kg, i.v.) and verapamil (two doses of 100 micrograms/kg each, i.v.) increased the cardiac cycle length (A-A interval) by 16% (429 +/- 20 to 496 +/- 21 ms) and 29% (470 +/- 33 to 605 +/- 54 ms), respectively. Nifedipine (100 micrograms/kg total) attenuated the initial vagally mediated prolongation of the A-A interval, from 474 +/- 19 to 369 +/- 42 ms above the basal A-A interval. Following the initial prolongation of the vagal effect, other A-A intervals were not affected. In contrast, verapamil potentiated the vagally mediated initial prolongation in cardiac cycle length at the first dose administered (100 micrograms/kg) from 492 +/- 17 to 561 +/- 14 ms, but other increases in dosages had no further effect. Thus these two calcium channel antagonists have different effects on the sinoatrial chronotropic responses caused by brief vagal stimulation.     

Effect of clonidine on cardiac baroreflex delay in humans and rats

The delay τ between rising systolic blood pressure (SBP) and baroreflex bradycardia has been found to increase when vagal tone is low. The α(2)-agonist clonidine increases cardiac vagal tone, and this study tested how it affects τ. In eight conscious supine human volunteers clonidine (6 μg/kg po) reduced τ, assessed both by cross correlation baroreflex sensitivity and sequence methods (both P < 0.05). Experiments on urethane-anaesthetized rats reproduced the phenomenon and investigated the underlying mechanism. Heart rate (HR) responses to increasing SBP produced with an arterial balloon catheter showed reduced τ (P < 0.05) after clonidine (100 μg/kg iv). The central latency of the reflex was unaltered, however, as shown by the unchanged timing with which antidromically identified cardiac vagal motoneurons (CVM) responded to the arterial pulse. Testing the latency of the HR response to brief electrical stimuli to the right vagus showed that this was also unchanged by clonidine. Nevertheless, vagal stimuli delivered at a fixed time in the cardiac cycle (triggered from the ECG R-wave) slowed HR with a 1-beat delay in the baseline state but a 0-beat delay after clonidine (n = 5, P < 0.05). This was because clonidine lengthened the diastolic period, allowing the vagal volleys to arrive at the heart just in time to postpone the next beat. Calculations indicate that naturally generated CVM volleys in both humans and rats arrive around this critical time. Clonidine thus reduces τ not by changing central or efferent latencies but simply by slowing the heart.     

Dynamics of the vagus effect on the cardiac rhythm during blockade of various subtypes of M-cholinoreceptors in cats

While single vagus bursts were used in cats with an incremental time delay following P-wave of the ECG, two zones were identified within the cardiac cycle differing from each other by their chronotropic responses. At the initial (approximately 120-130 ms) part of the cardiac cycle, an increase in the P-stimulus interval evoked a "moderate" (+8-16%) increment of the chronotropic response up to its maximal amplitude. Further increase of that interval provoked an "abrupt" (-80-90%) decrease of the vagus response. Block of M1-(pirenzepine), M2-(metoctramine and gallamine) or M3-(DAMP) cholinoreceptors diminished vagally-induced minimal and maximal prolongation of the ECG P-P interval and decreased the amplitude of its alterations associated with varying the position of vagus stimulus within the cardiac cycle. The coefficient delineating magnitude of the vagus effect over a zone with "moderate" changes of the chronotropic response was decreased after blocking the M1- and M2-cholinoreceptors, whereas duration of that zone was shortened following blockade of the M1- and M3-receptors. Velocity of the original vagus response and the rate of its subsequent decline decreased following blockade of the M1- and M2-subtypes of cholinoreceptors.     

A model of the phase-sensitivity of the pacemaking cell in the bullfrog heart.

In this study, mathematical models of the bullfrog sinus venosus (SV) pacemaker cell (Rasmusson et al., 1990, Am. J. Physiol. 259, H352-H369) and the ACh-sensitive K+ channel (Shumaker et al., 1990, Biophys. J. 57, 567-576) are combined to simulate the response of the SV myocyte to brief hyperpolarizing currents or acetylcholine (ACh) pulses. These simulations provide an ionic basis for the interpretation of the response of this pacemaker cell to either single perturbation or periodic stimuli. The model predicts that the effects of ACh stimulation on the pacemaker cycle length are dependent both on the phase and temporal characteristics of the [ACh] waveform. For example, the simulations show that (1) although ACh normally has an inhibitory effect on the pacemaker model, for cases where the rise time and duration of the [ACh] waveform are sufficiently brief, ACh can paradoxically accelerate the beat in which a single stimulus is given; (2) the SV pacemaker normally exhibits type 1 (odd) phase-resetting in response to ACh delivery, however type 0 (even) phase-resetting behavior may be exhibited when the [ACh] waveform is large enough and has a very fast rise time; and (3) the SV pacemaker may become phase-locked to a repetitive ACh stimulus applied with either a constant period or coupling interval. In the latter case, this entrainment phenomenon has implications for the control of the cardiac pacemaker by a neural oscillator (e.g. located in the medullary cardiovascular control center) which provides input to the pacemaker cell via the vagus nerve. In these regions of capture, repetitive ACh stimulation produces a well-known paradoxical accelerative effect on the SV pacemaker cell, similar to that seen in a variety of other species.     

Model of bidirectional interaction between myocardial pacemakers based on the phase response curve

As a basis for the study of sinus rhythm determination, a model is proposed of bidirectionally-coupled oscillators as a system of difference equations based on the phase response curve of sinoatrial pacemaker cells. Solutions corresponding to the one-to-one synchronization of the two pacemakers are obtained, and the relation among those solutions is examined: It is revealed that two different solutions with different cycle length coexist, and the synchronized frequency can be higher or lower than the original intrinsic frequencies of the two pacemaker cells. The experimental results of the cultured cells of cardiac pacemakers are interpreted by the analytical result of the model.     

Cardiac Arrest during Gamete Release in Chum Salmon Regulated by the Parasympathetic Nerve System

Cardiac arrest caused by startling stimuli, such as visual and vibration stimuli, has been reported in some animals and could be considered as an extraordinary case of bradycardia and defined as reversible missed heart beats. Variability of the heart rate is established as a balance between an autonomic system, namely cholinergic vagus inhibition, and excitatory adrenergic stimulation of neural and hormonal action in teleost. However, the cardiac arrest and its regulating nervous mechanism remain poorly understood. We show, by using electrocardiogram (ECG) data loggers, that cardiac arrest occurs in chum salmon (Oncorhynchus keta) at the moment of gamete release for 7.39±1.61 s in females and for 5.20±0.97 s in males. The increase in heart rate during spawning behavior relative to the background rate during the resting period suggests that cardiac arrest is a characteristic physiological phenomenon of the extraordinarily high heart rate during spawning behavior. The ECG morphological analysis showed a peaked and tall T-wave adjacent to the cardiac arrest, indicating an increase in potassium permeability in cardiac muscle cells, which would function to retard the cardiac action potential. Pharmacological studies showed that the cardiac arrest was abolished by injection of atropine, a muscarinic receptor antagonist, revealing that the cardiac arrest is a reflex response of the parasympathetic nerve system, although injection of sotalol, a β-adrenergic antagonist, did not affect the cardiac arrest. We conclude that cardiac arrest during gamete release in spawning release in spawning chum salmon is a physiological reflex response controlled by the parasympathetic nervous system. This cardiac arrest represents a response to the gaping behavior that occurs at the moment of gamete release.     

Afferentation of the heart in pathological states of the myocardium

The state of afferent connections between sinoatrial node zone (SNZ) in the normal and affected myocardium (reversible ischemia, necrosis) and rostral parts of the brain cortex (RPC) was studied with the help of evoked potentials in acute experiments on cats. Interruption of afferent connections (SNZ-RPC) during reversible ischemia and experimental myocardial necrosis of the right ventricle was revealed. Ultramicroscopic data of SNZ studies as well as electrophysiologic changes registered in RCP during irritation of the peripheral part of the right vagus nerve and local application of GABA on SNZ suggest possible interruption of cardiac afferentation at the level of SNZ due to the mechanism of presynaptic inhibition.     

Mechanisms of cutaneous-cardial interconnections

Using the method of double stimuli, some features of inhibitory influences of conditioning stimulation of cutaneous zones on the afferent transmission from the heart were studied in the experiments on cats. It has been shown that when the interval between testing and conditioning stimuli is from 50 to 150 ms, the cutaneous stimulations effectively block the appearance of evoked potentials in the rostral parts of the cerebral cortex, which were induced by the electrical stimulation of the sinoatrial nodal zone. Bilateral vagotomy completely eliminated this reaction. Stimulation of the peripheral part of the cut vagus also effectively blocked the transmission of the afferent impulses from the heart. It was concluded that inhibitory cutaneo-cardiac effects are determined by the mechanisms of presynaptic inhibition, which develop at the level of intracardiac nervous system.     

Phase resetting in a model of cardiac Purkinje fiber.

The phase-resetting response of a model of spontaneously active cardiac Purkinje fiber is investigated. The effect on the interbeat interval of injecting a 20-ms duration depolarizing current pulse is studied as a function of the phase in the cycle at which the pulse is delivered. At low current amplitudes, a triphasic response is recorded as the pulse is advanced through the cycle. At intermediate current amplitudes, the response becomes quinquephasic, due to the presence of supernormal excitability. At high current amplitudes, a triphasic response is seen once more. At low stimulus amplitudes, type 1 phase resetting occurs; at medium amplitudes, a type could not be ascribed to the phase resetting because of the presence of effectively all-or-none depolarization; at high amplitudes, type 0 phase resetting occurs. The modeling results closely correspond with published experimental data; in particular type 1 and type 0 phase resetting are seen. Implications for the induction of ventricular arrhythmias are considered.     

Controlled sinus arrhythmia during burst stimulation of the vagus nerve

In experiments on anesthesized cats and rats the desynchronization of the heart rate and burst stimulation of the vagus brought about severe sinus arrhythmia. Analysis of the functional dependence between the P--S interval (atrial wave of the ECG--moment of vagus stimulation) and the P--P interval showed periodical alterations in pacemaker sensitivity to the effect of the vagus during each cardiac cycle. It is supposed that natural vagus arrhythmia is the result of discoordination between heart automacy and efferent vagus bursts of central origin.     

Laryngeal motor control in frogs: Role of vagal and laryngeal feedback

Using decerebrate frogs (Rana catesbeiana), we investigated the role of vagal and laryngeal sensory feedback in controlling motor activation of the larynx. Vagal and laryngeal nerve afferents were activated by electrical stimulation of the intact vagal and laryngeal nerves. Pulmonary afferents were activated by lung inflation. Reflex responses were recorded by measuring efferent activity in the laryngeal branch of the vagus (Xℓ) and changes in glottal aperture. Two glottic closure reflexes were identified, one evoked by lung inflation or electrical stimulation of the main branch of the vagus (Xm), and the other by electrical stimulation of Xℓ. Lung inflation evoked a decrementing burst of Xℓ efferent activity and electrical stimulation of Xm resulted in a brief burst of Xℓ action potentials. Electrical stimulation of Xℓ evoked a triphasic mechanical response, an abrupt glottal constriction followed by glottal dilatation followed by a long-lasting glottal constriction. The first phase was inferred to be a direct (nonreflex) response to the stimulus, whereas the second and third represent reflex responses to the activation of laryngeal afferents. Intracellular recordings of membrane potential of vagal motoneurons of lung and nonlung types revealed EPSPs in both types of neurons evoked by stimulation of Xm or Xℓ, indicating activation of glottal dilator and constrictor motoneurons. In summary, we have identified two novel reflexes producing glottic closure, one stimulated by activation of pulmonary receptors and the other by laryngeal receptors. The former may be part of an inspiratory terminating reflex and the latter may represent an airway protective reflex. © 1997 John Wiley & Sons, Inc. J Neurobiol 33: 213–222, 1997     

降钙素基因相关肽对家兔离体窦房结电生理活动的影响

用常规微电极方法研究了降钙素基因相关肽（ＣＧＲＰ）对家兔窦房结起搏细胞的电生理作用,并进一步探讨这种作用与钙电流的关系。结果：⑴低浓度ＣＧＲＰ（１ｎｍｏｌ／Ｌ）对窦房结动作电位各参数无显著影响;中等浓度ＣＧＲＰ（１０ｎｍｏｌ／Ｌ）可增加最大舒张期电位、动作电位幅度、０期最大除极化速率和４期自动除极速率,缩短窦性周期、动作电位复极化５０％和９０％时间,这些作用经２０ｍｉｎ达到高峰;高浓度ＣＧＲＰ（２     

Sexual swarms in Daphnia magna, a cyclic parthenogen

SUMMARY. Dense swarms were observed in a population of Daphnia magna (Crustacea: Cladocera). The swarms appeared during a sexual period in which females were abundant but males were rare. Each swarm contained several males and large numbers of 'imminently sexual' females, a class of females that was rare except in the swarms. This implies that both sexes respond to attractant stimuli, and that the female response occurs only during a brief critical phase in the ovarian cycle.     

Cardiac cycle length variability in ponies at rest and during exercise.

We evaluated cardiac cycle length variability in ponies at rest and during strenuous exercise with and without premedication with atropine. In the absence of premedication, cardiac cycle length at rest was 1,112 +/- 53 ms, the individual cardiac cycle length standard deviation (SDCL) was 75 +/- 23 ms, and the individual cycle length coefficient of variation (CVCL) was 6.32 +/- 1.62. Exercise significantly decreased (P < 0.05) all three indexes (290 +/- 9 ms, 5 +/- 1 ms, and 1.65 +/- 0.20, respectively). Atropine premedication significantly reduced resting cardiac cycle length (685 +/- 46 ms), SDCL (10 +/- 2 ms), and CVCL (1.45 +/- 0.19) compared with nonpremedicated values. Cardiac cycle length was significantly decreased by exercise after atropine premedication, but no statistically significant changes occurred in SDCL or CVCL. Thus, although considerable cardiac cycle length variability exists in nonpremedicated ponies at rest, it is nearly completely abolished by strenuous exercise. The absence of significant differences between the indexes of variability during exercise without premedication, at rest after atropine, and during exercise after atropine indicates that cardiac cycle length variability in the pony is mediated primarily through activity of the parasympathetic system.     

Comparison of phenylephrine bolus and infusion methods in baroreflex measurements

Phenylephrine (PE) bolus and infusion methods have both been used to measure baroreflex sensitivity in humans. To determine whether the two methods produce the same values of baroreceptor sensitivity, we administered intravenous PE by both bolus injection and graded infusion methods to 17 normal subjects. Baroreflex sensitivity was determined from the slope of the linear relationship between the cardiac cycle length (R-R interval) and systolic arterial pressure. Both methods produced similar peak increases in arterial pressure and reproducible results of baroreflex sensitivity in the same subjects, but baroreflex slopes measured by the infusion method (9.9 +/- 0.7 ms/mmHg) were significantly lower than those measured by the bolus method (22.5 +/- 1.8 ms/mmHg, P less than 0.0001). Pretreatment with atropine abolished the heart rate response to PE given by both methods, whereas plasma catecholamines were affected by neither method of PE administration. Naloxone pretreatment exaggerated the pressor response to PE and increased plasma beta-endorphin response to PE infusion but had no effect on baroreflex sensitivity. Thus our results indicate that 1) activation of the baroreflex by the PE bolus and infusion methods, although reproducible, is not equivalent, 2) baroreflex-induced heart rate response to a gradual increase in pressure is less than that seen with a rapid rise, 3) in both methods, heart rate response is mediated by the vagus nerves, and 4) neither the sympathetic nervous system nor the endogenous opiate system has a significant role in mediating the baroreflex control of heart rate to a hypertensive stimulus in normal subjects.     

Cardiac responses to exercise in the dog before and after destruction of the sinoatrial node

Chronotropic and dromotropic responses to treadmill exercise were compared in conscious dogs prior to and following excision of the sinoatrial node (SAN). The initial junctional rhythm accompanying removal of the SAN region was replaced within hours to days by subsidiary atrial pacemaker (SAP) foci located in the inferior right atrium along the sulcus terminalis. With SAN intact, cardiac acceleration was immediate at onset of exercise and the tachycardia was directly proportional to work intensity. Atrioventricular (AV) conduction concurrently accelerated during exercise as manifest by shortening in P-R and atrioventricular (A-V) intervals. Following SAN excision, subsidiary atrial pacemaker foci likewise demonstrated prompt tachycardias during exercise, although heart rate was significantly reduced at rest and during steady state exercise. In the SAP state, tachycardia during exercise was related to work intensity and was mediated by changes in cardiac autonomic nerve activity. Combined propranolol-atropine blockade increased heart rate at rest in the SAP state, and significantly attenuated the tachycardia accompanying treadmill exercise. Following SAN excision the P-R (A-V) interval was significantly reduced in the resting animal. In response to exercise, AV conduction time decreased in the SAP state, though the absolute levels during steady state exercise were not significantly different from prior control runs with SAN intact. Blood pressure response to exercise was similar during both SAN and SAP states. We conclude that following an initial unstable period, SAP foci maintain adequate heart rate increases in response to dynamic exercise, primarily mediated via autonomic nerve regulation.     

HCN4 provides a 'depolarization reserve' and is not required for heart rate acceleration in mice

Cardiac pacemaking involves a variety of ion channels, but their relative importance is controversial and remains to be determined. Hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels, which underlie the I(f) current of sinoatrial cells, are thought to be key players in cardiac automaticity. In addition, the increase in heart rate following beta-adrenergic stimulation has been attributed to the cAMP-mediated enhancement of HCN channel activity. We have now studied mice in which the predominant sinoatrial HCN channel isoform HCN4 was deleted in a temporally controlled manner. Here, we show that deletion of HCN4 in adult mice eliminates most of sinoatrial I(f) and results in a cardiac arrhythmia characterized by recurrent sinus pauses. However, the mutants show no impairment in heart rate acceleration during sympathetic stimulation. Our results reveal that unexpectedly the channel does not play a role for the increase of the heart rate; however, HCN4 is necessary for maintaining a stable cardiac rhythm, especially during the transition from stimulated to basal cardiac states.     

Beat-to-Beat Variation in Periodicity of Local Calcium Releases Contributes to Intrinsic Variations of Spontaneous Cycle Length in Isolated Single Sinoatrial Node Cells

Spontaneous, submembrane local Ca²⁺ releases (LCRs) generated by the sarcoplasmic reticulum in sinoatrial nodal cells, the cells of the primary cardiac pacemaker, activate inward Na⁺/Ca²⁺-exchange current to accelerate the diastolic depolarization rate, and therefore to impact on cycle length. Since LCRs are generated by Ca²⁺ release channel (i.e. ryanodine receptor) openings, they exhibit a degree of stochastic behavior, manifested as notable cycle-to-cycle variations in the time of their occurrence.

Aim

The present study tested whether variation in LCR periodicity contributes to intrinsic (beat-to-beat) cycle length variability in single sinoatrial nodal cells.

Methods

We imaged single rabbit sinoatrial nodal cells using a 2D-camera to capture LCRs over the entire cell, and, in selected cells, simultaneously measured action potentials by perforated patch clamp.

Results

LCRs begin to occur on the descending part of the action potential-induced whole-cell Ca²⁺ transient, at about the time of the maximum diastolic potential. Shortly after the maximum diastolic potential (mean 54±7.7 ms, n = 14), the ensemble of waxing LCR activity converts the decay of the global Ca²⁺ transient into a rise, resulting in a late, whole-cell diastolic Ca²⁺ elevation, accompanied by a notable acceleration in diastolic depolarization rate. On average, cells (n = 9) generate 13.2±3.7 LCRs per cycle (mean±SEM), varying in size (7.1±4.2 µm) and duration (44.2±27.1 ms), with both size and duration being greater for later-occurring LCRs. While the timing of each LCR occurrence also varies, the LCR period (i.e. the time from the preceding Ca²⁺ transient peak to an LCR’s subsequent occurrence) averaged for all LCRs in a given cycle closely predicts the time of occurrence of the next action potential, i.e. the cycle length.

Conclusion

Intrinsic cycle length variability in single sinoatrial nodal cells is linked to beat-to-beat variations in the average period of individual LCRs each cycle.     

Nonlinear stiffness--force relationships in whole mammalian skeletal muscles

Small, sinusoidal length changes were superimposed on isometric contractions of fast- and slow-twitch mouse muscles, which were stimulated maximally via their nerves. Stiffness increased with increasing frequency of sinusoidal stimulation, but the relative time course of force and stiffness changes during twitch, tetanic, or partially fused contractions was quite invariant over a range of frequencies in both muscles. Typically, stiffness increases more rapidly than force during contraction and decreases less rapidly during relaxation. This pattern was observed at various temperatures and with various numbers of stimuli. It can be described by a nonlinear relation between stiffness and force with some hysteresis. The presence in the muscle of parallel and series elastic elements, whose stiffness varies with force, may account for the nonlinear relation. This nonlinearity can be used to relate the patterns for summation of force and stiffness observed with brief trains of stimuli under a variety of conditions.