Paradoxical Cardiac Rhythm in Rat Pups as a Possible Analog of the Sick Sinus Syndrome

Experiments with simultaneous recordings of ECG, electrogram of myocardial fibers of the right ventricle and neurogram of the right branch of vagus or sympathetic nerves were carried out on 3–18-day old rat pups. The electric activity recordings were performed under both in vivo and in situ conditions at unilateral or bilateral pneumothorax. It is established that the change in the cholinergic system activation level produced prior to pneumothorax decelerates the rhythm of cardiac contractions (CC) and induces periods of more frequent CC. After pneumothorax, this syndrome becomes more pronounced. The periods of alternation of the slow and fast rhythms may last for up to 1–2 min. The development of the pathological process leads to development of the sino-auricular and atrio-ventricular blocks. There appears a pattern of CC changes and complexes of the ventricular electric potentials that occur in decasecond and then in minute rhythms and are separated by periods of total asystole or preserved potentials of atrial excitation. When the electrograms of myocardium were recorded in rat pups under conditions of bilateral pneumothorax without any pharmacological intervention, it was possible to see a distortion of the sinus rhythm of CC developing for 2–2.5 h after respiration arrest and similar to the above-described distortion. The appearance and development of the phenomenon of the atypical cardiac rhythm is not directly related to the firing patterns of vagus and sympathetic nerves. At the same time, a correlation is clearly seen between the amplitude–frequency modulation of CC and discharges of vagus. A contraction of groups of respiratory muscles lasts for up to 30 min after pneumothorax, with occasional discharges seen in neurogram even after the complete immobilization of the animal. The development of the pathological process reveals a certain similarity with the phenomenon described in literature as the sinus node syndrome (tachy–bradycardia syndrome, TBS). Based on analysis of the cardiac rhythm transformation, it is suggested that the clinical TBS is a consequence of recapitulation, i.e., a successive release of ancient rhythms of excitation due to an impair of regulatory mechanisms.     

Change of Parameters of Functioning of the Cardiovascular and Respiratory Systems in Rats of Different Ages under Effects of Low Doses of the Cholinesterase Inhibitor Phosphacol

In the 4-day old rat pups and adult animals there were studied effects of subacute and acute phosphacol intoxication at the doses producing and not producing inhibition of aetylcholinesterase on ECG parameters and respiration rate. Analysis of the heart rhythm variability (HRV) was performed using an original program designed in the Labview media. The frequency diapason of cardiointervals was divided as follows: the high-frequency component (HF)-0.8-2.5 Hz, the low-frequency (LF, waves of the II order)-0.8-0.3 Hz; frequencies lower than 0.3 Hz-VLF (slow waves of the III order). Under the conditions of the subacute intoxication the heart rate frequency in the 4-day old and in adult rats increases by 36% and 13%, respectively, the respiration rate frequency increases by 73% in the newborn and does not change in adult rats. The VHR analysis indicates an increase of the tone of the parasympathetic nervous system. The level of the sympathetic activity somewhat increases in adults, but decreases in the 5-day old rat pups, which leads to an essential shift of the vagosympathetic balance towards predominance of parasympathetic influences. In adult rats, unlike the newborns, the role of the humoral-metabolic factors in regulation of the cardiac rhythm and vasomotor reactions increases significantly. Remarkably, the decrease of the nervous sympathetic effects in the rat pups leads to the positive chronotropic effect and stabilization of the cardiac rhythm. Acute phosphacol intoxication (doses of 0.25 and 1 µg/kg) is studied in the 4-day old rat pups. The low dose of the drug leads to development of a moderate bradycardia without disturbances of the cardiac rhythm. The high phosphacol dose produces pronounced bradycardia; on its background there develops a long-term transitory arrhythmia representing complexes of the heart rhythm, which alternate in the decasecond or nearminute rhythm and are separated by periodicity that is one order slower. We observed such rhythm earlier during activation of central N-cholinoreactive structures and development of bilateral pneumothorax. Results of the present study allow stating the paradoxical heart rhythm not as agonic, non-peculiar to the “living” organism, but as a special form of functioning of cardiac pacemakers due to disturbances of the heart regulatory mechanism.__________Translated from Zhurnal Evolyutsionnoi Biokhimii i Fiziologii, Vol. 41, No. 2, 2005, pp. 160–167.Original Russian Text Copyright © 2005 by Kuznetsov, Goncharov, Glashkina.     

Endogenous 24-hour cycle of core temperature and oxygen consumption in week-old Zucker rat pups

Summary Experiments were designed to test whether or not the 24-h core temperature fluctuations in week-old rat pups are of endogenous origin. Lean (Fa/-) Zucker rat pups born on the same day to mothers maintained in two different colonies with light/dark cycles 12 h out of phase with each other were mother-reared through the first 3–4 days of life and then artificially reared simultaneously in constant dim light. Continuous, automatic measurement of core temperature and oxygen consumption during artificial rearing showed clear 24-h rhythms in 5- to 8-day-old pups. Each rhythm reached a daily minimum at a time corresponding to the beginning of the light period in the colony of origin. The amplitude of these rhythms did not diminish during artificial rearing, nor did the phase difference between the rhythms of pups originating in the two colonies systematically change. The persistent 12-h phase differences between these two groups of pups prove that the observed rhythms are not caused by exogenous stimuli. We conclude that the rat pup possesses an endogenous time-keeping mechanism that permits the expression of overt rhythmicity at the age of 1 week.Abbreviations Tc core temperature - LD-pups born to a mother entrained to a 7:00 to 19:00 light cycle, then artificially reared in constant dim light - DL-pups born to a mother entrained to a 19:00 to 7:00 light cycle, then artificially reared in constant dim light - SCN suprachiasmatic nucleus     

Secondary Rhythms of Cardiac Activity in Rat Ontogeny

Endogenous periodic oscillations of the heart beat rate are described in rat pups aged 3–4, 7–8, 10–11, 13–14, 21–22 days and 1.5 month after birth. These oscillations have all characteristic features established earlier for the secondary rhythms of endogenous contractile activity in the wall of various regions of the gastrointestinal tract and for bursts of spontaneous somatomotor excitation in the early postnatal ontogeny of rats: a multi-stage organization, inconstancy, irregularity of components. In frequency spectra of secondary oscillations of the heart rate obtained by means of fast Fourier transform of R–R intervals of the periodogram, age-related changes of the spectral frequency power are demonstrated in 4 ranges, 0.01–0.03, 0.03–0.1, 0.1–1.0, and 1.0–2.5 Hz, which correspond to the about-one-minute, decasecond, and about-one-second waves of the heart rhythm oscillations and to sinus arrhythmia. It is shown that the dominating frequencies of the secondary rhythms in each range do not have regular age-related changes, which is characteristic of all endogenous secondary rhythms.     

Study of ECG, parameters of external respiration and motor activity of rats of different ages during acute poisoning with fluoroacetamide

Acute poisoning with fluoroacetamide (FAA), an efficient blocker of the tricarboxylic acid cycle, was studied in newborn and adult Wistar rats. FAA was administered once: to adult rats at a dose of 11 mg/kg peros, to the 7-day old rat pups at doses of 3 and 50 mg/kg subcutaneously. For 7 days after the poisoning, ECG, respiration, and motor activity were recorded. Based on the obtained data, clinical analysis of ECG and analysis of the heart rate variability (HRV) were performed to evaluate the state of the autonomic nervous system. Administration of FAA to adult rats leads to pronounced disturbances in activities of cardiovascular and respiratory systems. The most dangerous is development of acute pulmonary heart and of respiratory disorder. Characteristic is development of diffuse myocardial alterations. In newborn rat pups, statistically significant disturbances of cardiac activity and respiration are absent. The rat pup death occurs due to the total organism exhaustion accompanied by arrest of development. In adult rats, during intoxication, the appearance of bursts of combined cardiac and respiratory tachyarrhythmias can be observed, as well as of regular high-amplitude convulsive inhalations following in the decasecond rhythm. In rats of all age groups, injection of FAA, after a brief increase of role of humoral-metabolic and sympathetic activity, leads to the gradual steady predominance of parasympathetic effects on HRV. These data can serve as a confirmation of that under conditions of inhibition of the Kreps cycle enzymatic reactions there occurs activation of physiological processes resulted from activity of alternative metabolic pathways. This statement is also confirmed by the absence of the ontogenetically fixed inhibition of the spontaneous motor activity in the FAA-poisoned rat pups. This also confirms our earlier statement about importance of pentose phosphate cycle reactions for realization of the decasecond and minute rhythms and about the absence of rigid dependence of these rhythms on activity of neuronal structures.     

Cardiogenic reflexes after immune injury of the heart

Afferent and efferent spike activity from the parasympathetic (vagus) and sympathetic cardiac nerves were recorded simultaneously with ECG, and indices of heart function were measured in acute experiments on anesthetized dogs, which allowed us to study the modifications of cardio-cardiac reflex influences after a local immune heart injury. After an injury nidus has been formed in the heart, cardiogenic depressor reflexes evoked by an intracoronary application of veratrine or bradykinin were considerably suppressed or even abolished, and afferent spike activity in the vagus cardiac nerves noticeably decreased. At the same time, both the facilitation of activity in sympathetic afferent fibers and pressor reflex effects were preserved after the heart injury. Different localization of vagus and sympathetic afferent structures in the heart and their specialized sensitivity to the biologically active substances are suggested as the factors determining the pattern of cardiogenic reflex influences after a heart injury.Neirofiziologiya/Neurophysiology, Vol. 27, No. 1, pp. 18–25, January–February, 1995.     

Variations in heart rate and rhythm of harbor seal pups during rehabilitation

Heart rate and rhythm is regulated by the autonomic nervous system, which matures during the first months of life. Little is known about heart rate and rhythm development and potential arrhythmias in seal pups during rehabilitation in seal centers. Using an iPhone ECG device, 1 min ECGs were obtained from harbor seal pups admitted to a seal rehabilitation facility. ECGs were taken from 55 seals after admission, 53 seals after 14 d, and 52 seals prior to release. From 24 seal pups additional ECGs were taken daily for the first week of rehabilitation. At admission sinus rhythm with a median heart rate of 148 complexes per minute was detected, prior to release sinus bradycardia or sinus arrhythmia with a median heart rate of 104 complexes minute was present. P wave morphology was highly variable and single supra‐ and ventricular premature complexes were recorded in individual animals. The first 14 d were characterized by highly variable heart rates and rhythms, including episodes of sinus tachycardia and 2nd degree atrioventricular blocks. The reduction in heart rates and development of a regular heart rhythm during rehabilitation suggest adaptation to the unfamiliar environment, resolution of disease, and/or maturation of the autonomic nervous system.     

Action of novocain on cardiac and somatomotor autorhythmical functions in early postnatal ontogenesis in rats

In conscious rat pups aged 2–3, 10–11, and 22–23 postnatal days, it has been shown that intraperitoneal administration of 0.5% Novocain at a dose of 25 mg/kg body mass leads to phasic changes of the level of spontaneous periodic motor activity—from increased at once after the administration to depression and subsequent restoration. They are accompanied by the cardiac rhythm fluctuations that change with age their direction on the background of an increased motor activity from brady to tachycardia. All the changes are the most pronounced in newborns. At comparison of the results with the afferent impulsation level fluctuations recorded under the same conditions in the peripheral vagus segment, it is suggested that the revealed reactions are associated with changes of the interoceptive afferentation flow. The conclusion is made that at early stages of ontogenesis, interoception plays an important role in regulation of autorhythmical functions and that this role decreases with age.     

Early Effects of Neonatal Rat Desympathization on Secondary Heart Rhythms

Neonatal Wistar rats for the first 3 weeks of life were injected intraperitoneally with isobarine every other day. The single doze was 40 mg/kg. Control animals were injected with saline. Degenerative changes in sympathetic ganglia were evident as early as in the 10-day old animals and increased by 18–19 days. The heart rate in the desympathized animals was lower than in control from 10–11 to 18–19 days, but by the end of the 3rd week the differences were eliminated. The same occurred with respiration rate. At same terms there was an essential decrease of amplitude of the heart rate high-frequency fluctuations synchronous with respiration and of the periodogram slow waves with the period about 1 min. Using the method of fast Fourier transform, the power spectra of heart rate fluctuations (secondary heart rhythms) in 5 frequency ranges (0–0.01, 0.01–0.03, 0.03–0.1, 0.1–1.0, and 1.0–2.5 Hz) were calculated. Desympathization leads to a decrease of the fluctuation power in all ranges, but in the ultralow-frequency range this decrease is the least pronounced, which suggests the presence of non- sympathetic mechanisms in their genesis. The greatest changes occur in the middle-frequency area. In all cases, differences from control values increase from the 10–11th to the 18–19th days, after which a tendency for restoration is observed, in spite of an enhancement of processes of degeneration of sympathetic neurons. This indicates an activation of the compensatory mechanisms, due to which consequences of desympathization are partially smoothed at distant terms of studies.     

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.     

The spectrogram of heart rate in cats following burst stimulation of the vagus nerve

Denervation of the heart (bilateral vagotomy and propranolol) in artificially ventilated cats didn't remove respiratory peaks on the spectrogram of heart rate, while burst stimulation of vagus nerve increased or decreased them several times by synchronization of the heart and vagus rhythms, which in its turn was observed under the bradycardia only. At the same time, the desynchronization of rhythms provoked severe sinus arrhythmia which had a distinct periodic character. Under these conditions, there were high non-respiratory peaks appearing at the spectrogram of the heart rate that indicated existence of two vagus chronotropic effects: a well known tonic one and special intracycle synchronizing effect correcting duration of every cardiac cycle.     

Study of cardiac, respiratory, and motor activities in rat fetuses

Development of the cardiac, respiratory, and motor activities in rat fetuses with preserved placental circulation was studied at the 16th, 18th, and 20th gestation days. The presence of three main movement types has been found: complexes of generalized activity, local movements, and jerks. In development of respiratory function, there is observed a gradual transition from individual inspirations to series of breathing movements and then to formation of periodic breathing episodes. At the studied period, the heart rate has been found to increase. The existence of the slow-wave modulations of the heart rate with a period of 2040 s has been revealed. Analysis of interrelations between the respiratory and motor systems has shown that in the 16-day fetuses, each breathing movement is accompanied by extensor jerk. By the 20th days of embryonic development (E20), uncoupling of the respiratory and motor activities occurs. Comparison of the activity observed in the cardiac and somatomotor systems has shown that at E16, the cardiac rhythm fluctuations do not depend on the motor excitation jerks. In the 18-day fetuses, brief slowing down (decelerations) of the cardiac rhythm appeared during the motor activity jerks, whereas at E20, on the contrary, an increase of frequency (accelerations) of the cardiac rhythm occurred.     

Vagus nerve stimulation suppresses generalized seizure activity and seizure-triggered postictal cardiac rhythm changes in rats

In the present study, we investigated the effects of vagus nerve stimulation (VNS), a proposed treatment for patients with intractable epilepsy, on cardiac rhythm following seizures induced by pentylenetetrazole (PTZ) in Wistar rats. After a baseline recording of electroencephalogram (EEG), electrocardiogram (ECG) and blood pressure (BP), rats in the first group received a single convulsive dose of PTZ (70 mg/kg) (Group 1). In the other two groups, the Wistar rats were implanted with a cuff electrode on the left cervical vagus nerve. One day after surgery, rats in the second group were treated with VNS (Group 2), whereas rats in the third group were connected to the stimulator but did not receive VNS (Group 3). Ten minutes after VNS onset, 70 mg/kg dose of PTZ was injected. EEG, ECG and BP were continuously recorded during post-injection period. Seizure severity was scored behaviorally. Then, baseline, ictal and postictal periods were analyzed for cardiac rhythms, seizure severity and blood pressure variability. PTZ treatment induced tonic-clonic seizure activity in all animals of Group 1 and Group 3. In these groups a marked increase of mean arterial blood pressure (MABP) but a significant decrease in heart rate and PP interval fluctuations was observed at postictal period. However, in the VNS-treated group the seizure scores and cardiac parameter returned to the baseline level. Present results emphasize that VNS effectively reduces seizure severity and suppress the seizure-induced cardiac rhythm changes.     

Selective pharmacological blocking of synaptic transmission through the parasympathetic ganglia: Effects on the cardiovascular system

In acute experiments on rats and dogs, compounds IEM-1556 and IEM-1678, the blockers of transmission through the parasympathetic ganglia, reduced the negative chronotropic effect of stimulation of the vagus nerve (VN), while practically not changing the heart rate (HR). In chronic experiments on dogs, these compounds increased the HR, substantially reduced the respiratory heart arrhythmia, did not change the arterial blood pressure (AP), and reduced the chronotropic effects of VN stimulation. IEM-1556 exerted more strong and long-lasting blocking effects on vagal heart control than IEM-1678 did, but in anesthetized animals could evoke a drop in the AP. Acetylcholine, if administered during the action of the above compounds, inhibited heart activity. It is concluded that both IEM-1678 and IEM-1556 are selective parasympatholytics (although IEM-1556 may produce a side effect). The above compounds block synaptic transmission through the intracardiac parasympathetic ganglia and do not affect neuro-effector transmission in the heart.Neirofiziologiya/Neurophysiology, Vol. 28, No. 2/3, pp. 151–159, March–June, 1996.     

Effect of lithium preparations on cardiac arrhythmias resulting from ligation of the common carotid arteries

In experiments on cats with dissected vagus and aortal nerves under chloralose-urethane anesthesia, ventricular disorders of the cardiac rhythm were induced by ligation of the common carotid arteries. Appearance of arrhythmias was preceded by an increase in the sympathetic activity (recorded from the inferior cardiac or renal nerve) accompanied by a rise of the arterial blood pressure and of the heart rate. Intravenous injection of lithium chloride or hydroxybutyrate resulted in lowering of the sympathetic activity, arterial blood pressure, and heart rate, and led to the recovery of the sinus rhythm.     

Topico-anatomical relations among the structures of the caudal medullary region in early postnatal rats

Anatomical and topographical organization of the structures of the caudal medullary region was studied in 3- and 21-day-old rats. A system of stereotaxic coordinates was proposed, and brain maps for the animals of these age groups were plotted. Topographic characteristics were given for the nuclei forming the medullary respiratory center: the lateral reticular nucleus,n. ambiguus, and the nuclei of the solitary tract, vagus and hypoglossal cranial nerves. These structures differ from each other in their cytoarchitectonics and in the changes in number during their neurons' postnatal development. The differences are suggested to result from the differences in morphological maturation of the neurons at early ontogenic stages.Neirofiziologiya/Neurophysiology, Vol. 26, No. 4, pp. 243–250, July–August, 1994.     

Peculiarities of regulation of high-frequency oscillations of cardiac rhythm in rat ontogenesis

Dynamics of high-frequency components of heart periodogram whose main part is respiratory arrhythmia (RA) as well as consequences of vagotomy, block of M-cholinoreceptors by atropine and of β-adrenoreceptors by propranolol was studied in Wistar white rats in a large age diapason from 2–4 days to adults. It was established that results of the actions in immature rats did not essentially differ from those observed in adult rats and described in animals of other species and in human. In rats of young age, predominant in genesis of RA is peripheral mechanism. Vagotomy produces an elevation of the RA amplitude due to a sharp increase of the inspiration time as well as to deafferentation. Sympathetic nervous system produces restricting action on RA. This role is preserved in animals of all age groups. Participation of parasympathetic innervation in the RA genesis is revealed from the third week and continues increasing to the mature age. However, in adult rats, the peripheral mechanism of the RA formation is preserved, as disturbance of parasympathetic innervation leads not to the disappearance of RA, but only to a decrease of its amplitude.     

Peculiarities of regulation of high-frequency oscillations of cardiac rhythm in rat ontogenesis

Dynamics of high-frequency components of heart periodogram whose main part is respiratory arrhythmia (RA) as well as consequences of vagotomy, block of M-cholinoreceptors by atropine and of beta-adrenoreceptors by propranolol was studied in Wistar white rats in a large age diapason from 2-4 days to adults. It was established that results of the actions in immature rats did not essentially differ from those observed in adult rats and described in animals of other species and in human. In rats of young age, predominant in genesis of RA is peripheral mechanism. Vagotomy produces an elevation of the RA amplitude due to a sharp increase of the inspiration time as well as to deafferentation. Sympathetic nervous system produces restricting action on RA. This role is preserved in animals of all age groups. Participation of parasympathetic innervation in the RA genesis is revealed from the third week and continues increasing to the mature age. However, in adult rats, the peripheral mechanism of the RA formation is preserved, as disturbance of parasympathetic innervation leads not to the disappearance of RA, but only to a decrease of its amplitude.     

Pronounced juvenile circadian core temperature rhythms exist in several strains of rats but not in rabbits

Torpor-like circadian variations of core temperature are well documented for suckling-age Zucker rat pups. To determine (1) whether this juvenile circadian rhythm is as strongly expressed in other rat strains, and (2) whether a similar rhythm is expressed in rabbit pups, we recorded core temperature and metabolic rate of artificially reared pups. Wistar, Brown Norway, and Long Evans pups were studied for 30 h under moderate cold loads (ambient temperature=28°C) when 9–11 days old, i.e. at the age and ambient temperature for which the rhythm has been most thoroughly characterized in Zucker rats. Chinchilla bastard rabbit pups were studied under similar conditions when they were 3–8 days old, the youngest age at which the rhythm can be easily detected in rats. Rat pups of each strain showed clear circadian rhythms with sharp decreases of core temperature and metabolic rate in subjective morning. Core temperature amplitudes were in the order Wistar < Brown Norway < Zucker < Long Evans strain. In contrast, the rabbit pups maintained stable high levels of core temperature and metabolic rate throughout the day. A torpor-like decrease of core temperature in the morning is thus not a pecularity of the Zucker rat strain but also occurs in other pigmented rat strains, whereas rabbit pups at a similar developmental stage do not show a circadian core temperature rhythm.     

Self-regulation of respiratory sinus arrhythmia

Respiratory sinus arrhythmia (RSA) — the peak-to-peak variations in heart rate caused by respiration — can be used as a noninvasive measure of parasympathetic cardiac control. In the present study four strategies to increase RSA amplitude are investigated: (1) biofeedback of RSA amplitude, (2) biofeedback of RSA amplitude plus respiratory instructions, (3) respiratory biofeedback, and (4) respiratory instructions only. All four procedures produce a significant increase of RSA amplitude from the first physiological control trial compared to baseline. This increase is faster for the groups that received respiratory biofeedback and respiratory instructions only than for the two groups that received biofeedback of RSA amplitude, the increases being equivalent for the four groups in the third session. All subjects of the group that received biofeedback of RSA amplitude only reported respiratory strategies in order to achieve the increase in RSA. Possible clinical implications of these results for parasympathetic cardiac control and cardiovascular disorders are discussed.This research was supported by a grant to the first author from the University of Granada (Spain).