Obsidan induced age-dependent bradycardia in intact rats and in rats with chemical sympathetic denervation. Subsequent vagal stimulation produced further decrease in heart rate. Administration of atropine to obsidan-treated rats increased heart rate. Key Words: rat; vagus; chemical sympathetic denervation; cardiac rhythm; obsidanThe degree of postvagotomy tachycardia varies in different species [7]. In small rodents the vagal tone is the lowest [5]. Inhibition of cardiotropic adrenergic influences produces only minor changes in heart rate in humans [9] and dogs [10]. The release of norepinephrine is regulated by adrenoreceptors of various types. Down-regulation of this release is mediated by ot-adrenoreceptors, while up-regulation is activated by stimulation of 13-adrenoreceptors [6]. However, the direct effect of catecholamines on postsynaptic 13-adrenoreceptors in the pacemakers cannot be ruled out [8]. Different effects of vagotomy on the dynamics of variational pulsogram were revealed in rats of different ages subjected to sympathetic denervation (SD) [2]. At the same time, electrical stimulation of the vagi resulted in proIound tachycardia both in intact and SD-animals [3].Our aim was to study the dynamics of cardiac variational pulsogram parameters in rats of different ages with SD produced by pharmacological means during vagal stimulation after blockade of cardiac [3-adrenoreceptors by obsidan. MATERIALS AND METHODSThe study was carried out on 4, 6, 8, and 20-week-old random-bred albino rats. The test group consisted of Department of Anatomy, Physiology and Human Health Protection, State Pedagogical University, Kazan SD rats. Chemical SD was pertbrmed in neonatal rats by daily subcutaneous injections of guanethidine sulfate (10 ml/kg body weight) during a 28-day period. The control group consisted of rats of the same age which were kept under identical conditions.Both the vagi and the right femoral vein were dissected in narcotized rats (urethane 800 mg/kg) under a microscope. Obsidan (0.8 mg/kg) and atropine (0.6 mg/kg) were injected intravenously.Vagal electrical stimulation was performed with 5-V pulses generated by an ESL-2 stimulator. Repetition rate, duration, and delay of the pulses were chosen individually tor every rat.The signals from an EKSP-02 electrocardiograph were fed into a C1-83 oscilloscope via an original interface and into a microcomputer via an F707732 digitizer. This computer was also used to control the set-up.The ECG was recorded and processed on-line [l ]. The original software calculated 13 parameters of the variational pulsogram from the arrays of the cardiointervals [4]. Six parameters were analyzed statistically: mean cardiointerval (Xmean), mode, amplitude of the mode, variation range (AX), standard deviation (8), and tension index (TI). RESULTSIntravenous injection of obsidan produced pronounced bradycardia in all age groups of intact and SD-ani-0007-4888/98/0012-l 184520.00 ~
Atropine induced various changes in the heart rate in intact rats of various ages. In sympathectomized animals, changes in heart rate variability were less pronounced. Atropine blocked the inhibition of cardiac activity induced by electrical stimulation of the vagus nerve.Key Words: rat; heart rate; sympathectomy; atropine," vagus nerve Vagotomy sharply increases heart rate (HR) in dogs and cats [7]. Atropine, a nonspecific muscarinic receptor (MR) blocker, induces similar effects [2,8,9,11]. Experimental data suggest that the vagus nerves induce tonic effects on the chronotropic heart function. At the same time, some authors believe that the vagus nerves induce no inhibitory effects [6,9], and acceleration of HR after vagotomy and atropine administration is due to excitation of the sympathoadrenal system. Low doses of atropine cause bradycardia, but not tachycardia [10,11]. HR reactions to vagotomy and atropine considerably differ in various animal species [8]; postvagotomy tachycardia is least pronounced in small rodents [5]. Therefore, the effects of atropine on HR in rats are of considerable interest.The population of MR is heterogeneous [ 12]. The ratio between various MR subtypes changes with age probably due to the development of sympathetic cardiac innervation during the postnatal ontogeny [13,14]. Our previous experiments demonstrated age-related peculiarities of HR response to uni-or bilateral vagotomy in intact and sympathectomized animals [3]. Here we studied the role of MR in the development and regulation of HR in intact and sympathectomized rats during the postnatal ontogeny.Department of Anatomy, Physiology, and Health Care, Kazan State Pedagogical University MATERIALS AND METHODSExperiments were performed on 58 outbred albino rats aging 4, 6, 8, and 20 weeks. Sympathectomy was induced by administration of 10 ml/kg guanethidine sulfate to 36 newborn rats for 28 days. The rats were intraperitoneally narcotized with 25% urethane in a dose of 800 mg/kg. Atropine sulfate in a dose of 0.6 mg/kg was injected into the right femoral vein.Stimulation of the right and left vagus nerves (individual parameters for each animal) was performed before and after the injection of atropine at 15-min intervals using an ESL-2 device, Electrocardiogram (ECG) was visually controlled with an Sl-83 oscillograph [ 1 ]. We analyzed 8 of 13 parameters of R-R intervals [4] reflecting activity of the major regulatory mechanisms: mean cardiac interval (Xm), mode (M), mode amplitude (MA), variational range (AX), mean deviation (8), strain index, and HR.The results were analyzed by Student's t test.
Changes in cardiac rhythm induced by blockade of hyperpolarization currents with ZD 7288 depend on animal's age. The increase in cardiointerval duration is related to prolongation of T-P segment on ECG. It is hypothesized that the age-related changes in activity of hyperpolarization channels are determined by a modulating effect of the autonomic nervous system.
High incidence of cobalt deficiency (89%) was found in 7-8-year-old girls residents of the Republic of Tatarstan. Significant correlations were revealed between parameters of physical development and cardiovascular system and hair cobalt content. These data suggest that positive balance of cobalt is essential for normal growth and development of child's body and function of the cardiovascular system.
A comparative analysis of temporal variations of the heart rate variability after cutting and stimulation of the vagus in sympathectomized and intact rats showed that changes in heart rate after bilateral vagotomy in sympathectomized rats are opposite to those in the controls. It is postulated that postganglionic fibers modulate sympathetic influence on intracardiac parasympathetic neurons.Cutting of the vagus nerve leads to a marked decrease in heart rate (HR) in animals and humans [5], although in small rodents the postvagotomy tachycardia is very mild [4], suggesting that acceleration of cardiac rhythm after vagotomy is due to excitation of the sympathoadrenal system in response to stimulation of afferent nerve fibers within the vagus [8], rather than being caused by interruption of parasympathetic tonic activity. It should be noted that 48% of nerve fibers in rat vagus are sympathetic fibers [2], while the content of afferent nerve fibers in the vagus is 14-to 15-fold higher than that of efferent fibers [10].At the present time, there is controversy over the mechanisms of postvagotomy tachycardia. Some researchers believe that vagotomy leads to a faster HR by activating the sympathetic nervous system [3,8], while others claim that this system is not implicated in the development of postvagotomy tachycardia [7]. Previously, we showed that the initial transient rise of HR is followed by its normalization or even bradycardia. It is impossible to derive any unequivocal conclusion from studies in which the effect of vagotomy on HR in animals with interrupted sympathetic nerve supply after surgical or chemical sympathectomy was investigated [3,7].
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