Effect of vagal stimulation on cardiac rhythm in rats under blockade of β-adrenoreceptors by obsidan

Abstract

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 ~

Peculiarities of regulation of heart rate and stroke volume in rats during ontogeny

“…It was established that irrespective of animal age the heart rate remained unchanged under conditions of total adreno-and cholinergic blockade of the heart. Stroke volume under conditions of total adreno-and cholinergic blockade increased with age.Laboratory of Physiology of Physical Training, Tatar State Humani tarian Pedagogical University, Kazan Adrenergic and cholinergic mechanisms regulating pumping function of the heart during ontogeny under conditions of muscle training and hypokinesia are extensively studied [2][3][4][5][6]. At the same time, the degree of changes in functional state of adrenergic and cholinergic receptors during physical exercise, and changes in heart rate (HR), stroke volume (SV), and cardiac output (CO) at rest and under the effect of muscle training and hypokinesia requires further investigation.…”

“…Adrenergic and cholinergic mechanisms regulating pumping function of the heart during ontogeny under conditions of muscle training and hypokinesia are extensively studied [2][3][4][5][6]. At the same time, the degree of changes in functional state of adrenergic and cholinergic receptors during physical exercise, and changes in heart rate (HR), stroke volume (SV), and cardiac output (CO) at rest and under the effect of muscle training and hypokinesia requires further investigation.…”

Cardiac activity and blood pressure in rats during selective blockade of various subtypes of muscarinic cholinoceptors

“…Published data show that parasympathetic regulatory influences on the heart are realized via muscarinic M 2 AChR [10]. Recent studies indicate that muscarinic M 1 and M 3 AChR play a role in cholinergic regulatory influences on the heart [4,11,13,14].Study of vagal regulation of the heart in laboratory animals revealed significant interstrain differences in the mechanisms of cardiac rhythm regulation in rats [1,3,7]. The possibility of immediate and tonic vagal inhibition of cardiac activity in rats was hypothesized [2].…”

“…Study of vagal regulation of the heart in laboratory animals revealed significant interstrain differences in the mechanisms of cardiac rhythm regulation in rats [1,3,7]. The possibility of immediate and tonic vagal inhibition of cardiac activity in rats was hypothesized [2].…”

Parasympathetic cardiac effects in sympathectomized rats

“…Rats sympathectomized with guanethidine in the early period of life served as the model to study functional activity of the sympathetic nervous system. Most studies were devoted to estimate the effect of sympathectomy on HR [3,4]. Little is known about the response of HR and stroke volume (SV) in sympathectomized animals to various factors [1,9,10].…”