Effect of Exogenous Extracellular Nicotinamide Adenine Dinucleotide (NAD⁺) on Bioelectric Activity of the Pacemaker and Conduction System of the Heart.

In rat sinoatrial node, NAD(+) (10 µM) reduced the rate of spontaneous action potentials, duration of action potentials, and the velocity of slow diastolic depolarization, but the rate of action potential front propagation increases. In passed rabbit Purkinje fibers, NAD(+) (10 µM) reduced the duration of action potentials. Under conditions of spontaneous activity of Purkinje fibers, NAD(+) reduced the fi ring rate and the rate of slow diastolic depolarization. The effects of extracellular NAD(+) on bioelectric activity of the pacemaker (sinoatrial node) and conduction system of the heart (Purkinje fibers) are probably related to activation of P1 and P2 purinoceptors.

[Influence exogenous nicotinamide adenine dinucleotide (NAD+) on contractile and bioelectric activity of the rat heart].

This study is aimed to the investigation of the nicotinamide adenine dinucleotide (NAD+) effects and mechanisms of action in a heart. NAD+ (mcM) induces multiphase alternation of contractile activity of isolated rat heart: short positive inotropic action is followed by a negative inotropic phase. NAD+ (1-100 mcM) induces decreasing of action potential duration (APD) in rat atrial myocardium (from 45 +/- 0.82 ms in control experiments to 39 +/- 1.05 (n = 8) and 32 +/- 2 (n = 8) during application of 10 and 100 mcM of NAD+, respectively). Significant APD increase (from 45 +/- 0.82 ms to 74 +/- 1.89 (n = 8) ms) was observed during washing out of NAD+ (100 mcM). ATP or adenosine was unable to increase APD both during application or washing out. NAD+ induced APD decrease was not suppressed by P1-antagonist theophylline. P1-purinoreceptor and metabolite independent direct action of NAD+ in rat heart is suggested. Activation of P2X or P2Y receptors, cyclic ADP-ribose accumulation in cardiomyocytes is proposed as a main mechanism of NAD(+)-induced effects in the heart.

Effect of selective stimulation of muscarinic M3 cholinoceptors on electrical and contractile activity of rat ventricular myocardium.

We studied the effect of selective activation of muscarinic M3receptors on electrical activity in the isolated preparation of rat ventricular myocardium as well as contractile activity of the left ventricle of Langendorff-perfused isolated heart. Application of muscarinic agonist pilocarpine (10(-5)M) against the background of selective blockade of subtype 2 muscarinic receptors with methoctramine (10(-7)M) markedly shortened the duration of action potentials in the isolated ventricular myocardium and reduced the amplitude and maximum rates of left-ventricular pressure rise and decay in the isolated heart paced at a fixed rate. All these effects were significantly suppressed by selective M3receptor blocker 4-DAMP (10(-8)M), which attested to the involvement of M3muscarinic receptors.

Inotropic effects of gaseous transmitters in isolated rat heart preparation.

We studied the effects of carbon monoxide and sodium hydrosulfide, hydrogen sulfide donor, on contractile activity of the left ventricle in Langendorf-perfused isolated rat heart. Carbon monoxide 5×10(-5) M significantly accelerated sinus rhythm and left-ventricular pressure wave growth and decay. To the contrary, negative inotropic and chronotropic effects were observed at higher concentrations of carbon monoxide (10(-4), 3×10(-4) M). Sodium hydrosulfide (10(-4)-4×10(-4) M) decreased all the parameters of left-ventricular contractive activity and reduced contraction rate. Carbon monoxide and hydrogen sulfide, which together with nitrogen oxide are qualified as a new class of gaseous signal compounds, may substantially modulate pumping function of the heart.

[Investigation of pacemaker shift in the rabbit sinoatrial node using the optical mapping technique].

Changes of the activation sequence in the rabbit sinoatrial node under the influence of low temperature and I(f) selective blocker ivabradine have been studied using the optical mapping technique. Both factors caused a shift of the pacemaker within the sinoatrial node region. These results are compared with the data obtained recently in the investigation of pacemaker shift under the influence of cholinergic and adrenergic factors. Possible mechanisms of the pacemaker shift are discussed. The suppression of electric activity in the central part of the sinoatrial node during the action of acetylcholine, which is called cholinergic inexcitability, may be considered as one of the mechanisms of the pacemaker shift. It is shown that the main cause of cholinergic inexcitability is the activation of potassium acetylcholine-dependent current I(KACh).

[Mechanisms of functioning and regulation of mammalian sinoatrial node].

Actual data concerning mechanisms of automaticity in sinoatrial node, which acts as a primary pacemaker in mammalian heart, is reviewed. Studies dealing with ionic currents, maintaining automatic generation of excitation in the sinoatrial cells, and possible role of intracellular calcium turnover are discussed. Special attention is given to the differences between the central and peripheral parts of sinoatrial node, phenomenon of intranodal pacemaker shift resulting from that differences and possible role of pacemaker shift in the modulation of the sinus rhythm. Mechanisms of sinus rhythm regulation under the action of acetylcholine and noradrenalin are also discussed in detail.

[Temperature and cardiotropic effects of kyotorphin and neokyotorphin in hibernating and nonhibernating animals].

Physiological effects of kiotorphyn (KT) and neokiotorphyn (NKT) has been reviewed. These peptides were isolated from the brain of hibernating ground squirrel Citellus undulates (CU) and assumed to be endogenous regulators of hibernation NKT does not alter the process of entering to hibernation, but induces arousal from the sleeping. KT administration in squirrel delays the rise of heart rate and body temperature associeated with awakening provoked in the middle but not at the final stage of hibernation bout. Neither KT nor NKT induce physiological effects in rats and mice under normal environmental conditions. In mice with induced hypothermia (17-18 degrees C) NKT accelerates restoration of body temperature, whereas KT induces progressive hypothermia. In addition, NKT modulates cardiotropic effects of nucleotides found in physiologically active fractions of hibernating squirrel tissues. NKT as well as KT modify sensitivity of cardiovascular system to adrenaline and AMP in rats one day after hypothermia.

[Changes in the activation sequence in the rabbit sinoatrial node induced by adrenergic stimulation].

We used high resolution optical mapping for the study of changes of activation sequence of a rabbit sinoatrial node induced by adrenergic stimulation during natural and paced rhythm. Activation of adrenoreceptors with isoproterenol (10, 100 nanoM, 1 microM) as well as stimulation of intramural postganglionic sympathetic nerves caused pacemaker migration within sinoatrial node and increase of the rate of generation of excitation. Pacemaker migration in cases of pronounced acceleration of rhythm could proceed in two stages. Termination of adrenergic influences has been followed by restoration of initial chronotopography of excitation of sinoatrial node.

[The cholinergic non-excitability phenomenon in the atrial myocardium of lower vertebrates].

Changes of electric activity induced by acetylcholine were studied in atrial myocardium of fishes (cod and carp) and reptilians (lizard and grass-snake). Standart microelectrode technique and novel method of optical mapping were used in the study. Acetylcholine (1-50 microM) provoked decrease of the action potential amplitude down to full inhibition of electrical activity in wide regions of atrium of cod and carp. We define this phenomenon as cholinergic inexcitability. In other regions excitation persisted even during action of 500 microM acetylcholine. In atria of lizard and grass-snake acetylcholine caused shortening of action potential without changes in it's amplitude. Local cholinergic inexcitability, shown in the atrial myocardium of fishes, is quite similar to the phenomenon, that was described earlier in the atria of frogs. It presents the heart of fish as an interesting model for study of mechanisms of cholinergic atrial arrhythmias initiation.

[Changes of activation sequence in a rabbit sinoatrial node induced by cholinergic influences].

We used high resolution optical mapping for the study of activation sequence in a rabbit sinoatrial node in normal state and during artificial stimulation as well as changes of excitation chronotopography under cholinergic influences. In the norm excitation originates in central region of sinoatrial node. During acetylcholine action (10, 50, 100 M) and activation of intramural parasympathetic nerves suppression of electrical activity in the zone of initial pacemaker localization occurs. Suppression of activation is associated with pacemaker migration within sinoatrial node and lowering of the rate of generation of excitation. Termination of cholinergic influences has been followed by restoration of initial chronotopography of excitation. During artificial stimulation of sinoatrial node nonexitable (refractory) area has been formed in the region of initial localization of the natural pacemaker.

Modulation of rabbit sinoatrial node activation sequence by acetylcholine and isoproterenol investigated with optical mapping technique.

AIMS: Changes in the rabbit sinoatrial node (SAN) activation sequence with the cholinergic and adrenergic factors were studied. The correlation between the sinus rhythm rate and the leading pacemaker site shift was determined. The hypothesis concerning the cholinergic suppression of nodal cell excitability as one of the mechanisms associated with pacemaker shift was tested. METHODS: A high-resolution optical mapping technique was used to register beat-to-beat changes in the SAN activation pattern under the influence of the cholinergic and adrenergic factors. RESULTS: Acetylcholine (10 microm) and strong intramural parasympathetic nerve stimulation caused a pacemaker shift as well as rhythmic slowing and the formation of an inexcitable region in the central part of SAN. In this region the generation of action potentials was suppressed. The slowing of the sinus rhythm (which exceeded 12.8 +/- 3.1% of the rhythm control rate) always accompanied the pacemaker shift. Isoproterenol (10, 100 nm, 1 microm) and sympathetic postganglionic nerve stimulation also evoked a pacemaker shift but without formation of an inexcitable zone. The acceleration of the sinus rhythm, which exceeded 10.5 +/- 1.3% of the control rate of the rhythm, always accompanied the shift. CONCLUSIONS: Both cholinergic and adrenergic factors cause pacemaker shifts in the rabbit SAN. While modest changes in the sinus rhythm do not coincide with the pacemaker shift, greater changes always accompany the shift and may be caused by it, according to one hypothesis. The formation of an inexcitable zone at the place where the leading pacemaker is situated is one of the mechanisms associated with pacemaker shift.

[M3-cholinoreceptors in mammalian heart].

Experimental facts, proving the presence of M3-cholinoreceptros in parasympathetic regulation of heart, are reviewed. Results of molecular biological and biochemical studies, showing existence of M3-cholinoreceptors in myocardium, are discussed. Physiological data, that demonstrates changes of action potential configuration due to M3-receptors activation, specific potassium current, linked with M3-receptors stimulation, and other evidence of functional importance of M3-receptros in myocardium, is analyzed. Studies dealing with cardioprotective role of M3-receptors are reviewed with special attention. The review widens our knowledge about molecular mechanisms of parasympathetic heart regulation.

[Influence of ADP-ribose, AMP and adenosine on bioelectric activity of hibernating ground squirrel atrium and papillary muscle].

The aim of work was to investigate effects of adenosine, AMP and ADP-ribose (1x10(-5)) on bioelectric activity of atrium and papillary muscle of nonhibernating (rat) and hibernating (Yakutian ground squirrel) animals. Action potential (AP) was registered with use of standard microelectrode technique. AP duration (APD) at level of 90% repolarisation in rat atrium in control experiments was 30+/-5 ms, APD at level of 50% repolarisation was 12+/-2 ms. APD at level of 90% repolarisation in rat papillary muscle was 56+/-7 ms, at level of 50% repolarisation was 18+/-2 ms. APD at level of 90% repolarisation in ground squirrel atrium was 77+/-6, APD at level of 50% repolarisation was 38+/-6 ms. APD at level of 90% repolarisation in ground squirrel papillary muscle was 105+/-9 ms, APD at level of 50% repolarisation was 42+/-8 ms. Purine nucleotides and nucleoside, that were tested in work, except ADP-ribose, act as inhibitory factors and decrease APD both in rat and hibernating ground squirrel heart. ADP-ribose decreases APD in papillary muscle of hibernator but did not in its atrium. In ground squirrel atrium AMP and adenosine decrease APD at level of 50% repolarisation by 10+/-3% and 18+/-3% respectively. AMP and adenosine decrease APD at level of 90% repolarisation by 9+/-2% and 11+/-2% respectively. In ground squirrel papillary muscle ADP-ribose, AMP and adenosine decrease APD at level of 50% repolarisation by 26+/-8%, 23+/-8% and 26+/-7%. ADP-ribose, AMP and adenosine decrease APD at level of 90% repolarisation by 12+/-3%, 10+/-3%, 13+/-3%. Thus, decrease of APD in ground squirrel papillary muscle at level of 90% repolarisation during nucleotides and adenosine action was 2-2.5 fold less, than the rat.

[The influence of purine nucleotides and adenosine on bioelectrical activity of bat (Pipistrellus nathusii) heart].

The aim of work was to investigate effects of adenosine, AMP, GMP and ADP-ribose on bioelectric activity of bat heart. Purine nucleotides decreased action potential duration at level of 90% (APD90) repolarization in bat ventricular myocardium. When preparation of right ventricle was paced with frequency of 6 Hz, APD50 and APD90 were 7 +/- 2 ms and 60 +/- 5 ms, respectively. Adenosine decreased APD90 by 50 +/- 10% (n=6), AMP - by 30 +/- 10% (n=6), GMP - by 38 +/- 5% (n=6), ADP-ribose - by 24 +/- 6% (n=6) (concentration of all compounds - 10 ). Effects of purine nucleotides and nucleoside in bat ventricular myocardium exceed effects of same compounds in rat and ground squirrel ventricular myocardium. Inhibitory effects of purine nucleotides and adenosine in bat heart could be mediated by A1 adenosine receptors.

[A study of synchronization of the sinus unit upon stimulation from auricles].

The excitation of the sinoatrial unit from heart auricles of the frog has been studied. Potentials were recorded by means of microelectrodes inserted to pacemaker of the sinoatrial unit. It has been established that auricles can impart the rhythm to the sinoatrial unit due to electric and electromechanical influence, and electromechanical influence is of greater significance. Specific transitions accompanying the establishment of the stationary rhythm have been studied. A mathematical model of transitions of the establishment of the rhythm of the sinoatrial unit, which is based on diophant methods is offered. The catculations performed by means of the mathematical model coincide well with results of experimental studies. The stabilizing role of auricles in the formation of the rhythm of the sinoatrial unit has been established.

[Peculiarities of regulation of cardiac function in some mammals].

We studied acetylcholine induced changes of action potentials (AP) in ventricles and atria of bat, hedgehog, yakut ground squirrel, and laboratory mice. In atria of all these species of mammals acetylcholine caused pronounced dose dependent shortening of AP. However sensitivity of ventricles of studied animals to acetylcholine was different. Acetylcholine induced AP shortening was 29.4, 42.1, 6.6 and 11% in ventricles of bat, hedgehog, laboratory mice and ground squirrel, respectively. Ventricles of none of previously studied mammals had sensitivity to acetylcholine comparable by magnitude to acetylcholine sensitivity which we found in hedgehog and bat. It is suggested that these animals have unique nervous regulation of cardiac action, principally different from that of other mammals, which do not require parasympathetic regulation of ventricles.