G. S. Sukhova

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.

Non‐quantal release of acetylcholine from parasympathetic nerve terminals in the right atrium of rats

Acetylcholinesterase (AChE) inhibitors provoke typical cholinergic effects in the isolated right atrium of the rat due to the accumulation of acetylcholine (ACh). Our study was designed to show that in the absence of vagal impulse activity, ACh is released from the parasympathetic nerve fibres by means of non‐quantal secretion. The conventional microelectrode technique was used to study changes in action potential (AP) configuration in the right atrium preparation of rats during application of AChE inhibitors. Staining with the lipophilic fluorescent dye FM1‐43 was used to demonstrate the presence of endocytosis in cholinergic endings. The AChE inhibitors armin (10−7–10−5 m) and neostigmine (10−7 to 5 × 10−6 m) caused a reduction of AP duration and prolonged the cycle length. These effects were abolished by atropine and were therefore mediated by ACh accumulated in the myocardium during AChE inhibition. Putative block of impulse activity of the postganglionic neurons by tetrodotoxin (5 × 10−7 m) and blockade of ganglionic transmission by hexomethonium (2 × 10−4 m), as well as blockade of all forms of quantal release with Clostridium botulinum type A toxin (50 U ml−1), did not alter the effects of armin. Experiments with FM1‐43 dye confirmed the effective block of exocytosis by botulinum toxin. Selective inhibition of the choline uptake system using hemicholinium III (10−5 m), which blocks non‐quantal release at the neuromuscular junction, suppressed the effects of AChE inhibitors. Thus, accumulation of ACh is likely to be caused by non‐quantal release from cholinergic terminals. We propose that non‐quantal release of ACh, shown previously at the neuromuscular junction, is present in cholinergic postganglionic fibres of the rat heart in addition to quantal release.

Cholinergic modulation of activation sequence in the atrial myocardium of non-mammalian vertebrates

Cholinergic changes of electric activity were studied in isolated atrium preparations from fishes (cod and carp), amphibians (frog) and reptilians (lizard) using the microelectrode technique and high-resolution optical mapping. Perfusion of isolated atrium with acetylcholine (10(-6)-5.10(-5) M) caused gradual suppression of action potential generation and, eventually, completely blocked the excitation in a part of the preparation. Other regions of atrium, situated close to the sinoatrial and atrioventricular junctions, remained excitable. Such cholinergic suppression of electric activity was observed in the atrial myocardium of frog and in both fish species, but not in reptilians. Ba(2+) (10(-4) M), which blocks the acetylcholine-dependent potassium current (I(KACh)), prevented cholinergic reduction of action potential amplitude. In several preparations of frog atrium, cholinergic suppression of excitation coincided with episodes of atrial fibrillation. We conclude that the phenomenon of cholinergic suppression of electric activity is typical for atria of fishes and amphibians. It is likely to be caused by I(KACh) activation and may be important for initiation of atrial arrhythmias.

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 fi bers, NAD+ (10 μM) reduced the duration of action potentials. Under conditions of spontaneous activity of Purkinje fi bers, 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 fi bers) are probably related to activation of P1 and P2 purinoceptors.

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.

Non-quantal release of acetylcholine in rat atrial myocardium is inhibited by noradrenaline

•  What is the central question of this study? Is non‐quantal release of ACh in the heart regulated by noradrenaline or does it represent uncontrolled leakage of ACh from cardiac cholinergic fibres? •  What is the main finding and its importance? Cardiac non‐quantal ACh release can be negatively regulated by noradrenaline. This finding suggests that non‐quantal release has physiological relevance and is also important in the context of cholinergic–adrenergic interactions 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 M3 receptors 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 M3 receptor blocker 4-DAMP (10−8 M), which attested to the involvement of M3 muscarinic receptors.

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 If 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 IKACh.

Investigation of the synchronization of the sinoatrial node upon stimulation from atria

The excitation of the sinoatrial node from frog heart atria has been experimentally investigated. Potentials were measured by means of microelectrodes introduced in pacemaker cells of the sinoatrial node. It has been found that atria can modulate the rhythm of the sinoatrial node due to electric and electromechanical actions, among which the electromechanical action is more important. Specific transient processes accompanying the establishment of the stationary rhythm have been studied. A mathematical model of the transient processes of achieving the rhythm of the sinoatrial node is proposed on the basis of Diophantine methods. The calculations performed using the mathematical model satisfactorily agree with the experimental results. The stabilizing role of atria in forming the rhythm of the sinoatrial node is revealed.