Date Published: , 2011
Publisher: A.I. Gordeyev
Author(s): Yu.G. Odnoshivkina, A.M. Petrov, A.L. Zefirov.
The effects of the selective β2-adrenoreceptor agonist (fenoterol) on the functioning of mouse atrial were studied using both tensometry and fluorescent methods. It has been demonstrated that with the use of a high concentration of fenoterol (in the range of 1–50 µM), there is a more significant positive inotropic effect observed within a shorter period of time. In the case of relatively low doses of fenoterol (1 and 5 µM), its contractility effects are observed 20 min after the application of agonist, whereby in the case of high concentrations (25, 50 and 300 µM), the effects appear within the first minutes. During the first 10–15 min, 5 µM fenoterol causes an increase in the amplitude of Ca-signals in cardiomyocytes (this indicates an increase in the concentration of Ca ions during systole) and the activation of NO synthesis. However, after 20 min, the production of NO decreases; while the amplitude of Ca-signals remains high. The application of 50 µM fenoterol leads to a rapid increase in the amplitude of Ca-signals: at the same time, it causes a decrease in the production of NO, which we found to begin to increase after 10 min of agonist application. It is suggested that the dynamics of the positive inotropic effect occurring under pharmacological stimulation of β2-adrenoreceptors depend on the rate of increase in the amplitude of Ca-signals and on the degree of NO synthesis.
One of the mechanisms that regulate the action of the heart is based on the interaction of catecholamines (adrenaline and noradrenaline) with G protein-coupled β-adrenoreceptors in cardiomyocytes [1, 2]. Depending on their sensitivity to pharmacological agents and on their structural features, β-adrenoreceptors are divided into three types: β 1 , β 2 , and β 3 . All types of β-adrenoreceptors are expressed within the heart. The function of the β 1 -adrenoreceptors in the heart is well known. Meanwhile, β 2 -adrenoreceptors have been subjected to much less thorough study: only their functions in the regulation of the vascular tone in the heart and the skeletal muscles of bronchial lumen and the metabolism of a body have .
Isolated atria of white mice were used in the experiments. The standard Krebs solution for warm-blooded animals with the following composition was used: 144.0 mM NaCl, 5.0 mM KCl, 0.1 mM MgCl 2 , 2.0 mM CaCl 2 , 1.0 mM NaH 2 PO 4 , 2.4 mM NaHCO 3 , 11.0 mM glucose; the solution was saturated with oxygen. The pH value of the solutions was maintained at 7.2–7.4 at a temperature of 20°C. During the experiment, the specimen was stimulated by electrical pulses of suprathreshold amplitude at a frequency of 0.1–1 Hz via platinum electrodes. In the majority of experiments, the application (20 min) of (±)-fenoterol (Sigma, USA), a β 2 -adrenoreceptor agonist, at concentrations of 1–300 µM was used. In some cases, 0.1 µM ICI-118.551 (Tocris, USA), a selective blocker of β 2 -adrenoreceptors was applied.
Dose-Dependence of the Inotropic Effect of Fenoterol
The slow development and stability of the effects produced by 1 and 5 µM of fenoterol indicate the activation of the intracellular signal systems with a long-term effect, which is caused by the stimulation of β 2 -adrenoreceptors. In our opinion, the influence of fenoterol on the inotropic atrial function can be explained by the interaction of β 2 -adrenoreceptors with several intracellular signaling cascades that regulate the contractility.
Relying on the data obtained in this work, the following can be suggested. The activation of β 2 -adrenoreceptors with low doses of the agonist simultaneously initiates signal cascades, which have differently directed effects on atrial contractility. That is why the positive inotropic effect produced by the agonist does not occur at the beginning. However, the production of nitric oxide declines, while the amplitude of Ca-signals remains high; this leads to an increase in contractility. In the case of activation of β 2 -adrenoreceptors with high doses of the agonist, the pathway associated with a dramatic increase in the amplitude of Ca-signals is activated first, while the increase in the production of nitric oxide is “delayed”; that is why a significant, positive inotropic effect of the agonist is observed. The hypothetic two-component differently directed mechanism which underlies the changes in the atrial contractility occurring upon activation of β-adrenoreceptors by fenoterol (racemic) requires a more detailed experimental study. The pharmacological approach involving the application of nitric oxide synthesis blockers, the adenylate cyclase system, L-type calcium channels, ryanodine receptors, and probably endocytosis inhibitors would allow to shed more light on the pathways of the effects produced by the β 2 -adrenoreceptors of atrial cardiomyocytes, thereby providing an answer to the questions raised in this work.