Comparative Analysis of the Impact of α1- and α2-Adrenoreceptor Blockade on Cardiac Function in Rats during Postnatal Ontogeny

The Effect of α2-Adrenoreceptors Blockade on the Isolated Rat Heart during the Formation of Sympathetic Innervation

We studied the effects of the α₂-adrenergic receptor antagonist yohimbine (10^-9-10^-6 M) on inotropy, chronotropy, and coronary flow in the Langendorff-isolated heart from 3- and 6-week-old rats. Yohimbine affected all the studied functional parameters of the isolated heart. The force of contraction of the left ventricular myocardium, HR, and coronary flow decreased at all studied concentrations of the antagonist. The maximum chronotropic effect was observed in the heart from 6-week-old rats in the presence of 10^-6 M yohimbine in the perfused solution and in the heart from 3-week-old rats in the presence of 10^-9 M yohimbine. The minimum chronotropic and inotropic effect was recorded in the hearts from 6-week-old animals after application of 10^-9 M α₂-adrenergic receptor blocker and in the hearts from 3-week-old animals at yohimbine concentration of 10^-7 M. The least pronounced decrease in coronary flow in 3- and 6-week-old rats was observed at the minimum concentration of yohimbine.

Delayed Consequences of Experimental Myocardial Infarction: Functional Responses to Stimulation of α2-Adrenoreceptors in the Isolated Heart

We carried out a comparative analysis of the performance of the heart isolated from healthy rats and in 54 and 120 days after modeling of myocardium infarction. On day 120, the blood supply and heart contraction force increased, while HR did not change. Stimulation of α2-adrenoreceptors with clonidine hydrochloride (10^-9 M) reduced the force and rate of contraction and the blood flow in the isolated heart from healthy rats. Stimulation of α2-adrenoreceptors of the isolated heart on day 54 after modeling of myocardium infarction induced a positive inotropic response, bradycardia, and insignificant changes in the heart blood flow. On day 120, stimulation of α2-adrenoreceptors slightly reduced HR and the force of contraction and induced biphasic changes in the coronary flow of the isolated heart.

Blockade of α2-Adrenergic Receptors Inhibits Functional Parameters of Langendorff-Isolated Rat Heart

We studied the effect of α2-adrenoreceptor antagonist yohimbine in concentrations of 10^-9-10^-6 M on the inotropy, chronotropy, and coronary flow of the Langendorff-isolated rat heart. It was found that antagonist of α2-adrenoreceptors yohimbine changes all the studied parameters of the isolated heart. The force of left ventricular myocardial contraction, HR, and coronary flow decreased after application of all concentrations of α2-adrenoreceptor antagonist. The maximum inotropic, chronotropic, and vascular effects were observed when the antagonist was applied to the perfused solution in a concentration of 10^-9 M. The minimum decrease in coronary flow and HR was observed when yohimbine was applied in a concentration of 10^-8 M; minimum effect on myocardial contractility was revealed at α2-adrenoreceptor blocker concentration of 10^-7 M.

Effect of Clonidine Hydrochloride on Isolated Newborn Rat Heart

The concentration dependenies of the chronotropic response and changes in blood supply to the isolated heart of 7-day-old newborn rats induced by application of α2-adrenergic receptor agonist clonidine hydrochloride in concentrations of 10^-9-10^-6 M were revealed. The minimum concentration of α2-adrenergic receptor agonist caused tachycardia, while higher concentrations led to bradycardia. The maximum effect manifesting in a decrease in coronary flow was recorded at the minimum concentration of the agonist, while the highest concentration had no effect on the coronary flow. When comparing these results with those obtained in control adult rats, we found that the most pronounced differences in the chronotropic effects were observed after addition of the minimum concentration of the α2-adrenergic receptor agonist: bradycardia in adult rats and tachycardia in newborns. The maximum differences in coronary flow parameters were observed after addition of α2-adrenergic receptor agonist in the maximum concentration that induced a two-phase response in adult rats and had no effect on the blood supply in newborns.

Comparative Analysis of Cardiac Effects of α1A-Adrenoreceptor Stimulation In Vivo and Ex Vivo in Newborn Rats

The study examined the effects of α_1A-adrenoceptor stimulation on chronotropic function of Langendorff-perfused isolated heart ex vivo and on cardiac chronotropy in vivo in 7-day-old rats. α_1A-Adrenergic receptor agonist A-61603 reduced heart chronotropy only in the whole organism. No chronotropic effects of selective stimulation of α_1A-adrenergic receptors on isolated hearts were observed in ex vivo experiments. These findings suggest that α_1A-adrenergic receptors are not implicated in HR regulation in newborn rats. Bradycardia induced by activation of these receptors in vivo is most likely associated with reflex influences on the heart and changes in the vascular tone in the whole organism.

Effect of Stimulation of α2-Adrenergic Receptors on Action Potential of Working Cardiomyocytes in Rat Atrium

The study examined the effect of clonidine hydrochloride (10^-9-10^-5 М) on electrical activity of working cardiomyocytes in rat right atrium. Stimulation of α₂-adrenergic receptor with clonidine changed electrical activity of the heart. All tested concentrations of the agonist lengthened the action potential and decreased the firing rate of cardiomyocytes in a dose-dependent manner. The maximum effects of clonidine were observed at concentration of 10^-5 М.

Effect of α2-Adrenoceptor Stimulation on Functional Parameters of Langendorff-Isolated Rat Heart

We studied the effect of α₂-adrenoreceptor agonist clonidine hydrochloride in concentrations of 10^-9-10^-6 M on inotropy, chronotropy, and coronary flow in Langendorff-isolated heart of adult rats. It was found that α₂-adrenoreceptor agonist changed all studied parameters. Left ventricular myocardium contraction force decreased after application of all tested concentrations, the maximum effect was observed at a concentration of 10^-6 M. Stimulation of α₂-adrenergic receptors in concentrations of 10^-8, 10^-7, and 10^-6 M produced a two-phase effect (initial increase and a subsequent decrease) on the coronary flow. Clonidine hydrochloride in the maximum concentration (10^-6 M) caused a decrease in HR in one group and an increase in the other.

Selective Blockade of α2-Adrenoceptor Subtypes Modulates Contractility of Rat Myocardium

The study examined the dose-dependent effects of selective antagonists of α_2A/D-, α_2B-, and α_2C- adrenoceptors applied in concentrations of 10^-9-10^-5 M on atrial and ventricular contractility of rat myocardium in vitro. Selective blockade of each α₂-adrenoceptor subtype affected the contractile force of the atrial and ventricular strips. Various concentrations of α_2A/D- and α_2C-adrenoceptor antagonists produced positive inotropic effect on ventricular strips and negative effect on atrial strips. α_2B-Adrenoceptor blocker in the majority of the tested concentrations produced a positive inotropic effect in both atria and ventricles.

Peculiar Aspects in Influence of α1-Adrenoceptor Stimulation on Isolated Rat Heart

The study examined the effect of α₁-adrenoceptor stimulation with methoxamine on chronotropic function of isolated heart perfused ex vivo according to Langendorff and cardiac chronotropy in vivo. Stimulation of α₁-adrenoceptors in isolated heart induced gradually developing bradycardia, which progressed during several minutes. Similar stimulation in vivo produced a short-term bradycardia probably terminated by the compensatory influences in the whole organism. Comparison of the data obtained in both experimental paradigms during α1-adrenoceptor stimulation revealed unidirectional changes in cardiac chronotropy characterized with time-related peculiarities.

Effect of Selective Blockade of α2C-Adrenoceptors on Cardiac Activity in Growing Rats

Selective blockade of α2C-adrenoceptors had different effects on the cardiovascular system in rats of various age groups. Blockade of α2C-adrenoceptors in adult rats and 3-week-old animals produced the positive and negative chronotropic effects, respectively. HR in 1-week-old and 6-week-old rats did not change during α2C-adrenoceptor blockade. Selective blockade of α2C-adrenoceptors in adult rats and 3-week-old animals was followed by the increase in BP. BP in 6-week-old rats was shown to decrease under these conditions.

Blockade of different subtypes of α(1)-adrenoceptors produces opposite effect on heart chronotropy in newborn rats

We compared the effects of blockade of α(1A)-, α(1B)-, and α(1D)-subtypes of α(1)-adrenoceptors on the cardiac rhythm in newborn rats. Different responses of the heart were observed after blockade of several subtypes of α(1)-adrenoceptors. Administration of WB 4101, a selective blocker of α(1A)-adrenoceptors, increased heart rate, while blockade of α(1AD)-adrenoceptors with BMY 7378 decelerated of heart rhythm. Blockade of α(1B)-adrenoceptors with chloroethylclonidine produced no significant effects on heart chronotropy.