Opposite Changes in Cardiac Chronotropy Induced by Selective Blockade of α1A-Adrenoceptors in Rats of Different Age

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.

Age- and sex-related changes in heart rate variability under conditions of blockade or stimulation of peripheral adrenoceptor in outbred rats

Changes in heart rhythm variability were studied in male and female mature and 5-6-week-old rats under conditions of 7-day administration of β1-adrenoreceptor blocker atenolol (2.5 mg/kg) and α1-adrenoreceptor agonist phenylephrine (0.3 mg/kg). Atenolol administration to mature rats was followed by a slight deceleration of cardiac rhythm, a tendency to heart rate variability decrease in the HF range, and moderate increase in centralization of regulation. In 6-week-old rats, increased variability of cardiointervals and significant increase of centralization of the heart rhythm regulation due to an increase in the power of low-frequency waves (specifically VLF) were observed. In both mature and young rats, changes of heart rate frequency and variability in response to atenolol administration were more pronounced in females. Phenylephrine administration was followed by a significant heart rate deceleration, increase in cardiointerval variability and centralization of heart rate regulation in mature rats and by a decrease in heart rate variability in all frequency ranges in 6-week-old rats. In mature rats, changes in heart rate frequency and variability produced by phenylephrine administration were more pronounced in males; in young rats, the most strained heart rhythm developed in females.