Comparative Analysis of Cardiac Effects of [[alpha].sub.1A]-Adrenoreceptor Stimulation In Vivo and Ex Vivo in Newborn Rats

The study examined the effects of [[alpha].sub.1A]-adrenoceptor stimulation on chronotropic function of Langendorff-perfused isolated heart ex vivo and on cardiac chronotropy in vivo in 7-day-old rats. [[alpha].sub.1A]-Adrenergic receptor agonist A-61603 reduced heart chronotropy only in the whole organism. No chronotropic effects of selective stimulation of [[alpha].sub.1A]-adrenergic receptors on isolated hearts were observed in ex vivo experiments. These findings suggest that [[alpha].sub.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. Key Words: rat; heart; chronotropy; A-61603; [[alpha].sub.1A]-adrenoceptors