Intrinsic cardiac catecholamines help maintain beating activity in neonatal rat cardiomyocyte cultures

In the present study, we identify intrinsic cardiac adrenergic (ICA) cells in the neonatal rat heart using immunofluorescent histochemical staining techniques with antibodies that specifically recognize the major enzymes in the catecholamine biosynthetic pathway. ICA cells are most concentrated near the endocardial surface of ventricular myocardium, but are also found sporadically throughout the heart. In primary cultures of neonatal rat cardiomyocytes, ICA cells are closely associated with clusters of cardiomyocytes. To investigate a potential role for intrinsically produced catecholamines, we recorded beating rates in the presence and absence of the catecholamine-depleting agent reserpine or the adrenergic receptor blockers prazosin and timolol using videomicroscopy and photodiode sensors. Our results show that beating rates slow significantly when endogenous catecholamines are depleted or when their action is blocked with either a β- or an α₁-adrenergic receptor antagonist. These data indicate that intrinsic cardiac catecholamines help to maintain beating rates in neonatal rat cardiomyocyte cultures via stimulation of a ₁- and β-adrenergic receptors. This information should help to increase our understanding of the physiologic mechanisms governing cardiovascular function in neonates.