Cardiac atria and ventricles contain different inducible adrenaline synthesising enzymes.

STUDY OBJECTIVE - The aim of the study was to investigate adrenaline synthesis in atrial and ventricular homogenates. DESIGN - The study involved the use of a new assay which measures the rate at which tissue homogenates convert noradrenaline into adrenaline, or dopamine into N-methyldopamine. This was coupled with a sensitive assay for tissue catecholamines in an investigation of ventricular and atrial homogenates from rats exposed to adrenal demedullation and chemical depletion of cardiac catecholamines. MEASUREMENTS and RESULTS - Atrial and ventricular homogenates from 12 male Sprague-Dawley rats were investigated. Atrial adrenaline forming activity resembled adrenal phenylethanolamine-N-methyltransferase (PNMT) in its relatively high affinity for noradrenaline, substrate specificity for noradrenaline over dopamine, and inhibition by the PNMT inhibitor SKF 29661. Ventricular tissue nonspecifically methylated both noradrenaline and dopamine, and was less inhibited by SKF 29661. Adrenal demedullation induced activity of ventricular adrenaline forming enzyme. CONCLUSIONS - The cardiac atria and ventricles contain different inducible adrenaline forming enzymes. About one third of cardiac adrenaline may be synthesised by the heart itself. The ventricular enzyme can synthesise adrenaline from noradrenaline, and N-methyldopamine from dopamine.