in pups pretreated with bretylium , methylatropine , and hexamethonium , additional bradycardia was observed after administration of the beta-adrenergic receptor blocking agent atenolol , suggesting that circulating catecholamines from extra-adrenal sources ( e.g. , organ of zuckerkandl ) are important in control of heart rate during the neonatal period .
C0428977
bradycardia
C0004147
atenolol