to determine if decreased cardiac rate with increasing age in fischer-344 rats was due to changes in the heart itself , in adrenergic nerves innervating the heart or in both , we studied heart rate in vivo and in vitro , and atrial and ventricular pacemaker activity in vitro following atrioventricular block , in control and in chemically sympathectomized rats [ pretreated with 6-hydroxydopamine ( 6-ohda ) , 20 mg/kg , s.c. , 24 h prior to testing ] at ages 1 to 28 months .
C0018787
heart
C0018792
atrial