Effect of increasing age on adrenergic control of heart rate in the rat

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. With increasing age, heart rate (bpm) in vivo decreased from 440 ± 12 to 385 ± 10 in the control and from 403 ± 20 to 318 ± 11 in 6-OHDA pretreated rats; heart rate in vitro decreased from 353 ± 9 to 243 ± 8 in the control, and from 346 ± 15 to 214 ± 18 in 6-OHDA pretreated rats; the atrial rate (AR)_decreased from 304 ± 9 to 210 ± 8 in the control and from 288 ± 13 to 161 ± 32 in 6-OHDA pretreated rats while the ventricular pacemaker rate (VR) decreased from 121 ±8 to 92 ± 5 in the control, and from 100 ± 14 to 70 ± 7 in 6-OHDA pretreated rats. With age, AR decreased to a greater extent than VR and 6-OHDA had a greater effect in decreasing AR than VR. Using cardiac rate as a measure, it appears that with age changes in the pacemakers of the heart themselves (postjunctional) as well as in the adrenergic nerve endings innervating the heart (prejunctional) contribute to decreased cardiac rate and peacemaker activity in older rats.