EFFECTS OF PHYSIOLOGICAL AGING ON CARDIAC ELECTROPHYSIOLOGY IN PERFUSED FISCHER-344 RAT HEARTS

Aging effects on heart rate and atrioventricular (AV) conduction were studied in Langendorff-perfused hearts from 18 mature (4-6 mo), 12 middle-aged (12-14 mo), and 18 senescent (24-26 mo) Fischer 344 rats. Heart rate decreased with increasing age from 218 +/- 18 in mature to 196 +/- 27 (means +/- SD) beats/min in middle-aged rats to 183 +/- 22 beats/min in senescent rats (analysis of variance, P < 0.001). Spontaneous AV conduction time increased from 43 +/- 7 to 49 +/- 5 to 62 +/- 9 ms with aging (P < 0.0001). Paced AV conduction time also lengthened with aging, and AV Wenckebach block cycle length increased from 122 +/- 10 to 133 +/- 9 to 152 +/- 16 ms (P < 0.005). Intra-atrial conduction time was unaffected by age. Age differences in heart rate and AV conduction responses to isoproterenol (0.5 x 10(-9) to 1 x 10(-7)M) were noted with greater sensitivity at lower doses in hearts from younger rats. In separate experiments, 18 mature and 19 senescent Fischer 344 rats received reserpine (0.25 mg. kg-1.day-1 ip) for 6 days before study. Age differences in heart rate and AV conduction persisted (P < 0.0001). Histopathological examination of AV nodal and His-bundle tissues in three hearts from each age group showed increased intercellular collagen with advancing age. It was concluded that in Fischer 344 rat hearts, aging from 4 to 24 mo is associated with 1) decreased heart rate, 2) prolonged AV conduction times, 3) increased intercellular collagen in the AV node and bundle, 4) no change in intra-atrial conduction time, and 5) no change in qualitative responses to beta-adrenergic stimulation.