EFFECT OF TRAINING ON RESTING HEART-RATE OF RATS

Adult male rats were progressively trained 5 days/wk on a motor-driven treadmill. The training period lasted 12 wk and consisted of 60 min/day of wind-sprints and endurance work. No singificant difference in resting heart rates was observed between the control and exercise groups during week 1 (394 .+-. 7 vs. 388 .+-. 5). At week 12 the exercise group had a lower resting heart rate (359 .+-. 6 vs. 331 .+-. 4). Heart rates observed following saline, propranolol, atropine and propranolol plus atropine injections were lower in the exercise group in all cases. The difference in heart rates between the control and exercise groups was 19 beats/min following propranolol plus atropine which was less than the 28 beats/min difference observed under control conditions. With atropine and then with propranolol the differences were 33 and 27 beats/min. These heart rate differences were observed without the presence of cardiac hypertrophy as assessed from ventricle weights. The bradycardia resulting from exercise training is due primarily to changes other than neural influences on the heart.