Influence of atropine on heart rates of rats

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Abstract
Resting heart rates were obtained from unanesthetized and unrestrained male rats before and after an intraperitoneal injection of atropine sulfate (1 mg/kg). Recordings were made at 10-min intervals for a duration of 90 min. Half of the rats had been trained and were capable of running at treadmill speeds of 1.25 miles/hr for 1 hr/day. The results showed that the trained group had a significantly lower preinjection heart rate than the nontrained group and that atropine caused a greater cardiac acceleration in nontrained rats than in trained rats. The group differences were statistically significant. These results were explained by the hypothesis that trained populations have available more nonneural acetylcholine to compete against the atropine for the receptor sites.