The Effects of Chronic Exercise on Chronotropic and Inotropic Responses of Isolated Rat Atria to Nicotine

Abstract

Sprague-Dawley rats were divided into either a control group or one of three training regimens. Rats were conditioned by either motorized wheel running (Group I), a combination of running and forced swimming (Group II) and forced swimming only (Group III). The training consisted of two sixty minute sessions per day, seven days a week for six to eight weeks. At the conclusion of the exercise program the atria were removed and placed in a Krebs-Henseleit solution (pH of 7.2±.01) continuously oxygenated with 95% O_2 and 5% CO_2 and maintained at a temperature of 34.0±°C. The chronotropic and inotropic responses of the isolated atria to acetylcholine (1 x 10-6M, 1 x 10-7M) and nicotine (1 x 10-4M, 1 x 10-5M, 1 x 10-6M) was evaluated. Group II and Group III displayed a significant increase in mean ventricular and mean total heart weights. All exercised rats (Groups I, II and III) were compared to control rats with respect to isolated atrial rates. Exercised rat atria beat at a significantly slower rate. The mean body weight of group I and group II were significantly greater than their control counterparts at the termination of their exercise program. The atria of exercised rats were significantly less sensitive to acetylcholine and nicotine when chronotropic and inotropic responses were evaluated. The results suggest that exercise induced the atria to become less sensitive to acetylcholine both at the muscarinic (effector cell) and the nicotinic (postganglionic) receptor sites, presumably as a result of increased vagal impulse traffic.