The Effect of Exercise Training on the Response of Isolated Rat Atria to Acetylcholine at 30°C and 37°C

Abstract

The effect of exercise on heart physiology was investigated on thirty-six male Sprague-Dawley rats. Twenty-four were subjected to a daily treadmill exercise program over a fourteen week period. Body weights, and resting heart rates were measured in the intact rats. Atria were isolated in an organ bath containing Krebs-Henseleit solution. The chronotropic responses of the atria were tested in the presence of varying concentrations of acetylcholine at two temperature levels. Rats engaged in the exercise programs demonstrated a lower body weight, a lower resting heart rate and lower isolated atrial rates. The response of isolated atria from trained rats to three concentrations of acetylcholine was measured (1 x 10-5M, l x 10-6M and 1 x 10-7M) at both 30° C and 37° C. At 30°C, the addition of acetylcholine decreased the atrial rate in trained rats by an average of 40.19%, 32.21% and 28.38%, respectively. At 30° C, the addition of acetylcholine decreased the atrial rate in control rats by an average of 29.5%, 18.55% and 12.89%, respectively. At 37° C, the addition of acetylcholine decreased the atrial rate in trained rats by an average of 65.76%, 53.1%, and 44.08%, respectively. At 37° c, the addition of acetylcholine decreased the atrial rate in control rats by an average of 43.03%, 41.32% and 34.55%, respectively. It is concluded that trained rats have a lower resting heart rate and a lower intrinsic heart rate than control rats. The isolated rat atria of trained animals were more sensitive to acetylcholine than the control group at 37°C. Increased stores of acetylcholine in the region of the pacemaker may account for the lowered heart rate. The isolated rat atria of trained animals demonstrated a subsensitivity to acetylcholine at 30°C. The sensitivity to acetylcholine may be temperature related and probably related to the greater metabolic demands of exercised tissue. In addition, the trained rats demonstrated significantly larger heart to body weight ratios than did the control animals. This indicates that chronic vigorous exercise results in a relative cardiac hypertrophy. An increase in the ratio of heart weight to body weight should result in an increase in the maximum capacity to deliver blood to the working muscles.