Effect of an exercise run to exhaustion on cAMP in the rat heart.

The purpose of this investigation was to determine whether adenosine 3',5'-cyclic monophosphate (cAMP) content is increased in vivo in the heart as a result of exercise at a time when there is rapid cardiac glycogen utilization. Rats were run to exhaustion on a treadmill for a period of 164.5 +/- 9.5 min. Blood norepinephrine and epinephrine were significantly elevated approximately 2.5-fold above resting levels at the end of the treadmill run. Myocardial glycogen was reduced by 54.7% at exhaustion compared with control values. Myocardial cAMP was significantly elevated 88% above control levels as a result of the run. Associated with the depletion of myocardial glycogen and the elevation of cAMP was an activation of phosphorylase to its a form. These data suggest that myocardial glycogen metabolism during exercise is, in part, mediated by hormonal influences that are associated with increases in cAMP.