Structural changes in the rat myocardium after exhaustive exercise.

The ventricular myocardium of rats sacrificed immediately after exhaustive swimming exercise showed the following degenerative changes. The organization of the myofilaments varied from an accentuated hypercontraction of the sarcomeres to a disorganization of the same with loss of orientation of the myofilaments and blurring of the Z lines ("contraction bands"). The mitochondria showed an increased swelling along with disorganization of the cristae and narrow joints between the membranes. The nuclei showed an increase in the amount of heterochromatin. All these changes were evident in the first few minutes of the exercise and they reached their maximum level between the first and second hours, diminishing during the second and third hour. The effect produced in the myocardium by this exercise is reversible, since the appearance of the fibres in rats sacrificed at the twelfth, twenty-fourth, and forty-eight hour following the end of the exercise was approaching that of the controls.