Left ventricular perimysial collagen fibers uncoil rather than stretch during diastolic filling

The collagen fibers in the myocardium are initially wavy, suggesting that they may not be directly stretched for a portion of diastolic filling. To test whether the fibers gradually straighten and at what left ventricular (LV) pressure they become straight, 24 isolated, arrested rat hearts were fixed at physiologic diastolic LV pressures and changes in collagen structure were examined. As LV pressure increased, mean ( +/- SE) sarcomere length increased (1.80 +/- 0.02 to 1.88 +/- 0.02 from 0 mmHg to 26.3 +/- 4.1 mmHg) while the tortuosity of the perimysial fibers (fiber length/midline length) decreased (1.088 +/- 0.014 to 1.031 +/- 0.006 from 0 mmHg to 26.3 +/- 4.1 mmHg). Transmural variations in collagen structure paralleled the trends in sarcomere length (epicardial regions had longer sarcomeres and straighter collagen fibers than endocardial regions). These results indicate that there is a tight coupling between perimysial collagen fibers and myocytes, consistent with the nonlinear pressure-volume and pressure-sarcomere length relationships.