Effects of exercise and pressure-overload hypertrophy on the orientation of myocardial laminae

Systolic transverse shear strain is associated with sliding between myocardial laminae and may account for over half of normal ventricular wall thickening. The effects of hypertrophy on laminar remodeling, and thus the relationship between shear and wall thickening, were measured in three groups of wistar rats : (I) pressure overload due to thoracic aortic banding (ab} (28 days, n=l4), (2) swimming (aw] induced exercise hypertrophy (max : 180mins/day ; 28-30 days ; n=8), (3) control (ctl) (n=9). Methods : In sections from the anterior free wall of the left ventricle-perfusion fixed at 0 mmHg-the laminar orientation in the longitudinalcircumferential (LC ; circum=0°) (fiber angle), longitudinal-radial (LR ; rad=0°), and circumferential-radial (CR ; rad=0°) planes were measured. Results : Heart weight to body weight ratio increased for both ab and sw (p<O.OOOI). For sw, LC and LR showed no changes from control, and mean(±SD) CR fell from c//=53.21 + 10.02 to in'=27.68±14.7l (p<0.005) at the subendocardium (80% depth). For ub at the subendocardium LC, LR, CR changed from 44.65+10.72, -50.10+11.15, 53.21±10.02 to 21.63+11.07 , -12.50+12.68 , 56.76±37.2I, respectively ( p<0.02). Conclusion : Changes for sw were significant only at 70% and 80% depths in the CR plane. Because there was no changes in the LR plane for ,vn', the relationship between systolic shearing and wall thickening is not expected to change. In ab, the laminae approach the endocardium more radially, thus contributing to increased diastolic wall thickness and reducing the potential for shear to contribute to wall thickening in pressure overload hypertrophy. (NIH #95-5363 & NIH/NHLBI #07491 ).