Cardiac myocytes' dynamic contractile behavior differs depending on heart segment.
The purpose of this study was to measure myocardial cell and sarcomere lengths at the bases and apices of the right (RV) and left ventricle (LV) of 4 normal canine hearts. The RV and LV were simultaneously fixed with glutaraldehyde at zero transmural pressure. Sections of trabeculae carneae were taken from the 1 cm cut at the bases (free wall) of the RV and LV. They were further fixed with osmium tetroxide, embedded in Epon 812, sectioned at % to 1 n and stained with azure II and methylene blue. Measured from photomicrographs (X 1,000) taken with the phase microscope, mean cell length was 70.9 fx, ± 1.49 (SEM); no statistical difference existed between the cell lengths at the bases and apices of the RV and LV. From adjacent tissue, the measurements of sarcomere lengths made from photomicrographs (phase microscope, oil immersion lens, enlarged to X 4,000) were not statistically different from those made with the electron microscope (P > 0.9). The sarcomere lengths at the LV base were the shortest—2.16 yn ± 0.002 (SEM), followed by the LV apex, 2.28 ^±0.005 (SEM); RV base, 2.41 /A± 0.006 (SEM); and RV apex, 2.46 fj. ± 0.003 (SEM). These variations in sarcomere length show the importance of specifying the site of sampling of myocardial tissue. The sarcomere lengths were related inversely to the thickness of the ventricular wall. This observation may be considered the structural reflection of the lesser distensibility of the left base as compared with the right apex, and may be simply the manifestation of the fact that the left base contains more muscle fibers. ADDITIONAL