RELATION OF ULTRASTRUCTURE AND FUNCTION - SARCOMERE DIMENSIONS, PRESSURE-VOLUME CURVES, AND GEOMETRY OF THE INTACT LEFT-VENTRICLE OF THE IMMATURE CANINE HEART

In 18 immature mongrel dogs (3-4 weeks of age), anesthetized with chloralose and morphine, the hearts were arrested with potasium chloride and excised. After reproducible curves relating pressure and volume in the left ventricle (LV) had been obtained,each LV was fixed by coronary perfusion with glutaraldehyde while pressure was maintained at a predetermined value between 0 an30 mm Hg. Midwall sarcomere length in the LV free was determined by electron microscopy and was related to fixation pressure anto postfixation volume by silastic cats. Cats also were used to measure ventricular chamber dimension. Results were compared wih previous results for adult dogs. In the immature canine LV, the relation of sarcomere length and pressure is similar to that f the adult dog at physiological pressures. Above 15 mm Hg, sarcomeres resist stretch significantly with immature LV's fixed beween 15 and 30 mm Hg pressure with an average sarcomere length of 2.21 ± 0.50 μm in contrast to adult sarcomere length of 2.3± 0.01 μm (P<0.05). In the immature heart electron microscopy demonstrated reduced cell size and evidence of assembly of immaure sarcomeres from myofilaments. Compliance is reduced in the immature canine LV at pressures greater than 5 mm Hg as demonstrted in pressure-volume curves analyzed by exponential curve fitting. Measurements of silastic casts indicate a close geometric imilarity between the immature and adult LV. Thus, whereas the ultrastructural limits on performance are similar in the adult ad immature canine LV, the latter LV may be better protected against damage from volume overloads.