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 potassium 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 and 30 mm Hg. Midwall sarcomere length in the LV free was determined by electron microscopy and was related to fixation pressure and to postfixation volume by silastic casts. Casts also were used to measure ventricular chamber dimensions. Results were compared with previous results for adult dogs. In the immature canine LV, the relation of sarcomere length and pressure is similar to that of the adult dog at physiological pressures. Above IS mm Hg, sarcomeres resist stretch significantly with immature LVs fixed between 15 and 30 mm Hg pressure with an average sarcomere length of 2.21 ± 0.05 fim in contrast to adult sarcomere length of 2.33 ± 0.01 Jim (P < 0.05). In the immature heart, electron microscopy demonstrated reduced cell size and evidence of assembly of immature sarcomeres from myofilaments. Compliance is reduced in the immature canine LV at pressures greater than 5 mm Hg as demonstrated in pressure-volume curves analyzed by exponential curve fitting. Measurements of silastic casts indicate a close geometric similarity between the immature and adult LV. Thus, whereas the ultrastructural limits on performance are similar in the adult and immature canine LV, the latter LV may be better protected against damage from volume overloads. Ore Res 44: 879-691, 1979