Heterogeneity of myosin isozyme content of rabbit heart

A detailed study was carried out to measure the relative contents of V1 and V3 myosin isozymes in different regions of rabbit ventricle as a function of age, to assess animal-to-animal variability, and to compare different experimental approaches aimed at minimizing the effects of such variability. In addition, comparisons were made in normal developing hearts between ventricular isozyme composition and myofibrillar myosin calcium-stimulated adenosine triphosphatase. V1 isozyme predominated relative to V3 isozyme in the hearts of 2-week-old rabbits, decreasing to become a minor component in 10-week-old animals. Despite this trend, there was considerable variability in relative isozyme content of whole ventricular tissue among different rabbits of the same age. This variability was reduced in comparisons of littermates and by use of cardiac biopsies to measure changes in isozyme content in the same animal over time. Within different regions of a given heart, there also were small but significant differences in the percent V1 isozyme. The percent V1 was greatest for right ventricular papillary, followed by right ventricular free wall and then the left ventricle (free wall plus septum). There also were differences in the percent V1 within those regions, as exemplified by the significantly higher values for ventricular epicardium vs. endocardium. There was a linear correlation between the myofibrillar myosin calcium-stimulated adenosine triphosphatase and percent V1 of total isozyme for both right and left ventricles in normal and developing hearts. The regression lines for calcium-stimulated adenosine triphosphatase vs. percent V1 had a steeper slope in the left than in the right ventricle.