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We tested the hypothesis that hypertrophy of the human heart is associated with the redistribution of ventricular isomyosins. Human cardiac myosin was isolated from autopsy samples of left ventricular free wall of patients with cardiac hypertrophy and of fetal, young, and adult subjects without heart disease. The following parameters were studied: electrophoretic migration in denaturing and non-denaturing conditions; immunological cross-reactivities with three different types of antibodies; and early phosphate burst size and steady state ATPase activities stimulated by K+-EDTA, Ca++, Mg++, and actin. The antibodies were chosen for their ability to recognize selectively the rat V1 and V3 cardiac isomyosins. The first type was a monoclonal antibody, CCM-52, prepared against embryonic chick cardiac myosin, the second was an anti-beef atrial myosin, and the third was an anti-rat V1 myosin. CCM-52 reacted with a greater affinity with rat V3 than with rat V1, and was a probe of mammalian V3. Anti-beef atrial myosin and anti-rat V1 myosin both recognized specifically beef atrial and rat V1 myosins, and were thus considered as probes of mammalian V1. Under non-denaturing conditions, human myosins migrated as rat V3 isomyosin; under denaturing conditions, no difference was observed in any of the electrophoretic parameters between all samples tested, except for the fetal hearts which contained a fetal type of light chain. The immunological studies indicated that human myosins were composed mostly of a V3 type (HV3), but contained also some V1 isomyosin. A technique was developed to quantify the amount of human VI isomyosin which was found to range from almost 0 to 15% of total myosin, and to vary from one heart to the other, regardless of the origin of the heart. Enzymatic studies showed no significant difference between normal, hypertrophied, and fetal hearts in any of the activities tested. However, there was a significant correlation between Ca++-stimulated ATPase activities and HV1 amount (at 0.05 M KCl, n = 18, r2 equal 0.49, P less than 0.01; at 0.5 M KCl, n = 18, r 2 = 0.5, P less than 0.01). These data demonstrate the heterogeneity of human ventricular myosin, which appears to be composed, as in other mammalian species, of V1 and V3 isoforms of different ATPase activities (V1 greater than V3). However it seems that V1 to V3 shifts do not appear to be of physiological significance in the adaptation of human heart to chronic mechanical overloads.