Myofibrillar Mass in Rat and Rabbit Heart Muscle: CORRELATION OF MICROCHEMICAL AND STEREOLOGICAL MEASUREMENTS IN NORMAL AND HYPERTROPHIC HEARTS

Abstract

A sequestered fraction of myofibrillar magnesium presumably bound to thin filaments during polymerization of actin was used to determine myofibrillar mass in rat left ventricles and rabbit hearts. Myofibrillar volume was also determined by stereological measurements on electron micrographs of rat ventricles. Myofibrillar Mg (1.11 mmoles/kg dry ventricle) can be determined in glycerinated ventricle either by measuring 28 Mg-inexchangeable Mg or Mg remaining after extraction with EDTA, KCl, and a nonionic detergent. These measurements, combined with the Mg content of myofibrillar suspensions similarly extracted (3.2 mmoles/kg protein), indicated that myofibrillar mass was 347 g myofibrillar protein/kg dry ventricle. In rabbit hearts, increases in myofibrillar Mg per unit dry weight were as follows: auricular appendage < right ventricle < left ventricle ⋍ interventricular septum. After production of left ventricular hypertrophy in rats by constriction of the ascending aorta, both myofibrillar Mg and volume increased within 24 hours. After 10 or more days, myofibrillar Mg and myofibrillar volume increased proportionately more than tissue dry mass and cell volume, and the ratio of mitochondrial volume to cell volume decreased.