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. Myafibrillar 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, KC1, 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. Portions of this work have been presented previously to the American Society for Clinical Investigation (J Clin Invest 50:72a, 1971) and to the International Physiological Congress, Munich, 1971 (Proc Int Union Physiol Sci 8: 133-134).
KEY WORDSReceived October 18, 1971. Accepted for publication February 7, 1972. tissue may change. Although the myofibrillar proteins can be extracted and purified from homogenates of heart muscle, there are few reliable quantitative estimates of myofibrillar mass. The reason is that the procedures used for extraction may lead to an incorrect estimation, because the extraction is incomplete and contaminants from other cellular structures remain associated with the extracted components. These difficulties notwithstanding, the measurement of myofibrillar mass is crucial for the quantitative description of cardiac hypertrophy.For which is based on the determination of traces of magnesium sequestered in the myofibril, has the advantage that it is applicable to human material obtained at autopsy as well as experimental and biopsy material.
Methods
DETERMINATION OF MYOFIBRILLAR MASS FROM THE AMOUNT OF SEQUESTERED MYOFIBRILLAR MAGNESIUMIn one set of experiments, glycerinated unhomogenized left ventricles, for which the dry weight before glycerination was known, were minced; their membranous constituents and their content of Determination of Sequestered Myofibrillar Mg in Rat Ventricles.-The nonseptal portion of the rat's left ventricle, excised after stunning the animals, was cut away from the interventricular septum and divided into two roughly equal pieces. Both were immediately weighed. One piece was then dried to constant weight in a vacuum oven and reweighed to obtain the dry weight. After obtaining its wet weight, the other piece was plunged rapidly...