Muscle and nonmuscle cell RNA polymerase activities in early myocardial hypertrophy

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Abstract
RNA polymerase was solubilized from separated rat cardiac muscle and nonmuscle cells during the development of myocardial hypertrophy 1 or 3 days after sham operation or aortic constriction. Six fractions of enzymes designated IA, IB, IIA, IIB, IIIA, and IIIB were identified by DEAE-Sephadex chromatography in each cell population. Fractions designated IA and IB, IIA and IIB, and IIIA and IIIB were similar to RNA polymerase I, II, and III, respectively, found in other eukaryotes. Muscle cell enzyme activity from sham-operated and aortic-constricted rats did not significantly differ 1 day after intervention. By 3 days there was a significant increase in the activity of each enzyme fraction in muscle cells from aortic-constricted rats. There was a slight increase in IIA activity in nonmuscle cells from aortic-constricted rats at 1 day. By 3 days after aortic constriction RNA polymerase IIA, IIB, and IIIB activities increased in nonmuscle cells, but no change was noted in nonmuscle RNA polymerase IA, IB, and IIIA. Changes in RNA polymerase activities in cardiac muscle and nonmuscle cells occur during the development of myocardial hypertrophy but do not appear to correlate with reported changes in RNA synthesis. chromatin-template activity, however, did increase in cardiac muscle cells at both 1 and 3 days after aortic constriction. A similar increase was not seen in nonmuscle cell chromatin-template activity until 3 days. These data suggest that the initial increase in RNA synthesis during the development of myocardial hypertrophy is related to changes in chromatin-template activity in muscle cells. The activities of the RNA polymerase increase after a short delay and vary in their response.