The mechanisms by which the DNA content of the heart increases following acutely induced cardiac hypertrophy were investigated in mature SpragueDawley rats. Special attention was given to the cellular organization of the growth process. Autoradiographic studies provided conclusive evidence that the uptake of tritiated thymidine is completely limited to nonmuscular cellular elements, chiefly connective tissue cells. The frequency of labeled nuclei was increased by sixfold during hypertrophy. The thymidine pool was not appreciably different in the hypertrophied hearts. Connective tissue nuclei formed a larger proportion of the total nuclear population in hypertrophied hearts, and their distribution was less uniform than in the normal heart. Quantitative histologic studies also showed that the total number of left ventricular muscle cell nuclei did not increase during hypertrophy but rather may have decreased slightly. Both the concentration and the total amount of hydroxyproline increased in parallel with the proliferative changes in the connective tissue and provide further supportive evidence to the autoradiographic and histologic studies. ADDITIONAL KEY WORDSaortic banding DNA hydroxyproline content thymidine pool autoradiography quantitative histology • In the preceding paper (1) we reported that in hearts hypertrophying in response to experimentally produced aortic constriction the DNA content increases proportionally to the increase in heart weight in the early phase of hypertrophy. (The term cardiac hypertrophy will be used to mean cardiac enlargement without regard to possible hyperplasia.) In the chronic phase of hypertrophy, the DNA content falls below the level present in the acute phase but is still higher than in control animals. Although we found that polyploid frequencies in muscle nuclei more than doubled, the large increase in DNA content in hypertrophied hearts could not be accounted for by the increased polyploidy. No mitotic activity was observed in differentiated muscle cells, but the mitotic index in nonmuscular elements was greatly increased.These results indicate strongly that cell proliferation is predominantly, if not exclusively, limited to nonmuscular cells at least in the model of rapidly developing cardiac hypertrophy which was under examination. Similar results and conclusions have been obtained by Crane

Haem a and cytochrome c were isotopically labelled in mitochondria from rat heart and liver after injection of delta-amino[2,3-(3)H(2)]laevulate, a specific haem precursor. [guanido-(14)C]Arginine or l-[4,5-(3)H(2)]leucine were used to label mitochondrial proteins. Half-lives were measured from biological decay in vivo and were similar (5.5-6.2 days) for haem a, cytochrome c and [(14)C]arginine-labelled proteins. Labelling of hepatic mitochondrial proteins with [(3)H(2)]leucine resulted in a prolonged apparent half-life.

We studied the coordination of nuclear and mitochondrial gene expression during cardiac hypertrophy following aortic stenosis or thyroid hormone treatment in rats. We measured mRNA levels for representative subunits of cytochrome-c oxidase, two encoded by mitochondrial DNA and two encoded by the nucleus, as well as the levels of one mitochondrial rRNA. In both models of hypertrophy, an increase of total tissue RNA, reflecting mainly cytosolic ribosomes, accompanied the increase in ventricular weight. Relative levels of mitochondrial rRNA remained unchanged, indicating a net synthesis of mitochondrial ribosomes as well. In both models, cytochrome-c oxidase activity and nuclear-encoded mRNAs remained fairly constant, whereas levels of mitochondrial mRNAs were transiently decreased 24 h after the growth stimulus. We conclude that, in the initial phase of hypertrophy, the signal regulating the synthesis of mitochondrial rRNA is synchronized with nuclear gene expression, whereas the signal regulating mitochondrial mRNA synthesis is not. We postulate that differential regulation of mitochondrial transcription and premature termination of the polycistronic transcript (the latter giving rise to the mitochondrial rRNAs) account for the observed results.

The role of thyroid hormone on the heart in terms of contractility, induction of growth, and selective synthesis of cardiac isomyosins was studied. After transplanting rat hearts from inbred hypothyroid donors into the abdomen of hypothyroid recipients of the same strain, two hearts were obtained in the same animal, both having reduced heart rate (200-250 bpm), decreased maximum rate of force, and high predominance of V 3 isomyosin. The heart in situ carried a full load, while the transplant was denervated, beat isovolumically with minimum external work. After surgery, the recipient rats were put either on normal diet only (controls) or injected with a daily dose of T 3 (average 200 /xg/kg), which increased the heart rate to 340 bpm in 3 days (euthyroid level) and to 450 bpm in 7 days (hyperthyroid level). In T 3 -treated rats, the contractility of both hearts normalized in 7 days and showed hyperthyroid pattern in 14 days, while the mass of the in situ hearts increased to normal values in 7 days (+130 mg) and hypertrophied in 14 days (+ 340 mg), in contrast to the transplanted heart, which underwent atrophy ( -90 mg and -210 mg) similar to that of control group ( -225 mg). The predominant V 3 isomyosin was completely reversed to V] in two weeks in both hearts. Thus, T 3 can neither stimulate cardiac growth nor can it attenuate the rate of atrophy in the denervated "nonworking" heart in spite of its direct effect on contractility and synthesis of isomyosins, which was similar to that observed in the in situ heart. (Circulation Research 1987;60:824-830) C ardiac myosin is a member of a multigene family consisting of cross-striated muscles of at least 8 variants. 1 In the rat ventricle, 3 isomyosins can be resolved by electrophoresis in native state. Two of them, labelled as isomyosins V, and V 3> are homodimers of myosin heavy chains (MHC), a and /3, respectively, while the V 2 is a heterodimer. The activity of actin-activated ATPase of purified V! isomyosin is about 3 times that of the V 3 isoform, while the V 2 activity lies between that of V, and V 3 . Similarly, the maximum unloaded shortening velocity of skinned fibers and papillary muscles correlates with relative proportions of individual isomyosins. 4 " 6 Many experimental situations have shown results in change of ventricular isomyosin composition. Of those interventions examined, thyroid hormone (TH) is the most potent. 7 The hypothyroid state favors the expression of /3-MHC at the expense of a-MHC, while the opposite is true for hyperthyroidism. The change in gene transcription induced by thyroid hormone is quite rapid; for example, the concentration of a-MHC mRNA rises at about 2 hours after triiodothyronine (T 3 ) or thyroxine (T 4 ) administration to hypothyroid Received June 6, 1986; accepted January 28, 1987. animals, with consequent change in the isomyosin profile noticeable about 2 days later.The mechanism of thyroid hormone effect on the heart is not entirely clear. However, there is general agreement that its organ-specific responses a...

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