Two mitochondrial fractions, termed intermyofibrillar (IMF) and subsarcolemmal (SS), were isolated from skeletal muscle, and their biochemical properties were related to differences in respiration and mitochondrial protein synthesis. State III respiration was 2.3- to 2.8-fold greater in IMF than in SS mitochondria. Site 1 inhibition of respiration with rotenone reduced this difference to 1.4-fold. When sites 1 and 2 were inhibited with antimycin, the 1.4-fold differences remained. The activities of cytochrome-c oxidase (CYTOX) and succinate dehydrogenase (SDH) could account for some of these differences, since CYTOX was 20% greater (P < 0.05) in IMF mitochondria, and SDH was 40% greater (P < 0.05) in SS mitochondria. Cytochromes a, b, c, and c1 contents were similar in the two fractions. Cardiolipin (CL) content was higher (P < 0.05) in SS mitochondria, indicating a less dense mitochondrial fraction with respect to CL. In vitro [3H]leucine incorporation was 1.8-fold higher (P < 0.05) in IMF than in SS mitochondria. Thus compositional differences between IMF and SS fractions exist, perhaps representing mitochondria at different stages of biogenesis. The biochemical and functional differences could not solely be due to differences in mitochondrial protein synthesis but could also be due to nuclear-directed protein synthesis specific to each mitochondrial fraction.

The effect of thyroid hormone on the expression of mitochondrial proteins was evaluated during development by measuring cytochrome c oxidase (CYTOX) activity and cytochrome c protein and mRNA levels in heart and skeletal muscle of control and hypothyroid rats. Animals were killed at the late fetal, early, and late postnatal stages up to 56 days of age. In heart, CYTOX activity increased 2.3-fold above the fetal level throughout development, most of which occurred prior to 2 days of age. No increase was observed in muscle. CYTOX activity was reduced in hypothyroid animals throughout development in heart compared to controls (by 50% at 56 days), but in muscle no effect of hypothyroidism was observed. In muscle and heart 4- and 1.5-fold increases in cytochrome c above the fetal level were evident by 1 day of age, with further increases to 8.5- and 2.7-fold by 56 days, respectively. The increase in cytochrome c differed from the increase in CYTOX, indicating changes in mitochondrial composition. Hypothyroidism reduced cytochrome c in muscle by 30-35% at 56 days, but had no effect in heart, indicating a muscle type-specific effect of thyroid hormone on cytochrome c protein expression. Cytochrome c mRNA increased rapidly to 4-5 fold above the fetal level in both heart and muscle by 6 h post-partum. Between 7 and 56 days of age, further increases to 6- and 25-fold were observed in muscle and heart, respectively. In muscle, the 6-fold developmental increase in mRNA paralleled that of the protein, suggesting transcriptional regulation.(ABSTRACT TRUNCATED AT 250 WORDS)

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