EFFECT OF MICROGRAVITY ON THE EXPRESSION OF MITOCHONDRIAL-ENZYMES IN RAT CARDIAC AND SKELETAL-MUSCLES

The purpose of this study was to examine the expression of nuclear and mitochondrial genes in cardiac and skeletal muscle (triceps brachii) in response to short-duration microgravity exposure. Six adult male ra ts were exposed to microgravity for 6 days and were compared with six ground-based control animals. We observed a significant 32% increase i n heart malate dehydrogenase (MDH) enzyme activity, which was accompan ied by a 62% elevation in heart MDH mRNA levels after microgravity exp osure. Despite modest elevations in the mRNAs encoding subunits III, T V, and VIc as well as a 2.2-fold higher subunit IV protein content aft er exposure to microgravity, heart cytochrome c oxidase (CytOx) enzyme activity remained unchanged. In skeletal muscle, MDH expression was u naffected by microgravity, but CytOx activity was significantly reduce d 41% by microgravity, whereas subunit III, TV, and VIc mRNA levels an d subunit TV protein levels were unaltered. Thus tissue-specific (i.e. , heart vs. skeletal muscle) differences exist in the regulation of nu clear-encoded mitochondrial proteins in response to microgravity. In a ddition, the expression of nuclear-encoded proteins such as CytOx subu nit TV and expression of MDH are differentially regulated within a tis sue. Our data also illustrate that the heart undergoes previously unid entified mitochondrial adaptations in response to short-term micrograv ity conditions more dramatic than those evident in skeletal muscle. Fu rther studies evaluating the functional consequences of these adaptati ons in the heart, as well as those designed to measure protein turnove r, are warranted in response to microgravity.