Are there distinct subcellular populations of mitochondria in rainbow trout red muscle?

Ultrastructural analysis typically shows vertebrate striated muscles to possess mitochondria residing primarily in two locations. One population is interlaced throughout the myofibrils and another occurs directly beneath the cell membrane. The two populations of mitochondria can be separated and studied in vitro. Subsarcolemmal mitochondria (SSmt) are released by mechanical shearing of the tissue, whereas protease treatment is required to release the intermyofibrillar population (IMFmt). These methods were applied to rainbow trout (Oncorhynchus mykiss) red muscle to investigate the possible existence of distinct populations in this tissue. The two populations were very similar in mitochondrial DNA content (mtDNA mg-1 mitochondrial protein) and enzymatically (activities of carnitine palmitoyl transferase, &bgr ;-hydroxyacyl CoA dehydrogenase, complex I, citrate synthase, cytochrome c oxidase expressed per milligram of mitochondrial protein). Respiration rates were the same for pyruvate and succinate, but IMFmt oxidized palmitoyl carnitine 26 % faster than SSmt (P<0.05). Apart from these minor differences in fatty acyl carnitine oxidation rates, no differences in biochemical or genetic properties were detected between populations. The lack of distinct subcellular populations in fish, in contrast to the situation in mammalian striated muscle, probably relates to the high mitochondrial volume density in fish red muscle.

Inter-tissue differences in mitochondrial enzyme activity, RNA and DNA in rainbow trout (Oncorhynchus mykiss)

We examined whether the relationships between mitochondrial enzyme activity, mitochondrial DNA (mtDNA) and mitochondrial RNA (mtRNA) were conserved in rainbow trout (Oncorhynchus mykiss) tissues that differ widely in their metabolic and molecular organization. The activity of citrate synthase (CS), expressed either per gram of tissue or per milligram of total DNA, indicated that these tissues (blood, brain, kidney, liver, cardiac, red and white muscles) varied more than 100-fold in mitochondrial content. Several-fold differences in the levels of CS mRNA per milligram of DNA and CS activity per CS mRNA were also observed, suggesting that fundamental differences exist in the regulation of CS levels across tissues. Although tissues varied 14-fold in RNA g-1, poly(A+) RNA (mRNA) was approximately 2 % of total RNA in all tissues. DNA g-1 also varied 14-fold across tissues, but RNA:DNA ratios varied only 2.5-fold. The relationship between two mitochondrial mRNA species (COX I, ATPase VI) and one mitochondrial rRNA (16S) species was constant across tissues. The ratio of mtRNA to mtDNA was also preserved across most tissues; red and white muscle had 10- to 20-fold lower levels of mtDNA g-1 but 7- to 10-fold higher mtRNA:mtDNA ratios, respectively. Collectively, these data suggest that the relationship between mitochondrial parameters is highly conserved across most tissues, but that skeletal muscles differ in a number of important aspects of respiratory gene expression ('respiratory genes' include genes located on mtDNA and genes located in the nucleus that encode mitochondrial protein) and mtDNA transcriptional regulation.

Enzyme profiles in two wing polymorphic soapberry bug populations (Jadera haematoloma: Rhopalidae).

Wing-polymorphic insects are excellent models for asking questions about trade-offs in physiology and life-history because of the existence of multiple morphs exhibiting numerous strategies living in one environment. We measured activities of select key enzymes in oxidative metabolism (citrate synthase), glycolysis (hexokinase, pyruvate kinase) and fatty acid oxidation (ß-hydroxyacyl CoA dehydrogenase, HOAD) in flying and non-flying long-winged bugs from two populations (ancestral and derived) of the wing-polymorphic soapberry bug (Jadera haematoloma). In the ancestral population, flyers had significantly greater activities of citrate synthase, hexokinase, pyruvate kinase, and HOAD than non-flyers. In contrast, in the derived population, flyers and non-flyers showed no significant differences in the activities of any of the enzymes tested. There were no significant differences in activities of citrate synthase as a function of adult age (3-20 d) for either population. Short-winged bugs in the derived population had significantly lower citrate synthase activities than either of the two derived, long-winged morphs, however, they were analogous to the ancestral long-winged non-flyer. These results suggest an evolution of alternative flight strategies between the two populations.