Plasticy and control of mitochondrial metabolism in fish muscle

Problems associated with fuel preference, metabolic capacity and regulation were addressed using isolated mitochondria and enzyme analyses of cardiac and skeletal muscles of spiny dogfish (Squalus acanthias), common carp (Cyprinus carpio), skipjack tuna (Katsuwonus pelamis) and rainbow (Oncorhynchus my kiss). Chondrichthian muscle is unusual because it possesses no detectible activities of carnitine palmitoyl transferase (CPT), which is involved in transport of fatty acids into mitochondria, yet demonstrates activity of B-hydroxyacyl CoA dehydrogenase, a fatty acid B-oxidation enzyme. This paradox was addressed by searching for alternate pathways for mitochondrial fatty acid utilization possibly involving peroxisomal fatty acid processing. Dogfish red muscle and ventricle mitochondria did not utilize long, medium or short chain free fatty acids or fatty acyl carnitines under a variety of conditions. These substrates include the putative products of peroxisomal B-oxidation, which was not detectible in muscles of 2 chondrichthians. It was concluded that dogfish muscles do not utilize fatty acids by direct or indirect muscle pathways, but may rely on ketone bodies as "lipid" fuel. Carp and tuna red and white skeletal muscle, compact and spongy myocardium and atrium were compared to examine the importance of mitochondrial differences in determining tissue aerobic capacity. The spectrum of tissues examined possess a 26-fold difference in mitochondrial content, as indicated by citrate synthase activity. Higher aerobic capacities (tuna vs. carp) are attributed to a combination of three factors: i. greater recruitable muscle mass/kg body mass, ii. greater mitochondrial volume density/g tissue iii. increased mitochondrial specific activity. The relative importance of each factor varied between tissues. In general, the more aerobic tissues (ventricle, red muscle) differ primarily in recruitable mass/kg body mass. There was less than a 2-fold difference in mitochondrial content/g or mitochondrial specific activity between tuna and carp tissues. In less aerobic tissues (white muscle, atrium), a several fold greater mitochondrial content in tuna contributed to greater aerobic capacity. Coupled with the greater aerobic capacities of tuna muscles, was a shift in fuel preference toward fatty acid utilization, as indicated by CPT/mg mitochondrial protein and CPT/hexokinase ratios. Tuna ventricle mitochondria may operate close to their maximal in vitro rates (State 3) in situ at cardiac VO₂MAX. In contrast, trout white muscle mitochondria in vivo operate at a small fraction of their in vitro maximum. The maximal aerobic demands of white muscle probably occur during recovery from burst exercise, when a high lactate load is converted to glycogen in situ: This added cost of recovery represents an ATP demand that is only 3.5% of the maximal mitochondrial capacity. This capacity is suppressed in vivo by highly inhibitory ATP/ADP ratios and limiting phosphate. The primary signal for increased ATP synthesis associated with recovery is not ATP/ADP but probably phosphate, elevated due to phosphocreatine hydrolysis and adenylate catabolism in the purine nucleotide cycle. At low ADP availability and sub-optimal phosphate (