Relative contribution of quantitative and qualitative changes in mitochondria to metabolic compensation during seasonal acclimatisation of rainbow trout Oncorhynchus mykiss

This study examined whether changes in the properties of mitochondria from red muscle of rainbow trout Oncorhynchus mykiss are accompanied by ultrastr uctural changes during cold acclimatisation, We compared measurements at fi ve levels of organisation in red muscle of winter- (1 degrees C) and summer - (16 degrees C) acclimatised trout. We examined (1) maximal rates of pyruv ate and palmitoyl carnitine oxidation by isolated mitochondria, (2) enzymat ic activities [cytochrome c oxidase (CCO), citrate synthase (CS), carnitine palmitoyl transferase (CPT) and phosphofructokinase (PFK)] of the muscle a nd isolated mitochondria, (3) mitochondrial protein content in the muscle, (4) the ultrastructure of muscle fibres, and (5) the cristae surface densit y of the mitochondria. All variables were measured on each trout sampled, T he mitochondria from winter-acclimatised trout possessed higher maximal cap acities for the oxidation of pyruvate and palmitoyl carnitine than those fr om summer-acclimatised trout. Muscle activities of CCO, CS and CPT were gre ater in winter than in summer trout, whereas the levels of PFK did not diff er seasonally, Similarly, the mitochondria from winter trout had elevated l evels of CCO, CS and CPT compared,vith those isolated from summer trout. Th e cristae surface density of the mitochondria from winter trout (40.2+/-0.6 mu m(2)mu m(-3); mean +/- S.E.M.) was significantly higher than that from summer trout (36.4+/-1.2 mu m(2) mu m(-3)), whereas there was no difference in the mitochondrial volume densities of muscle fibres from winter and sum mer trout. Thus, the considerable compensation of muscle aerobic capacity a t low temperatures in trout is not accompanied by changes in mitochondrial volume density, but rather by shifts in enzyme levels and cristae surface d ensity.