The time course of changes in the properties of mitochondria from oxidative muscle of rainbow trout was examined during warm (15°C) and cold (5°C) acclimation. Mitochondrial oxidative capacities showed a biphasic response during thermal acclimation: at a given assay

temperature is of crucial importance to ectotherms, particularly for fish whose branchial respiration forces body temperature to follow that of the habitat. During an annual cycle, fish from temperate zones cope with a wide thermal range. To offset these thermal shifts, many species modify their biochemical and physiological properties (Hazel and Prosser, 1974). A common response in cold-active temperate-zone fish occurs at the level of the aerobic capacity of skeletal muscle. During cold acclimation, fish tissues can increase mitochondrial volume density (Jankowsky and Korn, 1965; Johnston and Maitland, 1980; Tyler and Sidell, 1984; Egginton and Sidell, 1989), activities of mitochondrial enzymes (Jankowsky and Korn, 1965; Hazel, 1972a,b; Shaklee et al., 1979; Johnston and Wokoma, 1986; Rodnick and Sidell, 1994; St. Pierre et al., 1998) or protein-specific oxidative capacities of their mitochondria (Guderley and Johnston, 1996; Guderley et al., 1997). In rainbow trout (Oncorhynchus mykiss), cold acclimation and acclimatisation decrease the ratio of phosphatidylcholine to phosphatidylethanolamine (PC/PE) in membranes from liver mitochondria (Miranda and Hazel, 1996), increase the capacity of skeletal muscle mitochondria to oxidise pyruvate and acyl carnitines and increase polyunsaturation of mitochondrial phospholipids (Guderley et al., 1997). Cold acclimation and acclimatisation also increase the activity of some mitochondrial enzymes: β-hydroxyacyl CoA dehydrogenase (Guderley and Gawlicka, 1992), cytochrome c oxidase (CCO), citrate synthase (CS) and carnitine palmitoyl transferase (CPT) (St. Pierre et al., 1998). Furthermore, the cristae surface density of mitochondria (St. Pierre et al., 1998) and the total mitochondrial volume in oxidative muscle fibres increase at low acclimatisation temperature (Egginton et al., 2000). However, cold acclimation of rainbow trout does not increase the proportion of oxidative fibre volume occupied by mitochondria (St. Pierre et al., 1998; Egginton et al., 2000). Little is known about the time course of thermal acclimation and, in particular, the time course of changes in mitochondrial capacities. During cold acclimation (from 15°C to 5°C) of goldfish (Carassius auratus), enzymatic activities initially decrease (between 6·h and 12·h) and subsequently stabilise between 48·h and 72·h (Lehmann, 1970). The PC/PE in plasma membranes of rainbow trout kidney decreases after 8·h of cold acclimation (from 20°C to 5°C; Hazel and Landrey, 1988a). 3455 The Journal of Experimental Biology 206, 3455-3465 © 2003 The Company of Biologists Ltd doi:10.1242/jeb.00578