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    Oxygen consumption, mitochondrial content and enzyme activities were determined in identified muscle fibre types of the ‘haemoglobin-less’ icefish Chaenocephalus aceratus Lonnberg. 2. Small bundles (2–12) of fast and slow fibres were isolated from the myotomal and superficial pectoral fin abductor muscles, respectively. At 0 degrees C the time to 50% peak force and the half-relaxation time of isometric twitches were, respectively, 18 +/− 1 and 38 +/− 4 ms for fast and 43 +/− 3 and 119 +/− 21 ms for slow muscle fibres (mean +/− S.E.). 3. Measurements of enzyme activities in homogenates suggest that phosphocreatine hydrolysis and oxidative phosphorylation are the main energy-supplying pathways in fast and slow muscles, respectively. Activities of glycolytic enzymes were relatively modest and showed no consistent differences between fibre types. 4. The relationship between oxygen consumption and mitochondria in slow muscle was also determined for a ‘red-blooded’ antarctic (Notothenia gibberifrons), a cold-temperate (Myoxocephalus scorpius) and a warm-temperate (Oreochromis niloticus) fish. Volume densities of mitochondria were as follows (mean +/− S.D.): C. aceratus, 0.50 +/− 0.08; N. gibberifrons, 0.30 +/− 0.10; M. scorpius, 0.23 +/− 0.05; and O. niloticus, 0.20 +/− 0.05. ADP-stimulated respiration rates were measured in isolated fibre segments. In spite of their different mitochondrial contents, slow fibres from the two antarctic fish utilized pyruvate and palmitoyl-1-carnitine at similar rates (1.0-1.2 mumol O2g-1 wet mass min-1 at 0 degrees C). This suggests that the high density of mitochondria in icefish muscle is related, in part, to diffusion limitations.

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