1. An in vitro preparation was used to measure rates of oxygen consumption, Na+, K+-ATPase-dependent respiration, [14C]phenylalanine incorporation and tyrosine release of skeletal (sternomandibuiaris) muscle from I0-21-d-old (three) and 7-month dairy (three) calves and control (CDM; four) and extreme double-muscled (EDM; two) calves.2. Rate of oxygen consumption was greatest (P < 0.001) for muscle from 10-21-d-old dairy calves and lowest (P < 0.05) for CDM calves.3. Ouabain 4. Na+, K+-ATPase-dependent respiration was similar for muscle from all calf groups except 10-21-d-old dairy calves which had a value 26% greater (P < 0.001) than that of older dairy calves.5. Na+, K+-ATPase-independent respiration was 16% greater (P < 0.001) for muscle from 10-21-d-old than that of older dairy calves while muscle from EDM calves had a value 11 % greater than that of CDM calves.6. The rate of [14C]phenylalanine incorporation was greater (P < 0.05) for muscle from IG21-d-old dairy than from older dairy calves, similar between older dairy and CDM calves, and decreased (P < 0.05) for EDM calves.7. Rate of tyrosine release was greatest (P < 0.05) for muscle from CDM and EDM calves; both dairy groups had similarly low rates of muscle tyrosine release. 8. The energy estimated to be kquired for peptide bond synthesis accounted for 2.0-3.3% of the 0, consumption of the muscle preparations.M) caused a 40% inhibition of muscle respiration.In order to more fully understand whole animal energy expenditure it is necessary first to identify the causes of metabolic energy expenditure, and then to determine their quantitative importance under a variety of physiological conditions. Active sodium ion-potassium ion transport, that is, the activity of the plasma membrane Na+, K+-ATPase (EC 3 . 6 . 1 .3) in counteracting transmembrane movement of Na+ and K+ along their concentration gradients, has been suggested to be a major component of the energy expenditure of animals and has been estimated to account for 2 M 5 % of the oxygen uptake of resting cells (Whittam, 1961). Protein synthesis has also been suggested to be a major energy cost of animals, accounting for up to 30% of the heat production of cattle (Lobley et al. 1980). The extent to which energy expended by processes such as active Na+-K+ transport and protein synthesis can vary between animals and is influenced by genetic and environmental factors is not clear. Evidence that exposure of animals to a cold environment selectively increased energy expenditure at the level of the Na+, K+-ATPase has been presented for muscle preparations from sheep (Gregg & Milligan, 1982).The objectives of this experiment were to obtain physiologically-realistic estimates of the magnitudes of the energy costs of active Na+-K+ transport and protein synthesis in skeletal muscle from calves and to examine the effects of breed and age on the relative costs of these two processes as components of background or maintenance energy expenditure of the tissue.

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EXPERIMENTAL Isocaloric diets containing various levels of protein supplied as commercially prepared defatted corn germ flour (CGF), CGF 'supplemented with 0.07% methionine and O.O2%*leucine (SCGF), casein (C) or lactalbumin (LA) were compared in this study. Weight gains of weanling rats fed various growth-limiting amounts of protein were used to determine a protein value, defined as the slope of the regression line relating weight gain to protein intake, for each of the protein sources. The PV of CGF was significantly different from that of LA, but did not differ significantly from the PV of SCGF and C. The relative protein value of CGF, the ratio of the PV of CGF to the PV of LA, is 0.62 + 0.06. These data indicate that the protein in commercially prepared CGF is of good quality and could be a valuable source of protein for human diets.