Water Content, Thermogenesis, and Growth Rate of Skeletal Muscles in the European Starling

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Abstract
The relationship between function, water content and growth rate of skeletal muscles was examined in the European starling (S. vulgaris). Function was measured as the ability of nestlings to increase their rate of O2 consumption in response to cold stress. Water content and growth rate of pectoral and leg muscles were determined for tissues dissected from a series of known-age nestlings. The maximum metabolic response to cold stress increased from 0 at 4 days of age to 4.5-5.5 cm3 O2/g per h at 15-16 days. During this period, the mass of the leg muscles increased 5-fold and that of the pectoral muscles, 12.5-fold. The water fraction (WF) of the pectoral muscles decreased from .apprx. 0.86-0.72, while that of the leg muscles decreased from 0.8-0.72. Of the variation in metabolic response to cold stress/g of muscle, 89% could be related to the WF of skeletal muscle by a linear relationship. Over the ages surveyed, the metabolic response varied from 0 at WF = 0.85 to an increase above resting metabolism amounting to 60 cm3 O2 .cntdot. gram of pectoral and leg/muscle per h at WF = 0.72. Also during this period, the growth rate of both muscle masses decreased with age and proportion of water from a mass-specific growth rate of 0.5/day at WF = 0.85 to near 0 at WF = 0.72. If these relationships represent a balance betweeen growth rate and functional maturity in developing tissues, then even small changes in function, associated with a change in water content of only a few percent, could have large consequences for the growth rate of the individual.