Changes in water, protein, sodium, potassium, and chloride in tissues with growth of the beagle

Changes in total body and tissue composition of 43 beagles were analyzed from 0 day (birth) to 1 year. The tissues studied were skeletal muscle, viscera (heart, lungs, gut, liver, kidneys), skeleton, skin, and brain, and the data were expressed as follows: fat-free tissue weight (FFTW) as a per cent of total fat-free wet weight (FFWW); water and protein in grams per kilogram FFTW; and Na, Cl, K in milliequivalents FFTW. The mass of skeletal muscle increased from 21% of FFWW at birth to 36% at 1 year while the contribution of the remainder of the tissues decreased: skeleton from 30 to 25%, viscera 23 to 15%, skin 18 to 13%, and brain 4 to 0.9%. Over the same period, total body water decreased from 780 g/kg to 665, water of skeletal muscle from 771 to 665, of viscera from 782 to 621, of skeleton from 644 to 424, of skin from 765 to 669, of brain from 853 to 692; Total protein increased from 113 g/kg to 196, in skeletal muscle from 122 to 253, in viscera from 114 to 195, in skeleton from 71 to 112, in skin from 170 to 227, and in brain from 63 to 164. Total Na was 84 mEq/kg throughout the first year of growth, 101 for skeleton, and 89 for skin, while Na increased in viscera from 66 to 75 and in brain from 63 to 77, but decreased in skeletal muscle from 75 to 59. Total K increased from 31 mEq/kg at birth to 62 at 1 year, and from 38 to 107 in skeletal muscle, from 49 to 78 in viscera, and decreased from 27 to 11 in skin, and 42 to 122 in brain. Total Cl decreased from 58 to 49, in skeletal muscle from 52 to 34, in skeleton from 43 to 33, while that in viscera increased from 56 to 78. The contribution of skeletal muscle and viscera (the major metabolic cell mass) to total FFWW increased from 44 to 52%, and it contributed over 50% of total water, protein, Cl, and 89% of K. Skeletal muscle accounted for the increases. Skin and skeleton contributed 38% of FFWW, 17% of water, 29% of Na, 19% of Cl, 16% of protein, and 10% of K. The rates of change in these parameters fell into three patterns: (1) the content of the chemical component did not change significanly in the first year of growth; (2) it increased or decreased at a constant rate; or (3) there were two rates at which the concentration changed; the break between them occurred between the third and fourth months and coincided with evidence of increasing sexual maturation. A specific pattern of change was characteristic of a particular tissue and appeared independent of that of the total dog and other tissues. These data support the conclusion that there are mechanisms intrinsic to each tissue which exert a degree of control during growth over its chemical composition; therefore, growth itself can be considered an intrinsic regulatory mechanism.