Volume and Composition of Fluid Compartments in Peripheral and Cardiac Muscles of Animals Born at Different Stages of Maturity

Abstract

The variable degree of somatic immaturity of newborns of different species is also reflected by the peculiar chemical compositin of their skeletal and heart muscle. The most striking difference was found in the degree of expansion of the extracellular (e.c.) fluid phase of the peripheral muscle: the muscle of the relatively mature newborn guinea-pig contains 50%, that of the puppy 200% and that of the most immature rat 1000% more e.c. fluid in terms of 100 g dry fat-free solids (DFFS) than the muscles of their respective adults. In contrast, intracellular (i.c.) fluid content is more constant, being only about 20% higher in the newborn rat. In sharp contrast to the peripheral muscle, e.c. fluid volume is in the heart only slightly expanded in the immature newborns. Potassium/100 g DFFS is considerably higher in the heart muscle of the more "im-maturely" born animals. This is mostly due to expanded i.c. fluid volume relative to DFFS, and less to the apparently only somewhat higher or equal i.c. potassium concentration. In immatures, compared to adults cardiac, and to a somewhat lesser extent, peripheral muscle potassium content appears to be greater relative to a unit of nitrogen than to a unit of DFFS. This may be connected with the "dilution effect" of the greater glycogen content of the infantile cells. The main differences in cell composition between "immature" newborns and their respective adults are the lower protein and the higher glycogen content. No striking difference in potassium concentration seems to be probable, i.c. Na-concentration defies calculation, there is some i.c. chloride. Plasma potassium, and also red cell potassium in the dog, is higher in newborns of the "immature species".