Postnatal changes in water content and proton magnetic resonance relaxation times in newborn rabbit tissues

In the present study, using proton nuclear magnetic resonance relaxation (H1 NMR) measurements, an attempt was made to quantitate water fractions with different mobility in the skin, skeletal muscle, and liver tissues obtained from New Zealand white rabbit pups. Serial studies were carried out at the postnatal age of 0-1, 24, 48, and 72 h in pups nursed with their mothers and suckling ad libitum (group I) and in those pups separated from their mothers and completely withheld from suckling (group II). Tissue water content (desiccation method) and T1 and T2 relaxation times (H1 NMR method) were measured. Free, loosely bound, and tightly bound water fractions were calculated by applying multicomponent fits of the T2 relaxation curves. It was demonstrated that skin water content and T1 and T2 relaxation times decreased with age (p < 0.01), the decrease in T2 proved to be more pronounced in group II than in group I (p < 0.05). Muscle and liver water, and T1 and T2 relaxation times did not change with age in the suckling pups. In response to with-holding fluid intake muscle water remained constant, liver water increased paradoxically (p < 0.05). T1 relaxation time showed no consistent change in either tissues, whereas T2 relaxation time decreased significantly (muscle, p < 0.01) or tended to decrease (liver, p < 0.06). Using biexponential analysis fast and slow components of T2 relaxation curve could be distinguished that corresponded to the bound and free water fractions. Bound water accounted for 42-47%, 50-57%, and 34-40% of total tissue water in the skin, skeletal muscle, and liver, respectively, regardless of age and fluid intake. Triexponential fits of the T2 relaxation curve made possible the further partition of tissue water into tightly bound (fast component), loosely bound (middle component), and free (slow component) water fractions. In all tissues studied, loosely bound fraction predominated (skin, 48-64%; muscle, 54-65%; liver, 45-63%), followed by the free (skin, 26-45%; muscle, 23-32%; liver, 20-25%) and the tightly bound water fraction (skin, 6-14%; muscle, 10-16%; liver, 14-33%). Postnatal age and fluid intake had no apparent influence on this pattern of distribution. It is concluded that the majority of neonatal tissue water is motion-constrained. The free, the loosely bound, and the tightly bound water fractions appear to be interrelated and dependent on age, fluid intake, the tissues studied, and their hydration.