Direct determination of daily water exchange between the environment and a mammalian organism is conceptually simple but technically difficult because sources other than water per se contribute to the body water reservoir. These sources include food, biological oxidation of hydrogen, and pulmonary and cutaneous exchange with the atmosphere. The practicability of estimating total water exchange solely by means of a tracer, however, has been elucidated and verified by several groups of investigators (Pinson and Langham, '57; Roberts, Fisher and Allen, '58).Pinson and Langham ('57) showed that tritium water (HTO) moves across barriers at the same rate as ordinary water. At any given time, the specific activity of water obtained from samples such as urine or sweat was the same as the specific activity of the body water. Pinson and Langham ('57) also found that after HTO equilibrated with body water, the logarithm of the HTO concentration in the body water decreased linearly with respect to the total accumulated volume of water entering the body water reservoir. When the intake rate was held constant at 2.7 liters per day, the logarithm of the HTO activity was a linear function of time. Roberts, Fisher and Allen ('58) described the functional relation between water intake and intake calculated from studies with deuterated water (HDO). The relation was linear with a slope of one and appeared to have consistent prediction validity within the limits of experimental accuracy. These workers also considered the exchange of body water in terms of the variables necessary to measure inflow and outflow rates for both HDO and HTO, when the body water was constant or changing in volume. The organic binding of tritium which occurs in animals represents only 0.5 to 1.0% of the total administered dose (Thompson, '52) and does not affect the validity of using an isotope of hydrogen to estimate total daily water exchange.As an adjunct to studies on the comparative metabolism of gamma-emitting radionuclides, the kinetics of body water retention were studied with tritium water in 7 mammalian species, including man. It was hoped that these studies on laboratory animals would yield information which would allow one to relate certain metabolic phenomena which occur among mammals on an interspecific basis. Such correlations are of considerable interest to the physiologist and may prove valuable in estimating the metabolic parameters which are used to determine maximum permissible exposure values for radionuclides.
METHODS AND MATERIALSTritiated water was administered to 7 species of adult mammals maintained on their normal dietary regimens throughout the experimental period. The usual feeding schedules were followed for dogs, horses, and human subjects, whereas food was available continuously to the mice, rats, rabbits, and Kangaroo rats. With the exception of the single case noted in table 1, all species were allowed water ad libitum. Table 1 gives details of the individual experiments as to species, sex, number of animals, initial body w...
