Effect of Age, Sodium Depletion and Sodium Repletion on the Retention of Sodium22 by Rats

A 4 π liquid scintillation counter was used to measure the whole body retention of Na22Cl for 173 days in young and adult rats fed either a ground commercial laboratory chow or a low sodium diet. Whole body retention of Na22 was measured also during a sodium repletion phase. Results of these studies indicated that radiosodium retention can be described by the sum of three exponential components. A slowly exchanging bone component with a biological half-time of 270 to 280 days was observed. Age-dependent differences in the retention patterns were noted. Sodium22 turnover was extremely slow in the sodium depleted animals. Only 4 and 16% of the administered activity were lost from the young and adult animals, respectively, at the end of 40 days. The amount of radiosodium bound in the slowly exchanging bone component of the sodium-repleted animals was larger by a factor of about 10 when compared with animals maintained with the normal diet. The slowly exchanging component, characterized by the 270- to 280-day biological half-time, was thought to represent the bone sodium pool that is not freely exchangeable with circulating sodium. This pool accounted for 44% of the total body sodium pool in the young rats and 27% in the adult rats. This work points out the effect of the nutritional state of an organism on the biological retention of a radionuclide and, therefore, on the radiation dose delivered to the animal. These implications should be considered by workers, especially clinicians, using Na23.