Influence of Sodium Intake on Exchangeable Sodium

Several methods for determining exchangeable sodium in human subjects by using the gammaray emitter Na22 (half-life 2.6 years) in conjunction with measurements of body radioactivity have previously been described (1-3). With these methods, it is possible to make repeated exchangeable sodium determinations over a period of several weeks or months after giving a single tracer dose of the radioisotope. Sodium metabolism can thus be studied in a way that has certain advantages over conventional metabolic balance techniques in that sodium losses from all sources are accounted for and the results obtained reflect specifically changes in the active sodium pool. For the determination of exchangeable sodium, a 24to 48-hour period is usually allowed for exchange of the tracer sodium with body sodium in adult subjects, but some observations have suggested that there is significant continuing exchange after this interval (1, 4-7). The fraction of body sodium not exchanged in 24 to 48 hours is estimated at 30% of total body sodium and is thought to consist mostly of sodium in bone (4, 5, 8-11). If these observations are correct, they would seriously limit the interpretation of longterm exchangeable sodium determinations after a single Na22 injection, for this slow exchange would result in a continually increasing calculated exchangeable sodium, and it would be difficult at any given time to determine how much of a measured increase in exchangeable sodium was due to the continuing internal exchange and how much