The quantities of body fluids and electrolytes, as well as the osmolality, are controlled within tight boundaries. It is generally thought that total body water (TBW) is essentially determined by total body Na+ (TBS) and total body K+ (TBP), since osmo- control adjusts TBW to the body's present content of the major cations, Na+ and K+ . However, a considerable portion of Na+ , e.g. Na+ bound to bone, is osmotically inactive. Recent studies suggest the amount of osmotically inactive Na+ is dynamically regulated by changes in TBS. We performed balance studies of 4 days duration in freely moving dogs to address this question. Alterations in TBS that covered the range from moderate deficit to large surplus were induced by 10 protocols. Changes in TBS were very often accompanied by changes in TBP, and the sum of individual changes in TBS and TBP correlated very well with TBW changes; it was always accompanied by osmotically adequate TBW changes. Calculations reveal that even moderate TBS changes often included substantial Na+ /K+ exchanges between extracellular and cellular space.

Conclusions: (i) osmocontrol effectively adjusts TBW to the body's present content of Na+ and K+ , thus, the amount of osmotically inactive Na+ is not dynamically controlled by TBS changes in dogs within 4 days, (ii) cells serve as a readily available, osmotically active Na+ store, (iii) the mechanisms that connect external Na+ /K+ balances with compartmental Na+ /K+ exchange are yet unknown.