Is the osmotically inactive sodium storage pool fixed or variable?

Recently, there is renewed interest in the role of osmotically inactive Na(+) storage during Na(+) retention. Although it is well accepted that a portion of the total exchangeable Na(+) reservoir is osmotically inactive, there is current controversy as to whether the osmotically inactive Na(+) storage pool is fixed or variable during Na(+) retention. In this article, we analyze the current scientific evidence to assess whether the osmotically inactive Na(+) storage pool can be dynamically regulated. Our analysis supports the assertion that the osmotically inactive Na(+) storage pool is fixed rather than variable.