The cell water problem posed by electron microscopic studies of ion binding in muscle.

The question whether cell K+ is free or bound in the striated frog muscle has been investigated during the last 10 years by different cryomethods and electron microscopy. The results support the view that most of cellular ions are osmotically inactive and that therefore the observed cell water activity must be explained by a model which assumes a specific cell water structure. According to the association-induction hypothesis, cell water is influenced by macromolecules and has low solubilities for Na+ and other solutes which therefore are partly excluded from cellular water. Autoradiography of frozen hydrated Na+ loaded muscles and microanalytical studies with freeze-dried cryosections of ouabain treated muscle support the view that cell water has the proposed Na+ exclusion property. It is concluded that problems such as cell volume regulation and muscle contraction cannot be understood completely without taking into account cellular ion binding and a specific cell water structure; in addition, mainly due to these cell properties it seems to be impossible to avoid volume changes of cells and subcellular compartments during conventional chemical fixation and dehydration of biological specimens.