An important advance towards an understanding of the inequalities in ionic distribution observed in muscle was made in 1941 when Boyle and Conway pointed out that the type of equilibrium for diffusible and nondiffusible ions characterized by Donnan might apply. In a Donnan system consisting of two compartments separated by a membrane, the concentration ratios for any diffusible ions must be equal at equilibrium, since the same membrane potential is common to all of them. Boyle and Conway (1941) showed experimentally that in frog sartorius muscle the relationship
K [Cl]o was duly obeyed, the ratio for K+ ions being the inverse of that for Cl_ ions because of their opposite charges. Subsequent observations by Hodgkin and Horowicz on the effect of sudden changes in [Cl]o on the membrane potential of frog muscle fibres have borne out their conclusions in every respect. A further requirement for the operation of a Donnan equilibrium is the presence of sufficient nondiffusible ions to achieve both an electrical balance between the anions and cations in each compartment, and an osmotic balance between the total solutes on the two sides. This can be met if the bulk of the anions inside the cell are unable to move outwards, and the principal cation in the external medium is unable to penetrate the cell. Boyle and Conway's proposition that K+ and Cl— could be regarded as diffusible ions and Na+ as nondiffusible therefore went an appreciable way towards explaining the observed facts.
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