Abstract
A comparison is made between the quantitative predictions of equilibrium ionic distribution in living cells according to the membrane theory (Donnan equilibrium) and according to the association-induction hypothesis. This comparison shows that both theories predict competitive effects of one permeant ion on the equilibrium concentration of another permeant ion; but within the limit of experimental accuracy only the association-induction model predicts quantitatively significant specific competition of one specified ion with the accumulation of another specified ion. The equilibrium distributions of K(+), Rb(+), and Cs(+) ions in frog sartorius muscle were studied and quantitatively significant specific competition was demonstrated; these results favor the association-induction hypothesis (adsorption on cell proteins and protein complexes and partial exclusion from cell water). Based on this model we estimated that at 257deg;C, the apparent association constants for K(+), Rb(+), and Cs(+) ion are 665, 756, and 488 (mole/liter)(-1). We found that the total concentration of adsorption sites (no less than 240 mmole/kg of fresh cells) agrees with the analytically determined concentrations of beta- and gamma-carboxyl groups of muscle cell proteins (260 to 288 mmole/kg).
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