A re-examination of the donnan distribution as a mechanism for membrane potentials and potassium and chloride ion distributions in crab muscle fibers

Abstract

1. 1. The adductor muscle in the meropodite of the walking leg of the blue crab, Callinectes sapidus, was studied with respect to the nature of the resting membrane potential of the muscle fiber and the ionic distributions that contribute to this potential. Equations describing a Donnan distribution for the crab muscle were utilized. Measurements of the changes in membrane potentials in response to alterations of the potassium, sodium and chloride contents of the external medium correlated well with predictions from the Donnan equations. 2. 2. Cable constants were determined on surface fibers of the adductor muscle. These fibers were found to have membrane properties intermediate between fast and slow crustacean muscle fibers. 3. 3. Ion analysis of whole adductor muscle showed a discrepancy between the experimental values for potassium and chloride and their values predicted from the Donnan equations. However, excellent agreement between the experimental and predicted values were obtained when the experimental values were corrected for “bound” potassium, “bound” chloride and non-solvent water. The results of experimental tests of some osmometric predictions of the Donnan equations are also described. 4. 4. The results of these electrophysiological, osmometric and ion analyses studies strongly support the hypothesis that the striated muscle fibers of the brackish-water crab, Callinectes sapidus, are characterized by a Donnan-like distribution.