[The fixed charge of protoplasm in barnacle muscle fiber; effect of the length of the sarcomeres].

The thermodynamically effective charge density (phi X) of myoplasm was measured on barnacle muscle fibers. A small section of a muscle cell formed a liquid junction between two electrolyte solutions and the electrical potential between the solutions permitted to evaluate phi X. If the sarcomere length was kept constant (Ls = 12.5 micron), phi X increased from 0.068 to 0.090 equiv./L when the water content was reduced from 75 to 65 g/100 g wet weight. This increment of X resulted from an increase of X when the concentration of macromolecules is raised in the sample of cytoplasm. But it also indicates that the counterions of the fixed charges concur less to the activity of the counterions when X is increased by lowering the water content. If the water content is maintained constant, phi X increased from 0.068 to 0.084 equiv./L for sarcomere lengths equal to 9.6 and 11.5 micron but phi X remained constant for Ls greater than 12 micron. Thus, when the length of the sarcomere is greater than 12 micron the contribution of the counterions of the fixed charges to the activity of the counterion is larger than for Ls = 9.6 micron.