Intracellular Na+, K+ and Cl− activity in tonic and phasic muscle fibers of the crabEriphia

(1) Intracellular activities of K⁺, Na⁺ and Cl^- were measured with ion-sensitive microelectrodes in four different types of muscle fibers in the closer muscle of the crabEriphia. (2) The membrane resting potentials of the tonic fibers were 9–15 mV more positive than those of phasic muscle fibers. This was due to higher permeability of the membranes of tonic fibers for Na⁺. (3) The intracellular Na⁺-activity of tonic fibers was 35–40% higher than that of phasic fibers. Also intracellular Cl^--activity was about 15–33% higher in tonic fibers. (4) No significant differences in K⁺-activities were found between physiologically different muscle fiber types. The K⁺-equilibrium potentials were always more negative than the resting potentials. In muscle fibers with inhibitory innervation, Cl^--equilibrium potentials were close to (phasic fibers) or slightly more negative (tonic fibers) than resting potentials.