Ionic Permeability of the Inhibitory Postsynaptic Membrane of Lobster Muscle Fibers

Reversal potentials (E_IPSP) of the inhibitory postsynaptic potential and the membrane resting potentials (E_M) of lobster muscle fibers were determined with intracellular recording under a variety of ionic conditions. E_IPSP is solely dependent on the electromotive force of anionic batteries; i.e., on the electrochemical gradient for a "mobile" fraction of intracellular Cl (Cl_i) which is considerably smaller than the total intracellular Cl. The active inhibitory membrane is more permeable to certain "foreign" anions in the order NO₃ > SCN > Br > Cl. The membrane is impermeable to BrO_s, isethionate, and methylsulfate, but is slightly permeable to acetate and propionate. The level of Cl_i appears to be determined in part by some active (pump?) process and most of the anions studied appear to interfere with the steady-state level of Cl_i.