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 NO(3) > 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).