Extrinsic charge movement in the squid axon membrane. Effect of pressure and temperature

The absorption of the lipophilic anions dipicrylamine (DPA-) and tetraphenylborate (TPhB-) by the lipid matrix of the squid axon membrane, and the kinetics of their translocation, were studied by the charge pulse relaxation technique. The axons were treated with tetrodotoxin (TTX) and 4-aminopyridine to block the ionic currents responsible for nerve excitation. At high enough concentrations of absorbed ions (approximately 10(-12) mol/cm2) the membrane voltage relaxation following a brief current pulse consisted mainly of two exponential components, whose time constants and relative amplitudes were used for estimating the translocation rate constant, K, and the density of absorbed ions, N. These measurements were performed at different hydrostatic pressures in the range 1-100 MPa (approximately 1,000 atm), and at different temperatures in the range 5 degrees C-20 degrees C. Both K and N were found to be little affected by pressure. The pressure dependence of K indicated that the translocation of lipophilic ions across the nerve membrane involves activation volumes of the order of 5 cm3/mol. In all experiments the passive membrane resistance was little affected by pressures up to 80 MPa. However, above 100 MPa it fell dramatically to low values, presumably because of phase separation phenomena between the membrane components. The temperature dependence of K, both for DPa- and TPhB-, implied an activation energy for ion translocation of the order of 60 kJ/mol, close to that measured in artificial lipid bilayers.(ABSTRACT TRUNCATED AT 250 WORDS)