Dipole moment changes and voltage dependent membrane capacity of squid axon

Changes in the membrane capacity of squid axons during hyper- and depolarizations are measured between --160 and +40 mV. After corrections for the series resistance and fringe effect, we found that the membrane capacity increased from 0.68 to 1.2 muF/cm2 with depolarization. It was further observed that tetrodotoxin the external medium eliminated the change in membrane capacity without affecting the conductivity. The voltage-dependent membrane conductivity is, in turn, greatley reduced by the internal cesium ion. These observations clearly indicate that the voltage-dependent membrane capacity and conductivity are closely related to ionic channels. Particularly, the increase in membrane capacity with depolarizations may be due to sodium channels. The change in the dipole moment associated with sodium sites was determined using values of alpham and betam at various depolarizations. We found, based on voltage clamp measurements, that the increase in the dipole moment of the sodium site between --40 and --5 mV is 1230 Debye units (D.U.) and 930 D.U. between --5 and +60mV, indicating that the depolarization of sodium channels may consist of two different steps.