The voltage dependence of membrane capacity.

1. Membrane capacity of sartorius muscle fibres has been measured at membrane potentials between -200 and +50 mV. Within this potential range the capacity is not independent of potential. Dielectric saturation is present at large negative and at positive internal potentials, indicating the presence in the membrane of permanent dipoles or movable charges. 2. In normally polarized fibres there is a sharp peak in the capacity-potential relation of about -50 mV; the capacity at this peak is 50% larger than the capacity at -90 mV. 3. In depolarized fibres this sharp peak of capacity is not present. Over the range -200 to +50 mV the capacity variation is about 10% with a broad maximum at about -80 mV. 4. The dielectric behaviour of muscle membrane is most simply explained by postulating two species of permanent dipoles or mobile charges: Charge 1 present in normally polarized fibres, but neutralized or immobilized in depolarized fibres; Charge 2 present in both polarized and depolarized fibres. The distribution of Charge 1 is more steeply voltage-dependent than is the distribution of Charge 2. 5. Movement of Charge 1 from one fully saturated configuration to the other involves a charge transfer across the membrane of between 20 and 30 nC/muF. Movement of Charge 2 in depolarized fibres requires a similar transfer of charge.