Permeation of barium and cadmium through slowly inactivating calcium channels in cat sensory neurones.

1. Instantaneous current‐voltage (I‐V) relations were measured from tail currents with 10 mM‐external calcium at 20 degrees C. The I‐V relations had a lower potential dependence than predicted by the Goldman‐Hodgkin‐Katz constant‐field equation. Previously proposed symmetric two‐site three‐barrier (2S3B) rate theory models were able to account for the I‐V relations reasonably well. 2. Reversal of the current flow through the calcium channels was recorded using 10 mM‐barium internally and 20 mM‐barium externally. The channels appeared to rectify at positive potentials, a property not consistent with symmetric rate theory models. 3. Externally applied cadmium ions blocked the calcium channels through at least two sites. One high‐affinity blocking site was located within the membrane electric field and had a dissociation constant of around 16 microM at O mV. Cadmium block at this site was relieved with hyperpolarization with a voltage dependence equivalent to a divalent cation moving through about 75% of the membrane electric field. 4. A low‐affinity potential‐independent blocking site also appeared to be present, having a dissociation constant of around 106 microM. 5. Cadmium had significant effects on the tail current kinetics at potentials close to 0 mV, presumably due to slow unblocking events. The rate at which cadmium ions left the calcium channel free to conduct was estimated to be about 3300 s‐1 at +10 mV.