Inactivation of single Ca2+ channels in rat sensory neurons by extracellular Ca2+.

1. Single Ca2+ channels conducting 20 mM Ba2+ from adult rat dorsal root ganglion cells were characterized using the two-electrode patch-clamp technique configuration. 2. Channels demonstrating specific characteristics of conductance, voltage dependence and dihydropyridine sensitivity were classified as high-threshold or L-type Ca2+ channels. 3. Mean single-channel current in 20 mM Ba2+ did not show inactivation, but inactivation occurred when using Ca2+ as a permeating ion. 4. Stimulus protocols were delivered alternately in the cell-attached and whole-cell electrode, while recording single-channel activity and total Ca2+ current simultaneously. 5. A mean single-channel Ba2+ current from a stimulated patch did not show inactivation. However, stimulation of a physiological whole-cell Ca2+ current induced a marked inactivation of mean single-channel Ba2+ current. 6. Complete Ca2+ current block by the addition of 200 microM Cd2+ in the external solution removed single-channel inactivation in patches stimulated through a whole-cell electrode.