High conductance anion channel in Schwann cell vesicles from rat spinal roots

Potassium uptake, possibly together with chloride, is one of the
presumed functions of Schwann cells in the peripheral nervous
system. However, the presence of chloride channels has not been
demonstrated in adult Schwann cells. We present here a new method
which allows single channel recordings to be made from Schwann
cells in situ without enzymatic treatment. Isolated rat spinal
roots were split mechanically into several bundles. Within about 30
min after this procedure small belb-like vesicles (20-30 m in
diameter) with a clean surface appeared at the edges of the fibre
bundles. Immunofluorescence microscopy with a surface marker for
Schwann cell membranes (monoclonal antibody O4) revealed that the
vesicles originate from Schwann cells. In standard patch clamp
recordings with symmetrical bath and pipette solutions (excised
inside-out configuration) an anion channel with the following
characteristics was mainly observed: (1) single channel slope
conductance of 337 ± 5 pS in 125 mM KCl and 209 ± 6 pS in 125 mM K+
methylsulphate; (2) ion permeability ratio: PCl/PK/Pgluconate =
1/0.12/0.06; (3) linear current-voltage relationship (range ± 60
mV) and (4) voltage- and time-dependent inactivation (the channel
was most active at potentials ± 20mV). Pharmacologically, the
channel was completely blocked with zinc (1 mM) and barium (10 mM).
A similar anion channel, showing characteristics 1 - (4), has been
described in cultured Schwann cells of newborn rats (Gray et al.,
1984). We now demonstrate that this channel is also present in
adult Schwann cells in situ.