GATING CHARACTERISTICS OF A STEEPLY VOLTAGE-DEPENDENT GAP JUNCTION CHANNEL IN RAT SCHWANN-CELLS

The gating properties of macroscopic and microscopic gap junctional cu rrents were compared by applying the dual whole cell patch clamp techn ique to pairs of neonatal rat Schwann cells. In response to transjunct ional voltage pulses (V(j)), macroscopic gap junctional currents decay ed exponentially with time constants ranging from < 1 to < 10 s before reaching steady-state levels. The relationship between normalized ste ady-state junctional conductance (G(ss)) and (V(j)) was well described by a Boltzmann relationship with e-fold decay per 10.4 mV, representi ng an equivalent gating charge of 2.4. At V(j) > 60 mV, G(ss) was virt ually zero, a property that is unique among the gap junctions characte rized to date. Determination of opening and closing rate constants for this process indicated that the voltage dependence of macroscopic con ductance was governed predominantly by the closing rate constant. In 7 8% of the experiments, a single population of unitary junctional curre nts was detected corresponding to an unitary channel conductance of ap proximately 40 pS. The presence of only a limited number of junctional channels with identical unitary conductances made it possible to anal yze their kinetics at the single channel level. Gating at the single c hannel level was further studied using a stochastic model to determine the open probability (P(o)) of individual channels in a multiple chan nel preparation. P(o) decreased with increasing V(j) following a Boltz mann relationship similar to that describing the macroscopic G(ss) vol tage dependence. These results indicate that, for V(j) of a single pol arity, the gating of the 40 pS gap junction channels expressed by Schw ann cells can be described by a first order kinetic model of channel t ransitions between open and closed states.