Calcium-activated potassium conductance in presynaptic terminals at the crayfish neuromuscular junction
Abstract
Membrane potential changes that typically evoke transmitter release
were studied by recording intracellularly from the excitor axon near presynaptic
terminals of the crayfish opener neuromuscular junction. Depolarization of the
presynaptic terminal with intracellular current pulses activated a conductance that
caused a decrease in depolarization during the constant current pulse. This
conductance was identified as a calcium-activated potassium conductance, g~c~), by
its disappearance in a zero-calcium/EGTA medium and its block by cadmium,
barium, tetraethylammonium ions, and charybdotoxin. In addition to gK~c,), a
delayed rectifier potassium conductance (gK) is present in or near the presynaptic
terminal. Both these potassium conductances are involved in the repolarization of
the membrane during a presynaptic action potential.