A slow potassium conductance after action potential bursts in rabbit vagal C fibers.

Sucrose gap recordings were made from vagus nerve in rabbit to examine the mechanisms underlying the generation of the hyperpolarization that follows a burst of evoked action potentials in unmyelinated C fibers. Analysis of the posttetanic hyperpolarization was made by fitting the membrane potential changes with the sum of two exponential components. The posttetanic hyperpolarization consisted of two separable components with time constants of approximately 0.5 and 30 s. The slower exponentially decaying component was dependent on an increase in electrogenic sodium pumping as shown by the effect of ouabain and changes in extracellular chloride. The faster-decaying exponential component was caused by a potassium conductance as shown by the effect of varied extracellular potassium. This potassium conductance appears to be novel as its dynamics vary with the frequency and duration of the burst yet increases in reduced calcium. It is suggested that this slow decaying and modifiable potassium conductance can play a role in modulation of preganglionic and presynaptic action potential conduction.