Hyperpolarizing afterpotentials in C fibers and local anesthetic effects of clonidine and lidocaine.

Effects of clonidine and lidocaine on the hyperpolarizing after-potential (HAP) and frequency-dependent block in C fibers were examined on desheathed rabbit vagus nerves, using the sucrose gap technique. A single action potential (AP) was followed by a fast and a slow HAP. Clonidine, at concentrations from 0.05 to 50 mumol/l, decreased the fast HAP, while the AP amplitude was unchanged. At a 500 mumol/l concentration of clonidine, the fast HAP amplitude was similar to control, the slow HAP was increased, and the AP amplitude decreased. Lidocaine at 500 mumol/l delayed and broadened the HAP, making a distinction between fast and slow HAP impossible, and decreased and delayed the AP amplitude. In the presence of lidocaine (500 mumol/l), clonidine at concentrations from 0.05 to 500 mumol/l decreased the HAP amplitude, without modifying the lidocaine-induced shape of the HAP. The modifications of the HAP, however, do not contribute to the local anesthetic effects of clonidine, as the addition of clonidine (0.5 and 500 mumol/l) to Locke or lidocaine (500 mumol/l) solution does not enhance the frequency-dependent block (3 and 10 Hz) observed with either Locke or lidocaine solution alone.