The effects of pH gradients on the action of procaine and lidocaine in intact and desheathed sciatic nerves.

Abstract

The local anesthetic action of procaine or lidocaine on intact frog sciatic nerves is increased at an alkaline pH. In completely desheathed frog sciatic nerves it is possible to demonstrate a transient potentiation of lidocaine or procaine block upon decreasing the pH of the solution from 9.2 to 7.2. However, at equilibrium the block by procaine is always greater at pH 9.2 than 7.2; whereas, block by lidocaine is about the same at the two pH9s. By contrast, in desheathed toad sciatic nerves, the block by lidocaine is greater, as is drug uptake, at an alkaline pH. Nerve block and lidocaine uptake in desheathed toad sciatic nerves at an alkaline pH can be increased by pre-equilibrating the nerves in neutral rather than alkaline anesthetic-free Ringer9s solution. These observations at first appear paradoxical but can be reconciled with the proposal that block by the tertiary amine local anesthetics procaine and lidocaine is effected primarily by the cationic form; whereas. the free base form primarily determines penetration of the local anesthetic into tissue.