The pH-dependent local anesthetic activity of diethylaminoethanol, a procaine metabolite.
To test whether the products of procaine hydrolysis have local anesthetic actions resembling those of procaine, the authors compared the ability of procaine and its metabolites diethylaminoethanol (DEAE) and para-aminobenzoic acid (PABA) to block compound action potentials in excised, desheathed frog and rat sciatic nerves. Studies were performed in solutions of impermeant buffers at pH 7.4 (corresponding to mammalian physiologic pH) and at pH 9.2 (close to the pKa of procaine and DEAE) to test for extracellular pH-dependent increases in drug permeation and potency. Both procaine and DEAE inhibited compound action potentials at pH 7.4 and 9.2 in a reversible and dose-dependent manner, and both were approximately ten-fold more potent at pH 9.2 than at pH 7.4, procaine inhibiting the action potential height by 50% at 0.15 mM (pH 9.2) and 1.1 mM (pH 7.4), DEAE at 4 mM (pH 9.2) and 70 mM (pH 7.4). In contrast, PABA at concentrations up to 25 mM and at either pH failed to inhibit compound action potentials, and did not modify the effects of DEAE when both drugs were given together. Procaine produced greater use-dependent block at the higher pH and at higher stimulation rates (100 Hz vs. 40 Hz); DEAE produced almost no use-dependent block. These observations suggest: 1) that DEAE might account for some of the neuropharmacologic activity of procaine in techniques that favor the accumulation of metabolites (such as those requiring large doses or prolonged infusions); and 2) that alkalinization of procaine and DEAE solutions appears to increase their potency for both resting and use-dependent block of action potentials.