The effects of extracellular pH with and without bicarbonate on intracellular procaine concentrations and anesthetic effects in crayfish giant axons

Abstract

Background: The potentiating effect of sodium bicarbonate on local anesthetic action is attributed to two mechanisms: (1) an increase in the un- ionized local anesthetic due to extracellular alkalinization, and (2) an accelerated conversion of local anesthetic from un-ionized to ionized form with intracellular acidification caused by bicarbonate. To evaluate these hypotheses, the intracellular pH, [intracellular ionized procaine concentration, and evoked action potentials were measured in crayfish giant axons. Methods: In all measurements, axon preparations from crayfish were perfused extracellularly for 15 min with either bicarbonate-containing solution at pH 7.6 (bicarb/7.6) or bicarbonate-free solution at pH 7.6 (nonbicarb/7.6) or pH 8.0 (nonbicarb/8.0). Intracellular pH was measured using a pH-sensitive microelectrode. Intracellular anesthetic concentration was measured using a specially designed procaine-sensitive microelectrode with each of three solutions containing 1 mM procaine hydrochloride. Membrane potential was measured and, as an index of anesthetic action, the dV/dt of evoked action potential was calculated during perfusion with procaine. Results: Mean intracellular pH was significantly lower in the bicarb/7.6 (7.16 ± 0.07) group than in the nonbicarb/7.6 (7.33 ± 0.09) and nonbicarb/8.0 (7.33 ± 0.12) groups (P < 0.01). The mean intracellular ionized procaine concentration was significantly higher in the bicarb/7.6 (0.53 ± 0.08 mM; P < 0.05) and nonbicarb/8.0 (0.58 ± 0.13 mM; P < 0.01) than in nonbicarb/7.6 (0.32 ± 0.14 mM) group but did not differ between the bicarb/7.6 and nonbicarb/8.0 groups. The mean percentage decrease in dV/dt(max) was approximately coincident with the mean intracellular procaine concentration in each solution. Conclusion: The presence of bicarbonate or extracellular alkalinization increased the intracellular concentration of ionized procaine and the anesthetic effect.