A reevaluation of calcium-local anesthetic antagonism

Abstract

Although it has been shown that Ca 2+ can inhibit the conduction blocking action of local anesthetic agents in lobster peripheral nerve, it is not known whether Ca 2+ directly competes for anionic membrane sites or indirectly alters the drug-membrane interaction. The present experiments were carried out during a sufficiently long time course to determine whether the blocking action of lidocaine, procaine, or tetracaine was reduced by Ca 2+ or whether Ca 2+ only delayed the onset of local anesthesia. Lidocaine HCl (5 m m) reduced the amplitude of the compound action potential by 50% in 10 min in low-Ca 2+ (5 m m) lobster physiologic saline, whereas in high-Ca 2+ (75 m m) saline, 5 m m lidocaine reduced the action potential amplitude 50% only after 25 min external bath perfusion. Similar results were seen with 2 m m procaine and 0.2 m m tetracaine. The data confirm the suggestion that Ca 2+ merely slows the passage of local anesthetic agents across glial membranes and are not compatible with a model that describes competition of Ca 2+ and local anesthetic agents for a specific binding site in nerve membranes.