Effects of Local Anesthetics and Hemicholinium-3 on 45Ca Efflux in Barnacle Muscle Fibers
Benzocaine, which occurs in the uncharged form in the physiological range of pH, caused inhibition of 45Ca efflux in barnacle muscle fibers. By contrast, in the presence of a low external Ca2+ concentration, it produced stimulation of the efflux. Both the inhibitory and stimulatory actions of benzocaine appeared to be less potent than those of procaine. Hemicholinium-3 (HC-3), on the other hand, which exists only in the charged form, caused a large stimulation of the 45Ca efflux following microinjection, and the potency of this action was found to be at least 10 times greater than that of procaine. External application of HC-3 produced inhibition occasionally. Effects of tetracaine were similar to those produced by procaine; however, its inhibitory action was greater in more alkaline solution, which is the opposite of that observed with procaine. Lidocaine produced a less consistent effect than procaine; the inhibitory action of the former was less potent but the stimulatory action of the two anesthetics were comparable. p-Aminobenzoic acid was without effect on 45Ca efflux. These results indicate that both the charged and uncharged forms of local anesthetics are capable of causing stimulatory and inhibitory effects on 45Ca efflux in barnacle muscle fibers, and that the inhibition produced is the result of action on the Ca–Ca exchange system whereas the stimulation is the result of release of Ca from internal storage sites.