Influence of voltage-sensitive Ca++ channel drugs on bupivacaine infiltration anesthesia in mice

Background: Local anesthesia has been traditionally associated with blockade of voltage-sensitive sodium (Na+) channels. Yet in vitro evidence indicates that local anesthetic mechanisms are more complex than previously understood. For example, local anesthetics bind and allosterically modify 1,4-dihydropyridine-sensitive Ca++ channels and can reduce Ca++ influx in tissues. The current study examines the influence of voltage-sensitive Ca++ channels in bupivacaine infiltration anesthesia. Methods: Baseline tail-flick latencies to radiant heat nociception were obtained before subcutaneous infiltration of bupivacaine and Ca++-modulating drugs in the tails of mice. No musculature is contained in the tail that could result in motor block. The magnitude of infiltration anesthesia over time, as well as the potency of bupivacaine alone or in the presence of Ca++-modulating drug, was assessed by obtaining test latencies. Results: The 1,4-dihydropyridine L-type Ca++ channel agonist S(-)-BayK-8644 reduced the duration of action and potency of bupivacaine anesthesia. In opposite fashion, nifedipine and nicardipine increased the effects of bupivacaine. Neither nifedipine nor nicardipine alone elicited anesthesia. Alternatively, the phenylalkylamine L-type blocker verapamil elicited concentration-dependent anesthesia. Other Ca++ channel subtype blockers were investigated as well. The N-, T-, P-, and Q-type channel blockers, omega -conotoxin GVIA, flunarizine, omega -agatoxin IVA, and omega -conotoxin MVIIC, respectively, were unable to modify bupivacaine anesthesia. Conclusions. These results indicate that heat nociception stimulates Ca++ influx through L-type channels on nociceptors in skin. Although other voltage-sensitive Ca++ channels may be located on skin nociceptors, only the L-type channel drugs affected bupivacaine in the radiant heat test.