LOCAL-ANESTHETICS INHIBIT SUBSTANCE-P BINDING AND EVOKED INCREASES ININTRACELLULAR CA2+

Background: During spinal and epidural anesthesia, local anesthetics r each concentrations in cerebrospinal fluid and spinal cord tissues at which their actions may extend beyond the classic blockade of sodium c hannels. This study examines the effects of several clinical and exper imental local anesthetics on the binding and actions of a peptide neur otransmitter, substance P, known to be important in nociceptive transm ission in the dorsal horn. Methods: The binding of radiolabeled (Bolto n-Hunter modified) substance P was studied in chick brain membranes in the presence of local anesthetics. The increase in intracellular calc ium [Ca2+](in) evoked by substance P was measured by the fluorescent i ndicator fura-2 loaded in a murine cell line expressing substance P (N K1) receptors. Cells were preincubated with bupivacaine before and dur ing the transient addition of substance P. Results: Both substance P b inding and Ca2+ increase were inhibited half-maximally by approximatel y 1 mM bupivacaine at pH 7.5, whereas tetracaine, lidocaine, and benzo caine were slightly less potent at inhibiting binding. Concentration-d ependent substance P-binding studies showed that bupivacaine's inhibit ion was not competitive. Inhibition of substance P binding by bupivaca ine Increased with increasing pH, but the protonated species appears t o have some inhibitory activity, and quaternary lidocaine also inhibit ed binding, There was no stereoselectivity to the binding inhibition. Conclusions: Because millimolar concentrations of local anesthetics ar e within the range measured in spinal cord during intrathecal and epid ural procedures, these results are consistent with a direct action of local anesthetics on tachykinin-mediated neurotransmission during regi onal anesthesia.