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SW Postma and WA Catterall
The effects of several local anesthetics on the binding of ligands to receptors associated with voltage-sensitive sodium channels in rat brain synaptosomes have been examined. In the presence of 0.3 microM scorpion toxin, the 13 local anesthetics tested inhibited the specific binding of [3H]batrachotoxinin A 20 alpha-benzoate [( 3H]BTX-B), a ligand which binds to a receptor site responsible for the activation of sodium channel ion flux, in a dose-dependent fashion, with KD values ranging from 1.2 microM for tetracaine to 1.58 mM for benzocaine. A plot of log KD from these binding experiments against log K0.5 for inhibition of sodium currents by local anesthetics from electrophysiological experiments yielded a regression line with a slope of 0.84 and a correlation coefficient, r, of 0.86, demonstrating that the inhibition of [3H]BTX-B binding by local anesthetics occurs within a concentration range of physiological relevance. Tetracaine had little effect on basal 125I-labeled scorpion toxin binding to synaptosomes in the absence of batrachotoxin. However, in the presence of batrachotoxin, tetracaine inhibited the batrachotoxin-dependent increase in scorpion toxin binding (KD = 2.0 microM) in a dose- dependent manner, suggesting that inhibition of [3H]BTX-B binding by local anesthetics does not occur through binding at the scorpion toxin binding site. The inhibition of [3H]BTX-B binding by lidocaine was reversible within 30 min when samples were diluted from 10(-3)M to 10(- 4) M lidocaine. Scatchard analysis of [3H]BTX-B binding to synaptosomes showed that bupivacaine and tetracaine reduced receptor affinity without decreasing maximal binding capacity. This reduction in receptor affinity in the presence of local anesthetics appears to be due, at least in part, to an increased rate of ligand dissociation from the receptor-ligand complex, suggesting an indirect allosteric mechanism for the inhibition of [3H]BTX-B binding by local anesthetics. Analysis of the effects of local anesthetics in terms of an allosteric model of drug action showed that they bind to inactive states of sodium channels with at least a 10-fold higher affinity than active states. A 7-fold difference in KD for inhibition of [3H]BTX-B binding between the local anesthetic stereoisomers RAC 109 I and RAC 109 II was observed. Similarly, the dissociation rate constant for the [3H]BTX-B/receptor complex was increased 9.3-fold in the presence of RAC 109 II and 4.3- fold in the presence of a comparable concentration of RAC 109 I.(ABSTRACT TRUNCATED AT 250 WORDS)
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