pH-dependent binding of local anesthetics in single batrachotoxin-activated Na+ channels. Cocaine vs. quaternary compounds.

The effects of internal and external pH on the binding kinetics of local anesthetics (LAs) were studied in single batrachotoxin-activated Na+ channels incorporated into planar bilayers. With internal quaternary QX-314 and RAC421-II drugs, the binding interactions were little affected by either external or internal pH. With tertiary cocaine, the binding kinetics were drastically altered by pH. A decrease in the internal pH from 9.3 to 6.2 decreased the apparent equilibrium dissociation constant (Kd) of internal cocaine by more than 100-fold. This increase in the binding affinity was mostly accounted for by an increase in the apparent cocaine on-rate constant (kon) of approximately 80-fold. The cocaine off-rate constant (koff) was little changed (between 3-4 s-1). These results demonstrate quantitatively that the charged form of cocaine is the active form for BTX-activated Na+ channels. Surprisingly, the apparent pKa of cocaine near its binding site was estimated to be 1.4 units lower than that in bulk solution (7.1 vs. 8.5), indicating that the LA drug encounters a relatively hydrophobic environment. Opposite to the internal pH effect, a decrease of external pH from 8.4 to 6.2 increased the Kd value of internally and externally applied cocaine by approximately 8- and approximately 25-fold, respectively. External pH effect was primarily mediated by modulation of kon; koff was again relatively unaffected. Our findings support a model in which neutral cocaine can readily cross the membrane barrier, but needs to be protonated internally to bind to its binding site.