ON THE MECHANISMS OF POTENTIATION OF LOCAL-ANESTHETICS BY BICARBONATEBUFFER - DRUG STRUCTURE-ACTIVITY STUDIES ON ISOLATED PERIPHERAL-NERVE

Impulse inhibition by local anesthetics (LAs) is potentiated by extrac ellular solutions containing HCO3-. CO2 (BC), relative to the inhibiti on in BC-free solutions at the same pH. We studied the mechanistic bas is of this potentiation by assaying compound action potential amplitud es in desheathed frog sciatic nerves with the sucrose-gap method. We c ompared the potencies of 12 different impulse-blocking agents in Ringe r's buffered with BC (BC-R) and in Ringer's containing only atmospheri c CO2 and buffered by a zwitterionic compound (3-(N-morpholino)propane sulfonic acid-Ringer's). The relative inhibition produced by an agent in BC divided by the inhibition produced in 3-(N-morpholino)propanesul fonic acid, was defined as the potentiation factor (PF). The organic g uanidinium blockers of sodium channels, tetrodotoxin and saxitoxin, wh ich act at a different site from that for LAs, were, by our definition , nominally potentiated (PF = 1.33 +/- 0.04, XBAR +/- SEM, n = 4, and 1.24 +/- 0.07, n = 10, respectively), implying that BC induces a decre ase in the safety margin for impulse conduction, a decrease that canno t itself alone account for the much larger potentiation (PF = 5-8) by BC observed with certain LAs. Only nominal potentiations occurred with charged LAs (PF = 1.15), showing that little direct potentiation of t he cationic LA species per se occurs. Inhibition by the permanently ne utral LA benzocaine had a significantly larger than nominal potentiati on (PF = 1.8) showing that BC can potentiate neutral LAs. Among the te rtiary amine LAs, potentiation of ester-linked drugs (procaine, RAG505 ; PF = 3.9, 5.4, respectively), exceeded that of their amide-linked ho mologues (procainamide, lidocaine; PF = 1.3, 2.8, respectively) which have higher pK(a) values. This result is consistent with an ion trappi ng mechanism whereby CO2 acidifies the axoplasm and thereby increases the concentration of protonated LA inside the nerve fibers. However, s light differences in the molecular structure of 3-degrees-amine LAs wi th similar pK(a) values resulted in significantly different potentiati ons (e.g., procaine, PF = 3.9; 2-chloroprocaine, PF = 8.7), suggesting that the HCO3- or CO2 molecules interact specifically with the LA mol ecule or with LA binding sites in the nerve membrane. Spectrophotometr ic measurements of the free [Ca2+] in Ringer's showed it to be similar (+/- 0.03 mM) for both buffers, obviating changes in extracellular Ca 2+ as a mechanism of BC potentiation. The resting potential of the ner ve was slightly more negative (approximately -4 mV) in BC-R, so membra ne depolarization cannot explain the potentiation. These results lead us to conclude that CO2.HCO3- potentiates the impulse-blocking action of LAs by the combined actions of (a) a nonspecific reduction in margi n of conduction safety, (b) ion trapping, and (c) direct modification of LA binding at sites on the Na+ channel.