Figure 6. A giant axon was initially perfused with bicarb/7.6 in the absence of procaine, then with nonbicarb/8.0 supplemented with 1 mM procaine for an additional 15 min, followed by bicarb/7.6 without procaine. Perfusion with procaine increased the cationic form of procaine (E^proc) from -59 to +17 mV without significantly altering membrane potential (E^m; from -92 to -90 mV). The difference between E^procand E^mwas increased by 74 mV in the nonbicarb/8.0 solution with 1 mM procaine, compared with bicarb/7.6 without procaine, and the intracellular concentration of cationic form of procaine was calculated as 0.46 mM (see Materials and Methods).