K-DEPENDENT AND HCO3--DEPENDENT ACID-BASE TRANSPORT IN SQUID GIANT-AXONS .2. BASE INFLUX()

We used microelectrodes to determine whether the K/HCO3 cotransporter tentatively identified in the accompanying paper (Hogan, E. M., M.A. C ohen, and W. F. Boron. 1995. Journal of General Physiology. 106:821-84 4) can mediate an increase in the intracellular pH (pH(i)) of squid gi ant axons. An 80-min period of internal dialysis increased pH(i) to 7. 7, 8.0, or 8.3; the dialysis fluid was free of K+, Na+, and Cl-. Our s tandard artificial seawater (ASW), which also lacked Na+, K+, and Cl-, had a pH of 8.0. Halting dialysis unmasked a slow pH(i) decrease. Sub sequently introducing an ASW containing 437 mM K+ and 0.5% CO2/12 mM H CO3- had two effects: (a) it caused membrane potential (V-m) to become very positive, and (b) it caused a rapid pH(i) decrease, because of C O2 influx, followed by a slower plateau-phase pH(i) increase, presumab ly because of inward cotransport of K+ and HCO3- ''base influx''). Onl y extracellular Rb+ substituted for K+ in producing the plateau-phase pH(i) increase in the presence of CO2/HCO3-. Mean fluxes with Na+, Li, and Cs+ were not significantly different from zero, even though V-m shifts were comparable forall monovalent cations tested. Thus, unless K+ or Rb+ (but not Na+, Li+, or Cs+) somehow activates a conductive pa thway for H+, HCO3-, or both, it is unlikely that passive transport of H+, HCO3- or both makes the major contribution to the pH(i) increase in the presence of K+ (or Rb+) and CO2/HCO3-. Because exposing axons t o an ASW containing 437 mM K+, but no CO2/HCO3-, produced at most a sl ow pH(i) increase, K-H exchange could not make a major contribution to base influx. Introducing an ASW containing CO2/HCO-3, but no K+ also failed to elicit base influx. Because we observed base influx when the ASW and DF were free of Na+ and Cl-, and because the disulfonic stilb ene derivatives SITS and DIDS failed to block base influx, Na+-depende nt Cl-HCO3 exchange also cannot account for the results. Rather, we su ggest that the most straight forward explanation for the pH(i) increas e we observed in the simultaneous presence of K+ and CO2/HCO3- is the coupled uptake of K+ and HCO3-.