K(+)- and HCO3(-)-dependent acid-base transport in squid giant axons II. Base influx.
We used microelectrodes to determine whether the K/HCO3 cotransporter tentatively identified in the accompanying paper (Hogan, E. M., M. A. Cohen, and W. F. Boron. 1995. Journal of General Physiology. 106:821-844) can mediate an increase in the intracellular pH (pHi) of squid giant axons. An 80-min period of internal dialysis increased pHi to 7.7, 8.0, or 8.3; the dialysis fluid was free of K+, Na+, and Cl-. Our standard artificial seawater (ASW), which also lacked Na+, K+, and Cl-, had a pH of 8.0. Halting dialysis unmasked a slow pHi decrease. Subsequently introducing an ASW containing 437 mM K+ and 0.5% CO2/12 mM HCO3- had two effects: (a) it caused membrane potential (Vm) to become very positive, and (b) it caused a rapid pHi decrease, because of CO2 influx, followed by a slower plateau-phase pHi increase, presumably because of inward cotransport of K+ and HCO3- ("base influx"). Only extracellular Rb+ substituted for K+ in producing the plateau-phase pHi increase in the presence of CO2/HCO3-. Mean fluxes with Na+, Li+, and Cs+ were not significantly different from zero, even though Vm shifts were comparable for all monovalent cations tested. Thus, unless K+ or Rb+ (but not Na+, Li+, or Cs+) somehow activates a conductive pathway for H+, HCO3-, or both, it is unlikely that passive transport of H+, HCO3-, or both makes the major contribution to the pHi increase in the presence of K+ (or Rb+) and CO2/HCO3-. Because exposing axons to an ASW containing 437 mM K+, but no CO2/HCO3-, produced at most a slow pHi increase, K-H exchange could not make a major contribution to base influx. Introducing an ASW containing CO2/HCO3-, 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 stilbene derivatives SITS and DIDS failed to block base influx, Na(+)-dependent Cl-HCO3 exchange also cannot account for the results. Rather, we suggest that the most straightforward explanation for the pHi increase we observed in the simultaneous presence of K+ and CO2/HCO3- is the coupled uptake of K+ and HCO3-.
Hogan
E M
EM
Department of Cellular and Molecular Physiology, Yale University School of Medicine, New Haven, Connecticut 06510, USA.
Cohen
M A
MA
Boron
W F
WF
D017878:4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid / Q000494:pharmacology; D012856:4-Acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic Acid / Q000494:pharmacology; D000136:Acid-Base Equilibrium / Q000502:physiology; D000143:Acids / Q000378:metabolism; D000468:Alkalies / Q000378:metabolism*; D000818:Animals; D001369:Axons / Q000378:metabolism*; D001639:Bicarbonates / Q000378:metabolism* / Q000494:pharmacology; D001692:Biological Transport / Q000187:drug effects / Q000502:physiology; D002245:Carbon Dioxide / Q000494:pharmacology / Q000502:physiology; D002352:Carrier Proteins / Q000494:pharmacology*; D002586:Cesium / Q000494:pharmacology; D049832:Decapodiformes; D005110:Extracellular Space / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D017136:Ion Transport / Q000502:physiology; D008094:Lithium / Q000494:pharmacology; D011188:Potassium / Q000378:metabolism* / Q000494:pharmacology; D012413:Rubidium / Q000494:pharmacology; D012964:Sodium / Q000494:pharmacology
D000143:Acids; D000468:Alkalies; D001639:Bicarbonates; D002352:Carrier Proteins; D002245:Carbon Dioxide; D002586:Cesium; D012856:4-Acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic Acid; D008094:Lithium; D012964:Sodium; D012413:Rubidium; D017878:4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid; D011188:Potassium