HCO3(-)-coupled Na+ influx is a major determinant of Na+ turnover and Na+/K+ pump activity in rat hepatocytes.
Recent studies in hepatocytes indicate that Na(+)-coupled HCO3- transport contributes importantly to regulation of intracellular pH and membrane HCO3- transport. However, the direction of net coupled Na+ and HCO3- movement and the effect of HCO3- on Na+ turnover and Na+/K+ pump activity are not known. In these studies, the effect of HCO3- on Na+ influx and turnover were measured in primary rat hepatocyte cultures with 22Na+, and [Na+]i was measured in single hepatocytes using the Na(+)-sensitive fluorochrome SBFI. Na+/K+ pump activity was measured in intact perfused rat liver and hepatocyte monolayers as Na(+)-dependent or ouabain-suppressible 86Rb uptake, and was measured in single hepatocytes as the effect of transient pump inhibition by removal of extracellular K+ on membrane potential difference (PD) and [Na+]i. In hepatocyte monolayers, HCO3- increased 22Na+ entry and turnover rates by 50-65%, without measurably altering 22Na+ pool size or cell volume, and HCO3- also increased Na+/K+ pump activity by 70%. In single cells, exposure to HCO3- produced an abrupt and sustained rise in [Na+]i from approximately 8 to 12 mM. Na+/K+ pump activity assessed in single cells by PD excursions during transient K+ removal increased congruent to 2.5-fold in the presence of HCO3-, and the rise in [Na+]i produced by inhibition of the Na+/K+ pump was similarly increased congruent to 2.5-fold in the presence of HCO3-. In intact perfused rat liver, HCO3- increased both Na+/K+ pump activity and O2 consumption. These findings indicate that, in hepatocytes, net coupled Na+ and HCO3- movement is inward and represents a major determinant of Na+ influx and Na+/K+ pump activity. About half of hepatic Na+/K+ pump activity appears dedicated to recycling Na+ entering in conjunction with HCO3- to maintain [Na+]i within the physiologic range.
Fitz
J G
JG
Department of Medicine and Liver Center, University of California, San Francisco.
Lidofsky
S D
SD
Weisiger
R A
RA
Xie
M H
MH
Cochran
M
M
Grotmol
T
T
Scharschmidt
B F
BF
D000818:Animals; D001639:Bicarbonates / Q000378:metabolism* / Q000493:pharmacokinetics; D001693:Biological Transport, Active / Q000187:drug effects / Q000502:physiology; D002352:Carrier Proteins / Q000187:drug effects / Q000502:physiology*; D002478:Cells, Cultured; D006863:Hydrogen-Ion Concentration; D008099:Liver / Q000166:cytology* / Q000378:metabolism / Q000502:physiology; D008297:Male; D010042:Ouabain / Q000494:pharmacology; D010100:Oxygen / Q000378:metabolism; D051381:Rats; D011919:Rats, Inbred Strains; D012414:Rubidium Radioisotopes / Q000493:pharmacokinetics; D012964:Sodium / Q000378:metabolism* / Q000493:pharmacokinetics / Q000502:physiology; D028021:Sodium-Potassium-Chloride Symporters
D001639:Bicarbonates; D002352:Carrier Proteins; D012414:Rubidium Radioisotopes; D028021:Sodium-Potassium-Chloride Symporters; D010042:Ouabain; D012964:Sodium; D010100:Oxygen