CO2 and bicarbonate exchange in the rat liver

Area of Science:

• Biochemistry
• Physiology
• Enzymology

Background:

• Carbonic anhydrase (CA) enzymes are present in hepatocytes, with distribution varying within the hepatic lobule.
• Sex steroid levels influence the predominant CA isoenzyme in animals.

Purpose of the Study:

• To investigate if hepatocyte CA activity is accessible to plasma for CO2 and HCO3- equilibration in isolated male rat livers.
• To determine the presence and accessibility of CA on the hepatocyte surface.

Main Methods:

• Isolated male rat livers were perfused with erythrocyte-free solutions.
• Labeled H14CO3- and 14CO2 were injected into the portal vein, and 14C emergence from the hepatic vein was monitored.
• Acetazolamide, a CA inhibitor, was infused to assess its effect on labeled compound distribution.

Main Results:

• H14CO3- emergence was slightly faster than 14CO2 emergence in control perfusions.
• Acetazolamide treatment confined H14CO3- to the extracellular space and prolonged 14CO2 outflow, indicating intracellular trapping.
• Infusion of erythrocyte CA normalized the outflow patterns of 14CO2 and H14CO3-.

Conclusions:

• A readily inhibitable CA isoenzyme is present on the hepatocyte surface, accessible to plasma HCO3- and acetazolamide.
• This surface CA facilitates the equilibration of CO2 and HCO3- during a single passage through the liver.
• An intracellular pH of 7.26 was calculated based on perfusate pH and tracer distribution.