(barrier2)

Two-barrier Model. If a tracer is transformed within the cells, or if its distribution is heterogeneous, two intracellular regions have to be considered, with a second barrier between them. 

Lactate

Lactate is a natural substrate of the liver. It is taken up by hepatocytes and is transformed to pyruvate by the action of the enzyme lactate dehydrogenase. The hydrogen atom bound to the central carbon atom of the molecule is transferred to NAD that is thus transformed to NADH. Various other dehydrogenases transfer this hydrogen to other metabolic intermediates who form a separate intracellular pool that can be assessed by the multiple indicator dilution method.

Experimental design

The following tracers were injected into the portal vein of an isolated buffer-perfused rat liver:

• [U^14C]Sucrose
• [U¹⁴C]Urea (a tracer that is distrubuted rapidly within the perenchymal cells)
• L-[2-³H]lactate (CH₃—C³HOH—COO^–)

Samples from the vena cava were analyzed for beta radioactivity, and the two isotopes where distinguished by scintillation spectrometry. The label contained in urea was separated from that in sucrose by hydrolyzing urea using the enzyme urease; the resulting [U¹⁴C]CO₂ was moved into the gas phase by acidification and trapped in phenyl ethyl amine.

Enalaprilat

Enalaprilat, (S)-1-[N-[1-(carbonyl)-3-phenylpropyl-L-alanyl]-L-proline, is a drug metabolite formed by hydrolysis of the antihypertensive drug [[http://www.rxlist.com/cgi/generic/enalap.htm| enalapril]]. It is taken up by hepatocytes, where, because of its low water solubility, it is bound to intracellular proteins. This results an additional intracellular pool of protein-bound enalaprilat. Enalaprilat is excreted predominatly into the bile.

Experimental design

The following tracers were injected into the portal vein of a perfused rat liver:

• [U¹⁴C]Sucrose
• [2-³H]Enalaprilat

Samples from the vena cava were analyzed for beta radioactivty. The two isotopes were distinguished by liquid scintillation spectrometry.

Calculations