Hepatic zonation of carbon and nitrogen fluxes derived from glutamine and ammonia transformations

Abstract

Background: Glutaminase predominates in periportal hepatocytes and it has been proposed that it determines the glutamine-derived nitrogen flow through the urea cycle. Glutamine-derived urea production should, thus, be considerably faster in periportal hepatocytes. This postulate, based on indirect observations, has not yet been unequivocally demonstrated, making a direct investigation of ureogenesis from glutamine highly desirable.

Methods: Zonation of glutamine metabolism was investigated in the bivascularly perfused rat liver with [U-14C]glutamine infusion (0.6 mM) into the portal vein (antegrade perfusion) or into the hepatic vein (retrograde perfusion).

Results: Ammonia infusion into the hepatic artery in retrograde and antegrade perfusion allowed to promote glutamine metabolism in the periportal region and in the whole liver parenchyma, respectively. The results revealed that the space-normalized glutamine uptake, indicated by (14)CO(2) production, gluconeogenesis, lactate production and the associated oxygen uptake, predominates in the periportal region. Periportal predominance was especially pronounced for gluconeogenesis. Ureogenesis, however, tended to be uniformly distributed over the whole liver parenchyma at low ammonia concentrations (up to 1.0 mM); periportal predominance was found only at ammonia concentrations above 1 mM. The proportions between the carbon and nitrogen fluxes in periportal cells are not the same along the liver acinus.

Conclusions: In conclusion, the results of the present work indicate that the glutaminase activity in periportal hepatocytes is not the rate-controlling step of the glutamine-derived nitrogen flow through the urea cycle. The findings corroborate recent work indicating that ureogenesis is also an important ammonia-detoxifying mechanism in cells situated downstream to the periportal region.

Figure 1

Schematic representation of some characteristics of the hepatic microcirculation and the experimental protocols. The arrows indicate the direction of flow. Legends: A the presinusoidal confluence of the arterial and portal bed; B the intrasinusoidal confluence of the arterial and portal bed.

Figure 2

Time courses of ammonia, urea and glucose productions and oxygen uptake during portal infusion of 0.6 mM glutamine. Livers from fasted rats were perfused as described in Materials and Methods. Glutamine was infused as indicated. Data are from 4 liver perfusion experiments and error bars are mean standard errors.

Figure 3

Time courses of the actions of arterially infused ammonia on urea and glucose productions from [U-¹⁴C]glutamine in antegrade and retrograde perfusion. Livers from fasted rats were perfused as described in Materials and Methods. [U-¹⁴C]Glutamine and ammonia were infused as indicated. Data are from 5 liver perfusion experiments and error bars are mean standard errors.

Figure 4

Time courses of the actions of arterially infused ammonia on ¹⁴CO₂ production from [U-¹⁴C]glutamine and the corresponding oxygen uptake increments in antegrade and retrograde perfusion. Livers from fasted rats were perfused as described in Materials and Methods. [U-¹⁴C]Glutamine and ammonia were infused as indicated. Data are from 5 liver perfusion experiments and error bars are mean standard errors.

Figure 5

Urea and glucose productions from glutamine and ammonia in 2 different zones along the hepatic parenchyma as a function of the extracellular ammonia concentrations. The corresponding experimental data J_ant and J_ret in Table 1 were used to calculate v_1R and v_2A using equations (3) to (5) and the numerical procedures described in the text; c_1R and c_2A were calculated from the rates of ammonia infusion, the total flow and the arterial flow as described in the text.