Fumie Suzuki-Kemmelmeier

The metabolism of fructose in the bivascularly perfused rat liver

The metabolism of fructose was investigated in the bivascularly and hemoglobin-free perfused rat liver. Anterograde and retrograde perfusions were performed. In anterograde perfusion, fructose was infused at identical rates (19 mumols min-1 g-1) via the portal vein (all liver cells) or the hepatic artery (predominantly perivenous cells); in retrograde perfusion fructose was infused via the hepatic vein (all liver cells) or the hepatic artery (only periportal cells). The cellular water spaces accessible via the hepatic artery were measured by means of the multiple-indicator dilution technique. The following results were obtained. (i) Fructose was metabolized to glucose, lactate and pyruvate even when this substrate was infused via the hepatic artery in retrograde perfusion; oxygen consumption was also increased. (ii) When referred to the water spaces accessible to fructose via the hepatic artery in each perfusion mode, the rate of glycolysis was 0.99 +/- 0.14 mumols min-1 ml-1 in the retrograde mode; and, 2.05 +/- 0.19 mumols min-1 ml-1 in the anterograde mode (P = 0.002). (iii) The extra oxygen uptake due to fructose infusion via the hepatic artery was 1.09 +/- 0.16 mumols min-1 ml-1 in the retrograde mode; and, 0.51 +/- 0.08 mumols min-1 ml-1 in the anterograde mode (P = 0.005). (iv) Glucose production from fructose via the hepatic artery was 2.18 +/- 0.18 mumols min-1 ml-1 in the retrograde mode; and, 1.83 +/- 0.16 mumols min-1 ml-1 in the anterograde mode (P = 0.18). (v) Glucose production and extra oxygen uptake due to fructose infusion did not correlate by a single factor in all perfusion modes. It was concluded that: (a) rates of glycolysis are lower in the periportal area, confirming previous views; (b) extra oxygen uptake due to fructose infusion is higher in the periportal area; (c) a predominance of glucose production in the periportal area could not be demonstrated; and (d) extra oxygen uptake due to fructose infusion is not a precise indicator for glucose synthesis.

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

BackgroundGlutaminase 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.MethodsZonation 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).ResultsAmmonia 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 14CO2 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.ConclusionsIn 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.

Zonation of gluconeogenesis from lactate and pyruvate in the rat liver studied by means of anterograde and retrograde bivascular perfusion

Gluconeogenesis from lactate and pyruvate and associated parameters were investigated in the bivascularly and hemoglobin-free perfused rat liver. The substrates were infused either via the portal vein (anterograde perfusion mode), via the hepatic vein (retrograde mode) or via the hepatic artery (anterograde and retrograde modes). The rates of lactate and pyruvate infusion were 10.3 and 3.5 mumol min-1 g-1, respectively. The metabolic rates measured when the substrates were infused into the hepatic artery were referred to the cellular spaces accessible in each perfusion mode. The following results were obtained when the substrates were infused into the hepatic artery: (1) gluconeogenesis from lactate was equal to 2.08 +/- 0.2 mumol min-1 ml-1 in the retrograde mode and 1.33 +/- 0.08 mumol min-1 ml-1 in the anterograde mode (P = 0.019); (2) gluconeogenesis from pyruvate was equal to 0.66 +/- 0.11 mumol min-1 ml-1 in the retrograde mode and 0.7 +/- 0.11 mumol min-1 ml-1 in the anterograde mode (P = 0.78); (3) oxygen uptake increase with lactate was 1.75 +/- 0.14 mumol min-1 ml-1 in the retrograde mode and 1.05 +/- 0.07 mumol min-1 ml-1 in the anterograde mode (P = 0.002); (4) oxygen uptake increase with pyruvate was equal to 0.59 mumol min-1 ml-1 in the retrograde mode and 0.57 +/- 0.05 mumol min-1 ml-1 in the anterograde mode (P = 0.73); (5) pyruvate production from lactate was 0.28 +/- 0.06 mumol min-1 ml-1 in the retrograde mode and 0.39 +/- 0.05 mumol min-1 ml-1 in the anterograde mode (P = 0.28); (6) lactate production from pyruvate was equal to 0.52 +/- 0.05 mumol min-1 ml-1 in the retrograde mode and 0.99 +/- 0.08 mumol min-1 ml-1 in the anterograde mode (P < 0.001). Since only periportal cells are supplied with substrates when they are infused via the hepatic artery in retrograde perfusion, these results allow the conclusion that gluconeogenesis from lactate predominates in periportal hepatocytes. When pyruvate is the sole substrate, however, gluconeogenesis in periportal and perivenous cells presents no difference.

Zonation of the action of glucagon on gluconeogenesis studied in the bivascularly perfused rat liver

We have measured the action of glucagon, infused into the hepatic artery, on gluconeogenesis from lactate in the rat liver, bivascularly perfused in both the anterograde and retrograde modes. Concerning glucose production and oxygen uptake per unit cell space, the response of the periportal cells reached via the hepatic artery in retrograde perfusion to glucagon is superior to the response of the cells reached via the same vessel in anterograde perfusion. This phenomenon, however, most probably reflects zonation of gluconeogenesis rather than zonation of the hormonal action. The latter conclusion is based on the observation that the fractional change caused by the hormone is the same for all liver cells.

Zonation of the metabolic action of vasopressin in the bivascularly perfused rat liver

Predominance of the vasopressin binding capacity in the hepatic perivenous area leads to the hypothesis that the metabolic effects of the hormone should also be more pronounced in this area. Until now this question has been approached solely by experiments with isolated hepatocytes where an apparent absence of metabolic zonation was found. We have reexamined this question using the bivascularly perfused liver. In this system periportal cells can be reached in a selective manner with substrates and effectors via the hepatic artery when retrograde perfusion (hepatic vein --> portal vein) is done. The action of vasopressin (1-10 nM) on glycogenolysis, initial calcium efflux, glycolysis and oxygen uptake were measured. The results revealed that the action of vasopressin in the liver is heterogeneously distributed. Glycogenolysis stimulation and initial calcium efflux were predominant in the perivenous area, irrespective of the vasopressin concentration. Oxygen uptake was stimulated in the perivenous area; in the periportal area it ranged from inhibition at low vasopressin concentrations to stimulation at high ones. Lactate production was generally greater in the perivenous zone, whereas the opposite occurred with pyruvate production. Analysis of these and other results suggests that at least three factors are contributing to the heterogenic response of the liver parenchyma to vasopressin: a) receptor density, which tends to favour the perivenous zone; b) cell-to-cell interactions, which tend to favour situations where the perivenous zone is amply supplied with vasopressin; and c) the different response capacities of perivenous and periportal cells.

Zonation of alanine metabolism in the bivascularly perfused rat liver.

Abstract: Aims/Background: Zonation of alanine metabolism was investigated in the bivascularly perfused rat liver, a technique in which a selective area of the periportal region can be reached via the hepatic artery.

Heterogenic response of the liver parenchyma to ethanol studied in the bivascularly perfused rat liver.

Zonation of ethanol oxidation and metabolic effects along the hepatic acini were investigated in the bivascularly perfused liver of fed rats. Ethanol was infused into the hepatic artery in antegrade and retrograde perfusion. Inhibition of glycolysis by ethanol, expressed as μmol min−1 (ml accessible cell space)−1, was more pronounced in the retrograde mode; the retrograde/antegrade ratio was equal to 1.63 for an ethanol infusion rate of 37.5 μmol min−1 g−1. Stimulation of oxygen uptake by ethanol was more pronounced in the retrograde mode; the retrograde/antegrade ratio was equal to 1.77. Diminution of the citrate cycle caused by ethanol was more pronounced in the retrograde mode; the retrograde/antegrade ratio was equal to 1.46. Transformation of arterially infused ethanol into acetate was more pronounced in retrograde perfusion; the retrograde/antegrade ratio was equal to 1.63. The increments in glucose release (glycogenolysis) caused by ethanol in the antegrade and retrograde modes were similar. It was assumed that the changes caused by arterially infused ethanol in retrograde and antegrade perfusion closely reflect a significant part of the periportal parenchyma and an average over the whole liver parenchyma, respectively. Under such assumptions it can be concluded that, in the perfused liver from fed rats, four related parameters predominate in the periportal region: ethanol oxidation, glycolysis inhibition, oxygen uptake stimulation and citrate cycle inhibition. One of the main causes for this predominance could be the malate/aspartate shuttle, which operates more rapidly in the periportal area and is essential for NADH oxidation.

The hemodynamic effects of ATP in retrograde perfusion of the bivascularly perfused rat liver.

Aims/Background: In the sinusoidal bed the distribution of water is flow‐limited, but it becomes partly barrier‐limited when adenosine triphosphate (ATP) is introduced. This effect could be exerted either directly by ATP or by substances released from presinusoidal regions. Furthermore, portally infused ATP seems to be able to diffuse in the direction of the arterial bed. It is not known if this diffusion route is specific. Answers to these questions can be obtained from indicator‐dilution experiments in retrograde perfusion.

Zonation of the action of ethanol on gluconeogenesis and ketogenesis studied in the bivascularly perfused rat liver.

Zonation of the actions of ethanol on gluconeogenesis and ketogenesis from lactate were investigated in the bivascularly perfused rat liver. Livers from fasted rats were perfused bivascularly in the antegrade and retrograde modes. Ethanol and lactate were infused into the hepatic artery (antegrade and retrograde) and portal vein. A previously described quantitative analysis that takes into account the microcirculatory characteristics of the rat liver was extended to the analysis of zone-specific effects of inhibitors. Confirming previous reports, gluconeogenesis and the corresponding oxygen uptake increment due to saturable lactate infusions were more pronounced in the periportal region. Arterially infused ethanol inhibited gluconeogenesis more strongly in the periportal region (inhibition constant=3.99+/-0.22mM) when compared to downstream localized regions (inhibition constant=8.64+/-2.73mM). The decrease in oxygen uptake caused by ethanol was also more pronounced in the periportal zone. Lactate decreased ketogenesis dependent on endogenous substrates in both regions, periportal and perivenous, but more strongly in the former. Ethanol further inhibited ketogenesis, but only in the periportal zone. Stimulation was found for the perivenous zone. The predominance of most ethanol effects in the periportal region of the liver is probably related to the fact that its transformation is also clearly predominant in this region, as demonstrated in a previous study. The differential effect on ketogenesis, on the other hand, suggest that the net effects of ethanol are the consequence of a summation of several partial effects with different intensities along the hepatic acini.

The action of glucagon infused via the hepatic artery in anterograde and retrograde perfusion of the rat liver is not a function of the accessible cellular spaces

The metabolic action of glucagon in the different spaces that can be reached via the hepatic artery in the bivascularly perfused rat liver of fed rats was investigated. When perfusion was performed in the anterograde mode, glucagon (10 mM) was infused either into the portal vein (type 1 experiment) or into the hepatic artery (type 2); in the retrograde mode, the hormone was infused either into the hepatic vein (type 3) or into the hepatic artery (type 4). The aqueous cell spaces were measured by means of the multiple-indicator dilution technique. Glucose release, oxygen uptake and glycolysis (lactate plus pyruvate production) were measured as metabolic parameters. The following results were obtained. (1) The aqueous cell space accessible via the hepatic artery in the type 2 experiment was 0.63 ml/g; in the type 4 experiment this space was 0.18 ml/g (only periportal cells); glucagon up to 10 nM did not affect these cellular spaces nor did it affect the vascular spaces. (2) The effects of glucagon on glucose release, oxygen uptake and glycolysis were practically the same in all types of experiment (1 to 4), i.e., the action of glucagon was not a function of the accessible cell spaces. (3) When the respiratory chain of the liver cells accessible via the hepatic artery in the type 4 experiment was inhibited by cyanide, glucagon still increased oxygen uptake; oxygen uptake stimulation by glucagon was completely blocked only when cyanide was given to all liver cells. (4) Calcium depletion did not affect the action of glucagon on glucose release and oxygen uptake in the type 4 experiment. It was concluded that, in addition to the receptor-elicited response, the action of glucagon can also be propagated by cell-to-cell communication.