Flow-limited tracer oxygen distribution in the isolated perfused rat liver: effects of temperature and hematocrit

The effect of old age on liver oxygenation and the hepatic expression of VEGF and VEGFR2

In old age, the liver contains less ATP and hypoxia-responsive genes are upregulated. Age-related changes in hepatic perfusion and the liver sinusoidal endothelial cell (LSEC) could contribute to this altered hepatic oxygen-dependent metabolism by causing intrahepatocytic hypoxia. Furthermore, age-related changes in the LSEC ('pseudocapillarization') have been partially induced by ATP depletion. To investigate whether there is intracellular hypoxia in the old rat liver, pimonidazole immunohistochemistry in intact livers and ATP levels in isolated LSECs were studied from young and old rats. There were no age-related changes. To determine whether defenestration of the LSEC could impair oxygen diffusion, pimonidazole immunohistochemistry was performed in rats treated with poloxamer 407. Despite defenestration, there was no change in pimonidazole staining. Immunohistochemistry was then performed to determine whether there are age-related changes in VEGF and VEGFR2. VEGF staining was not associated with age. However, there was an increase in perisinusoidal VEGFR2 expression with increasing age. In conclusion, liver hypoxia does not occur in old age and LSEC pseudocapillarization does not constitute an oxygen-diffusion barrier. There are no age-related changes in VEGF expression but an increase in perisinusoidal VEGFR2 expression, which has implications for the effects of aging on the hepatic sinusoid.

The Hepatic Sinusoid in Aging and Cirrhosis

The fenestrated sinusoidal endothelium ('liver sieve') and space of Disse in the healthy liver do not impede the transfer of most substrates, including drugs and oxygen, from the sinusoidal lumen to the hepatocyte. Plasma components transfer freely in both directions through the endothelial fenestrations and into the space of Disse. The endothelium is attenuated, there is no basement membrane and there is minimum collagen in the space of Disse, thus minimising any barriers to substrate diffusion. Both cirrhosis and aging are associated with marked structural changes in the sinusoidal endothelium and space of Disse that are likely to influence bulk plasma transfer into the space of Disse, and diffusion through the endothelium and space of Disse. These changes, termed capillarisation and pseudocapillarisation in cirrhosis and aging, respectively, impede the transfer of various substrates. Capillarisation is associated with exclusion of albumin, protein-bound drugs and macromolecules from the space of Disse, and the progressive transformation of flow-limited to barrier-limited distribution of some substrates. There is evidence that the sinusoidal changes in cirrhosis and aging contribute to hepatocyte hypoxia, thus providing a mechanism for the apparent differential reduction of oxygen-dependent phase I metabolic pathways in these conditions. Structural change and subsequent dysfunction of the liver sieve warrant consideration as a significant factor in the impairment of overall substrate handling and hepatic drug metabolism in cirrhosis and aging.

A novel method for determining hepatic sinusoidal oxygen permeability in the isolated perfused rat liver using [15O]O2

Measurement of hepatic sinusoidal permeability of oxygen and other substrates may help elucidate the mechanisms responsible for impaired liver function in cirrhosis. However studies of sinusoidal oxygen permeability in normal liver and various disease states have been limited due to the considerable technical difficulties involved in the use of standard techniques. We have developed a new method for measuring sinusoidal oxygen permeability in the isolated perfused rat liver that overcomes the difficulties of previous methods by using [(15)O]O(2) and an in-line fluid monitor. This method uses data obtained from impulse response curves of radiolabelled red cells, albumin and oxygen that are fitted mathematically using the axial dispersion model to yield rate constants that describe oxygen transit through the liver. We have demonstrated the utility and reproducibility of this method by comparing multiple injections and permeability determinations in the same preparation. This approach could be used in isolated perfused organs to study oxygen permeability in a range of disease states.

Changes in distribution spaces and cell permeability caused by ATP in the rat liver

AIMS/BACKGROUND

Cellular and extracellular volume changes caused by ATP were investigated in the liver as well as the possible formation of diffusion barriers, which could be responsible for some of its metabolic effects.

METHODS

The experimental system was the bivascularly perfused rat liver. [(14)C]Sucrose and [(3)H]water were simultaneously injected into either the portal vein or the hepatic artery. Mean transit times, distribution spaces, variances and linear superimpositions were calculated.

RESULTS

In the portal system, ATP reduced the transit time in the great vessels, had little or no effect on sinusoidal and cellular spaces, but impaired the flow-limited distribution of both [(14)C]sucrose and [(3)H]water. In the arterial bed ATP infused into either the portal vein or the hepatic artery produced vasodilation and increased the aqueous extra-sucrose space. These effects were inhibited by Nomega-nitro-L-arginine methyl ester infused into the hepatic artery.

CONCLUSIONS

Sucrose and extra-sucrose space changes caused in the arterial bed by portally infused ATP are most probably analogous to the transhepatic vasodilation effect already described for the rabbit liver. Impairment of flow-limited distribution of tracers in the sinusoidal bed indicates that ATP induces the formation of permeability barriers, which could be responsible for some of its metabolic effects.

Decreased splanchnic oxygen uptake and increased systemic oxygen uptake in cirrhosis are normalized after liver transplantation

The aim of this study is to (1) characterize the impact of orthotopic liver transplantation (OLT) on splanchnic and systemic oxygen uptake (VO(2)) in patients with liver cirrhosis, and (2) investigate possible influencing factors, as well as metabolic consequences, of reduced splanchnic VO(2) in patients with cirrhosis. Therefore, we measured systemic VO(2) (indirect calorimetry), portal pressure (hepatic venous pressure gradient), hepatic blood flow (HBF; primed continuous infusion of indocyanine green), and hepatic turnover (arteriohepatic venous concentration differences multiplied by HBF) of oxygen, glucose, free fatty acids (FFAs), and aromatic amino acids (AAAs) in 52 patients with advanced cirrhosis and 16 patients with a clinically stable long-term course after OLT. Systemic VO(2) was significantly increased in patients with cirrhosis (261 +/- 7 mL/min) and normalized after OLT (216 +/- 8 mL/min; P < .001). Arterial and hepatic venous oxygen saturation and splanchnic oxygen extraction (in percent) were not different between patients with cirrhosis and after OLT. Splanchnic VO(2) was decreased in patients with cirrhosis (41 +/- 3 mL/min, representing 16% +/- 1% of systemic VO(2)) and normalized after OLT (69 +/- 6 mL/min; P < .001, representing 32% +/- 3% of systemic VO(2); P < .001). In patients with cirrhosis, a decrease in HBF was associated with decreased splanchnic VO(2) (r = 0.74; P < .001). Conversely, decreased splanchnic VO(2) reflected a decrease in hepatic glucose production (r = 0.34; P = .01) and hepatic extraction of FFAs (r = 0.40; P < .01) and AAAs (r = 0.30; P < .05). These results show that (1) splanchnic and systemic VO(2) normalize after OLT, indicating correction of hepatic and extrahepatic metabolic derangements; (2) in cirrhosis, HBF becomes limiting for hepatic oxygen supply; and (3) impaired splanchnic VO(2) reflects a decrease in metabolic liver function.

Oxidative injury reproduces age-related impairment of oxygen-dependent drug metabolism

The Oxygen Diffusion Barrier Hypothesis states that aging in the liver is associated with restricted oxygen uptake that explains the age-related impairment of phase I drug clearance observed in vivo with preservation of in vitro phase I enzyme activity and in vivo phase II drug clearance. Aging in the liver may be secondary to oxidative stress. Therefore we examined the effects of oxidative injury on oxygen uptake, and phase I and phase II drug metabolism in the liver. Oxidative stress was induced in the perfused rat liver with hydrogen peroxide. The intrinsic clearances of propranolol and morphine were used as markers of phase I and phase II activity, respectively. Oxidative injury was associated with a 14+/-99% (P=0.03) reduction in oxygen uptake. The decrease in the intrinsic clearance of propranolol was greater than that of morphine (57+/-14% vs 34+/-7% P<0.005). This result supports the concept of a restriction of oxygen supply constraining hepatic drug metabolism following oxidative stress. This has implications for aging and hepatic drug metabolism.

Carbon monoxide disposition in the perfused rat liver

A simple method for determining carbon monoxide (CO) disposition in the rat liver perfused with erythrocyte-free buffer was developed. Wash-in experiments were performed with buffer containing tracer quantities of [14C]sucrose and 3H2O and equilibrated with CO. Outflow samples were collected into tubes containing human erythrocytes, which avidly bind CO. Outflow curves were analyzed using compartmental models. Fractional recovery of CO was 1.07 +/- 0. 17, and the apparent volume of distribution was 1.37 +/- 0.30 ml/g of liver (n = 8). A flow-limited model fitted the data most effectively, although estimates of the permeability-to-surface area product were attempted using a barrier-limited model. This technique will facilitate investigation of the effects of disease on gaseous substrate disposition in perfused organs.

The aging liver. Drug clearance and an oxygen diffusion barrier hypothesis

A change in drug clearance with age is considered an important factor in determining the high prevalence of adverse drug reactions associated with prescribing medications for the elderly. Despite this, no general principles have been available to guide drug administration in the elderly, although a substantial body of clearance and metabolism data has been generated in humans and experimental animals. A review of age-related change in drug clearances established that patterns of change are not simply explained in terms of hepatic blood flow, hepatic mass and protein binding changes. In particular, the maintained clearance of drugs subject to conjugation processes while oxygen-dependent metabolism declines, and all in vitro tests of enzyme function have been normal, requires new explanations. Reduction in hepatic oxygen diffusion as part of a general change in hepatocyte surface membrane permeability and conformation does provide one explanation for the paradoxical patterns of drug metabolism, and increased hepatocyte volume would also modify oxygen diffusion path lengths (the 'oxygen diffusion barrier' hypothesis). The reduction in clearances of high extraction drugs does correlate with observed reduction in hepatic perfusion. Dosage guidelines emerge from these considerations. The dosage of high clearance drugs should be reduced by approximately 40% in the elderly while the dosage of low clearance drugs should be reduced by approximately 30%, unless the compound is principally subject to conjugation mechanisms. If the hepatocyte diffusion barrier hypothesis is substantiated, this concept may lead to therapeutic (preventative and/or restorative) approaches to increased hepatocyte oxygenation in the elderly. This may lead to approaches for modification of the aging process in the liver.

Tracer oxygen distribution is barrier-limited in the cerebral microcirculation

The kinetics of tracer oxygen distribution in the brain microcirculation of the awake dog were investigated with the multiple indicator dilution technique. A bolus containing 51Cr-labeled red blood cells, previously totally desaturated and then resaturated with [18O]2 (oxygen), 125I-albumin, 22Na, and [3H]water, was injected into the carotid artery, and serial anaerobic blood samples were collected from the sagittal sinus over the next 30 seconds. The outflow recovery curves were analyzed with a distributed-in-space two-barrier model for water and a one-barrier model for oxygen. The analysis provided an estimate of flow per gram brain weight as well as estimates for the tracer water and oxygen rate constants for blood-to-brain exchange and tracer oxygen parenchymal sequestration. Flow to tissue was found to vary between different animals, in concert with parallel changes in oxygen consumption. The 18O2 outflow curves showed an early peak, coincident with and more than half the magnitude of its vascular reference curve (labeled red blood cells), whereas the [3H]water curve increased abruptly to a low-in-magnitude curve at low flow values and to a small early peak at high flow values. Analysis indicates that the transfers of both 18O2 and [3H]water indicators from blood to brain are barrier-limited, with the former highly so because of the large red blood cell capacity for oxygen, and that the proportion of the tracer oxygen returning to the circulation from tissue is a small fraction of the total tracer emerging at the outflow.

Clearance by the liver in cirrhosis. II. Characterization of propranolol uptake with the multiple-indicator dilution technique

We studied the steady-state hepatic extraction and single-pass hepatic uptake of propranolol in isolated perfused livers from normal rats and compared these values with those of rats with carbon tetrachloride-induced cirrhosis, rats treated with chlorpromazine (an inhibitor of propranolol metabolism) and rats with acute liver injury. The kinetics of propranolol transport in the liver were characterized by means of the multiple-indicator dilution technique, and estimates of cellular influx, efflux and sequestration rate constants were obtained with a computer fit to the model of Goresky. The outflow pattern of propranolol in the hepatic veins was then resolved into throughput material, which had swept past the hepatocytes along with albumin, and returning material, which had entered the cells but returned in the outflow after escaping metabolic sequestration. The steady-state extraction of propranolol was significantly decreased in the three experimental groups compared with that in controls, but the outflow profile differed within each group. In cirrhotic animals, influx was markedly decreased and the sequestration rate constant remained unchanged; most of the propranolol in the outflow consisted of throughput material. In rats treated with chlorpromazine, the sequestration rate constant was decreased, and propranolol in the outflow was mainly returning material. In rats with acute liver injury, both influx and sequestration rate constants were decreased. Indicator dilution curves for nonsequestered tracers showed a decreased transit time for red blood cells and abnormal diffusion of albumin and sucrose into the space of Disse in cirrhotic rats compared with the other groups.(ABSTRACT TRUNCATED AT 250 WORDS)