Role of the hepatic artery in canalicular bile formation by the perfused rat liver. A multiple indicator dilution study

A Comparative Study of Single and Dual Perfusion During End-ischemic Subnormothermic Liver Machine Preservation

Background

It remains controversial if arterial perfusion in addition to portal vein perfusion during machine preservation improves liver graft quality. Comparative studies using both techniques are lacking. We studied the impact of using single or dual machine perfusion of donation after circulatory death rat livers. In addition, we analyzed the effect of pulsatile versus continuous arterial flow.

Methods

Donation after circulatory death rat livers (n = 18) were preserved by 6 hours cold storage, followed by 1 hour subnormothermic machine perfusion (20°C, pressure of 40/5 mm Hg) and 2 hours ex vivo warm reperfusion (37°C, pressure of 80/11 mm Hg, 9% whole blood). Machine preservation was either through single portal vein perfusion (SP), dual pulsatile (DPP), or dual continuous perfusion (DCP) of the portal vein and hepatic artery. Hydrodynamics, liver function tests, histopathology, and expression of endothelial specific genes were assessed during 2 hours warm reperfusion.

Results

At the end of reperfusion, arterial flow in DPP livers tended to be higher compared to DCP and SP grafts. However, this difference was not significant nor was better flow associated with better outcome. No differences in bile production or alanine aminotransferase levels were observed. SP livers had significantly lower lactate compared to DCP, but not DPP livers. Levels of Caspase-3 and tumor necrosis factor-α were similar between the groups. Expression of endothelial genes Krüppel-like-factor 2 and endothelial nitric oxide synthase tended to be higher in dual perfused livers, but no histological evidence of better preservation of the biliary endothelium or vasculature of the hepatic artery was observed.

Conclusions

This study shows comparable outcomes after using a dual or single perfusion approach during end-ischemic subnormothermic liver machine preservation.

Application of the Dispersion Model to Describe Disposition Kinetics of Markers in the Dual Perfused Rat Liver

The liver receives two blood supplies, portal and hepatic, yet most in situ studies use only portal perfusion. A model based on dispersion principles was developed to provide baseline data of the dual perfused rat liver preparation by characterizing the temporal outflow profiles of noneliminated reference markers (vascular marker, red blood cells; extracellular markers, albumin, sucrose; and intracellular markers, urea, water). The model consists of two components: the common and a specific arterial space operating in parallel. The common space receives all the portal flow and some of the arterial flow; the remaining arterial flow perfuses the specific space. Each space is divided into three subspaces: vascular, interstitial, and intracellular. The extent of axial spreading of solute on passage through the common and specific spaces is characterized by their respective dispersion numbers, D(N). The model was fully characterized by analysis of the outflow data following independent bolus administration into the portal vein and hepatic artery. The model provided a good fit of the data for all reference compounds. The estimate of the fraction of the total space assigned to the specific arterial space varied from 4 to 11%, with a mean value of 9%. The estimated D(N) was always small (<0.25) and tended to be greater for the common space (0.08-0.23) than the specific space (0.05-0.12). However, for each space, there was no significant difference in the D(N) value among all reference markers; this is assumed to arise because all markers are reflecting a common feature, the heterogeneity of the microvasculature.

Estimation of hepatic distributional volumes using non-labeled reference markers

Hepatic distributional volumes were investigated in the in situ perfused rat liver. Perfusion experiments were conducted using Krebs bicarbonate buffer delivered via the portal vein in single-pass mode at a total flow rate of 15 mL/min. A bolus dose of normal erythrocytes (RBC, vascular marker) and Evans blue (EB, extracellular marker) respectively was administered in the presence and absence of protein. At the end of the experiment, liver total water content was determined by desiccation and freeze-drying methods. Similar moment analysis results and superimposable effluent curves were obtained in the presence (RBC, mean transit time [MTT]: 7.31 +/- 0.45 s and volume of distribution [V]: 0.17 +/- 0.01 mL/g; EB, MTT: 10.9 +/- 0.62 s and V: 0.25 +/- 0.02 mL/g) and in the absence (RBC, MTT: 7.55 +/- 0.84 s and V: 0.18 +/- 0.02 mL/g; EB, MTT: 9.24 +/- 0.77 s and V: 0.20 +/- 0.02 mL/g) of protein, which indicates that the hepatic distribution of RBC and EB within the liver is not influenced by protein. Furthermore, the almost identical results obtained with the desiccation and freeze-drying methods clearly suggest that the freeze-drying method can be used as an alternative to desiccation for the estimation of liver water content.

Effects of hypothermic machine perfusion on rat liver function depending on the route of perfusion

BACKGROUND AND METHODS

The aim of this study was to evaluate the efficacy of hypothermic machine perfusion (HMP) to preserve rat livers according to the route of perfusion, i.e., via portal vein, hepatic veins (retrograde), or hepatic artery. Livers were preserved for 24 or 48 hr by simple cold storage (SCS) or by HMP. Preservation solution was supplemented with (HMP) or without (SCS) hydroxyethyl starch. After preservation, grafts were reperfused for 2 hr with an oxygenated Krebs-Henseleit bicarbonate buffer.

RESULTS

After 24 hr of preservation, total glutathione concentrations in HMP livers were similar (1287+/-37, 1418+/-118, and 1471+/-62 nmol/g in hepatic artery, portal vein, and hepatic vein HMP livers, respectively) and higher than in the SCS (833+/-118 nmol/g, P<0.05) group. These higher total glutathione values were due to higher reduced glutathione concentrations. ATP concentrations in the liver tissue were similar in HMP groups (0.75+/-0.4, 0.64+/-0.1, and 0.77+/-0.1 micromol/g in hepatic artery, portal vein, and hepatic vein HMP livers, respectively) and higher than in SCS (0.32+/-0.06 micromol/g, P<0.05). After 2 hr of normothermic reperfusion, bile production in the HMP portal and HMP retrograde groups were similar (391+/-29 ml and 372+/-25 ml) and higher than in the HMP artery or SCS groups (275+/-25 ml and 277+/-32 ml, respectively; P<0.05). Aspartate transaminase, alanine transaminase, lactate dehydrogenase, and purine nucleoside phosphorylase release into the perfusate of HMP portal and HMP retrograde perfused livers was similar and significantly lower compared to the HMP artery and SCS groups. At the end of reperfusion, no statistical differences were found for glutathione concentration and energetic reserves in the livers of each group. After 48 hr of preservation, livers from the HMP portal and HMP retrograde groups did significantly better than livers from the HMP artery or SCS groups.

CONCLUSIONS

This study confirms the superiority of HMP over SCS to preserve the liver graft. It shows that retrograde perfusion is similar to PV perfusion and that perfusion by HA is less beneficial.

Bile duct epithelia regulate biliary bicarbonate excretion in normal rat liver

BACKGROUND & AIMS

A number of transporters and channels have been identified in cholangiocytes, but the role that bile ducts play in the formation of bile in vivo is unclear. We determined the contribution of cholangiocytes to bile flow and biliary bicarbonate excretion in normal rat liver.

METHODS

Bile flow and biliary bicarbonate were measured in isolated rat livers perfused via both the portal vein and the hepatic artery because the hepatic artery provides the blood supply to bile ducts. Livers were perfused with secretin or acetylcholine (ACh), which respectively increase either adenosine 3',5'-cyclic monophosphate (cAMP) or cytosolic Ca(2+) in cholangiocytes. Livers also were perfused with glucagon or vasopressin to instead increase cAMP or cytosolic Ca(2+) in hepatocytes.

RESULTS

Secretin increased biliary bicarbonate in a dose-dependent fashion and was much more effective when administered via the hepatic artery. Secretin did not affect bile flow. Similarly, ACh increased bicarbonate excretion when infused via the hepatic artery but not the portal vein. The effects of secretin were augmented by ACh, and this was prevented by cyclosporin A. The effects of ACh were blocked by 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS), 5-nitro2-(3-phenylpropylamino)benzoic acid (NPPB), or diphenylamine-2-carboxylic acid (DPC), and the effects of secretin were inhibited by NPPB or DPC and unaffected by DIDS. Neither glucagon nor vasopressin altered biliary bicarbonate.

CONCLUSIONS

Biliary bicarbonate is regulated by cholangiocytes rather than hepatocytes in normal rat liver. ACh-induced bicarbonate excretion depends on both chloride channels and bicarbonate exchange, whereas secretin-induced bicarbonate excretion is independent of bicarbonate exchange.

Estimation of aqueous distributional spaces in the dual perfused rat liver

1. The aim of this study was to estimate the aqueous distributional spaces of the liver as a function of the route of input: portal vein (PV) versus hepatic artery (HA). 2. Studies were performed in the situ single (PV) and dual (PV-HA) perfused rat liver (n = 6-10) using Krebs bicarbonate buffer at constant PV (12 ml min-1) and HA (3 ml min-1) flow rates. An impulse input-output response technique was employed, varying the route of input, using non-labelled erythrocytes (intravascular marker), 125I-albumin and [14C]sucrose (extracellular markers), and [14C]urea and 3H2O (total water markers) as the reference indicators. 3. Distributional spaces were estimated using two different methods, namely standard and specific. The standard method was applied to hepatic outflow data obtained from the single PV perfused liver. The specific method was used when operating in the dual perfused mode to provide an estimate of the excess space perfused solely by the HA input. Specific spaces, interstitial and intracellular volumes, were estimated by difference. 4. The results were evaluated by means of visual inspection of the outflow profiles and comparison of the distributional spaces. Different hepatic effluent profiles obtained as a function of the route of input indicated that these two inputs did not completely mix within the liver. Estimates of the distributional spaces supported this observation, and further suggested that the arterial input perfuses 9-12 % more hepatic tissue than the venous input. 5. The knowledge obtained from the existence of a specific arterial space can be extended to help make predictions about the fate of an eliminated compound following arterial administration. Any difference between the HA and PV in terms of hepatic recovery could be attributed to this excess space and its enzyme density.

Localisation of hepatic vascular resistance sites in the isolated dual-perfused rat liver

The locations of the vascular resistance sites which regulate vascular tone in the hepatic arterial and portal venous vasculatures of the rat liver were identified using a new, in vitro, dual-perfused liver preparation. Twelve livers of male Wistar rats were perfused via the hepatic artery and portal vein at fixed flow and at physiological pressure. Dose-related vasoconstriction to injections or infusions of noradrenaline was measured as transient or sustained increases in perfusion pressure, respectively, in the hepatic arterial and portal venous vasculatures. Direct injections/infusions of noradrenaline refer to those administered into the vasculature from which pressure was recorded, e.g., the effects of hepatic arterial (direct) injections/infusions of noradrenaline upon hepatic arterial perfusion pressure. Indirect injections/infusions of noradrenaline were those administered to the adjacent afferent vasculature, e.g., the effects of portal venous (indirect) injections of noradrenaline upon hepatic arterial perfusion pressure. The converse applies for recordings of portal venous perfusion pressure. The -log(M) ED50 values to direct (hepatic arterial) and indirect (portal venous) injections in the hepatic artery were 4.25+/-0.20 and 3.40+/-0.10, respectively, and were significantly different (P < 0.01, Student's unpaired t-test); the -log(M) ED50 values to direct (portal venous) and indirect (hepatic arterial) injections in the portal vein were 3.91+/-0.08 and 3.85+/-0.11, respectively, and were not significantly different (P > 0.05, Student's unpaired t-test). Similarly, the -log(M) ED50 values to direct (hepatic arterial) and indirect (portal venous) infusions in the hepatic artery were 5.28+/-0.11 and 3.75+/-0.12, respectively, and were significantly different (P < 0.01, Student's unpaired t-test); the -log(M) ED50 values to direct (portal venous) and indirect (hepatic arterial) infusions in the portal vein were 5.31+/-0.19 and 5.70+/-0.16, respectively, and were not significantly different (P > 0.05, Student's unpaired t-test). These results demonstrated that there is little transfer of noradrenaline from the portal venous to the hepatic arterial resistance sites, but significant transfer from the hepatic artery to the portal venous suggesting that; (a) the portal venous resistance sites are located at the sinusoidal or post-sinusoidal level; and (b) the hepatic arterial resistance sites are located at the pre-sinusoidal level.

Estimation of specific hepatic arterial water space

The aim of this study was to estimate the specific arterial water space and associated blood flow using statistical moments of the frequency versus time outflow profile, with a model with specific spaces for hepatic arterial (HA) and portal venous (PV) flows in parallel with a common space. Studies were performed in the in situ dual-perfused rat liver (n = 6-10), using Krebs-bicarbonate buffer with constant PV flow (12 ml/min) and various HA flow rates (3-6 ml/min). An impulse input-output technique was employed, varying the route of input, using [14C]urea as the reference indicator. Regardless of flow conditions, the frequency outflow profile after HA input was flatter and broader and the mean transit time longer than after PV input. Excellent recovery of marker was obtained in all cases. Applying the above model, the specific arterial space was estimated to be 9.7 +/- 2.3 of total water space and receives approximately 17% of the HA flow, with the remainder mixing with portal blood in the common space. The estimated total water content of liver (0.67-0.72 ml/g liver) agrees well with that determined by desiccation (0.72 +/- 0.01 ml/g liver).

Development of an optimal method for the dual perfusion of the isolated rat liver

The liver receives two blood supplies, the portal vein (PV) and hepatic artery (HA). The isolated perfused rat liver preparation (IPRL) is widely used to examine physiological factors affecting the hepatic disposition of compounds, but usually it is perfused via the PV only. In the development of a more physiological dual perfused system, we examined three surgical procedures for HA perfusion--cannulation through the gastroduodenal artery, the aorta, and the celiac artery--using 14C-urea as the reference marker. Similar efflux profiles for 14C-urea were obtained for all three procedures, with a clear difference between HA and PV administration; however, cannulation of blood vessels and isolation of the HA supply were the most reliable with the celiac artery cannulation, making it the preferred procedure.

Ursodeoxycholate-induced hypercholeresis in cirrhotic rats: further evidence for cholehepatic shunting

The aim of the investigation was to explore whether ursodeoxycholate, a tertiary bile acid with potential for treatment of chronic cholestasis in cirrhotic liver disease, has the same physiological effects in cirrhotic as in normal rats. Furthermore, we wanted to investigate whether ductular proliferation, as it occurred in this situation, increases the bicarbonate stimulatory effect of ursodeoxycholate. Rats (n = 16) were rendered cirrhotic by continuous exposure to phenobarbital-carbon tetrachloride; untreated animals (n = 13) served as controls. In cirrhotic rats in vivo, ursodeoxycholate (20 mumoles/min/kg) stimulated bile salt secretion and bile flow less than in controls. Nevertheless, the increment in ursodeoxycholate-induced biliary bicarbonate--the bicarbonate stimulatory potency--was increased by 29% in cirrhotic animals (0.55 +/- 0.08 mmol vs. 0.71 +/- 0.11 mmol; p < 0.05). This finding could be related to ductular proliferation because the volume fraction of bile ductules, determined stereologically, increased from 0.3% +/- 0.1% to 2.7% +/- 0.6% in cirrhotic rats (p < 0.005). To explore further the behavior of ductules during ursodeoxycholate stimulation, we carried out experiments in the in situ perfused rat liver. In the portally perfused organ, replacement of bicarbonate by tricine-acetate abolished ursodeoxycholate-induced hypercholeresis. In the dually perfused organ (perfusion of both portal vein and hepatic artery) perfusion of the hepatic artery with bicarbonate-containing buffer, ursodeoxycholate had a similar stimulatory effect as in vivo in both control and cirrhotic rats.(ABSTRACT TRUNCATED AT 250 WORDS)

An isolated dual-perfused rabbit liver preparation for the study of hepatic blood flow regulation

An original, isolated dual-perfused rabbit liver preparation was developed for investigations into mechanisms that control the hepatic vascular tone. The hepatic artery (HA) and portal vein (PV) were perfused at constant flows of 0.16 +/- 0.01 and 0.64 +/- 0.05 mL/g/min (n = 5), respectively. Responses of the hepatic arterial and portal venous vascular beds to noradrenaline (NA) were measured as changes in perfusion pressure. Noradrenaline injected directly into the hepatic artery and portal vein produced dose-dependent increases in pressure in the respective vascular beds, the maximum response in the hepatic arterial bed being two to three times greater than that in the portal venous bed. A restricted transmission of vasoconstrictor stimulus between the intrahepatic portal venous and hepatic arterial vasculature was demonstrated. The results demonstrate the suitability of the dual-perfused rabbit liver model for detailed studies of the control of hepatic vascular tone.

Hepatic circulation: potential for therapeutic intervention

In recent years, knowledge of the physiology and pharmacology of hepatic circulation has grown rapidly. Liver microcirculation has a unique design that allows very efficient exchange processes between plasma and liver cells, even when severe constraints are imposed upon the system, i.e. in stressful situations. Furthermore, it has been recognized recently that sinusoids and their associated cells can no longer be considered only as passive structures ensuring the dispersion of molecules in the liver, but represent a very sophisticated network that protects and regulates parenchymal cells through a variety of mediators. Finally, vascular abnormalities are a prominent feature of a number of liver pathological processes, including cirrhosis and liver cell necrosis whether induced by alcohol, ischemia, endotoxins, virus or chemicals. Although it is not clear whether vascular lesions can be the primary events that lead to hepatocyte injury, the main interest of these findings is that liver microcirculation could represent a potential target for drug action in these conditions.