The comparative effect of administration of substances via the hepatic artery or portal vein on hepatic arterial resistance, liver blood volume and hepatic extraction in cats

Computational Modeling in Liver Surgery

The need for extended liver resection is increasing due to the growing incidence of liver tumors in aging societies. Individualized surgical planning is the key for identifying the optimal resection strategy and to minimize the risk of postoperative liver failure and tumor recurrence. Current computational tools provide virtual planning of liver resection by taking into account the spatial relationship between the tumor and the hepatic vascular trees, as well as the size of the future liver remnant. However, size and function of the liver are not necessarily equivalent. Hence, determining the future liver volume might misestimate the future liver function, especially in cases of hepatic comorbidities such as hepatic steatosis. A systems medicine approach could be applied, including biological, medical, and surgical aspects, by integrating all available anatomical and functional information of the individual patient. Such an approach holds promise for better prediction of postoperative liver function and hence improved risk assessment. This review provides an overview of mathematical models related to the liver and its function and explores their potential relevance for computational liver surgery. We first summarize key facts of hepatic anatomy, physiology, and pathology relevant for hepatic surgery, followed by a description of the computational tools currently used in liver surgical planning. Then we present selected state-of-the-art computational liver models potentially useful to support liver surgery. Finally, we discuss the main challenges that will need to be addressed when developing advanced computational planning tools in the context of liver surgery.

A Hemodynamic Study to Evaluate the Buffer Response in Cirrhotic Patients Undergoing Liver Transplantation

The physiological regulation of the liver blood flow is a result of a reciprocal portal vein and hepatic artery flow relationship. This mechanism is defined as the hepatic arterial buffer response (HABR). This study was addressed to investigate whether HABR is maintained in denervated grafts in liver transplant recipients. Portal blood flow (PBF) and hepatic arterial resistance index (PI) were measured 6 months after transplantation using Doppler. In each patient we consecutively measured the vasodilator (Ensure Plus PO versus placebo) and vasoconstrictor (isosorbide dinitrate 5 mg SL versus placebo) stimuli. The meal ingestion caused a significant increase of both parameters, PBF (from 1495±260 to 2069±250 mL/min, P<0.05) and PI (from 0.7±0.2 to 0.8±0.2, P<0.05). By contrast, isosorbide dinitrate reduced PBF (from 1660±270 to 1397±250 mL/min, P<0.05) and PI (from 0.7±0.2 to 0.5±0.2, P<0.05). We show that PBF and PI are reciprocally modified with the administration of vasoconstrictor and vasodilator stimuli. These results suggest the persistence of the HABR in a denervated human model, suggesting that this mechanism is independent of the regulation from the autonomic nervous system.

Disposition kinetics of diclofenac in the dual perfused rat liver

This study investigates the hepatic disposition of diclofenac as a function of route of input: portal vein (PV) versus hepatic artery (HA) in the presence of its binding protein, albumin. The in situ dual perfused rat liver was performed using Krebs bicarbonate buffer containing human serum albumin (HSA, 0.25%-1%) at constant PV (12 mL/min) and HA (3 mL/min) flow rates. Bolus doses of [(14) C]-diclofenac and (125) I-labeled HSA were injected randomly into the HA or PV and then, after an appropriate interval, into the alternate vessel. Regardless of route of input and perfusion medium protein concentration, the hepatic outflow profile of diclofenac displayed a characteristic sharp peak followed by a slower eluting tail, indicating that its radial distribution is not instantaneous. Based on the estimated effective permeability-surface area product/blood flow ratio, hepatic uptake of diclofenac is governed by both perfusion and permeability. Fractional effluent recovery (F) increased as unbound diclofenac fraction in the perfusate decreased. Although no significant difference in hepatic clearance of diclofenac as a function of route of delivery at 0.5% and 1% HSA, it was demonstrable at 0.25% HSA (p < 0.001), when the extraction ratio is higher.

The role of ATP and adenosine in the control of hepatic blood flow in the rabbit liver in vivo

BACKGROUND: The role of adenosine and ATP in the regulation of hepatic arterial blood flow in the "buffer response" was studied in vitro and in a new in vivo model in the rabbit. The model achieves portal-systemic diversion by insertion of a silicone rubber prosthesis between the portal vein and inferior vena cava and avoids alterations in systemic haemodynamics. RESULTS: Hepatic arterial (HA) blood flow increased in response to reduced portal venous (PV) blood flow, the "buffer response", from 19.4 (3.3) ml min-1 100 g-1 to 25.6 (4.3) ml min-1 100 g-1 (mean (SE), p < 0.05, Student's paired t-test). This represented a buffering capacity of 18.7 (5.2) %. Intra-portal injections of ATP or adenosine (1 micrograms kg-1-0.5 mg kg-1) elicited immediate increases in HA blood flow to give -log ED50 values of 2.0 and 1.7 mg kg-1 for ATP and adenosine respectively. Injection of ATP and adenosine had no measurable effect on PV flow. In vitro, using an isolated dual-perfused rabbit liver preparation, the addition of 8-phenyltheophylline (10 MicroMolar) to the HA and PV perfusate significantly inhibited the HA response to intra-arterial adenosine and to mid-range doses of intra-portal or intra-arterial ATP (p < 0.001). CONCLUSIONS: It is suggested that HA vasodilatation elicited by ATP may be partially mediated through activation of P1-purinoceptors following catabolism of ATP to adenosine.

A model of dual circulation in liver acini with hypoxia regulated adenosine secretion

It was postulated by W.W. Lautt that the hepatic artery flow compensation for changes in portal vein flow (the 'hepatic arterial buffer response') is regulated through the portal blood washout of adenosine from the small fluid compartment that surrounds the hepatic arterial resistance vessels. It is presumed that the adenosine secretion there is constant and independent of oxygen supply or liver demand. It was reported by others that liver secretes variable quantities of adenosine and that secretion is related to the level of liver hypoxia. This paper is an attempt to describe a model of acinar circulation without sources of constant adenosine secretion. The presented model is based on the fact that portal blood enters acinar space near the vascular stalk in the zone 1, while most of the arterial branches empty one-third from the interlobular septa, at the beginning of the zone 2, just downstream from the zone 1. Another important characteristic of liver architecture is that near 5/9 of lobular volume is in the zone 1. Liver cells in zone 1 are well oxygenated by the portal blood and they have low adenosine secretion that might seem almost constant. Since most arterial branches empty more peripherally, the zone 1 normally does not depend on the arterial circuit and most of arterial branches are governed by the adenosine secretion from the upstream zone 1. Low portal flow, would increase adenosine secretion from the zone 1 and thus dilate numerous downstream arterial resistance vessels. An increased flow from these arterial vessels would compensate any decrease in the portal flow. Zones 2 and 3 probably have higher adenosine secretion rates since the oxygenation depends on the amount of added arterial blood and on the liver cell metabolism. Some of the arterial branches in those zones are probably open all the time, preserving them zones from hypoxic injury. Since the main point for arterial inflow is concentrated downstream from the zone 1, in cases of low portal pressures, or elevated upstream resistance, some of the arterial blood might leave the acinus in retrograde direction via the portal branch and enter some other acinus as a part of portal blood. These arterio-portal communications might be important in cases of low or none portal flow when zone 1 is in hypoxia. In the 3D liver space with tightly packed acini, very complex and ever-changing patterns of combined antegrade and retrograde flows can be expected.

Hepatic artery buffer response following left portal vein ligation: its role in liver tissue homeostasis

Occlusion of a lobar portal vein is known to induce atrophy of downstream liver lobes and hypertrophy of contralateral lobes. Changes in portal flow are known to be compensated by changes in hepatic arterial flow, thus defining the hepatic artery buffer response (HABR). To understand the role of liver flow in liver atrophy, we measured portal flow and hepatic artery flow after different degrees of left portal vein stenosis (LPVS). Surgery was performed to obtain 0, 43, 48, 59, 68, 72, 78, and 100% LPVS. Systemic and splanchnic blood flows were measured at 4 h or 7 days after surgery using radiolabeled microspheres. At 4 h, LPVS produced no changes in systemic hemodynamics. Increasing degrees of LPVS produced a significant decrease in left portal flow (P < 0.0001) and a fully compensatory increase in right portal flow (P < 0.0001) without significantly affecting total portal flow. Left hepatic artery flow increased by 210% (P = 0.002), and right hepatic artery flow decreased by 67% (P = 0.05) after full LPVS. There was a significant inverse correlation between portal and arterial flow changes induced by different degrees of LPVS in the left (r(2) = 0. 61) and right (r(2) = 0.41) lobes. Despite this HABR, we observed a reduction in left liver flow (-45%; P = 0.01) and an increase in right liver flow (+230%; P = 0.01) with 100% LPVS. At 7 days, a significant decrease in the weight of left liver lobes (-75%; P < 0. 0001) and a compensatory increase in the weight of the right lobes (+210%; P < 0.0001) were observed with 100% LPVS. Left and right liver flows were similar to results measured at 4 h, and HABR was still present. However, when expressed per gram of liver, liver flows were identical to results obtained with sham animals. Reduction in lobar portal flow is accompanied by an increase in ipsilateral hepatic artery flow and a compensatory increase in portal flow to the rest of the liver. In a given lobe, when compensatory HABR is overcome, liver weight changes occur so that at the end total liver flow per gram of liver tissue is restored. This suggests that in normal conditions liver flow is a major regulator of liver volume.

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.

Adenosine receptor antagonism affects regional resting vascular resistance during rat peritoneal sepsis

BACKGROUND

To identify vascular beds where endogenous adenosine plays a significant role as a mediator of resting perfusion alterations associated with sepsis, we tested the hypothesis that adenosine receptor blockade would cause differential regional increases in vascular resistance during intraperitoneal (ip) sepsis in the rat.

MATERIALS AND METHODS

Rats (250-350 g) were catheterized and randomized to septic or nonseptic groups. Sepsis was induced with an ip injection of cecal slurry (150 mg/kg in D5W; 5 ml/kg), and baseline hemodynamics, cardiac output (CO), and blood flows (microspheres) were measured 24 h later. Animals then received the adenosine receptor antagonist 8-phenyltheophylline (8-PTH; 10 mM, 1.5 ml/kg), its vehicle (1.5 ml/kg), or normal saline (1.5 ml/kg), iv, and measurements were repeated.

RESULTS

Septic animals treated with 8-PTH had a significant increase in skeletal muscle, hepatic portal, and cerebral vascular resistance with concomitant decreases in CO when compared with vehicle at 1 min. No significant resistance changes were observed in the renal, adipose, or coronary vasculatures. Adenosine receptor blockade caused a significant increase in +dP/dt and -dP/dt during sepsis, indicating that the reduced CO was not secondary to myocardial depression.

CONCLUSIONS

These data suggest that adenosine receptor-mediated actions during sepsis affect vascular beds selectively and indicate a significant role for adenosine in resting perfusion redistribution in sepsis.

The transhepatic response to noradrenaline in the rabbit liver: the influence of arterioportal pressure gradient

The dose-related responses of the hepatic arterial and portal venous vascular beds to bolus administration of noradrenaline (10(-10)-10(-4) mol), injected into the hepatic artery and portal vein, were studied in the isolated dual-perfused rabbit liver at both basal and raised tone. The transhepatic ratio, defined as the ratio between the intra-arterial molar ED50 dose and the intraportal dose required to give the same arterial response, was calculated for arterial and venous responses to noradrenaline. At basal tone, the transhepatic ratio for hepatic arterial vasoconstrictive responses was 500. Portal venous vasoconstrictive responses were similar in potency independent of injection site but differed significantly in analysis of dose-response slope and maximal response. At raised tone, the arterio-portal pressure gradient increased by 68.5 mmHg and there was a 10-fold increase in the transhepatic ratio for hepatic arterial responses, while the portal venous responses remained unchanged. These results demonstrate that arterio-portal pressure gradient has a powerful effect on transhepatic action of noradrenaline, and suggest a pre-sinusoidal site for the generation of both hepatic arterial and portal venous vascular resistance.

Hemodynamic changes in blood flow through the denervated liver in pigs

We investigated the effects of liver denervation on hemodynamic circulation in seven anesthetized pigs. Simultaneous measurements of the hepatic artery and portal vein were performed with an ultrasound Doppler flow meter before and after liver denervation. Neither resting systemic nor hepatic hemodynamics changed following liver denervation. However, temporary occlusion of the portal vein resulted in a significant increase in hepatic artery flow in the innervated liver (from 123 +/- 15 ml/min to 177 +/- 17 ml/min, p < .01), whereas, in the denervated liver, a significant decrease was observed (from 128 +/- 11 ml/min to 106 +/- 19 ml/min, p < .05). Thus, the reciprocity between the hepatic artery and portal vein in the hepatic artery flow during portal vein occlusion might intensify symptoms of portal vein thrombosis in liver transplantation. In the denervated liver, a significant decrease also occurred in systolic blood pressure and central venous pressure from 1 to 3 min after portal vein occlusion. Since the liver plays a crucial role in the maintenance of cardiovascular homeostasis during blood loss, it is likely that denervation at the porta hepatis induced a lack of vasoconstriction in the portal territory. Liver denervation might further exacerbate this response to hypotension. The current study confirms that the hepatic nerves play an important role in hepatic arterial and portal venous interactions aimed at maintaining a constant blood flow through the liver. We also suggest that the hepatic nerves are important for cardiovascular homeostasis.

Splanchnic and renal sympathetic activity in relation to hemodynamics during isoflurane administration in pigs

The aim of the present study was to investigate the impact of isoflurane on regional neurogenic mechanisms in the control of vascular tone. Therefore, regional determinations of sympathetic activity and hemodynamics were made in chloralose-anesthetized swine before and during administration of 1.4% isoflurane. Sympathetic activity was examined from spillover of norepinephrine (NE) into the circulation using an isotope dilution technique. Administration of isoflurane caused a marked decrease in mesenteric (65 +/- 9 pmol.min-1.100 g-1; P < 0.05) NE spillover. Renal NE spillover was moderately decreased (25 +/- 6 pmol.min-1.100 g-1; P < 0.05), whereas liver NE spillover did not change significantly during isoflurane administration, suggesting that liver sympathetic activity is maintained at this level of isoflurane anesthesia. Total body NE spillover decreased (13 +/- 2 pmol.min-1.100 g-1, P < 0.05). Thus, isoflurane affected sympathetic outflow in a regionally differentiated pattern. Significant correlations were found between total body, mesenteric, and renal NE spillovers and vascular resistances, supporting the concept that the observed reductions in vascular resistances in these circulations during isoflurane administration were in part a consequence of reduced sympathetic outflow. In the liver circulation, no correlation was found between NE spillover and liver portal or liver arterial vascular resistances. Liver arterial resistance was significantly reduced during isoflurane administration while liver portal resistance was unchanged. Administration of isoflurane caused reductions in cardiac output, renal, portal, hepatic arterial, and total hepatic blood flows, whereas mesenteric blood flow was unchanged. To summarize, isoflurane decreased mesenteric and renal NE spillover with concomitant reductions in vascular resistances.(ABSTRACT TRUNCATED AT 250 WORDS)

The transhepatic action of ATP on the hepatic arterial and portal venous vascular beds of the rabbit: the role of nitric oxide

1. The effect of bolus administration of adenosine 5'-triphosphate (ATP) into the portal vein on hepatic arterial pressure (the transhepatic action of ATP) and portal venous pressure, and the contribution of nitric oxide towards these responses, was studied in the in vitro dual-perfused rabbit liver. 2. At basal tone, hepatic arterial and portal venous vasoconstriction followed ATP injection, while at a tone raised with methoxamine (10(-6)-10(-5) M) ATP caused hepatic arterial vasodilatation, and a phasic vasodilatation followed by vasoconstriction in the portal venous vascular bed. 3. To determine whether the transhepatic arterial dilatation was due to the diffusion of nitric oxide (NO) from the portal venous vasculature, NG-nitro-L-arginine methyl ester (L-NAME, 100 microM), an inhibitor of NO synthesis, was infused selectively into the portal vein. L-NAME infusion potentiated portal venous vasoconstriction to ATP (-log M ED50 5.32 +/- 0.31 to 6.51 +/- 0.43, P < 0.05, Student's paired t test) indicating the possible inhibition of a NO-mediated vasodilator component of the portal venous response to ATP. There was, however, no demonstrable difference in the transhepatic arterial vasodilatation induced by ATP during this infusion. 4. Simultaneous perfusion of both the hepatic arterial and portal venous inflows with L-NAME (100 microM) resulted in a significant decrease in the amplitude of hepatic arterial responses to ATP demonstrating that these responses were ultimately mediated by an NO-dependent mechanism. 5. This study has thus demonstrated a vasodilator component of the portal venous response to ATP that is NO-mediated. It also provides evidence that it is not portally-derived NO, but NO released from the hepatic arterial vascular bed, that accounts for the hepatic arterial vasodilatation to intra-portal administration of ATP. This implies that ATP itself, and not a second messenger, diffuses from the portal venous to hepatic arterial vascular bed to elicit the hepatic arterial response.

An evaluation of whether duration of perfusion alters vascular responses in the isolated dual-perfused rabbit liver

The isolated dual-perfused rabbit liver has been used to characterize hepatic arterial vascular receptors. These hepatic arterial responses are reproducible for 2.5 hr. Further studies in relation to the assessment of portal venous responses using this model require longer periods of perfusion. This study was designed to determine if this model is suitable for the assessment of hepatic arterial and portal venous vascular responses over a 5-hr perfusion period. Hepatic arterial responses were consistent to all agents during 5 hr of perfusion. Portal venous responses to the direct smooth muscle acting agents, sodium nitroprusside and adenosine, were constant, but the vasoconstrictor responses to acetylcholine and adenosine 5'-triphosphate (agents that cause an endothelium-independent vasoconstriction and an endothelium-dependent vasodilatation) were potentiated. Coarse measurements of liver function were also performed and suggested that the liver remained viable for the 5 hr of perfusion. These results suggest a decrease in the ability of the endothelium of the portal venous vasculature to respond to vasoactive substances as duration of perfusion increases. The possible reasons for this are discussed.

Acute effects of sublingual nitroglycerin on hepatic blood flow in healthy volunteers

Duplex sonography was used to assess the effects on hepatic blood flow after administering 0.6 mg nitroglycerin (NTG) sublingually to ten healthy volunteers. The study was a randomized, placebo-controlled, cross-over study in which subjects were studied on three separate occasions. Each visit involved administering either placebo or NTG followed by estimation of blood flow through a particular branch of the hepatic artery, portal vein, and hepatic vein every minute for 15 minutes after NTG and placebo administration. Two hours later, subjects were crossed over to the other treatment and the same vessel branch was again examined for 15 minutes. Total blood flow increased 7% in the portal vein and 27% in the hepatic vein during NTG treatment, but did not change significantly in the hepatic artery. Vascular resistance was increased in the hepatic artery and decreased in the portal and hepatic veins after NTG. Qualitatively, flow changed dramatically in the hepatic vein after NTG with the disappearance of normal retrograde flow. The results indicate that nitroglycerin effects hepatic blood flow through the portal and hepatic veins with a decrease in vascular resistance in the portal and hepatic veins and an increase in resistance in the hepatic artery.

The actions of two sensory neuropeptides, substance P and calcitonin gene-related peptide, on the canine hepatic arterial and portal vascular beds

1. The two peptides, calcitonin gene-related peptide (CGRP) and substance P (SP) were administered individually as bolus injections into the separately perfused hepatic arterial and portal vascular beds of the anaesthetized dog to assess their actions and relative molar potencies at these sites. 2. CGRP caused an immediate dose-related increase in hepatic arterial flow when injected close-arterially, reflecting a fall in resistance. This vasodilator effect was slightly increased by the prior administration of the selective beta 2-adrenoceptor antagonist, ICI 118,551. 3. On a molar basis, CGRP was more potent as an hepatic arterial vasodilator than the non-selective beta-adrenoceptor agonist, isoprenaline (Iso). 4. Intra-portal injection of CGRP also evoked hepatic arterial vasodilatation unaccompanied by other cardiovascular changes. 5. CGRP in doses up to 10 nmol had no effect on portal vascular resistance when administered intra-portally. 6. SP evoked a rapid, dose-related increase in hepatic arterial flow when injected intra-arterially. The molar ED50 for this hepatic vasodilatation was 40.2 fmol, significantly less than the ED50 for either CGRP or Iso. SP was the most potent hepatic arterial vasodilator yet examined. The vasodilator effect of SP was slightly potentiated by prior beta 2-adrenoceptor blockade. 7. SP caused hepatic arterial vasodilatation when administered by intra-portal injection; its absolute and relative potency was much reduced. 8. SP when injected intra-portally caused a graded increase in hepatic portal inflow resistance. The molar potency for this portal vasoconstriction was significantly greater than that for noradrenaline (NA); however, the maximum increase in portal resistance was significantly less to SP than to NA.9. In view of the location of the peptides CGRP and SP within the afferent innervation of the liver, it is proposed that they play an important function in controlling the hepatic microvasculature in response to sensory stimuli, particularly those arising from changes in portal blood composition secondary to change in metabolic activity within the gastrointestinal tract (GIT).10. Since the peptides are released from the GIT into the hepatic portal inflow, they may modify hepatic arterial blood flow, the extent of which is related to events within the GIT.

Hemodynamic effects on hepatic blood flow of a selective beta 2-adrenoceptor agonist, clenbuterol, in rat

Acute clenbuterol administration (50 micrograms/kg, i.v.) to anesthetized normotensive rats, produce a marked reduction in the mean blood pressure, (MBP), about 58 mm Hg. Indocyanine Green clearance analysis (control, 1.83 +/- 0.15: clenbuterol, 1.10 +/- 0.20 ml/min/100 g, P < 0.05) showed that the action in the hepatic vascular bed is opposite to its systemic vasodilator effects. The hepatic blood flow (HBF) appears significantly reduced (control, 8.24 +/- 0.35: clenbuterol, 3.83 +/- 0.71 ml/min/100 g, P < 0.05) whereas the hepatic uptake and excretion proceedings were apparently not affected (control hepatic extraction coefficient, 0.225 +/- 0.024: clenbuterol, 0.300 +/- 0.04, NS). These findings show that a marked reduction in HBF follows systemic vasodilator effects produced by clenbuterol.

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.

Control of glycogenolysis and blood flow by arterial and portal adrenaline in perfused liver

In isolated liver from fed rats, simultaneously single-pass-perfused via both the hepatic artery (80 mmHg, 30-35% flow) and the portal vein (10 mmHg, 70-65% flow), adrenaline was infused either singly or jointly via the hepatic artery or the portal vein in the absence or presence of the alpha 1-blocker prazosin and the beta 2-blocker butoxamine. It was found that: (1) arterial adrenaline caused increases in glucose and lactate output which were slower in onset, smaller in peak height but longer in duration than did portal adrenaline; (2) arterial adrenaline elicited a much more pronounced decrease in flow and increase in pressure in the ipsilateral vessel than did portal adrenaline, and arterial, but not portal, adrenaline elicited qualitatively similar alterations also in the contralateral vessel; (3) arterial adrenaline caused metabolic changes mainly via alpha 1-receptors, with beta 2-receptors playing a permissive role via haemodynamic alterations, whereas portal adrenaline acted only via alpha 1-receptors; (4) arterial adrenaline decreased arterial flow via alpha 1-receptors counteracted via beta 2-receptors and operated on portal flow as portal adrenaline only via alpha 1-receptors; and (5) arterial adrenaline was extracted to a far greater extent than portal adrenaline. The results indicate that the hepatic artery and the portal vein can function as independent sites of hormonal signal input, which interact by complex, still undefined, mechanisms.

Control of glycogenolysis and blood flow by arterial and portal norepinephrine in perfused liver

In isolated rat liver single pass perfused via both the hepatic artery (80 mmHg, 30% flow) and the portal vein (10 mmHg, 70% flow), norepinephrine (NE) was infused either singly or jointly via the hepatic artery or the portal vein in the absence or presence of the alpha 1-blocker prazosin or the beta 2-blocker butoxamine. Arterial NE caused an increase in glucose output and a shift from lactate uptake to release that was slower in onset and smaller in peak height but longer in duration than the alterations affected by portal NE. The sum of the metabolic changes by arterial and portal NE was not equal to the changes by jointly applied arterial plus portal NE. The metabolic alterations by arterial NE were mediated via alpha 1-receptors, with beta 2-receptors probably having a permissive function, but those by portal NE were transmitted only via alpha 1-receptors. Arterial NE caused a strong decrease in arterial flow and contralaterally also a smaller reduction of portal flow. Portal NE decreased portal flow but did not significantly influence arterial flow. The sum of the alterations in flow by arterial and portal NE was not equal to the changes by jointly applied NE. The hemodynamic alterations in the artery by arterial NE were the results of actions via alpha 1-receptors and counteractions via beta 2-receptors, whereas the changes in the portal vein by arterial NE and portal NE were mediated via alpha 1-receptors. About 65% of arterial and only 30% of portal NE was extracted during a single path. The results indicate that the hepatic artery and the portal vein can function as independent sites of hormonal signal input, which interact by complex but still undefined mechanisms in the regulation of metabolism and hemodynamics.

The role of adenosine in the hyperaemic response of the hepatic artery to portal vein occlusion (the 'buffer response')

1. Adenosine has been shown to be responsible for the hyperaemic response of the hepatic artery to portal vein occlusion (the hepatic arterial 'buffer response'). 2. The effect of adenosine receptor blockade and of adenosine uptake inhibition on the hepatic arterial response to portal vein occlusion was investigated in three groups of anaesthetized dogs. 3. Venous return and arterial blood pressure were maintained during periods of portal occlusion by establishing a side-to-side portacaval shunt. Hepatic artery and portal vein blood flows were measured with electromagnetic flowmeters. 4. Hepatic arterial infusions of 8-phenyltheophylline (500 micrograms kg-1 and 3-isobutyl-1-methylxanthine min-1) and 3-isobutyl-1-methylxanthine (75 micrograms kg-1 min-1), doses sufficient to block the vasodilator response of the hepatic artery to exogenously applied adenosine, reduced the magnitude of the 'buffer response' by 50% and 75%, respectively. 5. Intravenous infusion of dipyridamole (100 micrograms kg-1 min-1), a dose sufficient to potentiate the vasodilator response of the hepatic artery to exogenously applied adenosine, had little effect on the 'buffer response'. 6. It is concluded that adenosine is an important, but not the sole, agent responsible for the hepatic arterial 'buffer response'.

The role of adenosine in the hyperaemic response of the hepatic artery to portal vein occlusion (the 'buffer response')

1. Adenosine has been shown to be responsible for the hyperaemic response of the hepatic artery to portal vein occlusion (the hepatic arterial 'buffer response'). 2. The effect of adenosine receptor blockade and of adenosine uptake inhibition on the hepatic arterial response to portal vein occlusion was investigated in three groups of anaesthetized dogs. 3. Venous return and arterial blood pressure were maintained during periods of portal occlusion by establishing a side-to-side portacaval shunt. Hepatic artery and portal vein blood flows were measured with electromagnetic flowmeters. 4. Hepatic arterial infusions of 8-phenyltheophylline (500 micrograms kg-1 and 3-isobutyl-1-methylxanthine min-1) and 3-isobutyl-1-methylxanthine (75 micrograms kg-1 min-1), doses sufficient to block the vasodilator response of the hepatic artery to exogenously applied adenosine, reduced the magnitude of the 'buffer response' by 50% and 75%, respectively. 5. Intravenous infusion of dipyridamole (100 micrograms kg-1 min-1), a dose sufficient to potentiate the vasodilator response of the hepatic artery to exogenously applied adenosine, had little effect on the 'buffer response'. 6. It is concluded that adenosine is an important, but not the sole, agent responsible for the hepatic arterial 'buffer response'.

The actions of human atrial natriuretic factor on hepatic arterial and portal vascular beds of the anaesthetized dog

1. The vascular actions of atrial natriuretic factor (ANF) have been assessed with other vasoactive agents on the hepatic arterial and portal vascular beds of the anaesthetized dog. 2. Intra-arterial bolus injections of ANF (0.1-50 nmol) caused graded increases in hepatic arterial blood flow representing a vasodilatation of relatively short duration. Vasoconstriction was never observed. 3. The maximum increase in hepatic arterial blood was the same for ANF and isoprenaline (Iso) i.e. approximately 60-70% increase over control flow. 4. On a molar basis, ANF was less potent than Iso although over the higher dose range (10(-9)-10(-7) mol) its vasodilator activity exceeded that of the endogenous vasodilator adrenaline. 5. Intraportal bolus injections (1.0-50 nmol) of ANF did not alter portal inflow resistance since no changes in portal inflow pressure occurred when the portal circuit was perfused at constant inflow volume. 6. This differential action of ANF on the hepatic arterial and portal vascular beds may provide a change in total liver blood flow in favour of the arterial component. 7. ANF, by altering hepatic haemodynamics to favour formation of trans-sinusoidal fluid exchange, may provide a temporary expansion of the extravascular fluid reservoir to buffer any increased venous pressure. However, chronically elevated plasma levels of ANF would encourage the formation of ascitic fluid.

Variation of incorporation of [3H]orotic acid into the nucleotide and RNA fractions of different parts of the same liver lobe in the rat

1. Anaesthetized rats were given [3H]orotic acid either intraperitoneally or via a catheter into the hepatic artery with or without degradable starch microspheres. 2. The radioactivity in the acid soluble and RNA fractions of five pieces of the left lateral liver lobes was determined. 3. A variation of the distribution of the precursor into the different parts of the same liver lobe was shown. 4. This variation was most pronounced (3000-17,000 cpm/micrograms in the acid soluble fraction) when the precursor was administered via the artery and without microspheres. 5. The correlation between the radioactivity in the acid soluble and RNA fractions within each liver piece was 0.85, 0.90 and 0.75 in the three groups respectively. 6. It is suggested that the variation of the distribution depends on circulatory differences within the liver.