Localization of intrahepatic portal vascular resistance

Quantitative modeling of the physiology of ascites in portal hypertension

Although the factors involved in cirrhotic ascites have been studied for a century, a number of observations are not understood, including the action of diuretics in the treatment of ascites and the ability of the plasma-ascitic albumin gradient to diagnose portal hypertension. This communication presents an explanation of ascites based solely on pathophysiological alterations within the peritoneal cavity. A quantitative model is described based on experimental vascular and intraperitoneal pressures, lymph flow, and peritoneal space compliance. The model's predictions accurately mimic clinical observations in ascites, including the magnitude and time course of changes observed following paracentesis or diuretic therapy.

Portal hypertension: pathophysiology, diagnosis, and treatment

Portal hypertension (PH) is the result of increased vascular resistance in the portal circulation, increased portal venous blood flow, or both. In veterinary medicine, where portal pressure is seldom measured directly, the diagnosis of PH often is inferred from identification of associated complications including multiple acquired portosystemic shunts, ascites, and hepatic encephalopathy. Likewise, treatment of PH primarily is aimed at controlling these complications. The goal of this review is to provide an update on the pathophysiology, diagnosis, and treatment of PH. The review draws from information in the veterinary hepatology literature, reviews, and consensus statements in human hepatology and the literature on experimental models of PH.

Enhanced expression of endothelin B receptor at protein and gene levels in human cirrhotic liver

Endothelin (ET) has been implicated in the regulation of hepatic microcirculation and development of portal hypertension. This study examined the localization of ETA receptor (ETAR) and ETB receptor (ETBR) in cirrhotic liver tissues from patients with hepatocellular carcinoma with hepatitis C-related cirrhosis, and normal liver samples from patients with metastatic liver carcinoma. Anti-ETAR and ETBR antibodies were used for immunohistochemistry and Western blot. Immunoelectron microscopy was conducted using immunoglobulin-gold and silver staining. For in situ hybridization (ISH), human ETAR and ETBR peptide nucleic acid probes were used with the catalyzed signal amplification system. In normal liver tissue, immunohistochemistry revealed that ETBR was predominantly expressed on hepatic sinusoidal lining cells, particularly on sinusoidal endothelial (SECs) and hepatic stellate cells (HSCs), and ETAR was scantily expressed. These findings were confirmed by Western blot and ISH. In cirrhotic liver tissue, overexpression of ETBR was demonstrated by Western blot and ISH. Morphometric analysis showed significant increase of ETBR expression on HSCs and SECs in cirrhotic liver, particularly on HSCs. ETAR expression was increased but remained low. Enhanced ETBR expression in cirrhosis may intensify the effect of endothelin on HSCs and increase hepatic microvascular tone.

Methods for assessing hepatic distending pressure and changes in hepatic capacitance in pigs

The equilibrium pressure obtained during simultaneous occlusion of hepatic vascular inflow and outflow was taken as the reference estimate of hepatic vascular distending pressure (P(hd)). P(hd) at baseline was 1.1 +/- 0.2 (mean +/- SE) mmHg higher than hepatic vein pressure (P(hv)) and 0.7 +/- 0.3 mmHg lower than portal vein pressure (P(pv)). Norepinephrine (NE) infusion increased P(hd) by 1. 5 +/- 0.5 mmHg and P(pv) by 3.7 +/- 0.6 mmHg but did not significantly increase P(hv). Hepatic lobar vein pressure (P(hlv)) measured by a micromanometer tipped 2-Fr catheter closely resembled P(hd) both at baseline and during NE-infusion. Dynamic pressure-volume (PV) curves were constructed from continuous measurements of P(hv) and hepatic blood volume increases (estimated by sonomicrometry) during brief occlusions of hepatic vascular outflow and compared with static PV curves constructed from P(hd) determinations at five different hepatic volumes. Estimates of hepatic vascular compliance and changes in unstressed blood volume from the two methods were in close agreement with hepatic compliance averaging 32 +/- 2 ml. mmHg(-1). kg liver(-1). NE infusion reduced unstressed blood volume by 110 +/- 38 ml/kg liver but did not alter compliance. In conclusion, P(hlv) reflects hepatic distending pressure, and the construction of dynamic PV curves is a fast and valid method for assessing hepatic compliance and changes in unstressed blood volume.

Active and passive liver microvascular responses from angiotensin, endothelin, norepinephrine, and vasopressin

Vasoconstrictor agents may induce a decrease in hepatic vascular volume passively, by decreasing distending pressure, or actively, by stimulating contractile elements of capacitance vessels. Hepatic venular resistance was estimated in anesthetized rabbits from hepatic venular pressure (P(mu hv); by servo-null micropipette), inferior vena cava pressure, and total hepatic blood flow (F(hv); by ultrasound flow probe). Changes in liver volume were estimated from measures of liver lobe thickness. Angiotensin (ANG) II, endothelin (ET)-1, norepinephrine (NE), and vasopressin (VP) were infused into the portal vein at a constant rate for 5 min. We conclude that ANG II and NE induced active constriction of hepatic capacitance vessels, because the liver lobe thickness decreased significantly even though P(mu hv) and portal venous distending pressure (P(pv)) increased. All four agents increased splanchnic and hepatic venous resistances in similar proportions. With VP, P(mu hv) and P(pv) decreased, but with ET-1, P(mu hv) and P(pv) increased. However, lobe thickness was not significantly changed by either drug during the infusion compared with the 2-min control period. Thus VP and ET-1 have only minor effects on hepatic capacitance vessels. ET-1, at 0.04 microg. min(-1). kg body wt(-1), caused an increase in systemic arterial blood pressure, but erythrocyte movement through the sinusoids in some animals stopped.

Impact of intrinsic blood flow regulation in cirrhosis: maintenance of hepatic arterial buffer response

The hepatic arterial buffer response (HABR) effectively controls total blood perfusion in normal livers, but little is known about blood flow regulation in cirrhosis. We therefore studied the impact of HABR on blood perfusion of cirrhotic livers in vivo. After 8-wk CCl(4) treatment to induce cirrhosis, 18 anesthetized rats (and 18 noncirrhotic controls) were used to simultaneously assess portal venous and hepatic arterial inflow with miniaturized ultrasonic flow probes. Stepwise hepatic arterial blood flow (HAF) or portal venous blood flow (PVF) reduction was performed. Cirrhotic livers revealed a significantly reduced total hepatic blood flow (12.3 +/- 0.9 ml/min) due to markedly diminished PVF (7.3 +/- 0.8 ml/min) but slightly increased HAF (5.0 +/- 0.6 ml/min) compared with noncirrhotic controls (19.0 +/- 1.6, 15.2 +/- 1.3, and 3.8 +/- 0.4 ml/min). PVF reduction caused a significant HABR, i.e., increase of HAF, in both normal and cirrhotic livers; however, buffer capacity of cirrhotic livers exceeded that of normal livers (P < 0.05) by 1. 7- to 4.5-fold (PVF 80% and 20% of baseline). Persistent PVF reduction for 1, 2, and 6 h demonstrated constant HABR in both groups. Furthermore, HABR could be repetitively provoked, as analyzed by intermittent PVF reduction. HAF reduction did not induce changes of portal flow in either group. Because PVF is reduced in cirrhosis, the maintenance of HAF and the preserved HABR must be considered as a protective effect on overall hepatic circulation, counteracting impaired nutritive blood supply via the portal vein.

Preserved arterial flow secures hepatic oxygenation during haemorrhage in the pig

1. This study examined the extent of liver perfusion and its oxygenation during progressive haemorrhage. We examined hepatic arterial flow and hepatic oxygenation following the reduced portal flow during haemorrhage in 18 pigs. The hepatic surface oxygenation was assessed by near-infrared spectroscopy and the hepatic metabolism of oxygen, lactate and catecholamines determined the adequacy of the hepatic flow. 2. Stepwise haemorrhage until circulatory collapse resulted in proportional reductions in cardiac output and in arterial, central venous and pulmonary wedge pressures. While heart rate increased, pulmonary arterial pressure remained stable. In addition, renal blood flow decreased, renal vascular resistance increased and there was elevated noradrenaline spill-over. Further, renal surface oxygenation was lowered from the onset of haemorrhage. 3. Similarly, the portal blood flow was reduced in response to haemorrhage, and, as for the renal flow, the reduced splanchnic blood flow was associated with an elevated noradrenaline spill-over. In contrast, hepatic arterial blood flow was only slightly reduced by haemorrhage, and surface oxygenation did not change. The hepatic oxygen uptake was maintained until the blood loss represented more than 30 % of the estimated blood volume. At 30 % reduced blood volume, hepatic catecholamine uptake was reduced, and the lactate uptake approached zero. 4. Subsequent reduction of cardiac output and portal blood flow elicited a selective dilatation of the hepatic arterial vascular bed. Due to this dilatation liver blood flow and hepatic cell oxygenation and metabolism were preserved prior to circulatory collapse.

Induction of haemodynamic oscillations in the perfused rat liver by K+ channel blockers

1. Exposure of the isolated perfused (constant flow) rat liver to the K+ channel blockers 4-aminopyridine (4-AP) or Cs+ causes the appearance of oscillations in portal pressure and oxygen uptake. The oscillations have a mean frequency of 0.035 Hz (2.1 cycles/min) and are fully reversible upon perfusion with blocker-free saline. Tetraethylammonium (0.17-24.7 mM) does not induce oscillatory behaviour. 2. Reversible block of the 4-AP-induced oscillations is caused by 2 mM-EGTA, or verapamil, chlorpheniramine, phentolamine or propranolol with IC50 values of 0.42, 13.5, 15 or 11.5 microM respectively. The oscillations are transiently blocked by atropine (IC50 = 8.3 microM at peak inhibition) and are not affected by 2.7 microM-tetrodotoxin. 3. Endothelium-dependent vasorelaxants, Kupffer cell activity modifiers, retrograde perfusion, or removal of the portal vein from the circuit do not modify the oscillation parameters. 4. Oscillations are also caused by infusion of physiological concentrations of adrenaline or phenylephrine, but not isoprenaline. 5. The results provide new evidence for the existence of intrahepatic voltage-sensitive Ca2+, and 4-AP- and Cs(+)-sensitive K+ channels. We propose that the K+ channel blockers reveal an intrinsic oscillator in the liver, and that phasic vasoactivity may involve a minor contribution from neurotransmitter and/or hormonal substances.

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.