Neuroendocrine innervation of the hepatic vessels in the rat and in man

Neural connections between the hypothalamus and the liver

After receiving information from afferent nerves, the hypothalamus sends signals to peripheral organs, including the liver, to keep homeostasis. There are two ways for the hypothalamus to signal to the peripheral organs: by stimulating the autonomic nerves and by releasing hormones from the pituitary gland. In order to reveal the involvement of the autonomic nervous system in liver function, we focus in this study on autonomic nerves and neuroendocrine connections between the hypothalamus and the liver. The hypothalamus consists of three major areas: lateral, medial, and periventricular. Each area has some nuclei. There are two important nuclei and one area in the hypothalamus that send out the neural autonomic information to the peripheral organs: the ventromedial hypothalamic nucleus (VMH) in the medial area, the lateral hypothalamic area (LHA), and the periventricular hypothalamic nucleus (PVN) in the periventricular area. VMH sends sympathetic signals to the liver via the celiac ganglia, the LHA sends parasympathetic signals to the liver via the vagal nerve, and the PVN integrates information from other areas of the hypothalamus and sends both autonomic signals to the liver. As for the afferent nerves, there are two pathways: a vagal afferent and a dorsal afferent nerve pathway. Vagal afferent nerves are thought to play a role as sensors in the peripheral organs and to send signals to the brain, including the hypothalamus, via nodosa ganglia of the vagal nerve. On the other hand, dorsal afferent nerves are primary sensory nerves that send signals to the brain via lower thoracic dorsal root ganglia. In the liver, many nerves contain classical neurotransmitters (noradrenaline and acetylcholine) and neuropeptides (substance P, calcitonin gene-related peptide, neuropeptide Y, vasoactive intestinal polypeptide, somatostatin, glucagon, glucagon-like peptide, neurotensin, serotonin, and galanin). Their distribution in the liver is species-dependent. Some of these nerves are thought to be involved in the regulation of hepatic function as well as of hemodynamics. In addition to direct neural connections, the hypothalamus can affect metabolic functions by neuroendocrine connections: the hypothalamus-pancreas axis, the hypothalamus-adrenal axis, and the hypothalamus-pituitary axis. In the hypothalamus-pancreas axis, autonomic nerves release glucagon and insulin, which directly enter the liver and affect liver metabolism. In the hypothalamus-adrenal axis, autonomic nerves release catecholamines such as adrenaline and noradrenaline from the adrenal medulla, which also affects liver metabolism. In the hypothalamus-pituitary axis, release of glucocorticoids and thyroid hormones is stimulated by pituitary hormones. Both groups of hormones modulate hepatic metabolism. Taken together, the hypothalamus controls liver functions by neural and neuroendocrine connections.

Anatomy of efferent hepatic nerves

The role of neural elements in regulating blood flow through the hepatic sinusoids, solute exchange, and parenchymal function is incompletely understood. This is due in part to limited investigation in only a few species whose hepatic innervation may differ significantly from humans. For example, most experimental studies have used rats and mice having livers with little or no intralobular innervation. In contrast, most other mammals, including humans, have aminergic and peptidergic nerves extending from perivascular plexus in the portal space into the lobule, where they course in Disse's space in close relationship to stellate cells (fat storing cells of Ito) and hepatic parenchymal cells. While these fibers extend throughout the lobule, they predominate in the periportal region. Cholinergic innervation, however, appears to be restricted to structures in the portal space and immediately adjacent hepatic parenchymal cells. Neuropeptides have been colocalized with neurotransmitters in both adrenergic and cholinergic nerves. Neuropeptide Y (NPY) has been colocalized in aminergic nerves supplying all segments of the hepatic-portal venous and the hepatic arterial and biliary systems. Nerve fibers immunoreactive for substance P and somatostatin follow a similar distribution. Intralobular distribution of all of these nerve fibers is species-dependent and similar to that reported for aminergic fibers. Vasoactive intestinal peptide and calcitonin gene-related peptide (CGRP) are reported to coexist in cholinergic and sensory afferent nerves innervating portal veins and hepatic arteries and their branches, but not the other vascular segments or the bile ducts. Nitrergic nerves immunoreactive for neuronal nitric oxide (nNOS) are located in the portal tract where nNOS colocalizes with both NPY- and CGRP-containing fibers. In summary, the liver is innervated by aminergic, cholinergic, peptidergic, and nitrergic nerves. While innervation of structures in the portal tract is relatively similar between species, the extent and distribution of intralobular innervation are highly variable as well as species-dependent and may be inversely related to the density of gap junctions between contiguous hepatic parenchymal cells.

Experimental monitoring of hepatic glucose, lactate, and glutamate metabolism by microdialysis during surgical preparation of the liver hilus

Mechanical liver manipulation can lead to hepatic microcirculation (MC) impairment. The pathobiochemical relevance of this phenomenon is not fully understood. Microdialysis (MD) allows a quantification of metabolic products in interstitial fluid, thus enabling analysis of the hepatic metabolic state during changes of liver perfusion. The aim of the study was to quantify the functional effects of standardized surgical liver preparation both on liver metabolism and microperfusion. Two groups of animals (pigs, n = 25) were formed: In the trial group (TG; n = 13) the liver was mobilized, followed by hilar preparation. In the control group (CG; n = 12) mobilization of the liver without hilar dissection was performed. Surgical manipulation was followed by an observation in both groups. Hepatic interstitial glucose, lactate, and glutamate concentrations were detected by MD and liver MC by thermodiffusion. During liver mobilization MC decreased significantly in both groups (TG; 86.7 +/- 2.0 to 73.4 +/- 2.3 ml/100 g min; and CG; 88.3 +/- 3.1 to 71.9 +/- 2.2 ml/100 g/min). In the trial group levels decreased further during hilar preparation reaching minimal values of 65.6 +/- 2.8. After preparation MC recovered to baseline. Glucose, lactate, and glutamate concentrations increased significantly during liver mobilization in the trial (glucose; 0.52 +/- 0.13 to 0.88 +/- 0.19 mmol/L; lactate; 0.34 +/- 0.07 to 0.54 +/- 0.07 mmol/L; glutamate; 34.5 +/- 3.6 to 52.6 +/- 8.0 micromol/L) and control group (glucose; 0.58 +/- 0.06 to 0.95 +/- 0.13 mmol/L; lactate; 0.30 +/- 0.06 to 0.49 +/- 0.07 mmol/L; glutamate; 32.9 +/- 2.36 to 56.1 +/- 5.12 micromol/L). Throughout hilus preparation maximum values could be measured in TG (glucose; 1.69 +/- 0.34; lactate; 0.90 +/- 0.18; glutamate; 63.5 +/- 7.2). After termination of mobilization or preparation baseline concentrations were reached again. MD allows monitoring of metabolic changes in hepatic parenchyma. Surgical liver preparation leads to changes of intrahepatic glucose, lactate, and glutamate levels (without alterations of parameters in systemic plasma) along with hepatic MC impairment. Reconstitution of hepatic MC was accompanied by rapid normalization of metabolic parameters. By measuring specific parameters, MD could prove to be of use for functional assessment of metabolic effects due to MC disturbances.

Vasoactive intestinal polypeptide is a potent regulator of bile secretion from rat cholangiocytes

BACKGROUND & AIMS

Vasoactive intestinal polypeptide (VIP) is a neuropeptide with diverse biological functions including stimulatory effects on bile secretion. The effects of VIP on bile secretion and its site of action were examined.

METHODS

Choleretic effects of VIP were examined using isolated perfused livers, hepatocyte couplets, isolated bile duct units, and cholangiocytes from rat liver.

RESULTS

VIP (100 nmol/L) produced a small increase in bile flow and bile salt output in taurocholate-supplemented isolated perfused livers but had no significant effect on bile flow in the absence of bile salt supplements or on fluid secretion in isolated hepatocyte couplets. In addition, VIP significantly increased bile pH, bicarbonate concentration, and output in the isolated perfused livers from both normal and 2 week bile duct-ligated rats, although bile flow increased only in the bile duct-ligated model. VIP also produced a dose-dependent increase in fluid secretion in isolated bile duct units, which was inhibited significantly by VIP antagonist, a specific VIP receptor inhibitor. This VIP-stimulated secretory response in isolated bile duct units was more potent than those produced by bombesin or secretin. Neither somatostatin nor substance P inhibited the VIP response in isolated bile duct units. In contrast to secretin, VIP had no significant effect on adenosine 3', 5'-cyclic monophosphate (cAMP) levels in isolated cholangiocytes.

CONCLUSIONS

VIP is a potent stimulant of fluid and bicarbonate secretion from cholangiocytes via cAMP-independent pathways, suggesting that this neuropeptide plays a major regulatory role in biliary transport and secretion.

Intracellular pH regulation in bombesin-stimulated secretion in isolated bile duct units from rat liver

Bombesin, a neuropeptide, stimulates fluid and HCO-3 secretion from cholangiocytes, but the underlying mechanisms are poorly understood. In this study, we aimed to examine the effects of bombesin on ion transport processes involved in the regulation of intracellular pH (pHi) and HCO-3 secretion in polarized cholangiocytes. Isolated bile duct units from normal rat liver were used to measure pHi by 2', 7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein 495 nm-to-440 nm dual ratio methods. Bombesin increased Cl--HCO-3 exchange activity but did not affect basal pHi or the activities of Na+/H+ exchange or Na+-HCO-3 symport. Depolarization of cholangiocytes increased basal pHi and the activity of Cl-/HCO-3 exchange, suggesting that an electrogenic Na+-HCO-3 symport might function as a counterregulatory pHi mechanism. Na+-independent acid-extruding mechanisms were not observed. We conclude that bombesin stimulates biliary secretion from cholangiocytes by activating luminal Cl-/HCO-3 exchange, which may be coupled to basolateral electrogenic Na+-HCO-3 symport.

Innervation of the liver: morphology and function

Although it has been known for many years that the liver receives a nerve supply, it is only with the advent of immunohistochemistry that this innervation has been analysed in depth. It is now appreciated not only that many different nerve types are present, but also that there are significant differences between species, especially in the degree of parenchymal innervation. This has stimulated more detailed investigation of the innervation of the human liver in both health and disease. At the same time, functional studies have been underlining the important roles that these nerves play in processes as diverse as osmoreception and liver regeneration. This article briefly reviews current understanding of the morphology and functions of the hepatic nerve supply.

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.

Immunohistochemical studies on the innervation of human transplanted liver

The changing pattern of innervation in the human transplanted liver was studied from the day of transplantation to 5 years later. Seven liver biopsies from non-transplant controls, 37 liver biopsies from 22 transplant patients, and one of these biopsied livers removed at retransplantation, were available for the study. Sections were immunostained for protein gene product 9.5 (PGP 9.5), neurone-specific enolase (NSE), S-100 protein, and vasoactive intestinal polypeptide. NSE and PGP 9.5 demonstrated nerves most successfully in our tissues. Staining for most small nerves was reduced by day 5 post-transplantation. Scanty fine nerves could be detected from day 13 to day 241 in occasional biopsies. Consistently identifiable immunostaining of PGP 9.5 and NSE nerve fibres was again apparent in portal areas after this time in all but one case. The findings indicate that in transplanted liver limited reinnervation can eventually take place. This could be due to either proliferation of intrinsic nerves, or regrowth of extrinsic nerves, or both.

[Regulation of liver functions by autonomic hepatic nerves]

The liver is the glucose reservoir of the organism and moreover an important blood reservoir, which takes up or releases glucose and blood depending on demand. Activation of the sympathetic nerves increases glucose release, shifts lactate uptake to output and reduces a.o. oxygen uptake. Moreover, it elicits a reduction of blood flow, and, by closing of sinusoids, an intrahepatic redistribution as well as a mobilization of blood. Activation of parasympathetic nerves enhances glucose utilization and causes a re-opening of closed sinusoids. The actions of sympathetic nerves can be modulated by hormones. Extracellular calcium as well as the mediators noradrenaline and probably also prostaglandins are involved in the signal chain. Intracellularly the signal chain is propagated by an increase of cytosolic calcium.