Adrenergic mechanisms in the hepatic microcirculation in the rat

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.

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.

A study of the inhibition of adrenaline-induced vasoconstriction in the isolated perfused liver of rabbit

We have studied the action of a series of vasoactive and antispasmodic agents on the intrahepatic vasoconstriction induced by adrenaline in the isolated perfused liver of rabbits. The arterial and portal venous resistance, oxygen consumption, liver weight and bile flow were investigated. The drugs used were as follows: nonspecific alpha-adrenergic antagonists (DH-ergocristine, dibenamine, phenoxybenzamine), vasodilators with a direct miscellaneous action (theophylline, papaverine, dipyridamole, glucagon, Aiu-cor by Instituto Gentilli, Italy [inosine, ATP, IPI, UTP]) and antispasmodics (piperylone, tropenziline, noraminophenazone). Adrenaline increased arterial and portal venous resistance followed by a diminution of oxygen consumption, liver weight and bile flow. alpha-Adrenergic antagonists inhibited the effects of adrenaline on portal venous resistance and oxygen consumption and especially the effects on hepatic arterial resistance. The most potent agent was phenoxybenzamine. In contrast to alpha-adrenoceptor blockade, the effects of other vasoactive agents were without a sustained influence on hepatic arterial resistance (excepting those of glucagon and dipyridamole). Some of them were effective as antagonists on responses in the portal venous bed (papaverine, Aiu-cor). Moreover, there were drugs exerting an enhancement of the vasoconstrictor responses of hepatic artery to low concentrations of adrenaline with no effect on the portal venous bed (piperylone, tropenziline). Theophylline and noraminophenazone exerted no effect either on the arterial or portal venous bed. No vasodilator agent antagonized the changes of the bile flow after adrenaline administration.

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.

Control of traumatic liver hemorrhage in the cirrhotic rat by intraportal infusion of norepinephrine

The effect of intraportal infusion of norepinephrine (NE) on primary hemostasis in the cirrhotic rat was investigated at standardized liver trauma. Cirrhosis was induced by simultaneous administration of increasing amounts of carbontetrachloride (CCl4) and phenobarbitone. Infusion of norepinephrine took place after cannulation of the gastroduodenal vein. Intraportal infusion of NE resulted in a significant increase in arterial blood pressure and portal pressure in all animals. No difference was observed between cirrhotic and control rats. Cirrhotic animals bled longer and more profusely as compared with the controls. Infusion of NE resulted in significant decrease in bleeding time and blood loss. NE did not affect hematocrit, hemoglobin, platelet, or white cell count. Platelet aggregation was not influenced by the compound. In conclusion, intraportal infusion of NE proved effective in decreasing hemorrhage at liver trauma in cirrhotic rats.

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.

Role of the central nervous system in acute-phase responses to leukocytic pyrogen

Intracerebroventricular injection of rabbit leukocytic pyrogen (LP) into conscious, healthy cannulated rabbits produced markedly enhanced febrile and acute-phase responses as compared with equivalent-dose, single bolus intravenous injection. The increased effectiveness in inducing granulocytosis and hypoferremia on intracerebroventricular injection was matched by changing the method of administration of intravenous LP from a single initial bolus to multiple fractional doses over a 2-h period. This suggested that augmentation for these parameters may have reflected only a "reservoir" function of the cerebral ventricles which prevented rapid clearance of LP from the blood. The ability of LP to induce hepatic synthesis of haptoglobin and C-reactive protein was so markedly enhanced by intracerebroventricular injection, however, that a role of the central nervous system in mediating or in modifying in an important way a non-neural mechanism for this mediation must be postulated.

Hepatic microvascular effects of terbutaline in experimental cardiogenic shock in rats

1. Experimental myocardial infarction was produced in rats by direct electrical cauterization of the myocardium of left ventricle. This produced cardiogenic shock with the accompanying haemodynamic changes of low cardiac output, low mean arterial pressure, raised central venous pressure and an absence of cardiac arrhythmias. 2. The liver microcirculation was observed using in vivo television microscope method. The diameter and erythrocyte flow velocity in the liver sinusoids were measured quantitatively. 3. During experimental cardiogenic shock 80% of the liver sinusoids were constricted; the remaining 20% showed dilatation. In all these liver sinusoids the erythrocyte flow velocity was only 50% of the pre-shock level. 4. Intravenous injection of the selective beta 2-adrenoceptor agonist terbutaline (0.15 mg/kg) restored the systemic arterial pressure to pre-shcok levels and partially raised the cardiac output. In the liver microcirculation terbutaline restored both constricted and dilated liver sinusoids to pre-shock calibres, but only partially raised erythrocyte flow velocity. 5. It is proposed that during experimental cardiogenic shock, terbutaline produces dilatation in the terminal liver microcirculation by opening sphincters of liver sinusoids and restores sinusoid diameters to pre-shock calibres. Therefore, terbutaline has the capacity to decrease peripheral resistance and unload the circulation during cardiogenic shock.

beta 2-Adrenoceptors in rat liver microcirculation

1. Rat liver sinusoids were observed by a microscopic in vivo transillumination method. The diameter of liver sinusoids and intrasinusoid erythrocyte flow velocities were measured quantitatively by a close-circuit television technique. 2. Both isoprenaline and the selective beta 2-adrenoceptor agonist terbutaline produced a concentration-dependent dilatation of liver sinusoids and slowed erythrocyte flow velocity. The effects of isoprenaline and terbutaline were antagonized by propranolol but not by the selective beta 1-adrenoceptor antagonist atenolol. Propranolol alone produced constriction of the liver sinusoids and increased erythrocyte flow velocity; these effects were not produced by atenolol. 3. The percentage dilatations produced by isoprenaline alone, isoprenaline in the presence of phenoxybenzamine and isoprenaline in the presence of phenylephrine-induced constriction were similar. 4. It is proposed that the beta-adrenoceptors in liver sinusoids are of the beta 2-type, and their physiological role was to counteract constrictor responses produced by alpha-adrenoceptor activity.

Stimulation and blockade of cholinergic receptors in terminal liver microcirculation in rats

This study was designed to establish the existence of cholinergic vascular receptors in the terminal portion of the rat liver microcirculation. The liver microcirculation was observed in vivo by a transillumination technique through a television microscope. The changes in the caliber of the liver sinusoids were measured directly on the television screen. Infusion of the parasympathetic neurotransmitter acetylcholine into the portal venous circulation caused a concentration-dependent dilation of liver sinusoids. Similar dilatation effects were observed for other cholinergic receptor agonists. Atropine, the specific cholinergic receptor blocker, inhibited this dilator effect, displacing the acetylcholine concentration-effect curve to the right. In contrast, physostigmine, the cholinesterase inhibitor, caused displacement of the curve to the left. In conclusion, cholinergic receptors are present in the terminal portion of the liver microcirculation, subserving the functional role of vasodilatation.