Evidence for the role of ammonia in the intracerebral transfer and metabolism of tryptophan.

After portacaval shunt in the rat, the transport of tryptophan and other neutral amino acids across the blood-brain barrier is enhanced. To determine the role of NH3 in the intracerebral transfer of tryptophan and serotonin metabolism, solutions containing either saline or NH3 or tryptophan or NH3 + tryptophan together were infused, respectively, into the internal carotid artery of rats in order to achieve blood levels similar to those observed after liver ischemia. After tryptophan infusion, a significant increase in the hypothalamic levels of tryptophan and 5-hydroxyindoleacetic acid was observed. A similar increment was found after NH3 infusion. NH3 + tryptophan infusion induced a significant increment in hypothalamic tryptophan and 5-hydroxyindoleacetic acid levels which were 2-fold greater than after tryptophan infusion. There was no significant change in 5-hydroxytryptamine levels in any of these experiments. Glutamine levels increased significantly after NH3 infusion. When tryptophan and NH3 were infused simultaneously, a significant reduction in glutamine levels occurred. These results cannot be explained by any modification of cerebral blood flow nor of the cerebral intercellular pH. Our data suggest that NH3 enhances the transfer of tryptophan across the blood-brain barrier and thus stimulates serotonin metabolism. The mechanism by which tryptophan transfer across the blood-brain barrier is facilitated is unknown. The reduction in glutamine levels in the hypothalamus when NH3 and tryptophan are infused together may be explained either by an inhibition of synthesis or by an intercellular influx of neutral amino acids and an efflux of glutamine as suggested by James et al.

Effect of the oral administration of branched chain amino acids on hepatic encephalopathy in the rat.

The aim of this study was to investigate the effect of oral administration of branched-chain amino acid (BCAA)-enriched diets after portacaval shunt (PCS) in rats. Fifty-one Sprague-Dawley male rats (200 gm) underwent PCS and 55 a sham operation. Half of the animals received BCAA 142 mg per day through a gastric tube; the other half underwent a sham procedure. Sleep disturbances were evaluated at 7, 14, and 28 days postoperatively by measurement of the excitability of the reticular brain-stem formation during the slow-wave sleep and paradoxical sleep. Animals were killed at the same intervals and liver/body weight ratio, plasma, and brain amino acids, brain norepinephrine, brain serotonin, 5-hydroxyindolacetic acid and histamine were measured. Each group of animals was matched with a similar group of sham-operated rats, i.e., receiving or not receiving BCAA. After PCS (as compared to sham-operated animals) a significant hyperexcitability of the reticular brain-stem formation was found during the slow-wave sleep. The liver/body weight ratio was significantly lower. Tryptophan (free tryptophan in the plasma), phenylalanine, tyrosine, and histidine increased in the plasma and the brain. Leucine and isoleucine were decreased in the plasma. After PCS, an initial decrease at 7 days postoperatively of brain norepinephrine and blockade of the intracerebral metabolism of tryptophan were observed. These changes were transient and progressively disappeared at 14 and 28 days postoperatively. Brain histamine remained at a very high level through the experiment. A good correlation was demonstrated between modification of the sleep disturbance and tryptophan (or 5- hydroxyindolacetic acid) and histamine brain levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Respective roles of ammonia, amino acids, and medium-sized molecules in the pathogenesis of experimentally induced acute hepatic encephalopathy.

Ammonia, amino acids (AA), and middle molecules (MM) have been implicated in the pathogenesis of experimentally induced acute hepatic coma in the pig. Hemodialysis (HD) using either a low- (Cuprophan = CU) or a high-permeability (polyacrylonitrile = AN 69) membrane has demonstrated the role of MM. Selective hemodialysis (SHD) of AA or NH3 and MM was performed by adding either NH3 (group I) or AA (group II) to the dialysate during AN 69 HD; for MM, SHD only was performed by adding NH3 and AA to the dialysate (group III). In group I the brain levels of tyrosine were similar to those in undialyzed animals with decreased striatal dopamine and decreased norepinephrine in the midbrain only. Brain tryptophan was higher than normal, but brain levels of 5-hydroxytryptamine and 5-hydroxyindoleacetic acid (5-HT, 5-HIAA) were within normal limits. In group II, despite an efficient NH3 clearance, brain NH3 levels were as high as in group I and did not correlate with plasma levels. Brain tyrosine (despite tyrosine overload of the dialysate) was lower than in group I; striatal dopamine decreased (but to a lesser extent than in group I), and norepinephrine was normal. Brain tryptophan was higher than normal, with an increase in brain 5-HT and 5-HIAA. In group III, results were similar to group I, except for a limited increase of 5-HT in the pons. Brain octopamine levels increased only in undialyzed and CU-HD animals, demonstrating a specific relation with MM. These experiments demonstrate the interrelationship between NH3 and neutral AA with regard to passage through the blood-brain barrier and to intracerebral metabolism.

Noninvasive measurement of nutrient portal blood shunting: an experimental study with [14C]ursodeoxycholic acid.

All of the methods proposed for measuring portal blood flow are either invasive, estimate total rather than nutrient flow, and none has proved reliable in cirrhotic patients. A method has been derived from pharmacokinetic principles used for the calculation of bioavailability of drugs according to the route of administration (i.v. or p.o.) and tested experimentally in 20 pigs. A tracer dose of [14C]ursodeoxycholic acid, a biliary acid with a high-liver first-pass effect, is administered in the duodenum, and serial peripheral blood samples are taken. Later, the same dose of the same drug is administered i.v. The shunt fraction of portal blood F is obtained by the ratio of the areas under the plasma level vs. time curves ("AUC") after p.o. and i.v. administrations: (see formula in text). The pigs were divided into three experimental groups. (i) Group I: undisturbed portal flow; (ii) Group II: total diversion of portal blood with an end-to-side portacaval shunt, and (iii) Group III: partial diversion of portal blood through a side-to-side portacaval shunt. Portal flow was measured during surgery with an electromagnetic flowmeter above and below the shunt and the degree of shunting calculated. Results show that the shunt fraction measured with ursodeoxycholic acid is well-correlated with hemodynamic data. No overlap between Groups I and III is observed. It is concluded that the shunt fraction of nutrient portal blood can be measured with this noninvasive method. Minute amounts of ursodeoxycholic acid were used in order to be completely metabolized by the liver, even in spite of hepatocellular dysfunction. Therefore, this method should be valid in cirrhotic patients and be useful to decide the type of portasystemic shunt to propose for the decompression of gastroesophageal varices.

Early changes in blood-brain barrier permeability after porto-caval shunt and liver ischaemia.

The brain oedema, distribution space (DS) and brain uptake index (BUI), of L-glucose, inulin, B12 vitamin and of three polypeptidic hormones of increasing molecular weight (angiotensin-I, gastrin and insulin) were measured in the rat after sham operation, porto-caval shunt (PCS) or liver ischaemia. At an early stage following PCS or liver ischaemia brain oedema was not constant, and was only demonstrable after liver ischaemia in a large number of animals. Substances without an active transport and with a low diffusion coefficient such as L-glucose and inulin had a very low BUI, unchanged even if the 3H2O brain content or the DS were modified. B12 vitamin, DS and BUI were very high and did not change after liver ischaemia or PCS. Insulin DS and BUI were low in the three groups of animals, whereas it decreased after PCS for gastrin. A significant increase of BUI and DS (without any cerebral oedema) was demonstrated for angiotensin-I, a polypeptidic hormone of molecular weight 1300. This polypeptidic marker is in the same range of MW as the preliminary recently recognized medium-sized molecules which may be involved in the pathogenesis of encephalopathy during experimental acute liver failure. However, not only the MW, but the nature of such polypeptides may be of importance in the genesis of this limited impairment of BBB permeability.

Relationship between plasma and brain tryptophan in pigs during experimental hepatic coma before and after hemodialysis with selective membranes.

Experimental acute liver ischemia in pigs induces an increment in plasma free tryptophan with decreased total tryptophan. Brain tryptophan is elevated in all brain areas. A slight, but significant increase of brain serotonin is demonstrated in the striatum only, while 5-HIAA (5-hydroxyindoleacetic acid) is significantly lower in the hypothalamus. Other brain areas do not show significant changes in serotonin and 5-HIAA levels. Neither the high plasma free tryptophan levels, nor the decreased sum of neutral competitive amino acids are consistent with such an elevation of brain tryptophan. Hemodialysis was carried out with two different kinds of membranes: cuprophan (with an efficient removal of molecules up to molecular weight 1300) and AN 69 polyacrylonitrile (efficient removal up to 15,000). Ammonia and aminoacid clearance are similar for both membranes. After AN 69, plasmatic free tryptophan and brain tryptophan are lower than after liver devascularization, but still higher than normal. Serotonin significantly increases in the cortex, midbrain and hypothalamus without concomitant rise of 5-HIAA levels. After cuprophan hemodialysis, plasma total tryptophan is lower than in normal and even comatose animals, whereas free tryptophan is normal. Intracerebral tryptophan is similar to AN 69 dialysed animals, but in the hypothalamus it is similar to nondialysed animals. Brain serotonin levels are not modified. 5-HIAA decreases in the hypothalamus. This finding suggests that middle molecules (which are not cleared out with cuprophan hemodialysis) are involved in the intracerebral transfer of tryptophan and the metabolism of serotonin, mainly in the hypothalamus.

An experimental study of survival after two hours of normothermic hepatic ischemia.

Total hepatic inflow occlusion is well tolerated in pigs with normothermia for as long as two hours, provided that splanchnic venous pooling is avoided by active pumping through a splenojugular bypass. Hepatic dysfunction after 60, 90 and, even, 120 minutes of hepatic ischemia is mild and transient. Complete return to normal liver function tests is rapid. Early microscopic alterations of the liver are moderate, and no late abnormalities, such as cirrhosis or vascular changes, were observed one to three months later. Conversely, interruption of hepatic blood flow for three hours is not compatible with life. In this study, a previously unsuspected resistance of the pig liver to warm ischemia is demonstrated. These findings corroborate and extend those of recent clinical studies in which a similar tolerance of the human liver to prolonged normotherthermic ischemia is reported, thus questioning the necessity for deliberate hypothermia in operations involving the liver.