Hepatic extraction of hyaluronic acid in porcine peritonitis

Decreased lung hyaluronan in a model of ARDS in the rat: effect of an inhibitor of leukocyte elastase

BACKGROUND

Hyaluronan (HA) is a component of the extracellular matrix in lung tissue and is normally present at low concentrations in blood. HA is rapidly cleared from blood by the liver. Increased concentrations of plasma HA have been found in patients with acute respiratory distress syndrome (ARDS). We investigated changes in HA levels in plasma, bronchoalveolar lavage fluid (BALF), and lung, and their relationship to pretreatment with a leukocyte elastase inhibitor in a rat model of ARDS.

METHODS

Rats were randomly assigned to three groups: control, thrombin, and thrombin plus elastase inhibitor. By use of a radiometric assay, HA was measured in lungs, BALF, and plasma. Tissue samples from the lungs were stained for HA and examined microscopically. Liver circulation and cardiac output were monitored using radiolabeled microspheres.

RESULTS

Infusion of thrombin produced a pronounced increase in wet weight to dry weight ratio, and relative lung water content. This increase was blunted by a leukocyte elastase inhibitor. A decrease in lung HA and increases in both BALF and plasma HA were found. The leukocyte elastase inhibitor counteracted not only the decrease in lung tissue HA, but also the increase in plasma HA. Histologically, there was decreased HA-staining of peribronchial and perivascular areas in the injured rat lung. Decreased liver perfusion was observed after infusion of thrombin.

CONCLUSIONS

The decrease in lung HA may be involved in the development of pulmonary edema in this ARDS model, and leukocyte elastase may be one cause of this decrease. In addition, an elevated plasma HA level may be an indicator of lung injury.

Liver endothelial cell function is depressed only during hypodynamic sepsis

Although studies have indicated that hepatocellular function is depressed early after the onset of sepsis, it remains unknown whether liver endothelial cell function is also compromised under such conditions. To study this, male rats were subjected to polymicrobial sepsis by cecal ligation and puncture (CLP), followed by administration of 3 ml/100 g body wt normal saline subcutaneously to these and to sham-operated animals. Blood samples (0.2-ml aliquots) were taken from the carotid artery, portal vein, and hepatic vein at 2, 5, 10 (i.e., hyperdynamic sepsis), or 20 hr (hypodynamic sepsis) after CLP, and plasma hyaluronic acid (HA) was determined using a Pharmacia assay kit. In addition, HA clearance was assessed at 5, 10, or 20 hr after CLP by injecting 30 micrograms/100 g body wt HA intravenously. Plasma HA was determined at 2-40 min after the administration of HA. The results indicate that plasma levels of HA in blood from three different sites did not increase significantly until 10 hr after CLP. Clearance of HA decreased only at 20 hr after CLP, compared to sham-operated animals. These results suggest that the increased plasma levels of HA at 10 hr after the onset of sepsis are solely due to the increased release/production of the polysaccharide. Since circulating HA is cleared exclusively by the liver endothelial cell, the results demonstrate that liver endothelial cell dysfunction (i.e., the increased circulating HA levels and decreased HA clearance) occurs only during the late, hypodynamic stage of polymicrobial sepsis.

Serum hyaluronan as a disease marker

Hyaluronan is a connective tissue polysaccharide which has also been found in blood serum in concentrations < 100 micrograms/L (average 30-40 micrograms/L in middle-aged persons). The serum level is regulated by the influx of the polysaccharide from the tissues via lymph and its receptor-mediated clearance by liver endothelial cells. Markedly high serum levels are noted in certain liver diseases, especially in patients with cirrhosis, when the clearance is impaired. In these cases serum hyaluronan can be used to follow the development of the disease. Serum hyaluronan is also a sensitive marker for impending rejection of liver transplants. Patients with rheumatoid arthritis constitute another major group with increased serum hyaluronan, but in this case the level varies markedly during the day corresponding to physical activity. There are good indications that in these subjects the excess hyaluronan comes from the joints. Under stringent sampling conditions of serum it should be possible to extract interesting information on the inflammatory joint process. Increased hyaluronan levels are also seen in other inflammatory diseases and it is of special interest that high hyaluronan levels in patients with septic conditions is a sign of poor prognosis. Certain tumours, notably Wilms' tumour and mesothelioma, produce factors which activate synthesis of hyaluronan and increase its serum level. Rare hereditary diseases with disturbances of hyaluronan metabolism and elevated blood levels have also been discovered, e.g. Werner's syndrome and cutaneous hyaluronanosis. Information accumulated during the last decade regarding the metabolism of hyaluronan has made this polysaccharide an interesting clinical marker for a number of pathological conditions.