The biological significance of lipoprotein lipase modulation by phenobarbital and heparin

High resting triacylglycerol turnover of rainbow trout exceeds the energy requirements of endurance swimming

Fish may use lipoproteins instead of albumin-bound fatty acids to fuel endurance exercise, but lipoprotein kinetics have never been measured in ectotherms. In vivo bolus injections of labeled very-low-density lipoproteins (3H-VLDL labeled in vivo from donor fish) and continuous infusions of Intralipid (3H-labeled artificial emulsion) were used to investigate the effects of prolonged exercise (6 h at 1.5 body length/s) and heparin (600 U/kg) on the turnover rate of circulating triacylglycerol (TAG) in rainbow trout. We hypothesized that swimming would stimulate TAG turnover rate to fuel working muscles and that heparin would reduce flux by releasing lipoprotein lipase (LPL) from endothelial cells. Results from both tracer methods show that the baseline TAG turnover rate of trout ranges from 24 to 49 μmol TAG·kg−1·min−1and exceeds all values measured to date in endotherms. More important, this high resting turnover rate is not stimulated during swimming, because it can already cover several times the energy requirements of locomotion. The fact that heparin causes a 50% decrease in baseline TAG turnover rate suggests that fish LPL must be bound to the endothelium for normal tissue uptake of fatty acids supplied by lipoproteins, as in mammals. We propose that the high resting TAG turnover rate of rainbow trout could be needed by ectotherms for rapid restructuring of membrane phospholipids. The continuous tracer infusion method implemented here could be a versatile tool to investigate the potential role of lipoproteins in providing fatty acids for rapid homeoviscous adaptation.

Optimization of rat hepatocyte culture in citrated human plasma

BACKGROUND

Maintenance of liver-specific functions in hepatocyte cultures during plasma exposure is critical for the clinical application of bioartificial liver assist systems. Sodium citrate is a common anticoagulant but has been shown to be cytotoxic to hepatocytes. We have tested the effect of various supplements on the viability and function of adult primary rat hepatocytes exposed to citrated plasma.

MATERIALS AND METHODS

Freshly isolated rat hepatocytes were cultured in the collagen gel sandwich configuration in culture medium for 6 days followed by exposure to citrated human plasma with various supplements for 1 week. Controls were left in culture medium throughout. Viability and synthetic functions were evaluated.

RESULTS

Hepatocytes exposed to unsupplemented citrated plasma lost significant viability and function within the first 2 days. Cells cultured in plasma supplemented with a fivefold concentrate of standard hepatocyte culture medium maintained urea (1. 2-2.1 micromol/day/10(6) cells) and albumin (51-62 microg/day/10(6) cells) synthesis rates equal to or higher than those of controls. Among the various components of the concentrated medium supplement, calcium chloride (1.8 mM), magnesium sulfate (0.8 mM), amino acids (fourfold Basal Medium Eagle amino acids including 4 mM glutamine), and glucagon (14 ng/ml) were found to be essential in maintaining urea synthesis. Maintenance of a high albumin synthesis rate also required the addition of hydrocortisone (7.5 microg/ml) and insulin (0.5 U/ml).

CONCLUSIONS

Appropriate metabolic and hormonal supplementation of citrated human plasma prevents its cytotoxic effects and may be used in conjunction with in vivo use of bioartificial liver assist systems.