Secretion of triglyceride lipase from rat hepatocytes in culture: modulation by insulin and phenobarbital

Abstract

Hepatic triglyceride lipase (HTGL) was measured in primary rat hepatocytes maintained for 3 days under three different culture conditions: basal medium, basal medium plus insulin, and basal medium plus insulin and phenobarbital. The activity of HTGL secreted by these cells was measured by treating intact cells with heparin; intracellular enzyme was subsequently measured in cell homogenates. Insulin stimulated intracellular triglyceride lipase activity by 48% and extracellular lipase by 30%. Phenobarbital, an enzyme-inducing drug, caused a further 15% increase in extracellular hepatic triglyceride lipase; whereas, the intracellular activity was reduced. The presence of insulin greatly stimulated the rate of enzyme secretion, and this rate was not notably affected by the presence of phenobarbital. After 3 days in culture, the short term (2–8 h) synthesis and secretion of enzyme from cultures treated with insulin or insulin plus phenobarbital were equally inhibited by cycloheximide. Monensin also inhibited enzyme secretion in both cultures and caused a similar increase in intracellular lipase activities. Insulin did not significantly affect the proportion of intracellular enzyme (17.7% basal vs. 15.8% insulin). On the other hand phenobarbital produced a 20–30% reduction in the proportion of intracellular enzyme (12.5 vs. 17.7% basal or 15.8% insulin). These findings suggest a drug-induced redistribution of triglyceride lipase.