抄録
To understand the role of hepatic and extrahepatic triglyceride lipase (TGL) in the metabolism of lipoproteins, we have reported the regulation of hepatic triglyceride lipase by insulin. The purpose of the present study was to determine an assay method of triglyceride lipase activity in rat heart muscle and to investigate the changes in plasma lipoproteins and triglyceride lipase activity in heart muscle of streptozotocin (STZ) induced diabetic rats. Three different methods of determining triglyceride lipase activity in rat heart muscle were studied utilizing the following preparations; 1) acetone-ether powder of heart muscle, 2) heart muscle slices, 3) heart muscle homogenates. Acetone-ether powder of rat heart muscle was prepared according to the method of Tan M. H. et al utilizing 0.05M NH4OH-NH4Cl buffer, pH 8.1. Release of TGL by heparin from 100mg of rat heart muscle slice was performed in 2.0ml of Krebs-Henseleit bicarbonate buffer, pH 7.4 with 2.1M glycine, 1.5% bovine serum albumin and heparin 50U/ml under 95% O2/5% CO2 at 37°C, based on the method of Lithell H. et al. Rat heart muscle, 200mg, was homogenized in either 4.0ml of 2.1M glycine buffer, pH 8.3, or 0.078M Tris-HCL buffer, pH 7.4 or 0.05M NH4OH-NH4Cl buffer, pH 8.1. Triglyceride lipase activity per mg protein of heart muscle was the highest in heart muscle homogenate utilizing 2.1 M glycine buffer, pH 8.3 among the assays investigated. TGL activity in rat heart muscle was activated by rat serum and heparin and inhibited by higher concentration of NaCl, 2.0M of which inhibited completely this activity, as is characteristic of lipoprotein lipase. Twelve-hour fasting increased heart muscle TGL activity from 2.53±0.77 to 4.09±1.28μmol FFA/hour/mg protein. However, heart muscle TGL activities in 48 hour and 72 hour-fasted rats (P<0.05) were lower than those in fed rats. TGL activities in heart muscle homogenates in diabetic rats either 3 days or 4 weeks after STZ injection, were decreased significantly compared with those of control rats. A significant negative correlation between heart muscle TGL activities and plasma triglyceride levels was observed in rats 4 weeks after STZ injection. Cholesterol, triglyceride (TG) and phospholipid in plasma, VLDL (d<1.006g/ml), LDL (1.006<d<1.063g/ml) and HDL (d>1.063g/ml) obtained from rats 4 weeks after STZ injection were studied. Cholesterol and phospholipid in plasma, VLDL and HDL of STZ diabetic rats were significantly higher than those of control rats. Plasma TG and VLDL-TG were extremely higher in diabetic rats than in control rats. Our data indicate the regulation of triglyceride lipase activities in rat heart muscle by insulin and a significant role of this enzyme in pathogenesis of hyperlipoproteinemia of diabetic rats.
