Abstract
Triacylglycerol lipase activities of homogenates and subcellular fractions of rat liver were measured under optimal conditions at pH 7.5 using emulsified tri[1-14C]oleoylglycerol as substrate. Twenty-four hr after administration of streptozotocin, hepatic alkaline lipase activity was 39% of normal, and this lower level of activity was observed at 72 hr and 7 days, after streptozotocin injection. After 24 hr of starvation, lipase activity also was significantly lower (35%) than normal. Insulin (35 U regular/kg body weight) had no acute (90 min) effect on the hepatic lipase activity of either normal or diabetic rats. Chronic insulin administration (4 subcutaneous injections of 10 U protamine zinc insulin/kg at 16-hr intervals) to normal rats provoked a 40% increase in hepatic lipase activity. Diabetic rats given the same insulin treatment showed lipase activity that was significantly higher (155%) than normal. Lipase activity fell to 65% of normal when insulin was withheld (32 hr) from diabetic rats given chronic insulin therapy. Intracardial injection of glucagon (1 mg/kg) into normal rats had no acute (30 min) effect on hepatic alkaline lipase activity. Hepatic alkaline lipase activity varied independently from the concentrations of either glucose or triacylglycerol in the plasma. However, there was an apparent negative correlation between this lipase activity and the concentration of fatty acids in the plasma; lipase activity was highest when fatty acid concentrations were lowest, and lowest when fatty acid concentrations were elevated. From these data we conclude: 1) changes in hepatic alkaline lipase activity ware provoked by chronic, but not acute, alteration of the hormonal and metabolic status of the rat, and 2) changes in hepatic alkaline lipase activity may be mediated through changes in the levels of circulating fatty acids presented to the liver, but the effect is not an immediate one.
Highlights
Triacylglycerol lipase activities of homogenates and subcellular fractions of rat liver were measured under optimal conditions at p H 7.5 using emulsified tri[l-'4C]oleoylglycerol as substrate
Based on the identity of hepatic alkaline triacylglycerol lipase with the post-heparin plasma activity of palmitoyl coenzyme A hydrolase (EC 3.1.2.2), which was significantly reduced after induction of diabetes, these authors concluded that hepatic triacylglycerol lipase activity was significantly reduced in the diabetic state
Nakai et al [2] measured the alkaline lipase activity of post-heparin plasma using emulsified ['4C]trioleoylglycerol as substrate. These authors distinguished heparin-released hepatic lipase from extrahepatic lipoprotein lipase by adding 1.0 M NaCl and deleting serum from the in vitro assays. They found that, whereas the total post-heparin lipolytic activity was significantly increased after induction of diabetes in rats by streptozotocin, hepatic triacylglycerol lipase activity in post-heparin plasma was significantly decreased from control
Summary
Triacylglycerol lipase activities of homogenates and subcellular fractions of rat liver were measured under optimal conditions at p H 7.5 using emulsified tri[l-'4C]oleoylglycerol as substrate. T o determine the effects of insulin, groups of normal and diabetic rats were given insulin and hepatic alkaline lipase activity was measured at various times thereafter.
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