Effects of insulin on the disposal of 14C-labelled very low density lipoprotein triglycerides in intact and hepatectomized rats

Abstract

Summary. In sham-operated rats, intravenous administration of 14C-very low density lipoprotein triglycerides (with labelled esterified fatty acids) caused an initial decrease and subsequent increase in plasma 14C-lipids of both very low density lipoproteins (VLDL) (density < 1.006) and lipoproteins of density > 1.019. There was a similar change in 14C-lipids in adipose tissue and heart whereas in kidney, spleen and liver, 14C-lipids increased initially and then decreased. Insulin treatment in sham-operated animals decreased circulating 14C-lipids in VLDL and in lipoproteins of density > 1.019, while intermediate density (1.006-1.019) lipoproteins increased. Insulin also enhanced the radioactivity retained in spleen. In functionally hepatectomized rats, 14C-lipids progressively increased in heart. Insulin treatment in these rats enhanced the disappearance from circulation of ~4C-VLDL and of lipoproteins of density > 1.019, as well as the appearance of 14C-intermediate density lipoproteins. The appearance of 14C-lipids in white adipose tissue also was augmented, while it decreased in heart and lung. Thus, in sham-operated animals, insulin apparently stimulates the uptake of products of VLDL metabolism by cells in the reticuloendothelial system, while in functionally hepatectomized rats there is increased heart utilization of VLDL triglycerides, and insulin enhances the net extrahepatic catabolism of these lipoproteins.Key words: Plasma lipoproteins, rat, insulin, spleen, hepatectomy.Insulin administration is known to reduce the elevated levels of circulating triglycerides and very low density lipoprotein (VLDL)-triglycerides in diabetic subjects [1] and it is well established that insulin enhances the deposition of triacyglycerol fatty acids in adipose tissue by increasing its lipoprotein lipase activity [2][3][4]. There are, however, situations in which insulin excess is associated with augmented circulating triglyceride concentrations [5][6][7], and the effects of insulin on lipoprotein lipase activity in other extrahepatic tissues do not follow the changes found in the adipose tissue enzyme [8,9]. As insulin also stimulates the secretion of newly synthesized VLDL-triglycerides by liver preparations [10,11], changes in circulating lipoproteins 'in vivo' produced by insulin are the result of a combination of hepatic and extrahepatic effects. In the present study, disassociation of these effects was achieved by determining the changes produced by insulin on the disposal of prelabelled VLDL-triglycerides administered to functionally hepatectomized rats and to sham-operated controls, to determine the effects of the hormone 'in vivo' on extrahepatic VLDL-triglyceride catabolism. Materials and MethodsFemale Wistar rats weighing 160-190 g and fed standard rat chow were maintained in a temperature (22_+ 2 ~ and light cycle (light from 9.00 to 21.00 h) controlled room. Animals were subjected to functional hepatectomy under sodium pentobarbital anaesthesia (40 mg/kg, IP) between 10.00 and 12.00 h, following the method of ...

The role of triacylglycerol in cardiac energy provision

“…Rat heart assimilation of VLDL-TAG was comparable to adipose tissue and skeletal muscle, and was not significantly affected by insulin, but hearts of functionally hepatectomised rats assimilated VLDL-TAG more rapidly [48].…”