Independent regulation of chylomicron lipolysis and particle removal rates: Effects of insulin and thyroid hormones on the metabolism of artificial chylomicrons

The processes of chylomicron lipolysis and removal from plasma were investigated by the intra-arterial infusion of doubly labeled artificial chylomicrons in rats. The rate of lipolysis was measured as a delipidation index (DI), which is the glyceryl-tri 9,10(N)-³H oleate (³H-TO) fraction removed from the particle as fatty acids, whereas the cholesteryl(1-¹⁴C) oleate (¹⁴C-CO) plasma disappearance rate measures the splanchnic organ particle uptake. In the alloxan-diabetic rats, despite a normal DI, the ¹⁴C-CO plasma residence time (RT) was longer than in control animals and remained longer after stimulation of the lipoprotein lipase by heparin. DI and ¹⁴C-CO removal rate were not significantly altered by insulin administration to glucose-supplemented control rats. Lipolysis was remarkable in propylthiouracil (PTU)-induced hypothyroidism, and yet the ¹⁴C-CO removal rate was retarded. In hypothyroidism, heparin enhanced the ¹⁴C-CO removal more than in the control group; however, after heparin, the ¹⁴C-CO RT still remained higher in the hypotyroid animals as compared with the control group. Hyperthyroidism lowered the DI; nevertheless, the ¹⁴C-CO disappearance rate was faster than in controls. In summary, lack or excess of thyroid hormone influences both the chylomicron lipolysis and removal systems, whereas lack of insulin impairs mostly the particle removal from plasma, and excess of insulin has no effect on the chylomicron metabolism.