Influence of components of oral contraceptive on lipid metabolism.

Oral contraceptives (OC) have been shown to enhance the risk of atherosclerosis. In the present study we sought to determine which component of the OC (containing 0.067 mg estrogen and 0.667 mg of progestin) counts for alteration in lipids profile. Female rats were administered with 0.067 mg of 17 beta-estradiol and 0.667 mg of norethindron acetate/kg body weight. Estrogen treatment exhibited higher levels of lipids in the serum and tissues. LDL-cholesterol increased by three folds but HDL-cholesterol decreased significantly, while progestin group showed decreased levels of lipids and LDL cholesterol. Elevated hepatic cholesterogenesis was observed as indicated by increased activity of HMG-CoA reductase and elevated incorporation of labelled acetate into liver cholesterol in estrogen group. On the other hand, progestin treatment did not alter the activity of HMG-CoA reductase and the rate of incorporation of labelled acetate into hepatic cholesterol. Hepatic degradation of cholesterol to bile acids however, decreased with estrogen treatment. No considerable changes were observed in hepatic bile acid levels in progestin group. Release of lipoprotein into circulation increased but their clearance from the circulation decreased as revealed by the activity of lipoprotein lipase (LPL) of extrahepatic tissues in estrogen group. With progestin treatment, activity of LPL increased significantly in adipose tissue. Activity of hepatic malic enzyme and glucose 6-phosphate dehydrogenase enhanced considerably in estrogen group, while activities of these enzymes were depressed with progestin administration. Thus results indicate that estrogen component of oral pills counts for major changes in lipid and lipoprotein metabolism favouring the development of atherosclerosis.

Role of oral contraceptive in atherosclerosis.

The possibility that risk of a atherosclerosis complication increases with oral contraceptive use was examined by studying the effect of oral pill containing 0.067 mg menstranol and 0.667 mg ehtynodiol diacetate/kg body weight on the metabolism of lipids in female rats fed a hypercholesterolemic diet for three months. Experimental group clearly exhibited higher levels of triglycerides and cholesterol in plasma and tissues, increase in aorta observed to be two folds. Increased hepatic cholesterogenesis was noted with treatment of oral contraceptive as indicated by higher activity of HMG-CoA reductase. Activity of lipoprotein lipase of extrahepatic tissue was depressed in experimental group. Activity of plasma LCAT, an enzyme involved in the transport of cholesterol from tissues, was also lower with treatment of oral contraceptive. However, activity of malic enzyme and glucose-6-phosphate dehydrogenase enhanced considerably with administration of oral pill. The increase in plasma and aortic cholesterol levels, increase in LDL+VLDL cholesterol and considerable decrease in HDL cholesterol in animals treated with oral contraceptives and fed with atherogenic diet, indicates that prolonged administration of oral pill may predispose towards atherosclerosis.

Mechanism of hyperlipidemia produced by estrogen-progestin formulation.

Rats administered estrogen-progestin formulation (0.667 mg of synthetic progestin and 0.067 mg of synthetic estrogen/kg body wt) showed increased hepatic cholesterogenesis, as evidenced by an increased activity of HMG-CoA reductase and increased incorporation of labelled acetate into liver cholesterol. Hepatic degradation of cholesterol to bile acids, however, was decreased. There was increased release of lipoproteins into the circulation but their clearance from the circulation was lower as revealed by a decreased activity of lipoprotein lipase of the extrahepatic tissues. Activity of plasma LCAT, which is involved in the transport of cholesterol from the tissues, was also decreased. The increase in serum and aortic cholesterol levels, increase in LDL cholesterol and decrease in HDL cholesterol in rats administered estrogen-progestin formulation suggest that prolonged administration of this formulation may predispose towards atherosclerosis.