Influence of low estrogen-containing oral contraceptives on lipoprotein phospholipid composition and mononuclear cell membrane fluidity

Since the effects of the new low estrogen-containing oral contraceptive (OC) preparations on lipoprotein phospholipid (PL) composition are unknown, we studied 3 groups of 10 young women before and after 6 months of use of 3 commonly prescribed agents containing almost identical amounts of ethinyl estradiol (0.30-0.35 microgram) and differing progestogens, and correlated these changes with their estrogen to progestin (E/P) ratio. The directional changes in both plasma neutral lipid and PL concentrations tended to correlate with the E/P ratio, with plasma HDL-cholesterol (HDL-C) falling slightly and the low density lipoprotein-cholesterol (LDL-C)/HDL-C ratio increasing in the women taking the OC with the lowest E/P ratio; in contrast, plasma HDL-C increased and the LDL-C/HDL-C ratio fell in those receiving the preparation with the highest E/P ratio. In HDL, the ratio of the 2 principal PL, sphingomyelin and lecithin, an index of lipid fluidity, tended to increase, suggesting that the surface of this lipoprotein class had become more rigid. This change was most apparent in women receiving the agent in which the progestin was predominant; in women receiving the preparations with the higher E/P ratios the sphingomyelin/lecithin ratio actually declined. The membrane fluidity of mononuclear cells obtained from five women taking an OC with a relatively high E/P ratio, however, was significantly increased (P less than 0.007) compared to that in normal women. These findings demonstrate that, even with substantial reductions in their estrogen content, the use of these newer OC is associated with quantitative and qualitative changes in lipoprotein PL composition that parallel their E/P balance and are associated with altered fluidity of mononuclear cell membranes.