The effect of oral hormone replacement therapy on lipoprotein profile, resistance of LDL to oxidation and LDL particle size.

ABSTRACT

OBJECTIVES: To disclose if oral estradiol (E(2)), alone or in combination with natural progesterone (P) or medroxyprogesterone acetate (MPA), may modify the oxidizability of low density lipoprotein (LDL), and if the effect is achieved at physiological dosages. LDL oxidizability was assessed by the resistance to oxidation by copper and by the particle size profile, since small particles have increased oxidation susceptibility. METHODS: Thirty-three women received two consecutive, two-month length doses of 1 and 2 mg/day of oral E(2). They were then randomly assigned to a fourteen-day treatment of 2 mg/day E(2) plus either 300 mg/day P or 5 mg/day MPA. A parallel group of experiments was performed on a pool of baseline plasma, where hormones were added at the desired concentration. Lipoprotein levels, resistance of LDL to oxidation, and LDL particle diameter, were measured at baseline and after each treatment. RESULTS: Estradiol reduced LDL levels and increased high density lipoprotein (HDL) and triglycerides. P abolished these changes, whereas MPA only reversed the increase of HDL. Estradiol protected LDL from oxidation in a dose-dependent manner, although only at pharmacological concentrations (1 microM or higher). Both P and MPA were inert at either physiological or pharmacological concentrations. The size of the LDL particles remained unaffected except under MPA, in which it was reduced. CONCLUSIONS: Estradiol has a protective effect against LDL oxidation, although only at pharmacological dosages. P and MPA did not limit the E(2) action. The size of the LDL particles remained unaltered after each E(2) dose, but MPA, and not P, was associated with a diminution.