Follicular fluid lipoproteins in the mare: evaluation of HDL transfer from plasma to follicular fluid

Abstract

Using a density gradient ultracentrifugal procedure, we have separated equine plasma and follicular fluid high-density lipoproteins (HDL). The density distribution of the follicular fluid HDL was clearly displaced towards the highest densities in comparison with that of plasma HDL. Similarly, an analysis of size distributions showed a decrease in follicular fluid HDL diameters (4.2 to 9.2 nm) compared to plasma HDL (5.5 to 9.5 nm). HDL were isolated into three subfractions on the basis of the disposition of the Sudan Black stained bands in the centrifuge tubes. Concentrations of each subfraction were clearly lower in the follicular fluid, and the relative percentages with regard to the plasma equivalents were inversely proportional to the molecular weights (23.8% for HDL-1, 49.9% for HDL-2 and 63.7% for HDL-3). The cholesterol/phospholipid molar ratio and the esterified/free cholesterol molar ratio were clearly increased in the follicular HDL-2 and HDL-3 subfractions. The apolipoprotein distribution in follicular fluid HDL was very close to that in plasma HDL. LCAT activity measured in human as well as equine samples was weaker in follicular fluid compared to plasma in both species (4.0 nmol of free cholesterol esterified per h per ml vs. 24 nmol per h per ml). Theoretical concentrations of follicular fluid HDL were calculated assuming that the HDL particles would be merely a filtration product undergoing no detectable metabolic modifications. Biochemical measurements showed that the lightest particles (HDL-1) were less numerous than suggested by the theoretical calculation. Thus, although follicular fluid HDL appear to be a filtration product of plasma HDL, they undergo metabolic transformations that we suggest may be linked to hormonal synthesis and reverse cholesterol transport.