Decline in Fluorescent Low Density Lipoprotein (LDL) Uptake by Small and Large Porcine Luteal Cells with Advancing Age of the Corpus Luteum1

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Abstract
The present study was designed to test the hypothesis that the ability of small luteal cells (SLC) and/or large luteal cells (LLC) to take up low density lipoprotein (LDL) declines with advancing age of the CL. Ovaries from 100–110-kg gilts were classified as early (Days 4–6; n = 5), mid (Days 8–12; n = 6)-, or late (Days 15–18; n = 5) cycle on the basis of gross morphology. Multiple CL from each ovary were pooled and enzymatically dissociated. An aliquot of dispersed luteal cells was reserved for cell culture. Remaining cells were incubated (~ 4 × 105 cells/0.25 ml Dulbecco’s Modified Eagle’s Medium [DMEM] + 0.1% BSA) for 20 min at 37°C with human LDL (10 μg/ml) tagged with the fluorescent probe, Dil (Dil-LDL). Washed and fixed cells were then isolated by flow cytometry into SLC and LLC subpopulations on the basis of forward and 90° light scatter. Cellular fluorescence was analyzed within each subpopulation. The percentage of fluorescent, i.e., Dil-LDL-positive (+), SLC did not differ between early (29.8 ± 5.9%) and mid (40.5 ± 6.8%)-cycle, but declined (p < 0.01) in late CL (7.0 ± 1.6%). Similarly, the percentage of Dil-LDL-(+) LLC was unchanged between early (80.5 ± 2.0%) and mid (78.6 ± 4.2%)-cycle, but diminished (p < 0.01) in late (40.2 ± 1.9%) CL. Moreover, the percentage of total cells isolated in the LLC subpopulation declined dramatically (p < 0.01) between mid (8.0 ± 0.9%)- and late (1.6 ± 0.2%) cycle, but the percentage of SLC did not change. Unsorted dispersed cells (4 × 104/0.2 ml) were cultured in DMEM/F-12 in the presence or absence of hLDL (50 μg/ml). LDL increased (p < 0.05) progesterone (P) production by early and mid-luteal cell cultures but failed to enhance P secretion in late cultures. The data indicate that LDL uptake by SLC and LLC diminishes with advancing age of the CL. Reduced ability of luteal cells to bind and/or internalize LDL-associated cholesterol may contribute to declining P production in regressive CL.

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