The present studies were conducted to evaluate whether apoptosis occurs during spontaneous and prostaglandin F2α (PG F2α)-induced lu-teolysis and, if so, to determine the relationship between the onset of luteolysis and oligonucleosome formation (a characteristic of apoptosis). In the first study, nine normally cycling heifers were ovariecto-mized (ovx) during the midluteal phase (day 10 or 15; day 0 = estrus) or after luteal regression (day 19; n = 3/time point). While there was no evidence of oligonucleosome formation in DNA from corpora lutea (CL) collected on days 10 and 15, each CL collected on day 19 exhibited DNA fragmentation, represented by distinct bands of DNA in approximately 185-basepair multiples. In the second study, heifers were ovx (n = 5; controls) or given 25 mg PG F2α 15-16 days after estrus. Heifers receiving PG F2α were subsequently ovx 4, 8, 12, 24, or 48 h (n = 5/time point) after the injection of PG F2α. The concentration of progesterone in venous sera collected at ovx was not different (P > 0.20) in control and 4 h groups, but was decreased (P < 0.01) in the 8, 12, 24, and 48 h groups. Total CL weight (mean ± SEM; grams) did not change (P > 0.10) from 0 h (controls) to 24 h after injection (range, 3.2 ± 0.5 to 4.1 ± 0.6), but decreased (P < 0.06) to 2.0 ± 0.3 at 48 h. With ethidium bromide (EtBr) staining, no oligonucleosome formation was detected in CL collected from 0-12 h after PG F2α injection. However, pronounced oligonucleosome formation was observed in all 10 CL collected 24 and 48 h after the injection of PG F2α. The absence of oligonucleo-somes in 0 and 4 h samples was confirmed by the more sensitive technique of 3'-end labeling of DNA fragments. Some samples in both the 8 and 12 h groups had slight oligonucleosome formation, while all samples in the 24 and 48 h groups showed evidence of intense oligonucleosome formation. Histological analysis of tissue sections indicated an increase (P < 0.001) in the percentage of degenerated luteal cells in the 24 and 48 h groups compared to that in the 0-12 h groups. These data indicate that apoptosis occurs during both spontaneous and PG F2α (-induced luteal regression in cattle; however, apoptosis, as indicated by oligonucleosome formation, is not apparent until after serum progesterone concentrations have begun to decrease. While the present results argue against apoptosis being a proximal cause of functional luteal regression, the relationship between the onset of oligonucleosome formation and the decline in CL weight suggests a possible role for apoptosis in structural regression of the CL.
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