Oxytocin secretion by cells from bovine corpora lutea destined to be normal or short-lived
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The working hypothesis for this study was that oxytocin (OT) production is greater in bovine luteal cells destined to be short-lived compared with cells from corpora lutea (CL) of a normal estrous cycle, and that OT secretion is dependent on the lipoprotein environment. Cultures of luteal cells were established from CL obtained on Day 4 or 10 of a normal estrous cycle (Day 0, day of behavioral estrus), and from CL destined to be short-lived, 4 days after ovulation induction. Mixed luteal cells and enriched cultures of large luteal cells (LLC) were incubated in control medium or medium containing (1) low-density lipoproteins (LDL), (2) high-density lipoproteins (HDL), or (3) both LDL and HDL. Addition of lipoproteins had no effect on OT secretion, regardless of stage of estrous cycle or cell type. Mixed luteal cell cultures obtained from Day 4 short-lived and Day 4 normal CL secreted similar quantities of OT, and both secreted less (P < 0.01) OT than mixed luteal cells from Day 10 normal CL. At 48 h in culture, Day 10 mixed luteal cells secreted more (P < 0.01) OT than Day 4 mixed cells from both short-lived and normal CL. Similarly, enriched cultures of LLC obtained from Day 4 normal CL secreted less (P < 0.01) OT than enriched cultures of LLC from Day 10 normal CL. Accumulation of OT at 48 h was greater (P < 0.01) from Day 10 than from Day 4 enriched cultures of LLC; however, Day 4 large cells secreted more OT late in the culture period (day X time, P < 0.001). The similar secretion profiles exhibited by mixed luteal cells obtained from short-lived and normal CL provide evidence that increased basal oxytocin secretion in vitro is not associated with CL destined to be short-lived; however, an increased capacity to secrete oxytocin by Day 4 enriched cultures of LLC late in the culture period suggests involvement of OT in the luteinization process.