Endothelin-1 (ET-1) a vasoactive peptide, is synthesized and secreted by endothelial cells. In the bovine corpus luteum (CL), endothelial cells constitute a major proportion (53.5%) of total CL cells. This study was designed to examine the effects of ET-1 on bovine luteal cell functions and its involvement in the action of PGF2alpha. To better define the cells implicated in this process, we used CL slices, whole CL-derived cells, and steroidogenic large (LLC) and small (SLC) luteal-like cells. High affinity binding sites for ET-1 (K(d), approximately 0.3 x 10(-9)) were present in both steroidogenic luteal cells. The binding affinity of ET-1 was 3 orders of magnitude higher than that of ET-3, and a selective ETA receptor antagonist (BQ123) competed similarly to ET-1, suggesting the presence of ETA receptors. The lack of effect of ET-3 on CL-derived cells further supported this conclusion. Both basal progesterone secretion and bovine LH (5 ng/ml)-stimulated progesterone secretion from CL-derived cells were significantly inhibited by ET-1 in a dose-dependent manner, whereas preincubation of these cells with ETA receptor antagonist prevented the inhibitory effect of added ET-1. Incubation of LLC with 10(-8) M ET-1 inhibited their progesterone secretion (114.8 vs. 176.7 ng/10(5) cells-20 h; P < 0.05). On the other hand, ET-1 did not affect progesterone production from SLC despite the presence of ET-binding sites. PGF2alpha only inhibited LH-stimulated progesterone secretion by luteal slices. This antisteroidogenic effect of PGF2alpha could be prevented by the addition of a selective ETA receptor antagonist. Luteal tissue and microvascular endothelial cells isolated from bovine CL produced ET-1; in contrast, the peptide was undetectable in the culture medium or in cell extracts of either LLC or SLC. These data support the concept that ET-1 may play a paracrine regulatory role in bovine luteal function and propose a novel role for this peptide in mediating PGF2alpha-induced luteal regression.

Prostaglandin F2alpha (PGF2alpha) has been recognized as the physiological luteolysin in ruminants and other species for more than three decades; however, the mechanisms involved in its action are poorly understood. We previously have shown that endothelin-1 (ET-1) mediates, at least in part, the action of PGF2alpha, and the current study examines the effect of PGF2alpha on the expression of ET-1 in bovine corpus luteum (CL). Endothelins (ETs) were extracted from CL, collected at various times of the estrous cycle, and highest levels were found during luteolysis. The expression of prepro-ET-1 was also highest in regressing CL, suggesting that PGF2alpha may have elevated ET-1 expression. This was confirmed by demonstrating that administration of PGF2alpha to heifers at midcycle elevated luteal ET-1 expression. Levels were induced as soon as 2 h after PGF2alpha treatment and 24 h later were 7-fold higher than preinjection levels. Endothelial cells isolated from bovine CL produced ET-1, and addition of PGF2alpha, oxytocin (OT), and vasopressin-augmented ET biosynthesis. Induction of ET-1 expression by PGF2alpha in these cells was evident after a short incubation time (15-90 min). Taken together, these data suggest that stimulation of luteal ET-1 expression by PGF2alpha may be achieved by several nonmutually exclusive mechanisms: 1) by acting directly on luteal endothelial cells; 2) indirectly, via OT release from large luteal cells; and 3) by causing hypoxia in the CL (as a result of ET-1-induced vasoconstriction). The latter mechanism may serve to augment ET-1 secretion in a positive-feedback process.