Abstract
We have examined the genes of the endothelin system that are targets for regulation by prostaglandin F 2α (PGF 2α). The effects of a luteolytic dose of PGF 2α on the mRNA encoding endothelin converting enzyme-1 (ECE-1), pre-pro endothelin-1 (pp ET-1) and the ET receptors ET A, ET B, in bovine corpus luteum (CL) during the early (days 1 and 4), mid (day 10) or late (day 17) luteal phases were examined. The effect of the PGF 2α treatment on ECE-1 protein, Big ET-1 and the biologically active mature ET-1 peptide were also examined. Most importantly, the direct ECE-1 activity was determined. Before day 10 of the cycle, in a PGF 2α-independent manner, the amounts of mRNA encoding ET-1, ECE-1, ET A, and ET B were increased steadily from day 1. After day 10 of the cycle, expression of mRNA encoding pp ET-1 and ET A acquired responsiveness to exogenous PGF 2α and both genes were up-regulated by the PGF 2α treatment. This effect of PGF 2α was also detected for the proteins corresponding to the mature ET-1. The enzymatic activity of ECE-1 remained unchanged throughout the lifespan of the CL in spite of the detected changes in mRNA and protein. The results suggest that the luteal endothelin system is regulated in a PGF 2α-independent and -dependent manner. Importantly, an alteration in luteal ET-1 availability is most likely achieved by modulating the expression of mRNA encoding pp ET-1 and not by the amount or activity of ECE-1. This interpretation is supported by the observation that the activity of ECE-1 remained unchanged throughout the ovarian cycle. The combined effects of greater ET-1 availability and gene expression encoding the ET A receptor in the late luteal phase could render the CL, at this developmental stage, more sensitive or responsive to ET-1. If the luteal tissue is responsive to the available ET-1 during the early phase of the ovarian cycle, an additional role for ET-1 should be considered beyond mediating the luteolytic actions of PGF 2α. Agents blocking the actions of ET-1 might be the best approach to interfere with the luteal ET system and test its physiological role(s) in vivo.
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