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Developmental Sensitivity of the Bovine Corpus Luteum to Prostaglandin F₂ (PGF₂) and Endothelin-1 (ET-1): Is ET-1 a Mediator of the Luteolytic Actions of PGF₂ or a Tonic Inhibitor of Progesterone Secretion?¹

Department of Biology,³ Eberly College of Arts and Sciences, West Virginia University, Morgantown, West Virginia 26506-6057 Division of Animal and Veterinary Sciences,⁴ College of Agriculture, Forestry and Consumer Sciences, West Virginia University, Morgantown, West Virginia 26506-6057

We examined the responsiveness of large luteal cells (LLC), small luteal cells (SLC), and endothelial cells of the Day 4 and Day 10 bovine corpus luteum (CL) to prostaglandin (PG) F₂ and endothelin (ET)-1. Using a single-cell approach, we tested the ability of each agonist to increase the cytoplasmic concentration of calcium ions ([Ca²⁺]_i) as function of luteal development. All tested concentrations of agonists significantly (P = 0.05) increased [Ca²⁺]_i in all cell populations isolated from Day 4 and Day 10 CL. Day 10 steroidogenic cells were more responsive than Day 4 cells to PGF₂ and ET-1. Response amplitudes and number of responding cells were affected significantly by agonist concentration, luteal development, and cell type. Response amplitudes were greater in LLC than in SLC; responses of maximal amplitude were elicited with lower agonist concentrations in Day 10 cells than in Day 4 cells. Furthermore, on Day 10, as the concentration of PGF₂ increased, larger percentages of SLC responded. Endothelial cells responded maximally, regardless of agonist concentration and luteal development. In experiment 2, we tested the developmental responsiveness of total dispersed and steroidogenic-enriched cells to the inhibitory actions of PGF₂ and ET-1 on basal and LH-stimulated progesterone accumulation. The potency of PGF₂ steroidogenic-enriched cells on Day 4 was lower than on Day 10; in contrast, the potency of ET-1 was not different. Therefore, ET-1 was a tonic inhibitor of progesterone accumulation rather than a mediator of PGF₂ action. The lower efficacy of PGF₂ in the early CL more likely is related to signal transduction differences associated with its receptor at these two developmental stages than to the inability of PGF₂ to up-regulate ET-1.

¹ Supported in part by USDA/CREES award 2002-35203-12230 to E.K.I. and J.A.F.

² Correspondence: Jorge A. Flores, Department of Biology, Eberly College of Arts and Sciences, West Virginia University, P.O. Box 6057, Morgantown, WV 26506-6057. FAX: 304 293 6363; jflores{at}wvu.edu

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