Ovary

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?

Abstract
We examined the responsiveness of large (LLC), small (SLC) and endothelial cells of the Day-4 and -10 bovine CL to PGF₂ and 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 concentrations of agonists tested significantly (P = 0.05) increased [Ca²⁺]_i in all cell populations isolated from Day-4 and -10 CL. Day-10 steroidogenic cells were more responsive to PGF₂ and ET-1 than Day-4 cells. Response amplitudes and number of responding cells were significantly affected 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 from Day-10 than in Day-4 cells. Furthermore, on Day-10, as 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 in Day-4 steroidogenic-enriched cells was lower than on Day-10; in contrast, the potency of ET-1 was not different. Therefore, ET-1 was a tonic inhibitor of P₄ accumulation rather than a mediator of PGF₂ action. The lower efficacy of PGF₂ in the early CL is more likely 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.

Key words: Ovary • Calcium • Corpus luteum function • Progesterone • Signal transduction

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