Direct Effect of PGF2.ALPHA., TPA and lonophore A23187 on Progesterone Release from Microdialyzed Corpus Luteum inthe Cow.

Abstract

There is general agreement that prostaglandin F2α (PGF2α) is a physiological luteolysin in the cow. Contrary to this fact, PGF2α has been shown to stimulate the progesterone (P) release from luteal cells in vitro, through the activation of intracellular protein kinase C (PKC) and free calcium ion (Ca2+). To examine the possibility that an essential factor or function(s) may be lost through the process of preparing cells from the corpus luteum (CL), we evaluated the direct effect of PGF2α, TPA and ionophore A23187 on local P release within the CL in the cow, by utilizing in vivo microdialysis system (MDS). Normal cyclic Holstein cows (n=4) were superovulated with FSH and the PGF2α analogue, and on Day 8 after estrus, they were surgically implanted with the MDS (cut-off Mr=1000 kDa) into multiple CL (1-3 line/CL). The system was continuously perfused with Ringer's solution from immediately after the surgery. On Days 10-11, several CL were assigned to the control, PGF2α (10-5 M), TPA (10-5 M), A23187 (10-5 M), PGF2α+A23187 and TPA+A23187. The stimulants were infused into the CL using the MDS for 6 h, and each line of the MDS was further perfused with Ringer's solution for 26 h. A 6-h infusion with PGF2α stimulated P release during infusion, but after infusion the P release was again stable around the baseline. A 6-h infusion with TPA or A23187 strongly stimulated the P release. Similarly, PGF2α+A23187 or TPA+A23187 stimulated the P release. During the last 8 h of the experiment, A23187 alone, PGF2α+A23187 or TPA+A23187 inhibited the P release, whereas TPA as well as PGF2α maintained the same level as in the control. The results indicate that the direct exposure of the microenvironment in the bovine CL to PGF2α in vivo does not result in the suppression of P release until 26 h after stimulation, and that the activation of cytosolic free calcium is more effective in inhibiting the P release than the stimulation of PKC during the following period. Thus, the results support the concept that PGF2α needs some local luteolytic intermediator(s) that may enhance the intracellular calcium concentration.