Regulation of Prostaglandin Biosynthesis by Estrogen and Progesterone in Simian and Ovine Endometrium: a Thesis

Abstract

Endometrial prostaglandins (PGs) play a role in menstruation in primates and in luteolysis in nonprimates. Their biosynthesis is regulated by estrogen (E) and progesterone (P) in a manner not fully understood. The purpose of this thesis research was to (1) study the effects of E and P, both in vivo and in vitro, on basal endometrial PG output in vitro during the course of the artificial menstrual cycle in the rhesus monkey, and (2) further to examine the cellular mechanisms of P action in vivo on PG output using an ovine model system. To carry out the first objective, ovariectomized rhesus monkeys (n=39) were maintained on either a standard or manipulated artificial menstrual cycle (SAMC and MAMC, respectively) and endometrial biopsies were obtained at precise times in separate cycles on: cycle day 9 (mid-proliferative), 13 (mid-cycle E peak), 14 (one day post E peak), and 23 (mid-secretory). PGF2α was the most abundant PG produced in vitro by endometrial organ cultures, the levels of which changed most dramatically throughout the SAMC. Within the first 24 hours of organ culture, PGF2α accumulation was low on day 9 and rose significantly (p<0.01) on day 13, indicating a stimulatory effect of E in vivo. However, E added in vitro, at either physiologic or supraphysiologic concentrations, to endometrial cultures did not stimulate PGF2α accumulation on any cycle day examined. On day 14, just one day post E peak, there was a dramatic fall in PGF2α accumulation which appeared to be due to both a decline in stimulatory E in vivo and a rise in inhibitory P in vivo. Basal PGF2α accumulation in vitro by day 23 endometrial cultures was 10-fold higher (p<0.01) compared to days 9 and 14. This high level of PGF2α output on day 23 appeared to be caused by a paradoxical priming effect of P in vivo and also a slight enhancement by the mid-cycle peak of E in vivo. Padded in vitro, at a physiologic concentration, to day 23 endometrial cultures markedly inhibited (p<0.01) the high level of PGF2α accumulation, suggesting that P withdrawal in vivo promotes the rise in endometrial PGF2α production in vivo at the time of menstruation in primates. An ovine model system was further used to investigate the cellular mechanisms of P action in vivo. Ovariectomized sheep (n=8) were administered an infusion regimen of either E and P, or E and P vehicle alone, to examine the effects of P in vivo on PGF2α production in vitro by endometrial explants during short-term incubations. P in vivo increased the mass amount of stimulated PGF2α output by both physiologic and pharmacologic mechanisms. In addition, P did not appear to significantly alter the sensitivity of the endometrium to stimulatory levels of oxytocin in vitro indicating that the cellular events accounting for the P priming effect, in part, may occur independent of the oxytocin receptor closer to the PG biosynthetic pathway. In P-primed endometrium, the mass amount of PGF2α stimulated by a calcium-ionophore (A23l87) was less than that stimulated by OT suggesting the involvement of calcium-insensitive mechanisms in PGF2α synthesis.