Identification of functional high and low affinity states of the prostaglandin F2 alpha receptor in the ovine corpus luteumin vivo and their role in hormone pulsatility

Abstract

Equivocal evidence has accumulated for the presence of high and low affinity receptors for PGF_2α in the corpus luteum based on binding affinities of³H-PGF_2α to cell membranes or separated whole cells. Some studies report only high affinity sites, while others report the occurrence of both high and low affinity sites. We have previously demonstrated, using subluteolytic levels of PGF_2α, the existence of functional high affinity luteal PGF_2α receptors which show desensitization and recovery after 6 to 9 h. The present study, using direct intra-arterial infusions of PGF_2α into the autotransplanted ovary in conscious sheep, was designed to probe for the existence of functional high and low affinity states of the PGF_2α receptor in the corpus luteumin vivo. Subluteolytic and luteolytic concentrations of PGF_2α (100 pg/min and 2500 pg/min, respectively) were infused sequentially, each for 2 h, into the ovary during the luteal phase (n=7 sheep). The same low and high concentrations of the inactive metabolite of PGF_2α (PGFM) were given over the same time periods as negative controls (n=4 sheep). During the 2 h intra-arterial infusion of 100 pg/min of PGF_2α the secretion rate of oxytocin increased (P<0.01) while the secretion rate of progesterone was unaffected. In contrast, during the 2 h intra-arterial infusion of 2500 pg/min of PGF_2α, secretion rate of oxytocin increased (P<0.01) and secretion rate of progesterone now began to decline (P<0.05). During the 2 h infusions of identical concent-rations of PGFM, the secretion rate of oxytocin and progesterone remained unchanged. These results indicate the existence of functional high and low affinity states of the PGF_2α receptor within the ovine corpus luteumin vivo.