IDENTIFICATION OF FUNCTIONAL HIGH AND LOW-AFFINITY STATES OF THE PROSTAGLANDIN-F2-ALPHA RECEPTOR IN THE OVINE CORPUS-LUTEUM IN-VIVO AND THEIR ROLE IN HORMONE PULSATILITY

Equivocal evidence has accumulated for the presence of high and low af finity receptors for PCF2 alpha in the corpus luteum based on binding affinities of H-3-PCF2 alpha to cell membranes or separated whole cell s. Some studies report only high affinity sites, while others report t he occurrence of both high and low affinity sites. We have previously demonstrated, using subluteolytic levels of PCF2 alpha, the existence of functional high affinity luteal PCF2 alpha receptors which show des ensitization and recovery after 6 to 9 h. The present study, using dir ect intra-arterial infusions of PGF(2 alpha) into the autotransplanted ovary in conscious sheep, was designed to probe for the existence of functional high and low affinity states of the PGF(2 alpha) receptor i n the corpus luteum in vivo. Subluteolytic and luteolytic concentratio ns of PGF(2 alpha) (100 pg/min and 2500 pg/min, respectively) were inf used sequentially, each for 2 h, into the ovary) during the luteal pha se (n = 7 sheep). The same low and high concentrations of the inactive metabolite of PCF2 alpha (PGFM) were given over the same time periods as negative controls (n = 4 sheep). During the 2 h intra-arterial inf usion of 100 pg/min of PCF2 alpha the secretion rate of oxytocin incre ased (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 PCF2 alpha, secretion rate of oxytocin increased (P<0.01) and secreti on rate of progesterone now began to decline (P<0.05). During the 2 h infusions of identical concentrations of PGFM, the secretion rate of o xytocin and progesterone remained unchanged. These results indicate th e existence of functional high and low affinity states of the PCF2 alp ha receptor within the ovine corpus luteum in vivo.