INVIVO DESENSITIZATION OF A HIGH-AFFINITY PGF2-ALPHA RECEPTOR IN THE OVINE CORPUS-LUTEUM

The corpus luteum (CL) of the sheep exhibits a differential sensitivity to PGF2-alpha in vivo in terms of an increase in oxytocin (OT) secretion and a decrease in progesterone secretion, pointing to the presence in vivo of both high and low affinity receptors for PGF2-alpha. The presence of the high affinity PGF2-alpha receptor was assessed by monitoring the secretion rate of OT from the ovine CL in response to subluteolytic infusions of PGF2-alpha. Rapid desensitization to PGF2-alpha occurred after only one hour of infusion, while a minimum rest period of six hours was required to restore sensitivity. The possibility that these findings could be explained by the depletion and resynthesis of OT was excluded by demonstrating an increase in OT secretion rate with supra-physiological levels of PGF2-alpha two hours after desensitization. Collectively, these results indicate the presence of a high affinity receptor for PGF2-alpha in the ovine CL which exhibits desensitization and recovery in vivo. The temporal nature of the desensitization and recovery of the high affinity PGF2-alpha receptor controlling luteal OT secretion may contribute to the pulsatile nature of PGF2-alpha release from the ovine uterus.