Baboon corpus luteum: presence of oxytocin receptors

To determine whether the effect of oxytocin (OT) on progesterone production in primate luteal tissue may be receptor-mediated, we used biochemical procedures to examine the CL of the baboon (Papio anubis) for the presence of OT receptors (OT-R). CL (n = 5) were obtained by luteectomy from the early (LH + 1-5 days), mid- (LH + 6-10 days), and late (LH + 10-15 days) luteal phases. Biopsies from the uterine fundus were also obtained at the same time. Total receptors were measured by incubating the membrane fractions (120,000 x g fraction) with increasing concentrations of [3H]oxytocin (0.08-1.6 nM) for 30 min at 22 degrees C with a protein concentration of 200 micrograms per tube. Nonspecific binding was evaluated by the addition of excess nonradioactive OT. The receptor-bound tritiated ligand was separated from the nonreceptor-bound ligand through use of polyethylene glycol. For the midluteal phase CL, the affinity Kd was 1.70 +/- 0.03 nmol/L and the mean receptor number (RT) was 122.6 +/- 8.3 fmol/mg protein. In comparison, myometrial tissue examined at the same phase of the cycle had a Kd of 1.3 +/- 0.1 nmol/L and an RT of 205.0 +/- 49.7 fmol/mg protein. Receptors were not detectable in the late luteal phase CL and were significantly lower in the early luteal phase CL (50.7 +/- 6.9 fmol/mg protein), with no difference in the Kd value. Competitive studies with structurally related and nonrelated peptides indicated the presence of specific receptors for OT in baboon CL.(ABSTRACT TRUNCATED AT 250 WORDS)