Up-regulation of oxytocin receptors in porcine endometrium by adenosine 3',5'-monophosphate

Porcine endometrial cells (a mixture of epithelial, stromal, and glandular cells) were examined for the presence of oxytocin (OT) receptors using a cell culture system and a ¹²⁵I-labeled OT antagonist. Binding specificity was tested in displacement studies with various related peptides. Scatchard analyses revealed the presence of a binding site with a dissociation constant (K(d)) = 0.9 nM and a capacity of 1.9 fmol/10⁵ cells. These cells, which were obtained from prepubertal gilts and thus had not been exposed to endogenous ovarian steroids, were used as a model to evaluate the possible action of ovarian steroids and intracellular cAMP on OT receptors. Although ovarian steroids showed no effect on OT receptors, forskolin (an adenylate cyclase activator) and dibutyryl cAMP caused 1.5- to 1.6-fold increases in specific binding of OT without changing the binding affinity. When the endometrial cells were exposed to OT (0.1-1000 nM) in combination with arachidonic acid (10 μM), OT stimulated prostaglandin F(2α) secretion in a dose-dependent manner. These results indicate the presence of functional OT receptors in prepubertal porcine endometrial cells and suggest that the concentration of OT receptors may be regulated by one or more substances that raise intracellular cAMP levels.