ENDOMETRIAL RESPONSIVENESS TO OXYTOCIN DURING DIESTRUS AND EARLY-PREGNANCY IN PIGS IS NOT CONTROLLED SOLELY BY CHANGES IN OXYTOCIN RECEPTORPOPULATION-DENSITY

These studies were performed to test the hypotheses that: 1) endometri al responsiveness to oxytocin (OT) in pig endometrium is associated wi th changes in OT receptor (OTr) population density resulting from corr esponding regulation of OTr gene transcription, 2) endometrial respons iveness to OT is controlled solely through a mechanism involving chang es in OTr population density, and 3) OTr population density and endome trial responsiveness to OT differ between diestrus and early pregnancy in pigs. In experiment 1, OTr population density and dissociation con stant (K-d) in cyclic pigs were constant on Days 10-16 but increased ( p < 0.05) between Days 10 and 12 of pregnancy before decreasing (p < 0 .05) through Day 16. OT induced phosphoinositide (PI) hydrolysis and p rostaglandin (PC) F-2 alpha secretion in cyclic pigs only on Day 16 (p < 0.05), and during pregnancy only on Day 12 (p < 0.05). Activation o f G protein by aluminum fluoride (AlF4-) treatment maximally stimulate d (p < 0.05) PI hydrolysis and PGF(2 alpha) secretion in cyclic pigs o n all days, indicating that downstream from the OTr, the PGF(2 alpha) secretory pathway was fully functional. During pregnancy, PI hydrolysi s and PGF(2 alpha) secretion in response to AlF4- decreased (p < 0.01) on Days 14 compared to Days 10 and 12, and AlF4- did not stimulate PG F(2 alpha) release on Day 16. In experiment 2, abundance of OTr mRNA i n cyclic pigs decreased between Days 0 and 5 before increasing between Days 5 and 12 (p <: 0.05), but it was higher (p < 0.05) on Days 10-15 of pregnancy than on equivalent days in cyclic gilts. These results i ndicate that control of PCF2 alpha secretion in cyclic pigs appeared t o occur primarily at the level of OTr coupling to G protein because ch anges ire OTr number were not associated with increased sensitivity to OT or G-protein activation by AlF4-. During pregnancy, control was ex erted at multiple levels, which included the OTr, G protein, phospholi pase C, and subsequent aspects of the secretory pathway. The present s tudy also indicated that endometrium was responsive to OT during luteo lysis in cyclic pigs but not during corpus luteum maintenance in pregn ant pigs.