OXYTOCIN-INDUCED AND ALUMINUM FLUORIDE-INDUCED PHOSPHOLIPASE-C ACTIVITY AND PROSTAGLANDIN-F2-ALPHA SECRETION DURING THE OVINE LUTEOLYTIC PERIOD

A series of studies was conducted to characterize changes in component s of the cell signalling cascade that mediates oxytocin-induced prosta glandin F-2 alpha (PGF(2 alpha)) synthesis at the onset of luteolysis in sheep. In the first experiment, caruncular tissue was dissected fro m 20 ewes on days 12-15 of the oestrous cycle, and incubated for the m easurement of phospholipase C (PLC) activity or secretion of PGF(2 alp ha). Activation of GTP-binding proteins with aluminium fluoride stimul ated both inositol phosphate accumulation and PGF(2 alpha) secretion o n all days examined. However, oxytocin did not stimulate PLC activity or PGF(2 alpha) accumulation until day 13. While the ability of oxytoc in to stimulate PLC activity increased after day 13, oxytocin-induced PGF(2 alpha) secretion declined slightly from day 13 to 15, suggesting that cell signalling components downstream from PLC modulate the resp onse to oxytocin after day 13. Oxytocin failed to stimulate PGF(2 alph a) synthesis on day 14 after oestrus. Secretion of endogenous luteal o xytocin may have rendered uterine tissues collected on day 14 refracto ry to oxytocin in vitro. Therefore, a second study was conducted in ov ariectomized, steroid replaced ewes. Ovarian steroids were administere d to mimic endogenous changes in progesterone and oestradiol. The temp oral patterns of PGF(2 alpha) synthesis in response to oxytocin and ph armacological agents were similar to uterine tissues from cyclic ewes in the first experiment; however, the magnitude of the response was le ss. These data suggest that oxytocin receptors are absent or are not c oupled to PLC until day 13 after oestrus.