Oxytocin- and aluminium fluoride-induced phospholipase C activity and prostaglandin F(2α) secretion during the ovine luteolytic period

A series of studies was conducted to characterize changes in components of the cell signalling cascade that mediates oxytocin-induced prostaglandin F(2α) (PGF(2α)) synthesis at the onset of luteolysis in sheep. in the first experiment, caruncular tissue was dissected from 20 ewes on days 12-15 of the oestrous cycle, and incubated for the measurement of phospholipase C (PLC) activity or secretion of PGF(2α). Activation of GTP-binding proteins with aluminium fluoride stimulated both inositol phosphate accumulation and PGF(2α) secretion on all days examined. However, oxytocin did not stimulate PLC activity or PGF(2α) accumulation until day 13. While the ability of oxytocin to stimulate PLC activity increased after day 13, oxytocin-induced PGF(2α) secretion declined slightly from day 13 to 15, suggesting that cell signalling components downstream from PLC modulate the response to oxytocin after day 13. Oxytocin failed to stimulate PGF(2α) synthesis on day 14 after oestrus. Secretion of endogenous luteal oxytocin may have rendered uterine tissues collected on day 14 refractory to oxytocin in vitro. Therefore, a second study was conducted in ovariectomized, steroid replaced ewes. Ovarian steroids were administered to mimic endogenous changes in progesterone and oestradiol. The temporal patterns of PGF(2α) synthesis in response to oxytocin and pharmacological agents were similar to uterine tissues from cyclic ewes in the first experiment; however, the magnitude of the response was less. These data suggest that oxytocin receptors are absent or are not coupled to PLC until day 13 after oestrus.