Preovulatory biosynthesis and granulosa cell secretion of immunoreactive oxytocin by goat ovaries

Summary. Oestrous cycles of goats were synchronized hormonally. Immunoreactive oxytocin was undetectable (< 0·1 ng/mg protein) in media from granulosa cells isolated before the LH surge for small (1–2 mm), medium (3–5 mm) and large >5 mm diameter) follicles when cultured for 24 h without or with added hormones. Granulosa cells from large and medium, but not small, follicles isolated 6–12 h after spontaneous preovulatory LH surges secreted high concentrations of oxytocin (4–12 ng/mg protein). Addition of PGE-2 (1 μg/ml) caused a further significant (P < 0·05) increase in oxytocin secretion by cultured granulosa cells, whereas PGF-2α, FSH and LH were ineffective when added to culture media. Ovarian venous blood and granulosa cells were collected at 0, 6, 12 or 18 h after GnRH injection in hormonally synchronized goats. Peripheral serum LH values were increased significantly in all but 2 of 22 goats within 2 h of GnRH injection. At the earliest sampling time after GnRH (6 h), ovarian venous levels of oxytocin were increased significantly from basal levels of 0·4 pg/ml to 2·4 pg/ml. Oxytocin concentrations in follicular fluid increased from a basal value of 67 pg/ml to 155 pg/ml by 6 h and to 372 pg/ml by 18 h after GnRH injection. Oxytocin secretion by cultured granulosa cells was not increased significantly by 6 h (0·1 ng/mg protein) but rose to 1·4 and 3·5 ng/mg protein at 12 and 18 h, respectively. Approximately parallel increases occurred in progesterone in ovarian venous blood and granulosa cell culture media over the same time period. Oestradiol secretion by granulosa cells cultured with androstenedione as an aromatizable substrate was essentially unchanged when obtained from 0 to 12 h after GnRH injection (31–37 ng/mg protein), but fell significantly to 7·4 ng/mg in cells obtained 18 h after GnRH injection, indicative of decreasing aromatase activity as luteinization progresses during the preovulatory period. It is concluded that granulosa cells, in response to the preovulatory LH surge, acquire the ability to synthesize and secrete oxytocin in parallel with the increased progesterone and decreased oestradiol secretion which occurs during luteinization.