Oxytocin is not important for the control of gonadotrophin secretion in the late follicular phase of the cycle.

Objective: To study the role of oxytocin in basal and GnRH-induced gonadotrophin secretion in normal women.

Design: Normal women were studied in three cycles. When the diameter of the leading follicle was 15-16 mm, the women were infused intravenously (i.v.) for 3 h with normal saline (cycle 1), atosiban (cycle 2) or oxytocin (cycle 3).

Patients: The study included 12 normally cycling women aged 23-38 years.

Measurements: After cessation of treatment, two injections of GnRH, 10 microg each, were administered i.v. 2 h apart and blood samples were collected every 30 min for a total of 240 min. The 30-min pituitary response (sensitivity) to a single GnRH injection (10 microg i.v.) was investigated thereafter every 12 h from the end of the 3-h infusion until the day of LH surge onset.

Results: No significant differences in LH and FSH response to GnRH (sensitivity and reserve) during the 240-min experiment were found between the three cycles. The time of LH surge onset from the initiation of the infusion was similar in the three cycles. Also similar in the three cycles were oestradiol (E2) and gonadotrophin levels as well as the 30-min response to GnRH for 48 h following the 3-h infusion.

Conclusions: The present study demonstrates that neither exogenous oxytocin administration nor blockage of endogenous oxytocin action influences pituitary sensitivity to GnRH in cycling women.