Acute and chronic elevations in serum levels of FSH after unilateral ovariectomy in the cyclic hamster.

During the normal 4-day estrous cycle of the hamster, 12.0 +/- 0.4 (n = 34) ova were shed by the paired ovaries on Day 1. When unilateral ovariectomy (ULO) was performed at 0900 h on any one of the first 3 days of the cycle, approximately the same number of ova were released from the remaining ovary on Day 1 of the ensuing postoperative cycle (compensatory ovulation), except after ULO on Day 4. In view of these results, the principal aim of the present study was to determine if ULO affected acute (24 h postoperative period) and chronic (postoperative cycles) alterations in serum levels of immunoreactive FSH. Multiple bleedings were obtained by cardiac puncture without anesthesia. The acute alteration in FSH levels was first examined at 24-h intervals following ULO at 0930 h on any one day of the cycle, and significant elevations in these levels were found 24 h postoperatively in all but Day-2 hemicastrates. Several experiments were then performed to determine the time-sequence for this 24-h response to ULO. The serum level of FSH was measured at 4-h intervals during the remainder of the cycle in which ULO was done at 0900 h on any one day. The results demonstrated that an acute but transient increase (approximately 100 ng FSH/ml serum) occurred either 7 h or 11 h postoperatively in all the hemicastrates. These transient elevations prior to Day 4 were interpreted to be possible causative factors for the subsequent compensatory ovulation. That they were not the sole cause was evident from the results of another experiment, where compensatory ovulation occurred after ULO at 0030 h on Day 4 (but to a reduced degree; 9.0 +/- 0.6 ova), although serum levels of FSH were not acutely altered. The capability of the remaining ovary to undergo compensatory ovulation after ULO on Day 4 was lost between 0300 h and 0230 h (6.7 +/- 0.8 ova). The serum profile of FSH during the second postoperative cycle after ULO showed a prolonged elevation of FSH levels during the first two days of the cycle compared to the intact estrous cycle. Specifically, the level of FSH gradually declined to 400 ng/ml by 2000 h on Day 1 of the intact (preoperative) cycle but did ont reach this concentration until 2000 h on Day 2 of the postoperative cycle. It is suggested that the prolonged elevated levels of FSH in chronic hemicastrates effect development of a greater number of follicles in the early growth stages in the remaining ovary than the number of follicles per ovary that would otherwise develop in the intact hamster.