Ovariectomy of Young Adult Rats has a Sparing Effect on the Ability of Aged Rats to Release Luteinizing Hormone 1 , 2

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Abstract
The effects of age and length of time of ovariectomy on the plasma luteinizing hormone (LH) responses to ovariectomy and subsequent tratement with estradiol benzoate (EB), progesterone (Prog) and LHRH were investigated. Rats were kept in a room with the lights on from 0500 to 1900 h daily. Groups consisted of females ovariectomized (OVX) at 2 mo. of age and used at 4, 15 or 24 mo. of age, as well as rats ovariectomized at 13 or 22 mo. of age and used at 15 or 24 mo. of age, respectively. At day 0, rats were bled at 7-min intervals for 84 min through an atrial cannula and then injected s.c. with 50 .mu.g EB in oil. At day 3, 25 mg of Prog was injected s.c. in oil at 0900 h. Blood samples were collected at hourly intervals between 1100 and 2300 h on days 2 and 3. At day 4, blood samples were collected just prior to and after i.v. injection of 10 ng LHRH at 1730 h. In 4-mo.-old:2-month-OVX controls, plasma LH concentration fluctuated in a pulsatile rhythm on day 0 at levels about 20-fold higher than that observed in intact cyclic rats during diestrus or estrus. The EB lowered the plasma LH levels and then caused a surge in plasma LH during the afternoon on day 2. The Prog lowered the plasma LH levels further than that observed after injection of EB, and a surge in plasma LH occurred during the afternoon of day 3. As age of ovariectomy increased, the postovariectomy rise in plasma LH concentration was suppressed because of a decreased amplitude of LH pulses; EB was ineffective in eliciting LH surges; EB-Prog-induced LH surges were of reduced magnitude; and the plasma LH response to LHRH decreased. The plasma LH concentration on day 0 was highest in the 15-mo.-old:13-mo.-OVX group but low in the 24-mo.-old:22-mo.-OVX group. On day 2, the 15-mo.-old:13-mo-OVX rats had an LH surge about 1/2 that of the controls while the 24-mo.-old:22-mo-OVX group had LH surges of greater magnitude and duration than those of the controls. On day 3, small and large LH surges were observed in these 2 groups, respectively. All rats except those in the 24-mo.-old:2-mo.-OVX group had a large increase in plasma LH levels in response to LHRH on day 4. Apparently, early ovariectomy has a temporary sparing effect on the loss of ability of aged rats to maintain high plasma LH concentrations after ovariectomy, as well as a slowly developing sparing effect on the loss of ability of aged rats to respond to EB with an LH surge and the decrease in ability of aged rats to respond to EB and Prog with an LH surge. Primarily, these sparing effects appear to concern hypothalamic LHRH secretion.