Development of pulsatile LH release as a function of time following ovariectomy in the rat

Since pulsatile patterns of luteinizing hormone (LH) secretion have been reported in long-term ovariectomized (OVX) rats, but not intact rats, the patterns of tonic LH secretion were studied in intact female rats and 2, 4 and 8 days after OVX. Blood samples were taken every 10 min for 2 h from intact rats, catheterized the previous day, during the late morning of diestrus-1, proestrus and estrus. Mean LH concentrations were 55 +/- 7 ng/ml and did not differ among days. A single increase of circulating LH suggestive of one pulsatile release was seen in 4 of the 14 rats, but no animal showed the regular, repeated increases characteristic of long-term OVX animals. 2 days after OVX, mean LH concentrations were 156 +/- 10 ng/ml and patterns characteristic of regular pulsatile releases of LH were seen in 2/6 animals, with occasional pulsatile discharges in another 3. By 4 days after OVX, all 6 animals showed pulsatile patterns of LH, and mean LH concentrations were 190 +/- 32 ng/ml. By day 8, mean LH levels had increased significantly, to 357 +/- 44 ng/ml, and peak and nadir levels, as well as LH pulse amplitude, had all risen significantly. No significant difference in frequency of LH pulses was noted after OVX, although they seemed to occur more frequently at 8 days. These results suggest that pulsatile discharges of LH in the intact rat are either heavily suppressed or infrequent but that they become a prominent feature of LH secretion soon after OVX.