Figure 2 depicts the daily patterns of serum LH levels in rats implanted with P4 for 3 wk as compared to regularly cyclic controls. During the estrous cycle, serum LH values at noon fluctuated between 103 ± 11 and 125 ± 5 ng/ml. Similarly low levels of LH were observed in P4-implanted rats. To examine in more detail the patterns of LH secretion in these groups, serial samples were obtained every 3 h between 0800 and 2000 h (Fig. 3). While cyclic controls displayed a distinct preovulatory LH surge on proestrus, P4-implanted rats showed consistently low plasma LH levels similar to those seen in cyclic controls on diestrous Day 1. Figure 2 also depicts the daily patterns of serum FSH in both groups of animals. birth control pills
During the estrous cycle, serum FSH values at noon were significantly (p < 0.05) higher on estrus than at other stages, whereas P4-implanted females exhibited low serum FSH values throughout the 3-wk period. Figure 3 (bottom panel) further reveals the plasma patterns of FSH during a 12-h period in these three groups of rats. While cyclic controls exhibited a distinct preovulatory FSH surge on proestrus, P4-implanted rats showed relatively stable low levels of plasma FSH, similar to that observed on diestrous Day 1. Thus, P4-implanted females did not exhibit cyclic increases in gonadotropin secretion during hormone treatment, consistent with the persistently low levels of E2 and elevated P4 observed in these animals.
FIG. 2. Daily patterns of plasma gonadotropin concentrations on different days of treatment with P4 implants (left) and at 1200 h on each day of the estrous cycle in control females (right) (n = 6-10 rats per group). Diestrous Days 1 (D-1) and 2 (D-2), proestrus (PRO), and estrus (EST).
FIG. 3. Levels of circulating LH (top) and FSH (bottom) from 0800 to 2000 h on proestrus (triangles) and diestrous Day 1 (squares) in control rats (n = 5 per group), and on Day 11 after placement of P4 implants in hormone-treated females (circles, n = 8 rats). Serial blood samples were obtained through chronic jugular vein catheters, and plasma was assayed for LH and FSH concentrations.