Serum Patterns of P, E, and Gonadotropins in P4-Implanted Rats and in Cyclic Females
Figure 1 depicts the daily patterns of serum E2 in rats during 3 wk of treatments with P4 implants (left panel), compared to those in cyclic females on different days of the estrous cycle (1200 h; right panel). Cyclic females exhibited elevated serum E2 (86 ± 6 pg/ml) at noon on pro-estrus. In young rats, s.c. placement of a P4 implant resulted in a rapid and consistent decrease in serum E2, ranging between 19 ± 1 and 27 ± 7 pg/ml, similar to that previously reported. These low E2 values in P4-implant-ed rats were comparable to the levels seen on estrus and diestrous Day 1 in cyclic females, but were substantially lower than proestrous E2 concentrations. Thus, placement of P4 implants prevented the cyclic increases in E2 observed in control females.
Figure 1 also compares the patterns of serum P4 between P4-implanted females and cyclic control rats. During the estrous cycle, serum P4 levels fluctuated at noon between 25 ± 3 and 40 ± 4 ng/ml. Placement of P4 implants in female rats produced a rapid and marked rise in serum P4 (139 ± 13 ng/ml) within 8 h, followed by a gradual decline that reached basal levels by Day 12. These data demonstrate that circulating concentrations of P4 were significantly elevated during the first half of P4 implant treatments, as compared to the levels observed at noon during the estrous cycle.
FIG. 1. Patterns of plasma steroid concentrations on different days of treatment with P4 (P on figure) implants (left) and at 1200 h on each day of the estrous cycle in control females (right) (n = 6-10 rats per group). Samples were obtained from control females on diestrous Day 1 (D-1), diestrous Day 2 (D-2), proestrus (PRO), and estrus (EST).