Follicular Steroidogenesis: Effect of Reproductive Condition

Follicular steroidogenesis has been investigated extensively in nonpregnant animals. To determine if follicular steroidogenesis is altered during pseudopregnancy or pregnancy, large Graafian follicles (1.5–2.0 mm) were isolated from nonpregnant (NP), pseudopregnant (PSP) Days 7 (7PSP) and 15 (15PSP)] and pregnant (P) [Days 7 (7P), 15 (15P) and 28 (28P)] rabbits and placed in perifusion chambers. After the establishment of a basal steroid output, follicles were stimulated with LH (100 ng, NIH-S20):FSH (50 ng, 50.4 × NIH-FSH-S1; Papkoff). Samples were collected continuously at 20 min intervals and assayed for progesterone (P4), testosterone (T) and estradiol-17β (E). Steroid concentrations are expressed as pg/follicle/20 min. The secretion of P4 by unstimulated follicles was similar for all groups; however, differences were observed after LH:FSH stimulation. With increasing length of PSP or P, follicles secreted significantly less P4. The values for NP, 7PSP and 7P were different (P<0.01) from those of 15PSP and 15P, and 15P was different from 28P (P<0.025), whereas E secretion by NP was greater (P<0.001) than for PSP and P groups. After treatment with LH:FSH, T and E secretion by NP was elevated (P<0.025) over PSP and P. There were no differences among PSP and P groups, except that stimulated follicles of 28P group secreted significantly less T than did those of 7P or 15P groups (P<0.005). Serum P4 for NP was 0.21 ng/ml, whereas PSP and P rabbits had concentrations ranging from 6.83 to 17.86 ng/ml. Luteal P4 content was not significantly different for any groups, varying from 77.4 to 161.3 ng/mg protein. To ascertain if the presence of elevated serum P4 during PSP and P alters follicular steroidogenesis, follicles of NP rabbits were incubated with various concentrations of P4 (101 ng/ml−104 ng/ml) or its vehicle, then placed in a perfusion system and basal and LH-stimulated P4 and E secretion determined. Incubation of follicles in 101 or 102 ng/ml P4 did not alter subsequent follicular secretion of P4 from that of controls; 103 and 104 ng/ml P4 significantly suppressed P4 secretion by perifused follicles. In contrast, LH-stimulated E secretion was suppressed by all concentrations of P4. The higher concentrations of P4 (103 and 104 ng/ml) also resulted in decreased basal E secretion. These results indicate that the follicles removed from PSP and P rabbits do have different profiles of basal and gonadotropin-stimulable P4, T and E secretion. Evidence presented here suggests that elevated P4 concentrations, present during PSP and P, may modulate follicular steroidogenesis.