Pulsatile patterns of gonadotropins and ovarian steroids during estrus and pseudopregnancy in the rabbit

Consecutive daily plasma levels of luteinizing hormone (LH), follicle-stimulating hormone (FSH), estradiol-17 beta (E2), progesterone (P4) and 20 alpha-hydroxypregn-4-en-3-one (20 alpha-OHP) were monitored in estrous rabbits and in these same doses during pseudopregnancy (PSP); these daily hormone levels, as well as the immediate post-coital changes in gonadotropin secretion, were similar to those in previous reports. To examine the pulsatile patterns of the gonadotropins and ovarian steroids, sequential, 10-min plasma samples were collected for 6 h from estrous does and on Days 3, 10, and 17 of PSP. All five hormones were measured in the serial samples from estrous and PSP Day 10 does; LH and FSH only were assayed in the remaining sequential samples. The amplitude and frequency of FSH pulses did not differ between any of these stages. In marked contrast, LH pulse amplitudes, and even pulse frequencies in Day 17 does, were profoundly increased during PSP above those in estrous does. Progestin secretions, both P4 and 20 alpha-OHP, also were sharply elevated in PSP Day 10 does as compared with those in estrous rabbits; the pulse amplitudes of both progestins were severalfold higher during PSP. P4 pulse frequencies were also increased at this time. Conversely, the parameters of E2 secretion did not differ between estrous and PSP Day 10 animals. In PSP Day 10 does, high amplitude pulses of both P4 and 20 alpha-OHP occurred simultaneously with high amplitude LH pulses. Simultaneous E2 and P4 pulses were evident in these same sequential plasma samples, and this E2-P4 pulse association was greater than that of 20 alpha-OHP pulses with E2 pulses. Our findings failed to identify conclusively the trophic stimulus for the progestin pulse patterns, but the mechanism may involve the coordinated action of LH and E2. The results do demonstrate that each gonadotropin and ovarian steroid is secreted in a pulsatile manner in both estrous and pseudopregnant rabbits. There are altered profiles in LH and progestin pulses, without major changes in FSH and E2 patterns, between the stages of estrus and PSP. The causes and consequences of these divergent endocrine shifts cannot be deduced from these data.