Direct luteotrophic effect of oestradiol-17β on pig corpora lutea

Five gilts each received intraluteal implants releasing about 4.4 ± 1.1 (Group 1), 15.0 ± 1.1 (Group 2) or 22.4 ± 1.0 (Group 3) μg oestradiol/day as determined by in-vitro incubation of implants of similar weight and oestradiol content. On Day 11 of the oestrous cycle (Day 0 = first day of oestrus), 3 CL in one ovary received oestradiol implants, 3 CL in the other ovary received vehicle implants, and all other CL in both ovaries served as uninjected control CL. An additional group of 6 animals served as controls and included 4 animals receiving bilateral vehicle implants (3 CL per ovary) on Day 11 and 2 unoperated gilts. All animals were slaughtered on Day 19 of their oestrus cycle, and the weight, progesterone content and concentration of each CL were determined. In Group 3 gilts, luteal weight, progesterone content and concentration were greater by 68.7 ± 24.0 mg, 6.54 ± 1.33 μg and 7.54 ± 2.00 ng/ml respectively (P < 0.01) in oestradiol-implanted CL than in vehicle-implanted CL, which appeared to be similar to uninjected control CL. No differences were seen between oestradiol-17β and vehicle-treated CL in CL from gilts in Groups 1 or 2. All CL of Group 3 gilts were heavier and contained a greater content and concentration of progesterone (P < 0.01) than gilts in Groups 1 and 2, and in controls for which the values were similar. These results demonstrate that exogenous oestradiol can directly stimulate the growth of an individual CL; however, the oestradiol release rate must be high enough to maintain all CL bilaterally before this effect is seen.