Regression of induced corpora lutea in mature cyclic gilts by human chorionic gonadotropin

This study was conducted to determine whether chronic hCG treatment would cause regression of induced corpora lutea (CL) in mature cyclic gilts. Thirty-two mature gilts that had displayed one or more estrous cycles of 18 to 22 d were used. Sixteen gilts were hysterectomized (HYSTX) on d 6 to 9 (d 0 = onset of estrus) and their CL were marked with charcoal (spontaneous group). Sixteen gilts (induced group) were injected with 1,500 IU of pregnant mare's serum gonadotropin (PMSG) on d 6 and 500 IU of hCG on d 9 (day of HCG = d 0 of the induced cycle). Ovulation was assumed to occur on d 2 of the induced cycle. Induced gilts were HYSTX on d 8 to 9 (d 17 to 18 of the original spontaneous cycle) and their CL were marked with charcoal. Only gilts (n = 14) in which induced CL were present and in which the original CL had regressed were then subjected to treatment with saline or hCG. From d 10 to 29, gilts with spontaneous CL were injected daily with 500 IU of hCG (n = 8) or saline (n = 8). From d 10 to 29 of the induced cycle, induced gilts were injected daily with 500 IU of hCG (n = 6) or saline (n = 8). Jugular blood samples were collected every other day from all gilts beginning on the 1st d of daily hCG treatment and quantified for estradiol and progesterone by RIA. On the day after the last hCG injection, the number of charcoal-marked CL and charcoal-marked corpora albicantia (CA) were determined. All spontaneous CL were maintained. Six of eight gilts with induced CL that received saline maintained CL, whereas only one of six (P < .05) gilts with induced CL that received daily hCG maintained CL. All gilts that were treated with hCG had numerous accessory CL. Serum estradiol concentrations were < 5 pg/ml throughout the sampling period in gilts receiving daily saline injections. Serum estradiol concentrations in the hCG-treated gilts with spontaneous CL increased by d 12 and remained elevated. In the hCG-treated gilts with induced CL, serum estradiol concentrations were elevated from d 12 to d 22 but returned to control concentrations on d 24. These results indicate that daily administration of hCG caused regression of induced CL of mature cyclic gilts.