Influence of gonadotropin treatment interval on follicular maturation, in vitro fertilization, circulating steroid concentrations, and subsequent luteal function in the domestic cat

The impact of the ECG-hCG interval on in vitro fertilization (IVF), endogenous hormonal patterns, and luteal integrity was studied in the domestic cat. Adult cats with inactive ovaries were given eCG (i.m.) and then hCG (i.m.) 80, 84, 88, 92, or 96 h later. Oocytes were aspirated 25-27 h after hCG and co-cultured with swim-up-processed cat spermatozoa. Blood samples were collected daily from 2 days before eCG treatment (Day -2) through Day 14, and sera were analyzed for estradiol-17 beta and progesterone. The mean number of oocytes recovered from the 80-92-h groups (range, 17.2 +/- 2.1 to 21.1 +/- 3.0) did not differ (p > 0.05); however, oocyte number was reduced (p < 0.05) in the 96-h group (10.3 +/- 2.1). The proportion of all oocytes classified as mature was greater (p < 0.05) when HCG was given 80, 84, or 88 h compared to 92 or 96 h after eCG. Delaying hCG treatment until 96 h caused more than 25% of all oocytes to degenerate, which was a greater rate (p < 0.05) than in all other groups. The IVF rate at 80 (57.1%), 84 (56.5%), 88 (65.0%), and 92 (52.5%) h was greater (p < 0.05) than that observed at 96 h (33.6%). Circulating estradiol-17 beta concentrations began to rise above nadir within 24 h of ECG injection in all interval groups. On the basis of areas under the curve, cats in the 80- and 84-h treatments produced more (p < 0.05) estradiol-17 beta than other groups. Circulating progesterone rose above nadir within the first 24 h of follicular aspiration, and there were no marked temporal differences among groups. Mean CL number on Day 14 ranged from 11.8 in the 96-h group to 20.3 in the 84-h group, but was not different (p > 0.05) because of wide variation among individual animals. Mean luteal weight per ovary was higher (p < 0.05) in the 80-92-h intervals (range, 0.13 +/- 0.01 to 0.21 +/- 0.14 g/ovary) than in the 96-h group (0.07 +/- 0.01 g/ovary), resulting in greater overall progesterone content per ovary (p < 0.05). However, neither progesterone concentration per unit mass of luteal tissue nor luteal histology differed (p > 0.05) among interval groups. Although cat oocytes normally reside within Graafian follicles for unpredictable intervals awaiting copulation, these oocytes are sensitive to the timing of the hormonal stimulus that triggers final maturation, but only if the interval between the FSH-like and LH-like stimulus exceeds 88 h. The effect is expressed by variation in the number and maturity of oocytes recovered and by as yet undefined alterations that influence fertilizability in vitro but not CL composition or function.