Superovulation of cows with PMSG: Variation in plasma concentrations of progesterone, oestradiol, LH, cortisol, prolactin and PMSG and in number of preovulatory follicles

Abstract

Forty-two heifers, exhibiting normal oestrous cycles, were treated with 2500 I.U. PMSG and 2 ml prostaglandin (PG) at day 10 and 12 of the oestrous cycle, respectively. In ten heifers progesterone, oestradiol, LH, prolactin, cortisol and PMSG levels were estimated until 10 days after the initiation of superovulation. In 32 heifers the occurrence of the LH surge was determined with a rapid radioimmunoassay, and 22–30 h after the LH surge the heifers were ovariectomized. The numbers of large follicles (> 10 mm) and ovulations on the collected ovaries were counted and the levels of progesterone, oestradiol, PMSG and LH in the peripheral blood where estimated. The sum of large follicles and ovulations was assumed to represent the number of preovulatory follicles. In 16.7% of the heifers no LH surge was detected. This failure of the LH surge after superovulation appeared not to be caused by significantly different levels of cortisol, prolactin and PMSG as compared to those of cows responding with an LH surge.

There was quite some variability in the interval between the PG injection and the maximum of the LH surge. The mean interval was 43.9 ± 1.5 h (SEM; n = 28). This interval PG-LH was negatively correlated with the number of preovulatory follicles (r= −0.483; P < 0.01). The interval between the onset of oestrus and the maximum of the LH surge was 1.96 ± 0.54 h (n = 22).

The oestradiol concentration during the preovulatory LH surge and the number of preovulatory follicles were positively correlated (r= 0.732; P < 0.01). Progesterone levels were significantly increased after the initiation of superovulation. There was no significant correlation between the progesterone level before or after the administration of PMSG and the number of preovulatory follicles. This number was also not correlated with the concentration of PMSG in the peripheral blood shortly after injection of PMSG or shortly after injection of PG. The results indicate that the marked variability in response to PMSG/PG superovulation is not due to variations of hormone concentrations during the stages of preovulatory follicular development.