160 EFFECT OF cAMP MODULATORS ON IN VITRO MATURATION OF BOVINE OOCYTES

Abstract

The efficiency of in vitro production (IVP) has remained low due to an inadequate in vitro maturation (IVM) system. Thus far, the most promising methods of improving IVM utilise cAMP modulators to slow the maturation process by keeping cAMP high. This experiment compares standard IVM to an extended IVM as previously described by Albuz (2010 Hum. Reprod. 25, 2999–3011). The extended IVM consists of a 2-h pre-IVM phase with FSK (an adenylate cyclase activator) and 3-isobutyl-1-methylxanthine [IBMX, a nonspecific phosphodiesterase (PDE) inhibitor], followed by a 31-h IVM phase with cilostamide (a PDE3 inhibitor). 3-Isobutyl-1-methylxanthine inhibits all PDE in both the oocyte and CC, whereas cilostamide inhibits PDE3 only in the oocyte, allowing a gradual reversal of inhibition. Bovine oocytes (n = 363) were obtained by transvaginal ultrasound-guided aspiration of both Brahman and Angus cattle over 4 collection days. Oocytes from each cow were divided into 2 groups. The first group was placed in standard 23-h IVM medium composed of TCM-199 supplemented with 10% fetal bovine serum, sodium pyruvate, penicillin and streptomycin, glutamine, and FSH, and cultured in 5% CO₂ at 39°C. A subset of oocytes was removed from maturation at 8, 13, 18, and 23 h. The second group was placed into a pre-IVM medium of HEPES-TALP supplemented with 100 μM FSK and 500 μM IBMX for 2 h at 39°C, and then moved into standard maturation medium supplemented with 20 μM cilostamide for 31 h (5% CO₂, 39°C). Oocytes were sampled at 8, 13, 18, 23, 28, and 33 h. Oocytes were stained with 1% orcein and nuclear status was examined for each sample time (Table 1). Data were analysed using a chi-squared test. At 8 h, there was a significant difference (P < 0.001) between GV stage of IVM and extended IVM (7.1 v. 73.2%), and between metaphase I (MI) stage IVM and extended IVM (76.2 v. 4.9%). At the 23-h time sample, there was a significant difference between metaphase II (MII) IVM and MII extended IVM (78.9 v. 30.6%; P < 0.001). There was also a significant difference between MII oocytes at 23 h IVM and MII oocytes at 33 h extended IVM (78.9 v. 33.3%; P < 0.001). These results are consistent with the hypothesis that cAMP modulators slow the nuclear maturation process. However, results also suggest that the inhibitory effect of the cAMP modulators is not completely reversible. Oocytes appear to arrest at MI by 23-h extended IVM and do not progress to MII at the same rate as standard IVM.

Table 1.  Nuclear status of bovine oocytes after standard IVM and extended IVM with cAMP modulators