Effect of follicle size and dibutyryl cAMP on the cAMP content and gap junctional communication of porcine prepubertal cumulus - oocyte complexes during IVM

Abstract

The lower ability of oocytes from prepubertal pigs to yield viable embryos than those from adult pigs appears, in part, a consequence of their reduced ability to accumulate cAMP during IVM. The present study examined the cAMP content of oocytes and cumulus–oocyte complexes (COCs), cumulus expansion and gap junctional communication (GJC) in COCs from 3- and 5–8-mm follicles during IVM. The effect of 1 mm dibutyryl cAMP (db-cAMP) treatment for the first 22 h of IVM was also examined for both follicle size classes. The cAMP concentration of oocytes from 5–8-mm follicles was threefold greater than that in oocytes from 3-mm follicles following 11 h of IVM (11.9 ± 5.9 v. 3.6 ± 1.8 fmol, respectively; P < 0.05). In the presence of db-cAMP, the cAMP content of oocytes from 3- and 5–8-mm follicles was no longer significantly different at 11 h IVM. The cAMP concentration of intact COCs from 5–8-mm follicles was significantly higher than that in COCs from 3-mm follicles at 11 h (1110.6 ± 318.0 v. 116.9 ± 55.7 fmol, respectively; P < 0.05). Despite maturation with db-cAMP, the cAMP content in COCs from 3- and 5–8-mm follicles at 11 h of IVM remained significantly different (15.1 ± 4.9 v. 196.2 ± 33.3 fmol, respectively; P < 0.05). The COCs from 3-mm follicles displayed lower cumulus expansion than did COCs from 5–8-mm follicles at both 11 h (cumulus expansion index (CEI) 1.0 ± 0.1 v. 1.8 ± 0.1, respectively; P < 0.01) and 22 h (CEI 1.9 ± 0.3 v. 2.9 ± 0.2, respectively; P < 0.05) of IVM. The level of cumulus cell–oocyte GJC decreased during IVM, with the number of GJC significantly greater in COCs from 3-mm compared with 5–8-mm follicles at both 6 h (613 ± 55 v. 304 ± 44 fluorescence intensity (FI), respectively; P < 0.05) and 11 h (644 ± 99 v. 337 ± 38 FI, respectively; P < 0.05) of IVM. By 22 h of IVM, the GJC of COCs from 3-mm follicles had decreased (227 ± 18 FI) and was no longer significantly different to that of COCs from 5–8-mm follicles (139 ± 15 FI; P > 0.05). Dibutyryl cAMP had no effect on the cAMP content, cumulus expansion or GJC of the whole COC. In conclusion, the results of the present study demonstrate that COCs from 3-mm follicles accumulate less intraoocyte and inter-COC cAMP, display lower cumulus expansion and maintain their cumulus cell–oocyte GJC for longer during IVM than do COCs from 5–8-mm follicles.