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Introduction
Immature equine oocytes are capable of completing meiosis in vitro, but subsequent in vitro fertilization (IVF) and embryonic development of those oocytes are questionable (Dell'Aquila et al., 1997; Zhao et al., 2001; Hinrichs, 2010). The effect of growth hormone (GH) on mammalian oocytes and embryo development in vitro has been studied in human (Hassan et al., 2001), rat (Yoshimura et al., 1994) and bovine (Kolle et al., 2001; 2004). In addition, studies have also demonstrated that equine oocytes resumed meiosis in the presence of equine GH (eGH) in vitro (Marchal et al., 2003; Pereira et al., 2012). Recently, we have reported the presence of eGH-R in equine ovarian follicular structures such as cumulus cells and oocyte, which may mediate a positive effect when eGH is used in culture during equine oocyte maturation in vitro (Pereira et al., 2013a). Furthermore, investigations are needed for a better understanding of the developmental competence of equine oocytes when eGH is added during in vitro maturation (IVM) to improve the efficiency and the use of assisted reproductive technologies (ART) in the horse.
Studies have demonstrated the critical roles of GH action, GH receptor (GH-R) and IGF-I in the mediation of cell growth (Liu & LeRoith, 1999; Lupu et al., 2001). The effects of rbGH on the IVM of bovine oocytes and cumulus cell expansion inhibition were exerted through kinase activity, thus indicating that rbGH action is not mediated by a tyrosine kinase pathway in bovine (Izadyar et al., 1997a). In cattle, GH action on oocytes is not mediated by IGF-I but is mediated by the cyclic AMP (cAMP) signal transduction pathway (Izadyar et al., 1997b). There have been no published reports on the level at which the eGH could be inhibited during IVM of equine oocytes, with this in mind we investigated if the possible eGH effect is mediated through the cAMP or adenylate cyclase (AC) cascade.
cAMP plays an important role in the control of oocyte maturation and development in mammals (Raper et al., 1995; Liu et al., 2013). The AC is composed of two cytoplasmic domains and two membrane-spanning domains, which interact with G-protein...