238 THE ROLE OF PHOSPHODIESTERASE (PDE) 5A AS MEDIATOR OF LIPOLYSIS IN BOVINE OOCYTES AND ITS EFFECT ON THE DEVELOPMENT AND LIPID CONTENT OF EMBRYOS PRODUCED IN VITRO

Abstract

Intracellular levels of cyclic adenosine monophosphate modulators (cAMP) and cGMP, in adipocytes, are important for the regulation of the lipolysis rate. The phosphodiesterases (PDE) control cGMP and cAMP levels by degradation. Different PDE isoforms are expressed in bovine oocytes and cumulus cells. Previously, we found that using an inhibitor of PDE5A (sildenafil, SILD) increased cGMP levels in bovine oocytes during in vitro maturation (IVM). In the current study we investigated if inhibition of PDE5A during maturation reduces the lipid content in IVF embryos. For this, oocytes were cultured for 24 h in maturation medium with 10% FCS and 10^–7 M SILD (treatment I), 10% FCS (treatment II) and 0.4% BSA (control; N ± 160 COC/groups submit to IVF). After COC were in vitro fertilized, cleavage (Day 4) and blastocyst rates (Day 7) were measured. Blastocysts were stained with Nile Red (1 μg mL^–1) for lipid content quantification, by mean fluorescence intensity per μm², measured in the ImageJ program (fluorescence intensity, f.i.). Four replicates were transformed to log₁₀ and subjected to statistical analysis using the SAS system (SAS Institute Inc., Cary, NC, USA) by ANOVA followed by Tukey test with a significance level of 5%. No difference in cleavage (Day 4) and blastocyst (Day 7) rates were observed in all groups (82 and 41.9%, respectively), showing that presence of FCS, SILD, or both in IVM medium did not affect embryo development. Treatment I had higher lipid content (40.35 f.i.) than treatment II (31.12 f.i.), which in turn was also superior to control (22.31 f.i.). According to the results, the presence of FCS in IVM media generates embryos with higher lipid content, and association of FCS and SIL further increased lipid content. Although inhibition of PDE5 increases cGMP levels and leads to higher lipolysis, such an effect was not observed when SIL was used as the PDE5 inhibitor. Reasons for such findings are still unclear, but a possibility would be the activation of a negative feedback mechanism by the increased cGMP generated by SIL, because this nucleotide activates PKG, which in turn inhibits cGMP synthesis by guanylate cyclase. During development the lower cGMP levels could reduce lipolysis, resulting in increased lipid accumulation in embryos. Further studies are needed to address this possibility.