Nuclear maturation, steroid metabolism and lipid droplets in porcine oocytes cultured with IBMX or dbcAMP

We previously reported in porcine oocytes that the activities of some hydroxysteroid dehydrogenases (HSDs) and the size of lipid droplets in the cytoplasm decreased as nuclear maturation progressed, and that the progression of nuclear maturation, the decrease of HSD activities and the reduction in the size of lipid droplets did not occur in those treated with olomoucine. From these results, we suggested that the changes in the steroid metabolism and the size of lipid droplets in the cytoplasm were closely associated with nuclear maturation. However, whether olomoucine directly acts on the cytoplasm to inhibit such changes could not be determined. In the present investigation, the activities of some HSDs and Sudanophilic lipid droplets were histochemically demonstrated in porcine oocytes in which the nuclear maturation was suppressed and the high cAMP level in the cytoplasm was maintained by the treatment of IBMX or dbcAMP, in order to clarify the relationship between nuclear maturation and the metabolism of steroids and the number of lipid droplets in the cytoplasm. Of the oocytes cultured with IBMX or dbcAMP for 22 hrs, 97 and 100% were in the germinal vesicle (GV) stage, respectively. The percentages of oocytes in the GV stage were significantly higher in both the treated oocytes than in control oocytes. The rates of the treated oocytes showing the activities of deltasup(5)-3beta-HSD (using pregnenolone and 17 alpha-hydroxypregnenolone as the substrates), 17 beta-HSD (estradiol-17 beta), 20 alpha-HSD (20 alpha-hydroxyprogesterone) and 20 beta-HSD (17beta-hydroxyprogesterone) did not differ from those of control oocytes. Also, there were no differences in the number of lipid droplets of different sizes between IBMX- or dbcAMP-treated oocytes and control oocytes. From these findings, it was suggested that the changes in the steroid metabolism and the size of lipid droplets in the cytoplasm with oocyte maturation depend on the cAMP level in their cytoplasm rather than the progression of nuclear maturation.