Aquaporin and proteolysis of yolk and ovarian follicles play essential roles in the regulation of oocyte hydration during oocyte maturation in the Japanese eel (Anguilla japonica)
To elucidate mechanisms of oocyte cytoplasmic maturation, the present study examined the effects of the presence of ovarian follicles, an uncoupler of gap junction (carbenoxolone), inhibitors of aquaporin water permeability (HgCl2) and yolk proteolysis (bafilomycin A1) on the hydration of oocytes in the Japanese eel during in vitro oocyte maturation. Human chorionic gonadotropin (HCG) and 17, 20β-dihydroxy- 4-pregnen-3-one (DHP) caused an increase in the follicle-enclosed oocyte (follicle) diameter but not in the de-folliculated oocyte diameter. Carbenoxolone did not influence HCG-induced increase in the follicle diameter. Addition of HgCl2 and bafilomycin A1 to the incubation media inhibited HCG- and DHP-induced increase in the follicle diameter in a dose-dependent manner. These data indicate that the ovarian follicles may play an essential role in HCG- and DHP-induced oocyte hydration. Furthermore, aquaporin facilitates water uptake as a water channel, and yolk proteolysis is essential for the water influx into oocytes via osmotic mechanisms.