Water permeability in rat oocytes at different maturity stages: aquaporin-9 expression

Important functional and structural modifications occur in mammalian oocytes during their arrival to maturity. In this process, oocytes switch from a high activity level, implying an important metabolic rate and a coordinated movement of water and solutes, to a lower functional state. The aim of this work was to study the mechanisms involved in water movements during oocyte arrival to maturity. Volume changes, induced by an osmotic gradient, were followed by video microscopy in rat oocytes. The water osmotic permeability (P(osm)) of immature oocytes (proestrus) was sensitive to HgCl(2) and phloretin. In contrast, mature oocytes (estrus) had a reduced P(osm) that was not sensitive to these compounds. When proestrus oocytes were incubated in vitro at 37 degrees C they spontaneously arrived at maturity and its P(osm) decreased between four and six hours of incubation. RT-PCR experiments were performed using specific primers for all rat aquaporins that had been cloned. We found that aquaporin-9 transcript (AQP9) is present in proestrus oocytes but not in estrus oocytes. AQP9 has been recently described as a "broad selective channel" responsible for solute and water transfers in highly active cells. Our experiments showed that proestrus oocytes, but not estrus, are permeable to mannitol. It is concluded that during the process of maturation, P(osm) decreases and AQP9 transcripts disappear. We report here the first study correlating water permeability and aquaporin mRNA expression in mammalian oocytes.