Channel-Dependent Permeation of Water and Glycerol in Mouse Morulae

Thecryosensitivity of mammalian embryos depends on the stage of development. Because permeability to water and cryoprotectants plays an important role in cryopreservation, it is plausible that the permeability is involved in the difference in the tolerance to cryopreservation among embryos at different developmental stages. In this study, we examined the permeability to water and glycerol of mouse oocytes and embryos, and tried to deduce the pathway for the movement of water and glycerol. The water permeability (LP, µm min^-1 atm^-1) of oocytes and four-cell embryos at 25°C was low (0.63-0.70) and its Arrhenius activation energy (Ea, kcal/mol) was high (11.6-12.3), which implies that the water permeates through the plasma membrane by simple diffusion. On the other hand, the Lp of morulae and blastocysts was quite high (3.6-4.5) and its Ea was quite low (5.1-6.3), which implies that the water moves through water channels. Aquaporin inhibitors, phloretin and p-(chloromercuri) benzene-sulfonate, reduced the Lp of morulae significantly but not that of oocytes. By immunocytochemical analysis, aquaporin 3, which transports not only water but also glycerol, was detected in the morulae but not in the oocytes. Accordingly, the glycerol permeability (PGLY, x 10^-3 cm/min) of oocytes was also low (0.01) and its Ea was remarkably high (41.6), whereas PGLY of morulae was quite high (4.63) and its Ea was low (10.0). Aquaporin inhibitors reduced the PGLY of morulae significantly. In conclusion, water and glycerol appear to move across the plasma membrane mainly by simple diffusion in oocytes but by facilitated diffusion through water channel(s) including aquaporin 3 in morulae.