Channel-dependent permeation of water and glycerol in mouse morulae

The cryosensitivity 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 (L-p, mu m min(-1) atm(-1)) of oocytes and four-cell embryos at 25 degrees C was low (0.63-0.70) and its Arrhenius activation energy (E-a, 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 L-p of morulae and blastocysts was quite high (3.6-4.5) and its E-a 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 L-p 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 (P-GLY, x 10(-3) cm/min) of oocytes was also low (0.01) and its E-a was remarkably high (41.6), whereas P-GLY of morulae was quite high (4.63) and its E-a was low (10.0). Aquaporin inhibitors reduced the P-GLY 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.