Down-regulation of epithelial water permeability by basolateral hypotony

Recent molecular characterizations of aquaporins have raised the prospect that H₂O permeability could be regulated. Functional evidence supporting this idea was obtained from measurements of unidirectional ³H₂O fluxes across various epithelia isolated in Ussing-type chambers. We addressed how osmolarity changes to either the apical or basolateral surfaces affected the diffusional water permeabilities of epithelia from frog bladder and cornea, and rabbit conjunctiva and ciliary body. In all cases, basolateral hypotony (tonicity to 17% of control in frog tissues and 37% in rabbit) reduced diffusional H₂O fluxes (J_dw) reversibly by 25-30%. From effects on electrical parameters and ³H-mannitol fluxes, it was clear that the reduced H₂O flow occurred transcellularly. An Arrhenius plot in cornea suggested closure of water channels. Apical hypotony elicited less extensive or insignificant changes, as did hypertonic shifts with 50 mM mannitol or 110 mM sucrose. Since epithelia have higher H₂O permeabilities basolaterally, water channel regulation at this surface may contribute towards cell volume maintenance with increases in volume triggering the response.