REDUCTION IN WATER PERMEABILITY OF THE RABBIT CONJUNCTIVAL EPITHELIUMBY HYPOTONICITY

The effects of unilateral exposure to hypotonic media on the diffusion al water permeability of the isolated rabbit conjunctiva were determin ed. For these experiments, a segment of the bulbar-palpebral conjuncti va was mounted between Ussing-type hemichambers under short-circuited conditions. Unidirectional diffusional water fluxes (J(dw)) were measu red in either direction by adding (H2O)-H-3 to one hemichamber and sam pling from the other. Electrical parameters were measured simultaneous ly, J(dw) were determined in control isosmotic conditions and after di lution of one of the bathing solutions from 290 to 108 mOsMolar. This hypotonic condition reduced J(dw) by 25-30% (n = 17) when applied baso laterally and by 25% (n = 6) apically. The effects were reversible and were also obtained when the opposite bathing solution contained ampho tericin B, selectively permeabilizing the contralateral cell surface. From concomitant changes in transepithelial electrical resistance as w ell as C-14-mannitol fluxes completed under identical conditions, argu ments are presented that the above effect is best explained as a cell regulated reduction in membrane water permeability. Presumably both ap ical and basolateral membranes can down-regulate their water permeabil ities. This response, suggesting a protective mechanism to help mainta in cell volume from hypotonicity, was also seen in other studies using the amphibian bladder and the frog cornea, in which the effect was on ly obtained basolaterally. Thus, regulation of epithelial water permea bility appears to he a basic trait common to both amphibians and mamma ls, although tissue differences exist. (C) 1998 Academic Press Limited .