Evidence for permanent water channels in the basolateral membrane of an ADH-sensitive epithelium

The transepithelial water permeability in frog urinary bladder is believed to be essentially dependent on the ADH-regulated apical water permeability. To get a better understanding of the transmural water movement, the diffusional water permeability (Pd) of the basolateral membrane of urinary bladder was studied. Access to this post-luminal barrier was made possible by “perforating” the apical membrane with amphotericin B. The addition of this antibiotic increasedPd from 1.12±0.10×10−4 cm/sec (n=7) to 4.08±0.33×10−4 cm/sec (n=7). The effect of mercuric sulfhydryl reagents, which are commonly used to characterize water channels, was tested on amphotericin B-treated bladders. HgCl2 (10−3m) decreasedPd by 52% andpara-chloromercuribenzoic acid (pCMB) (1.4×10−4m) by 34%. The activation energy for the diffusional water transport was found to increase from 4.52±0.23 kcal/mol (n=3), in the control situation, to 9.99±0.91 kcal/mol (n=4) in the presence of 1.4×10−4mpCMB. Our second approach was to measure the kinetics of water efflux, by stop-flow light scattering, on isolated epithelial cells from urinary bladders.pCMB (0.5 or 1.4×10−4m) was found to inhibit water exit by 91±2%. These data strongly support the existence of proteins responsible for water transport across the basolateral membrane, which are permanently present.