Probing the water permeability of ROMK1 and amphotericin B channels using Xenopus oocytes

Abstract

Water permeability of ion channels in the plasma membrane of Xenopus oocytes was studied by simultaneously measuring the membrane conductance under two-electrode voltage-clamp and the cell size by video-imaging technique. The basal level of osmotic water permeability of oocyte plasma membrane was 15.9±0.98 μm/s (SE, n=5). Extracellular application of pore-forming antibiotic amphotericin B at 5 μM developed macroscopic conductance of 995±70 μS ( n=5) and increased the osmotic water permeability of cell membrane by 44.9±4.1 μm/s. Meanwhile, after expressing ROMK1 channels, originally cloned from kidney, virtually no increase in the water permeability was observed even at the conductance level as high as 1113±47 μS ( n=5). This result suggests that even though potassium channels, like any others, are considered to be water-filled pores, K +-selective ion-transporting pathway remains virtually water-impermeable in physiological conditions, such as in kidney epithelia where huge water transport takes place at both apical and basolateral sides.