PROBING THE WATER PERMEABILITY OF ROMK1 AND AMPHOTERICIN-B CHANNELS USING XENOPUS OOCYTES

Water permeability of ion channels in the plasma membrane of Xenopus o ocytes was studied by simultaneously measuring the membrane conductanc e under two-electrode voltage-clamp and the cell size by video-imaging technique. The basal level of osmotic water permeability of oocyte pl asma membrane was 15.9 +/- 0.98 mu m/s (SE, n = 5). Extracellular appl ication of pore-forming antibiotic amphotericin B at 5 mu M developed macroscopic conductance of 995 +/- 70 mu S (n = 5) and increased the o smotic water permeability of cell membrane by 44.9 +/- 4.1 mu m/s. Mea nwhile, 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 mu S (n = 5). This resul t suggests that even though potassium channels, like any others, are c onsidered to be water-filled pores, K+-selective ion-transporting path way remains virtually water-impermeable in physiological conditions, s uch as in kidney epithelia where huge water transport takes place at b oth apical and basolateral sides. (C) 1998 Elsevier Science B.V.