CELL SWELLING ACTIVATES A POORLY SELECTIVE MONOVALENT CATION CHANNEL IN THE APICAL MEMBRANE OF TOAD URINARY-BLADDER

We measured the effects of conditions that increase cell volume on ion movements through the Ca2+-blockable poorly selective monovalent cation channel of the apical membrane of the toad urinary bladder. Three conditions were studied: dilution of the basolateral solution, basolateral perfusion with solutions prepared with solutes of low reflection coefficient and dilution of the apical solution in bladders treated with oxytocin. All three procedures markedly increased K+ movements and elevated the plateau of the Lorentzian component of the power spectrum by enhancing ion currents through the Ca2+-blockable pathway. Simultaneously there was a large increase in Ca2+-sensitive conductance. The magnitude of this increased conductance strongly suggests that the stimulation of ion flow is due to increased ion permeability and not solely to increases in driving force across the apical membrane. We were not able to detect an increase in the movements of alkali-earth ions induced by the conditions that increase cell volume. We speculate that activation of the Ca2+-blockable channel may play an important role in the regulation of cell volume and/or in K+ homeostasis.