Amiloride, Animals, Barium, Cell Membrane, Female, Hydrogen-Ion Concentration, In Vitro Techniques, Ion Channel Gating, Kidney Cortex, Kidney Tubules, Collecting, Kinetics, Male, Mathematics, Membrane Potentials, Microscopy, Electron, Scanning, Models, Biological, Potassium Channels, Rats, Rats, Sprague-Dawley