Channels in epithelial cell membranes and junctions

Epithelia may be classified as "tight" or "leaky," depending on whether there is a significant pathway for transepithelial ion permeation via the junctions and bypassing the cells. The resistance of this paracellular channel may depend partly on structures visible in the electron microscope, partly on wall charge. Permeability determinations in the leaky junctions of gallbladder epithelium, using many different organic cations, suggest that the critical barriers barriers to ion permeation are 5--8 A in radius and bind cations by up to four strongly proton-accepting oxygens. The apical cell membrane of tight epithelia contains a Na+-selective channel that is blocked by amiloride and Ca2+, subject to negative feedback control by the Na+ pump in the basolateral membrane, and somehow promoted by aldosterone. To determine the permeabilities of these two channels (the junctional channel of leaky epithelia, and the Na+ channel of tight epithelia) to water and nonelectrolytes remains a major unsolved problem.