Electrogenic proton transport in epithelial membranes
Certain polar epithelial cells have strong transport capacities for protons and can be examined in vitro as part of an intact epithelial preparation. Recent studies in the isolated turtle bladder and other tight urinary epithelial indicate that the apical membranes of the carbonic anhydrase-containing cell population of these tissues contain an electrogenic proton pump which has the characteristics of a proton-translocating ATPase. The translocation of protons is tightly coupled to the energy of ATP hydrolysis. Since the pump translocates protons without coupling to the movement of other ions, it may be regarded as an "ideal" electrogenic pump. The apparent simplicity of the functional properties has led to extensive studies of the characteristics of this pump and of the cellular organization of the secondary acid-base flows in the turtle bladder. Over a rather wide range of electrochemical potential gradients, for protons (delta approximately microH) across the epithelium, the rate of H+ transport is nearly linear with delta approximately microH. The formalisms of equivalent circuit analysis and nonequilibrium thermodynamics have been useful in describing the behavior of the pump, but these approaches have obvious limitations. We have attempted to overcome some of these limitations by developing a more detailed set of assumptions about each of the transport step across the pump complex and to formulate a working model for proton transport in the turtle bladder than can account for several otherwise unexplained experimental results. The model suggests that the real pump is neither a simple electromotive force nor a constant current source. Depending on the conditions, it may behave as one or the other.