H + Transport in Urinary Epithelia

This review of urinary acidification is primarily based on studies in isolated epithelia such as the turtle bladder. Despite the lack of unambiguous proof, the wealth of indirect evidence suggests that the cause of bicarbonate absorption is H+ secretion into the lumen. The mechanisms that regulate H+ transport are discussed. The electrochemical gradient for protons across the membrane is found to be the most fundamental regulator not only of passive movement but also of active transport. CO2 and aldosterone stimulate H+ transport, the latter by a mechanism apparently separate from the effect of this hormone on sodium transport. Although carbonic anhydrase activity is important for optimal function of the H+ pump, the results with carbonic anhydrase inhibitors need to be interpreted with caution. The evidence for Na:H exchange is reviewed and found to be not very persuasive, The metabolic pathways that fuel H+ transport are found to be all the major energy-yielding reactions in the cell, but particular prominence is given to the new discovery of the role of the pentose shunt in energizing transport. Finally, I discuss the important role H+ transport in energy transduction in subcellular organelles.