The Relation Between Transport Rate and ASH

�A number of tight urinary epithelia, as exemplified by the turtle bladder, acidify the luminal solution by active transport ofH* across the luminal cell membrane . The rate of active H+ transport (JH) decreases as the electrochemical potential difference for H+ [o~H = AH(lumen) - ~H(serosa)] across the epithelium is increased . The luminal cell membrane has a low permeability for H' equivalents and a high electrical resistance compared with the basolateral cell membrane . Changes in j,4 thus reflect changes in activeH+ transport across the luminal membrane . To examine the control ofJH by A/base conditions or the luminal pH was varied at 04, = 0 and constant serosal Pco s and pH . When the luminal compartment was acidified from pH 7 to 4 or was made electrically positive, JH decreased as a linear function of D~H as previously described . When the luminal compartment was made alkaline from pH 7 to 9 or was made electrically negative, JH reached a maximal value, which was the same whether the AAIi was imposed as a ApH or a Dip. The nonlinearJH vs . Alai relation does not result from changes in the number ofpumps in the luminal membrane orfrom changes in the intracellular pH, but is a characteristic of the H' pumps themselves . We propose a general scheme, which, because of its structural features, can account for the nonlinearity oftheJH vs . A~H relations and,more specifically, for the kinetic equivalence of the effects of the chemical and electrical components of D~H . According to this model, the pump complex consists of two components : a catalytic unit at the cytoplasmic side of the luminal membrane, which mediates the ATP-driven H* translocation, and a transmembrane channel, which mediates the transfer of H+ from the catalytic unit to the luminal solution . These two components may be linked through a buffer compartment for H+ (an antechamber) .