Intrinsic characteristics of the proton pump in the luminal membrane of a tight urinary epithelium. The relation between transport rate and delta mu H.

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Abstract
A number of tight urinary epithelia, as exemplified by the turtle bladder, acidify the luminal solution by active transport of H+ across the luminal cell membrane. The rate of active H+ transport (JH) decreases as the electrochemical potential difference for H+ 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 JH thus reflect changes in active H+ transport across the luminal membrane. To examine the control of JH by .DELTA.~.mu.H in the turtle bladder, transepithelial electrical potential differences (.DELTA..psi.) were imposed at constant acid-base conditions or the luminal pH was varied at .DELTA..psi. = 0 and constant serosal PCO2 [CO2 partial pressure] 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 .DELTA.~.mu.+H as previously described. When the luminal compartment was made alkaline from pH 7to 9 or was made electrically negative, JH reached a maximal value, which was the same whether the .DELTA.~.mu.H was imposed as a .DELTA.pH or a .DELTA..psi.. The nonlinear JH vs .DELTA.~.mu.H relation does not result from changes in the number of pumps in the luminal membrane or from changes in the intracellular pH, but is a characteristic of the H+ pumps themselves. A general scheme was proposed, which because of its structural features, can account for the nonlinearity of the JH vs. .DELTA.~.mu.H relations and, more specifically, for the kinetic equivalence of the effects of the chemical and electrical components of .DELTA.~.mu.H. According to this model, the pump complex consists of 2 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 2 components may be linked through a buffer compartment for H+ (an antechamber).