Control of active proton transport in turtle urinary bladder by cell pH.

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Abstract
The rate of active H+ secretion (JH) across the luminal cell membrane of the turtle [Pseudemys scripta] bladder decreases linearly with the chemical (.DELTA.pH) or electrical potential gradient (.DELTA..PSI.) against which secretion occurs. To examine the control of JH from the cell side of the pump, acid-base changes were imposed on the cellular compartment by increasing serosal [HCO3-] at constant pCO2 or by varying pCO2 [partial pressure of CO2] at constant [HCO3-]. When serosal [HCO3-] was increased from 0 to 60 mM, cell [H+] decreased, as estimated by the 5,5-dimethyloxazoladine-2,4-dione method. JH was a saturable function of cell [H+], with an apparent Km of 25 nM. When pCO2 was varied between 1-20% at various serosal [HCO3-], the pCO2 required to reach a maximal JH increased with [HCO3-] so that JH was a function of cell [H+] rather than of cell [HCO3-] or CO2. The proton pump was controlled asymmetrically with respect to the pH component of the electrochemical potential for protons, ~.mu.H. On the cell side of the pump, a .DELTA.pH of < 1 U was required to vary JH between maximal and zero values, whereas on the luminal side a .DELTA.pH of 3 U was required. Cell [H+] regulates JH by determining the availability of H+ to the pump in a relationship resembling Michaelis-Menten kinetics. Increasing luminal [H+] generates an energy barrier at a luminal pH near 4.4 that equals the free energy (per H+ translocated) of the metabolic driving reaction.