Inhibition of the Bicarbonate Exit Step in

the isolated turtle bladder involves H+ secretion by a pump at the luminal membrane. The OH- dissociated in this process reacts with CO2 and forms HCO- which moves passively out of the cell across the serosal cell membrane. In the present study, this exit step for HCO3 was inhibited by serosal addition of the disul-fonic stilbene, SITS, an agent which is thought to bind to a transport protein at the serosal cell membrane. 90 min after serosal addition of 0.5 mM SITS, H+ secretion decreased by> 80%. In contrast, luminal addition of SITS had no effect. During inhibition of H+ secretion by serosal SITS, overall cell pH, measured by the 5,5-dimethyl-2,3-oxazolidinedione method, increased from 7.48+0.03 to 7.61+0.02. This increase of0. 13+0.02 pH U was associated with a much larger regional pH increase as judged from the decre-ment in the attainable pH gradient across the epi-thelium. After serosal SITS, this gradient was reduced from 2.88+0.06 to 2.09±0.11 pH U. In the absence of evidence for increased H+ permeability or a change in the force of the H+ pump, the gradient decrement of 0.79±0.08 U reflects a similar pH increment on the cytoplasmic side of the pump. SITS inhibits the exit of bicarbonate across the serosal cell membrane and, thereby, creates a com-partment of high alkalinity in series with the pump. The increased electrochemical gradient across the active transport pathway is the primary factor in the inhibition of urinary acidification