Acetate Transport in the $3 Segment of the Rabbit Proximal Tubule and its Effect on Intracellular pH

We moni to red intracellular p H (pH.~ in isolated perfused $3 segments o f the rabbi t proximal tubule, and s tudied the effect o f acetate (Ac-) transpor t on p H i. p h i was calculated from the absorbance spect rum of 4' ,5'-dimethyl5-(and 6) carboxyfluorescein t r apped intracellularly. All solutions were nominally HCO~-free. Removal o f 10 mM Acf rom bath and lumen caused pH~ to rapidly rise by ~0.2, and then to decline more slowly to a value 0 . 3 5 below the initial one. Removal o f only luminal Accaused pH~ changes very similar to those resulting f rom bilateral removal o f Ac. When Acwas removed from bath only, p H i rose rapidly at first, and then cont inued to rise more slowly. Readdl t ion o f Acto bath caused p H i to rapidly fall to a value slightly higher than the one prevailing before the removal o f Acfrom the bath. In exper iments in which Acwas first removed from both bath and lumen, readdl t ion o f 10 mM Acto only lumen caused a rapid but small acidification, followed by a slower alkalinization that b rought the pH~ near the value that prevailed before the bilateral removal o f Ac. The alkalinizing effects caused by the readdl t ion o f 10 or 0.5 mM Acwere indistinguishable. When Acwas re tu rned to only lumen in the absence o f luminal Na +, there was a small and rapid p h i decrease, but no pH~ recovery. Removal of Na + from bath did not affect the pHi transients caused by the addi t ion o f Acto lumen. In exper iments in which Acwas first removed bilaterally, readdl t ion of Acto only bath caused a large and sustained d rop in pHi, whereas the subsequent removal o f Acf rom the bath caused a slight alkalinization. These p h i changes caused by readdi t ion or removal o f Acfrom baths were unaffected by the absence o f external Na +. We conclude that there is a Na+/Ac co t ranspor te r at the luminal membrane , and pathways for acetic acid t ranspor t at both luminal and basolateral membranes. The net effect of Act ranspor t on p H i is to alkalinize the cell as a result o f the luminal entry o f Na+/Ac -, which is followed by the luminal and basolateral exit o f acetic acid. I N T R O D U C T I O N Near ly 70% o f the r eabsorp t ive work o f the k idney involves the t r a n s p o r t o f solutes by the p rox ima l tubule . Monoca rboxy l i c acids a n d o t h e r metabo l ic subs t ra tes that Address reprint requests to Dr. Walter F. Boron, Department of Cellular and Molecular Physiology, Yale University School of Medicine, 333 Cedar Street, P.O. Box 3333, New Haven, CT 06510-8026. J. GEN. PHYSIOL. O The Rockefeller University Press 9 0022-1295/88/09/0395/18 $2.00 Volume 92 September 1988 395-412 395 on Jne 1, 2017 D ow nladed fom Published September 1, 1988