Effect of Carbonic Anhydrase , Na ' - H + Exchange Inhibitors and the Loop Diuretics Furosemide and

We microperfused the loop of Henle (LOH) to assess its contribution to urine acidification in vivo. Under control conditions (Na HCO3= 13 mM, perfusion rate 17 nl/min-') net bicarbonate transport (JHCO-) was unsaturated, flowand concentration-dependent, and increased linearly until a bicarbonate load of 1,400 pmols minwas reached. Methazolamide (2-10-4 M) reduced JHCO3 by 70%; the amiloride analogue ethylisopropylamiloride (EIPA) (2. 10-4 M) reduced JHCO3 by 40%; neither methazolamide nor EIPA affected net water flux (Jv). The H+-ATPase inhibitor bafilomycin Al (10-i M) reduced JHCO3 by 20%; the C1channel inhibitor 5-nitro-2'(3-phenylpropylamino)-benzoate (2 10-' M) and the Cl-base exchange inhibitor diisothiocyanato-2,2'-stilbenedisulfonate (5. 10-5 M), had no effect on fractional bicarbonate reabsorption. Bumetanide (10-' M) stimulated bicarbonate transport (net and fractional JHCO3) by 20%, whereas furosemide (10-4 M) had no effect on bicarbonate reabsorption; both diuretics reduced Jv. In summary: (a) the LOH contributes significantly to urine acidification. It normally reabsorbs an amount equivalent to 15% of filtered bicarbonate; (b) bicarbonate reabsorption is not saturated; (c) Na'-H' exchange and an ATP-dependent proton pump are largely responsible for the bulk ofLOH bicarbonate transport. (J. Clin. Invest. 1991. 88:430-437.)