Abstract
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 approximately 17 nl/min-1) net bicarbonate transport (JHCO3-) was unsaturated, flow- and concentration-dependent, and increased linearly until a bicarbonate load of 1,400 pmol.min-1 was reached. Methazolamide (2 x 10(-4) M) reduced JHCO3 by 70%; the amiloride analogue ethylisopropylamiloride (EIPA) (2 x 10(-4) M) reduced JHCO3 by 40%; neither methazolamide nor EIPA affected net water flux (Jv). The H(+)-ATPase inhibitor bafilomycin A1 (10(-5) M) reduced JHCO3 by 20%; the Cl- channel inhibitor 5-nitro-2'-(3-phenylpropylamino)-benzoate (2 x 10(-4) M) and the Cl(-)-base exchange inhibitor diisothiocyanato-2,2'-stilbenedisulfonate (5 x 10(-5) M), had no effect on fractional bicarbonate reabsorption. Bumetanide (10(-6) 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 of LOH bicarbonate transport.