Chloride transport by rat renal proximal tubule: effects of bicarbonate absorption

In the early part of the proximal tubule chloride concentration is increased above that in plasma, a change attributed to preferential absorption of bicarbonate with sodium and water. To determine the mechanism(s) of the preferential absorption of HCO3 over chloride, in vivo paired perfusions were done in surface proximal tubules of the rat kidney. Each tubule was perfused with a control bicarbonate solution, similar in ionic composition to the solution normally present in the early part of the proximal tubule, and one of the experimental solutions that were modified by either replacing all of bicarbonate with sulfate, N-2-hydroxyethylpiperazine-N'-ethanesulfonate (HEPES), or all bicarbonate with sulfate and sodium with lithium, or addition of acetazolamide (ATZ), or cyanide (CN). Sufficient mannitol was added to reduce net fluid flux to zero. When sulfate replaced bicarbonate in luminal fluid, net chloride absorption was enhanced, a change associated with low lumen HCO3 and low pH. ATZ inhibited HCO3 absorption, whereas net Cl absorption increased, an effect associated with high HCO3 and low pH. With HEPES-plus-ATZ solutions, chloride absorption also increased but, in contrast to SO4 or ATZ solutions, pH increased. With sulfate and lithium replacement chloride absorption also increased despite net sodium secretion. CN, a metabolic inhibitor, inhibited HCO3 and sodium absorption but stimulated net chloride absorption. The data indicate that HCO3 absorption inhibits chloride absorption, a process independent of luminal HCO3 per se, luminal pH, sodium absorption, and transepithelial volume flux and voltage.