HETEROGENEITY OF CHLORIDE BASE-EXCHANGE IN RABBIT SUPERFICIAL AND JUXTAMEDULLARY PROXIMAL CONVOLUTED TUBULES

Active transcellular NaCl transport in the proximal convoluted tubule (PCT) is via apical parallel Na/H and Cl/base exchange. The mechanism of Cl/base exchange remains unclear. The present in vitro microperfusion study examined the mechanism of Cl/base exchange in superficial and juxtamedullary PCT by examining the rate of change in intracellular pH in response to luminal Cl removal. In superficial PCT the rate of Cl/base exchange was 24.0 +/- 2.3 without formate, 36.4 +/- 6.6 with 10 mu M formate (P < 0.05), and 43.6 +/- 2.8 pmol.mm(-1).min(-1) (P < 0.001) with 1 mM luminal formate. Cl/base exchange was inhibited by luminal 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) in the presence and absence of formate. In juxtamedullary PCT, Cl/base exchange was 22.2 +/- 3.8 without formate and 25.0 +/- 5.4 pmol.mm(-1).min(-1) in the presence of 1 mM luminal formate [P = not significant (NS)]. Cl/base exchange was inhibited by luminal DIDS in juxtamedullary PCT. The rates of Cl/base exchange in both superficial and juxtamedullary PCT were not affected by 0.1 mM acetazolamide and 2 mM cyanide and were the same in the presence and absence of HCO3/CO2, consistent with Cl/OH rather than Cl/HCO3 exchange. To examine the effect of formate on PCT transport, tubules were perfused with a high-Cl solution without organics simulating late proximal tubular fluid. In superficial PCT net volume absorption (J(V)) was 0.00 +/- 0.05 in the absence of formate and 0.14 +/- 0.06 nl.mm(-1).min(-1) in the presence of 1 mM formate (P < 0.05). In juxtamedullary PCT J(V) was 0.20 +/- 0.08 in the absence of formate and 0.17 +/- 0.07 nl.mm(-1) min-l in the presence of 1 mM formate (P = NS). In conclusion, superficial PCT have both a Cl/OH and Cl/formate exchanger. Cli formate exchange is present with physiological concentrations of formate. Addition of 1 mM formate stimulates Cl transport in superficial PCT. Juxtamedullary PCT have Cl/OH exchange but do not have Cl/formate exchange. Juxtamedullary PCT have Cl transport in the absence of formate, and the rate is not affected by formate.