Sodium-dependent chloride transport in basolateral membrane vesicles isolated from rabbit proximal tubule

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Abstract
The mechanisms for Cl transport across basolateal membrane vesicles (BLMV) isolated from rabbit renal cortex were examined by using the Cl-sensitive fluorescent indicator 6-methoxy-N-(3-sulfopropyl)quinolinium (SPQ). The transporters studied included Cl/base exchange, Cl/base/Na cotransport, K/Cl cotransport, and Cl conductance. Initial rates of chloride influx (JCl) were determined from the measured time course of SPQ fluorescence in BLMV following inwardly directed gradients of Cl and gradients of other ions and /or pH. For a 50 mM inwardly directed Cl gradient in BLMV which were voltage and pH clamped (7.0) using K/valinomycin and nigericin, JCl was 0.80 .+-. 0.14 nmol s-1 (mg of vesicle protein)-1 (mean .+-. SD, n=8 separate preparations). In the absence of Na and CO2/HCO3 in voltage-clamped BLMV, JCl increased 56% .+-. 5% in response to a 1.9 pH unit inwardly directed H gradient; the increase was further enhanced by 40% .+-. 3% in the presence of CO2/HCO3 and inhibited 30% .+-. 8% by 100 .mu.M dihydro-4,4''-diisothiocyanostilbene-2,2''-disulfonic acid. Na gradients did not increase JCl in the absence of CO2/HCO3; however, an outwardly directed Na gradient in the presence of CO2/HCO3 incrased JCl by 31% .+-. 8% with a Na KD of 7 .+-. 2 mM. These results indicate the presence of Cl/OH and Cl/HCO3 exchange, and Cl/HCO3 exchange trans-stimulated by Na. There was no significant effect of K gradients in the presence or absence of valinomycin, suggesting lack of significant K/Cl cotransport and Cl conductance under experimental conditions . The Na-dependent Cl/HCO3 echanger may provide an important route for Cl exit across the proximal tubule basolateral membrane.