Active NaCl transport in the cortical thick ascending limb of Henle's loop of the mouse does not require the presence of bicarbonate.

The aim of the present study was to investigate whether bicarbonate buffer (CO2 + HCO3-) is required to sustain maximal NaCl transport in the cortical thick ascending limb of Henle's loop (cTAL) of the mouse. Transepithelial Na+ and Cl- net fluxes (JNa, JCl, pmol min-1 mm-1), measured by electron microprobe analysis, were similar irrespective of the presence or absence of CO2 + HCO3- in luminal and bathing solutions (JNaCl with CO2 + HCO3- = 203 +/- 25 pmol min-1 mm-1; JNaCl without CO2 + HCO3- = 213 +/- 13 pmol min-1 mm-1, n = 14). Furthermore the transepithelial potential difference, Vte, the transepithelial resistance, Rte, and the basolateral membrane potential, Vbl, were unaffected by CO2 + HCO3-. In the absence of CO2 + HCO3-, Vte was +17.0 +/- 1.7 mV (n = 9) (lumen positive), Rte was 28 +/- 2 omega cm2 (n = 9) and Vbl was -76 +/- 4 mV (n = 6). In the presence of CO2 + HCO3-, Vte, Rte and Vbl were +15.9 +/- 1.5 mV, 29 +/- 1 omega cm2 and -73 +/- 5 mV, respectively. 4-Acetamido-4'-isothiocyanatostilbene-2,2'-disulphonic acid (SITS; 0.1 mmol l-1) and amiloride (1 mmol l-1) added to the (CO2 + HCO3-)-containing lumen perfusate were without effect on Vte and Rte. Finally, the effect of furosemide (0.1 mmol l-1) on Vte and Vbl in the presence of CO2 + HCO3- was investigated. Furosemide reversibly decreased Vte from +13.7 +/- 1.1 mV to +1.7 +/- 0.7 mV (n = 6) and hyperpolarized Vbl from -70 +/- 1 to -89 +/- 3 mV (n = 5), suggesting passive distribution of Cl- across the basolateral membrane. In conclusion, these data suggest that active NaCl transport in the cTAL of the mouse does not require the presence of CO2 + HCO3-.