Axial heterogeneity of rabbit proximal tubule luminal H⁺ and basolateral HCO₃^- transport

The present in vitro microperfusion study examined whether the rates of the apical membrane Na⁺-H⁺ antiporter and basolateral membrane Na(HCO₃)₃ symporter vary along the length of the proximal tubule. Initial proximal convoluted tubules (PCT obtained within 0.5 mm from the glomerulus), mid-PCT (PCT without glomerular attachments) and cortical proximal straight tubules were examined. The rate of either the apical or basolateral membrane acidification mechanism was measured from the initial rate of change of intracellular pH after a change in either the luminal or bathing solution. Intracellular pH was measured fluorometrically using the pH-sensitive dye (2',7')-bis(carboxyethyl)-(5,6)-carboxyfluorescein. The rate of bicarbonate exit across the basolateral membrane was examined by imposing either a sodium or bicarbonate gradient. There was no difference between initial and mid-PCT, but the rate of change in cell pH was 30% slower in PST in both series. The rate of sodium-dependent apical proton secretion was examined by changing the sodium concentration in the lumen. There was no difference in sodium-dependent apical proton secretion in initial vs. mid-PCT, but the rate fell by 70% in the proximal straight tubule (PST). These differences were not due to a difference in buffer capacity in these segments. These data are consistent with a homogeneous rate of apical Na⁺-H⁺ antiporter and basolateral Na(HCO₃)₃ activity along the rabbit PCT, but a lower rate in the PST.