Coupled Na+/H+ exchange in rat parotid basolateral membrane vesicles.

pH gradient-dependent sodium transport in highly purified rat parotid basolateral membrane vesicles was studied under voltage-clamped conditions. In the presence of an outwardly directed H+ gradient (pHin = 6.0, pHout = 8.0) 22Na uptake was approximately ten times greater than uptake measured at pH equilibrium (pHin = pHout = 6.0). More than 90% of this sodium flux was inhibited by the potassium-sparing diuretic drug amiloride (K1 = 1.6 microM) while the transport inhibitors furosemide (1 mM), bumetanide (1 mM), SITS (0.5 mM) and DIDS (0.1 mM) were without effect. This transport activity copurified with the basolateral membrane marker K+-stimulated p-nitrophenyl phosphatase. In addition, 22Na uptake into the vesicles could be driven against a concentration gradient by an outwardly directed H+ gradient. pH gradient-dependent sodium flux exhibited a simple Michaelis-Menten-type dependence on sodium concentration consistent with the existence of a single transport system with KM = 8.0 mM at 23 degrees C. A component of pH gradient-dependent, amiloride-sensitive sodium flux was also observed in rabbit parotid basolateral membrane vesicles. These results provide strong evidence for the existence of a Na+/H+ antiport in rat and rabbit parotid acinar basolateral membranes and extend earlier less direct studies which suggested that such a transporter was present in salivary acinar cells and might play a significant role in salivary fluid secretion.