Modulation of Na+-H+ exchange activity by intracellular Na+, H+, and Li+ in IMCD cells

In the present study, cultured inner medullary collecting duct (IMCD) cells loaded with the pH-sensitive dye 2',7' bis(carboxyethyl)-carboxyfluorescein were used to study the interaction of H+, Na+, and Li+ with the intracellular face of the Na+-H+ exchanger. The interaction of Na+ with the external face of the exchanger followed simple saturation kinetics with an apparent affinity for Na+ of 19.5 +/- 2 mM (n = 3) at an extracellular pH of 7.4. Extracellular H+ and Li+ inhibit Na+o-H+i exchange, mainly due to interference with Na+ binding to an external site or the same enzyme intermediate species in the turnover cycle of the Na+-H+ exchanger. The interaction of H+ with the internal face of the exchanger does not follow simple saturation kinetics. The calculated Hill coefficient for H+ interaction with intracellular sites was 1.92. Na+ and Li+ also interact with the intracellular face of the Na+-H+ exchanger. This interaction of Li+ and Na+ with internal sites results in inhibition of Na+-H+ exchange activity in the forward direction (Na+ influx/H+ efflux). The inhibitory effect of intracellular Na+ and Li+ is pHi dependent and most apparent under physiological conditions, i.e., at an intracellular pH of 7.1, an extracellular pH of 7.4, and an extracellular Na+ of 140 mM. This study shows the importance of intracellular Na+ concentration in determining exchanger activity.