New drugs for the Na+/H+ exchanger.: Influence of Na+ concentration and determination of inhibition constants with a microphysiometer

The NHE-1 isoform of the Na+/H+ exchanger is excessively activated in cardiac cells during ischemia. Hence NHE-1 specific inhibitors are being developed since they could be of beneficial influence under conditions of cardiac ischemia and reperfusion. In this study, the Cytosensor(TM) microphysiometer was used to measure the potency of four new drug molecules, i.e., EMD 84021, EMD 94309, EMD 96785 and HOE 642 which are inhibitors of the isoform 1 of the Na+/H+ exchanger. The experiments were performed with Chinese hamster ovary cells (CHO K1) which are enriched in the NHE-1 isoform of the Na+/H+ antiporter. The Na+/H+ exchanger was stimulated with NaCl and the rate of extracellular acidification was quantified with the Cytosensor. The proton exchange rate was measured as a function of the NaCl concentration in the range of 10-138 mM NaCl stimulation. The proton exchange rate followed Michaelis-Menten kinetics with a K-M = 30 +/- 4 mM for Na+. Addition of either one of the four inhibitors decreased the acidification rate. The IC50 values of the four compounds could be determined as 23 +/- 7 nM for EMD 84021, 5 +/- 1 nM for EMD 94309, 9 +/- 2 nM for EMD 96785 and 8 +/- 2 nM for HOE 642 at 138 mM NaCl, in good agreement with more elaborate biological assays. The IC50 values increased with the NaCl concentration indicating competitive binding of the inhibitor. The microphysiometer approach is a fast and simple method to measure the activity of the Na+/H+ antiporter and allows a quantitative kinetic analysis of the proton excretion rate.