SELECTIVITY OF INHIBITION OF NA+ CA-2+ EXCHANGE OF HEART-MITOCHONDRIA BY BENZOTHIAZEPINE CGP-37157

The objective was to determine if the benzothiazepine compound CGP-37157 selectively inhibits the Na+-Ca2+ exchanger of cardiac mitochondria without affecting the L-type voltage-dependent calcium channel, the Na+-Ca2+ exchanger, or the Na+-K+-ATPase of the cardiac sarcolemma, or the Ca2+-ATPase of the cardiac sarcoplasmic reticulum. Mitochondrial Na+Ca2+ exchange activity was determined by monitoring intramitochondrial free [Ca2+] in isolated heart mitochondria loaded with the Ca2+-sensitive fluorophore fura-2. CGP-37157 inhibited the activity of mitochondrial Na+-Ca2+ exchange in a dose-dependent manner (IC50 0.36 muM. Calcium currents were recorded by whole-cell voltage clamp in isolated neonatal ventricular myocytes. Diltiazem was able to block the recorded current completely, thus confirming the current to be exclusively L-type. CGP-37157 had no effect on the calcium current recorded under identical conditions. CGP-37157, at concentrations less-than-or-equal-to 10 muM, had no effect on the activities of the Na+-Ca2+ exchanger and Na+-K+-ATPase in isolated cardiac sarcolemmal vesicles or on activity of the Ca2+-ATPase in isolated cardiac sarcoplasmic reticulum vesicles. The data suggest that CGP-37157 is a potent, selective, and specific inhibitor of mitochondrial Na+-Ca2+ exchange at concentrations less-than-or-equal-to 10 muM.