| Abstract: | The dihydropyridine calcium channel blocker, nitrendipine, was studied for its effects on the sodium current of single cultured ventricular cells from neonatal rats. The patch-clamp method of recording whole cell currents was used, and sodium currents were isolated by suppressing potassium and calcium currents. Potassium currents were blocked by replacing potassium with cesium in the internal and external solutions and by adding tetraethylammonium chloride and 4-aminopyridine in the external solution; calcium current was blocked by replacing calcium with cobalt in the external solution. At low frequencies (0.1 Hz), nitrendipine reduced sodium currents without any significant change in the current-voltage relation. The block was dose dependent, and assuming a single occupancy model with complete block, had a half-maximum value of 3 X 10(-6) M at a holding potential of -80 mV where half the sodium channels are activatable. This value is within the range of the Kd's that have been reported for low-affinity dihydropyridine-binding sites found in cardiac sarcolemmal vesicles. In the presence of nitrendipine, the inactivation curve was shifted to hyperpolarized potentials. The block was greater with pulse intervals shorter than 1000 msec, and repriming was prolonged in the presence of the drug. These effects are similar to those of local anesthetics of the tertiary amine class, such as lidocaine. The block was relieved by the dihydropyridine agonist Bay K8644. The results are interpreted as indicating that dihydropyridines react with sodium channels. |
