CALCIUM AND SODIUM CURRENTS EVOKED BY ACTION-POTENTIAL WAVE-FORMS IN RAT SYMPATHETIC NEURONS

1. Calcium channel currents evoked by action potential waveforms were recorded from rat sympathetic neurones using the whole-cell variant of the patch-clamp technique. A voltage template was created in which the unmodified action potential (AP) was played back followed by a rectangular pulse. 2. The inhibitory effects of noradrenaline (NA) and neuropeptide Y (NPY) on the AP and rectangular pulse-evoked currents, were compared. The percentage inhibition of the Ca2+ current produced by NA and NPY was significantly greater in the case of currents evoked using AP waveforms. 3. A train of APs was applied in the voltage command (40 or 75 Hz) and the inhibition produced by NPY on the AP-evoked Ca2+ currents was examined. The inhibitory effect of NPY on the Ca2+ currents evoked by the high frequency APs did not change significantly during the train. 4. The AP falling phase was artificially prolonged and the resulting Ca2+ currents were compared. AP prolongation increased the amount of Ca2+ entering into the cell. However, the peak value of the Ca2+ current was primarily determined by the AP height. The AP plateau phase was also prolonged in defined voltage ranges. Prolongation in the positive voltage region was most effective in increasing the Ca2+ current. 5. Ion substitution studies were used to isolate Na+ and Ca2+ currents evoked by AP waveforms. The inhibitory effects of NA and oxotremorine (OXO-M) on Ca2+ currents evoked by AP waveforms were examined in the presence and absence of the Na+ current. The presence of the Na+ current did not significantly alter the inhibitory effects of NA and OXO-M on the Ca2+ current.