Ionic currents during the action potential in the molluscan neurone with the self-clamp technique

The ionic currents during the action potential in the F1 neurone of Helix aspersa were investigated, using the Self-Clamp Technique. A spontaneous action potential was recorded and then replayed, both in its direct and in its inverted form, to the same cell in voltage clamp and in control conditions. Under various experimental conditions such as treatment with the specific ionic channels blockers tetrodotoxin, lanthanum, 4-aminopyridine or tetraethylammonium, as well as low sodium and low calcium external media, the single ionic currents were detected by stimulating the membrane with the direct pulse only. The Self-Clamp Technique allowed the measuring of the following parameters, in their real time course during the action potential: a) the total action currents; b) the pharmacologically blocked ionic components; c) the ionic components which remained insensitive to the agents used (residual currents). These data were compared with those obtained by applying conventional rectangular pulses in voltage clamp. The membrane capacity was measured with the Self-Clamp Technique and the recorded currents were normalized assuming a specific capacity of 4 mu F/cm(2). The isolated ionic components were directly compared with the total action currents to evaluate the degree to which blockage was complete. The electric charge transported by each ionic specimen was evaluated as well as the individual ionic amounts. The sodium influx was 3.18 +/- 0.55 pM/cm(2) per impulse (9 cells), calcium influx 1.03 +/- 0.37 pM/cm(2) per impulse (10 cells). A value of 6.37 +/- 1.03 pM/cm(2) per impulse was found for the potassium outflux, with a probable overestimation of about 1 pM/cm(2) per impulse (9 cells).