Calcium inward currents in internally perfused giant axons

Voltage clamp experiments were carried out on squid axons perfused with an isotonic solution of 25 mM-CsF + sucrose and placed in a Na-free solution of 100 mM-CaCl(2) + sucrose.2. Depolarizing voltage steps produced inward currents of 4-6 muA/cm(2) peak amplitude which decayed slightly during a 60 msec pulse; the inward current disappeared when the internal potential reached +50 to +60 mV and became outward for larger depolarizations.3. Tetrodotoxin completely blocked the inward current and part of the outward current. No inward currents were seen with 100 mM-MgCl(2) + sucrose as the external solution. Substituting acetate for external Cl(-) did not abolish the tetrodotoxin-sensitive outward currents.4. It is concluded that the inward current is carried by Ca and the tetrodotoxin-sensitive outward current by Cs ions, both moving through the Na channel.5. The reversal potential of the tetrodotoxin-sensitive current was in the average +54 mV. Raising the external Ca concentration or adding NaCl to the external solution increased the reversal potential; lowering the external Ca concentration or replacing the internal CsF by a Na salt decreased the reversal potential.6. From the reversal potentials of the tetrodotoxin-sensitive current measured with varying external and internal solutions the relative permeabilities of the Na channel were calculated as P(Ca)/P(Cs) = 1/0.6, P(Ca)/P(Na) = 1/10 to 1/7 and P(Cs)/P(Na) = 1/22 to 1/9 by means of the constant field equation. The permeability ratios suggest that under these experimental conditions the Na channel is still primarily permeable to Na ions, although its selectivity is relatively small.7. The time course of the tetrodotoxin-sensitive Ca inward current was different from the time course of the Na inward current. The Na current consisted of an initial peak followed by a more slowly decaying component, the Ca current showed only the slow component.8. The slowly inactivating tetrodotoxin-sensitive Ca inward currents give rise to the long lasting action potentials which have first been observed by Tasaki and coworkers under similar conditions.