Calcium currents in internally perfused nerve cell bodies of Limnea stagnalis

1. When K(+) is removed from both sides of the somal membrane of Limnea neurones, time-dependent, voltage-dependent outward currents are observed at positive potentials. These currents can be carried by Tris(+) and tetraethylammonium (TEA(+)), as well as Cs(+), but the Cs currents are several times larger. The Cs currents are not affected by external or internal TEA, but are strongly reduced by 4-aminopyridine (4-AP) and all Ca blockers tried.2. The presence of these non-specific outward currents and their sensitivity to all treatments that eliminate the Ca currents prevent the complete isolation of Ca currents. The non-specific outward currents are most prominent at large positive potentials and as slow tail currents on stepping back to the holding potential.3. Ca currents are ;washed out' in well perfused cells. Typically the Ca current has decayed to less than one tenth of its original size after (1/2) h of perfusion. This wash-out is specific for the Ca current; Na and K currents persist for several hours.4. Once the Ca current has completely decayed, it is possible to study one type of non-specific current without overlapping inward currents. This current activates between 0 and +30 mV and appears to reverse near 0 mV.5. In spite of the probable presence of slowly activating outward currents, the net inward currents measured show little apparent inactivation. In all the cells studied the inward current evoked at +20 mV has never decayed by more than 50% during a 60 ms pulse. So the true inactivation of these Ca currents must be quite slow, with time constants of the order of 100 ms and larger.6. The activation of the Ca current agrees with m(2) kinetics. The rate of activation is the same for Ba currents as for Ca currents.7. When the membrane potential is stepped back to the holding level (-50 mV), the Ca current turns off quite rapidly with a time constant of about 100 mus (25 degrees C). The time constant for turning off the Ca current is not related to the time constant for turning on the Ca current at the same voltage as expected for m(2) kinetics in the Hodgkin and Huxley model. At -30 mV the tau(m) for turn-on is eight times larger than the tau(m) for turn-off.