Calcium currents in internally perfused nerve cell bodies of Limnea stagnalis

Abstract

When K+ is removed from both sides of the somal membrane of Lymnea neurons, time-dependent, voltage-dependent outward currents are observed at positive potentials. These currents can be carried by Tris+, tetraethylammonium (TEA+), and 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 and all Ca blockers tried. 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. Ca currents are washed out in well perfused cells. Typically the Ca current has decayed to < 1/10 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. Once the Ca current has completely decayed, it is possible to study 1 type of non-specific current without overlapping inward currents. This current activates between 0 and +30 mV and appears to reverse near O mV. The net inward currents measured show little apparent inactivation. The inward current evoked at +20 mV never decayed by > 50% during a 60 ms pulse. The true inactivation of these Ca currents must be quite slow, with time constants of the order of .gtoreq. 100 ms. The activation of the Ca current agrees with m2 kinetics. The rate of activation is the same for Ba currents as for Ca currents. When the membrane potential is stepped back to the holding level (-50 mV), the Ca current turns off rapidly with a time constant of .apprx. 100 .mu.s (25.degree. 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 m2 kinetics in the Hodgkin and Huxley model. At -30 mV the .tau.m for turn-on is 8 times larger than the .tau.m for turn-off.