Na and Ca channels in a transformed line of anterior pituitary cells

The ionic conductances of GH3 cells, a transformed line from rat anterior pituitary, have been studied using the whole-cell variant of the patch-clamp technique (Hamill et al., 1981). Pipettes of very low resistance were used, which improved time resolution and made it possible to control the ion content of the cell interior, which equilibrated very rapidly with the pipette contents. Time resolution was further improved by using series resistance compensation and "ballistic charging" of the cell capacitance. We have identified and partially characterized at least three conductances, one carrying only outward current, and the other two normally inward. The outward current is absent when the pipette is filled with Cs+ instead of K+, and has the characteristics of a voltage-dependent potassium conductance. One of the two inward conductances (studied with Cs+ inside) has fast activation, inactivation and deactivation kinetics, is blocked by tetrodotoxin (TTX), and has a reversal potential at the sodium equilibrium potential. The other inward current activates more slowly and deactivates with a quick phase and a very slow phase after a short pulse. Either Ca++ or Ba++ serves as current carrier. During a prolonged pulse, current inactivates fairly completely if there is at least 5 mM Ca++ outside, and the amplitude of the current tails following the pulse diminishes with the time course of inactivation. When Ba++ entirely replaces Ca++ in the external medium, there is no inactivation, but deactivation kinetics of Ca channels vary as pulse duration increases: the slow phase disappears, the fast phase grows in amplitude. Inactivation (Ca++ outside) is unaltered by 50 mM EGTA in the pipette: inactivation cannot be the result of internal accumulation of Ca++.