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

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Abstract
The ionic conductances of GH3 cells, a transformed line from rat anterior pituitary, were studied using the whole-cell variant of the patch-clamp technique. 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. Three conductances were identified: 1 carrying only outward current and the other 2 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 K conductance. One of the 2 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 Na 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 Ca2+ or Ba2+ serves as current carrier. During a prolonged pulse, current inactivates fairly completely if there is at least 5 mM Ca2+ outside and the amplitude of the current tails following the pulse diminishes with the time course of inactivation. When Ba2+ entirely replaces Ca2+ 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 (Ca2+ outside) is unaltered by 50 mM EGTA [glycoletherdiamine-N,N,N'',N''-tetraacetic acid] in the pipette: inactivation cannot be the result of internal accumulation of Ca2+.