Currents carried by sodium ions through transient calcium channels in clonal GH3 pituitary cells

The transient or T-type Ca channel currents of voltage-clamped clonal (GH3) pituitary cells were studied after blockade of Na and K channels. Removal of Ca ions from the bathing medium abolished the Ca currents and allowed Na ions to convey current through the transient channels. These Na currents (I Na,t) were activated at threshold potentials of ca. -85 mV, reached peak amplitudes (up to 2,000 pA) at -60 mV, and were 50% inactivated at holding membrane potentials of -110 to -120 mV. The time constant for inactivation of the maximal I Na,t was 12 +/- 2.4 msec (compared with 22.1 =/- 0.85 msec for maximal I Ca,t). The I Na,t was insensitive to tetrodotoxin (1 micron) and to nimodipine (200 nM) but was abolished by 100 micron external [Ca2+]. It is suggested that high affinity Ca binding sites in the channel modulate the ion selectivity and the current flow through T-type Ca channels.