Figure 1. Expression of T-type Ca²⁺ channels in neonatal chromaffin cells.

A, representative macroscopic sodium (I_Na) and calcium (I_Ca) currents recorded in neonatal chromaffin cells subjected to whole-cell patch clamp during a depolarization to +20 mV from a holding potential of −80 mV. The decay of the tail current generated on repolarization to −70 mV reflects the closing time course of the channels open during the pulse. B, single (red (fast) or green (slow)) exponential functions fitted to the Ca²⁺ tail current. The fast and slow time constant values in this example are, respectively, 0.12 and 2.08 ms. C, family of Ca²⁺ current traces recorded in isolation after replacement of external Na⁺ with N-methyl-d-glucamine. The holding potential was −80 mV and the depolarization voltages (mV) are indicated near each trace. The arrows indicate the duration of the pulse (100 ms). Note that a measurable inward current was already appreciable at −40 mV. D, Ca²⁺ current–voltage relations obtained during depolarization ramps from −100 to +40 mV lasting 500 ms. The activation threshold of the current is at ∼−50 mV and a typical ‘shoulder’, due to the activation of low voltage-activated Ca²⁺ channels, appears preceding activation of a larger population of high voltage-activated channels. This ‘shoulder’ is abolished by application of Ni²⁺ (50 μm) to the external solution. E, selective inhibition of the slow component of the tail current by Ni²⁺. F, quantitative analysis of the selective effect of Ni²⁺ on the amplitude (current density) of fast and slowly-deactivating components of tail currents. *Statistically significant difference, P < 0.05; n= 8 cells.