N-Type Calcium Channel Inactivation Probed by Gating-Current Analysis

Abstract

N-type calcium channels inactivate most rapidly in response to moderate, not extreme depolarization. This behavior reflects an inactivation rate that bears a U-shaped dependence on voltage. Despite this apparent similarity to calcium-dependent inactivation, N-type channel inactivation is insensitive to the identity of divalent charge carrier and, in some reports, to the level of internal buffering of divalent cations. Hence, the inactivation of N-type channels fits poorly with the "classic" profile for either voltage-dependent or calcium-dependent inactivation. To investigate this unusual inactivation behavior, we expressed recombinant N-type calcium channels in mammalian HEK 293 cells, permitting in-depth correlation of ionic current inactivation with potential alterations of gating current properties. Such correlative measurements have been particularly useful in distinguishing among various inactivation mechanisms in other voltage-gated channels. Our main results are the following: 1) The degree of gating charge immobilization was unchanged by the block of ionic current and precisely matched by the extent of ionic current inactivation. These results argue for a purely voltage-dependent mechanism of inactivation. 2) The inactivation rate was fastest at a voltage where only approximately (1)/(3) of the total gating charge had moved. This unusual experimental finding implies that inactivation occurs most rapidly from intermediate closed conformations along the activation pathway, as we demonstrate with novel analytic arguments applied to coupled-inactivation schemes. These results provide strong, complementary support for a "preferential closed-state" inactivation mechanism, recently proposed on the basis of ionic current measurements of recombinant N-type channels (Patil et al., . Neuron. 20:1027-1038).

Systematic Identification of Splice Variants in Human P/Q-Type Channel α12.1 Subunits: Implications for Current Density and Ca2+-Dependent Inactivation

“…Population means for these relationships confirmed a clearly deeper decay of r 800 with Ca 2ϩ versus Ba 2ϩ , and the unmistakable U shape of the relationship with Ca 2ϩ provided a classic hallmark for a Ca 2ϩ -driven process. The comparatively modest decline of the Ba 2ϩ relationship is believed to reflect a separate voltage-dependent inactivation process (Jones, 1999), so the distance between Ca 2ϩ and Ba 2ϩ relationships at ϩ10 mV, f, furnished a convenient index of pure CDI. Overall current size was assessed by averages of peak Ca 2ϩ current amplitude plotted as a function of the test pulse voltage.…”

Cumulative inactivation of N-type CaV2.2 calcium channels modified by alternative splicing

“…Closed-state inactivation is the mechanism thought to underlie cumulative inactivation of high-voltage-activated neuronal channels when induced by trains of action potential waveforms (5,6 (15).…”

“…Cumulative inactivation, a feature of both native and cloned Ca V 2 calcium channels, has been studied in detail because of its importance in regulating the short-term dynamics of synaptic efficacy (2)(3)(4)(5)(6)(7)(8). Inactivation that accumulates during brief, repetitive stimulation is thought to result from a process known as closed-state inactivation (5,6).…”

“…Inactivation that accumulates during brief, repetitive stimulation is thought to result from a process known as closed-state inactivation (5,6). That is, voltage-gated calcium channels undergo inactivation in response to depolarizations that are insufficiently large to open the channel.…”

Schwann Cell-Derived Factors Modulate Synaptic Activities at Developing Neuromuscular Synapses

“…This may reflect the property of developing synapses because the dependency on extracellular Ca 2ϩ and the releasable vesicle pool for evoked and spontaneous synaptic transmission are different during the early development of Xenopus neuromuscular synapses (Evers et al, 1989). Furthermore, the elevated cytoplasmic Ca 2ϩ concentration induced by SC-CM may diffuse from the soma to the nerve terminal and increase the Ca 2ϩ concentration in the nerve terminal, which in turn can cause the Ca 2ϩ -dependent Ca 2ϩ channel inactivation (Jones et al, 1999). In addition, the potentiated spontaneous synaptic release can enhance the depolarization of the nerve terminal via autoreceptors located on the nerve terminal (Fu et al, 1998).…”