Ca2+-dependent modulation of voltage-gated Ca2+ channels

Abstract

Background: Voltage-gated (Cav) Ca2+ channels are multi-subunit complexes that play diverse roles in a wide variety of tissues. A fundamental mechanism controlling Cav channel function involves the Ca2+ ions that permeate the channel pore. Ca2+ influx through Cav channels mediates feedback regulation to the channel that is both negative (Ca2+-dependent inactivation, CDI) and positive (Ca2+-dependent facilitation, CDF).

Scope of review: This review highlights general mechanisms of CDI and CDF with an emphasis on how these processes have been studied electrophysiologically in native and heterologous expression systems.

Major conclusions: Electrophysiological analyses have led to detailed insights into the mechanisms and prevalence of CDI and CDF as Cav channel regulatory mechanisms. All Cav channel family members undergo some form of Ca2+-dependent feedback that relies on CaM or a related Ca2+ binding protein. Tremendous progress has been made in characterizing the role of CaM in CDI and CDF. Yet, what contributes to the heterogeneity of CDI/CDF in various cell-types and how Ca2+-dependent regulation of Cav channels controls Ca2+ signaling remain largely unexplored.

General significance: Ca2+ influx through Cav channels regulates diverse physiological events including excitation-contraction coupling in muscle, neurotransmitter and hormone release, and Ca2+-dependent gene transcription. Therefore, the mechanisms that regulate channels, such as CDI and CDF, can have a large impact on the signaling potential of excitable cells in various physiological contexts. This article is part of a Special Issue entitled Biochemical, biophysical and genetic approaches to intracellular calcium signaling.

Figure 1

Characterization of CDI of Ca_v2.1 channels in voltage-clamp recordings. a) Comparison of inactivation of I_Ca (black trace) and I_Ba (grey trace) in HEK293T cells transfected with Ca_v2.1 (α₁2.1, β_2A and α2δ). Currents are evoked by 1-s pulses from −80 mV to 0 mV. Extracellular solution contains 10 mM Ca²⁺ or Ba²⁺ and intracellular solution contains 0.5 mM EGTA. Right, inactivation is measured by dividing residual current amplitude (I_res) by the peak amplitude (I_peak). b,c) Double pulse protocol for measuring CDI. Left, I_Ca is evoked by a 10-ms test pulse after a conditioning prepulse to various voltages as indicated. Right, normalized I_Ca for the test and prepulse are plotted against prepulse voltage. With physiological Ca²⁺ buffering (0.5 mM EGTA, b) but not with higher levels of Ca²⁺ buffering (10 mM BAPTA, c), maximal CDI of the test current occurs at prepulse voltages evoking peak inward current.

Figure 2

CDI of Ca_v1 channels. I_Ca (black) or I_Ba (grey) were evoked by 1-s depolarizing pulses in HEK293T cells transfected with Ca_v1.3 (a) or Ca_v1.4 (c) or in mouse inner hair cells (IHC), which express predominantly Ca_v1.3 (b).

Figure 3

Characterization of CDF using repetitive and double pulse voltage-clamp protocols. (a) I_Ca is evoked in mouse cerebellar Purkinje neurons by a train of action potential (AP) waveforms at 100 Hz. CDF is evident as an initial increase in the amplitude of I_Ca above the baseline level (dashed line). (b) CDF is measured by plotting the peak amplitude of test currents normalized to the first in the train for I_Ca(filled circles) or I_Ba (open circles). Shown are results from AP trains in mouse cerebellar Purkinje neurons and Ca_v2.1-transfected HEK293T cells, and square test pulses (−80 mV to +10 mV, 100 Hz) for Ca_v2.1-transfected HEK293T cells. (c) Double-pulse protocol measures CDF of Ca_v2.1 in transfected HEK293T cells. Left, I_Ca was evoked by test currents evoked before (P1) or after (P2) a conditioning prepulse (Pre). The prepulse-induced current is not sampled in the representative current trace. The ratio of P2:P1 current amplitude (Fractional current) is plotted against prepulse voltage for I_Ca (filled circles) and I_Ba (open circles).