L-type calcium channels regulate a diverse array of cellular functions within excitable cells. Of the four molecularly defined subclasses of L-type Ca channels, two are expressed ubiquitously in the mammalian nervous system (Ca V 1.2␣ 1 and Ca V 1.3␣ 1 ). Despite diversity at the molecular level, neuronal L-type channels are generally assumed to be functionally and pharmacologically similar, i.e., high-voltage activated and highly sensitive to dihydropyridines. We now show that Ca V 1.3␣ 1 L-type channels activate at membrane potentials ϳ25 mV more hyperpolarized, compared with Ca V 1.2␣ 1 . This unusually negative activation threshold for Ca V 1.3␣ 1 channels is independent of the specific auxiliary subunits coexpressed, of alternative splicing in domains I-II, IVS3-IVS4, and the C terminus, and of the expression system. The use of high concentrations of extracellular divalent cations has possibly obscured the unique voltage-dependent properties of Ca V 1.3␣ 1 in certain previous studies. We also demonstrate that Ca V 1.3␣ 1 channels are pharmacologically distinct from Ca V 1.2␣ 1 . The IC 50 for nimodipine block of Ca V 1.3␣ 1 L-type calcium channel currents is 2.7 Ϯ 0.3 M, a value 20-fold higher than the concentration required to block Ca V 1.2␣ 1 . The relatively low sensitivity of the Ca V 1.3␣ 1 subunit to inhibition by dihydropyridine is unaffected by alternative splicing in the IVS3-IVS4 linker. Our results suggest that functional and pharmacological criteria used commonly to distinguish among different Ca currents greatly underestimate the biological importance of L-type channels in cells expressing Ca v 1.3␣ 1 .

type calcium channels couple membrane depolarization in neurons to numerous processes including gene expression, synaptic efficacy, and cell survival. To establish the contribution of L-type calcium channels to various signaling cascades, investigators have relied on their unique pharmacological sensitivity to dihydropyridines. The traditional view of dihydropyridine-sensitive L-type calcium channels is that they are high-voltage-activating and have slow activation kinetics. These properties limit the involvement of L-type calcium channels to neuronal functions triggered by strong and sustained depolarizations. This review highlights literature, both long-standing and recent, that points to significant functional diversity among L-type calcium channels expressed in neurons and other excitable cells. Past literature contains several reports of low-voltageactivated neuronal L-type calcium channels that parallel the unique properties of recently cloned Ca V 1.3 L-type channels. The fast kinetics and low activation thresholds of Ca V 1.3 channels stand in stark contrast to criteria currently used to describe L-type calcium channels. A more accurate view of neuronal L-type calcium channels encompasses a broad range of activation thresholds and recognizes their potential contribution to signaling cascades triggered by subthreshold depolarizations. L-type calcium channels regulate numerous neuronal functionsL-type calcium channels are perhaps the best characterized of the voltage-gated calcium channels. They were first recognized as essential for coupling excitation to contraction in skeletal, cardiac, and smooth muscle cells (Beam et al. 1989; Franzini-Armstrong and Protasi 1997; Reuter 1985;Schneider and Chandler 1973; Tanabe et al. 1990). L-type calcium channels are also expressed in neurons and endocrine cells where they regulate a multitude of processes including secretion of neurohormones and transmitters, gene expression, mRNA stability, neuronal survival, ischemic-induced axonal injury, synaptic efficacy, and the activity of other ion channels (Ashcroft et al. 1994; Bading et al. 1993; Bean 1989; Charles et al. 1999; Christie et al. 1997; De Koninck and Cooper 1995; Deisseroth et al. 1998; Dunlap et al. 1995; Finkbeiner and Greenberg 1998; Fuchs 1996; Galli et al. 1995; Heidelberger and Matthews 1992; Kamsler and Segal 2003; Lei et al. 2003; Marrion and Tavalin 1998;Marshall et al. 2003;Murphy et al. 1991;Norris et al. 1998;Ouardouz et al. 2003; Sand et al. 2001;Schorge et al. 1999;Shinnick-Gallagher et al. 2003; Smith et al. 1993; Tachibana et al. 1993; Thaler et al. 2001; Thibault et al. 2001; Weisskopf et al. 1999; Wiser et al. 1999;Zhang and Townes-Anderson 2002). The unique pharmacological sensitivity of L-type calcium channels to dihydropyridine agonists and antagonists has proved critical for their identification in physiological assays and also for their biochemical isolation (Kanngiesser et al. 1988). Biochemical purification of the dihydropyridine receptor from skeletal muscle was the essenti...

Neuronal L-type calcium channels are essential for regulating activity-dependent gene expression, but they are thought to open too slowly to contribute to action potential-dependent calcium entry. A complication of studying native L-type channels is that they represent a minor fraction of the whole-cell calcium current in most neurons. Dihydropyridine antagonists are therefore widely used to establish the contribution of L-type channels to various neuronal processes and to study their underlying biophysical properties. The effectiveness of these antagonists on L-type channels, however, varies with stimulus and channel subtype. Here, we study recombinant neuronal L-type calcium channels, Ca V 1.2 and Ca V 1.3. We show that these channels open with fast kinetics and carry substantial calcium entry in response to individual action potential waveforms, contrary to most studies of native L-type currents. Neuronal Ca V 1.3 L-type channels were as efficient as Ca V 2.2 N-type channels at supporting calcium entry during action potential-like stimuli. We conclude that the apparent slow activation of native L-type currents and their lack of contribution to single action potentials reflect the state-dependent nature of the dihydropyridine antagonists used to study them, not the underlying properties of L-type channels.