Weifengxu.mit.edu
L-Type Calcium Channels: The Low Down
Diane Lipscombe, Thomas D. Helton and Weifeng Xu
J Neurophysiol
92:2633-2641, 2004.
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J Neurophysiol 92: 2633–2641, 2004;10.1152/jn.00486.2004.
L-Type Calcium Channels: The Low Down
Diane Lipscombe, Thomas D. Helton, and Weifeng Xu
Department of Neuroscience, Brown University, Providence, Rhode Island 02912
Submitted 9 May 2004; accepted in final form 5 June 2004
Lipscombe, Diane, Thomas D. Helton, and Weifeng Xu. L-type
agonists and antagonists has proved critical for their identifi-
calcium channels: The low down.
J Neurophysiol 92: 2633–2641,
cation in physiological assays and also for their biochemical
2004; 10.1152/jn.00486.2004. L-type calcium channels couple mem-
isolation (Kanngiesser et al. 1988). Biochemical purification of
brane depolarization in neurons to numerous processes including gene
the dihydropyridine receptor from skeletal muscle was the
expression, synaptic efficacy, and cell survival. To establish the
essential step in cloning the first voltage-gated calcium channel
contribution of L-type calcium channels to various signaling cascades,
(Tanabe et al. 1987). Sequence information from this landmark
investigators have relied on their unique pharmacological sensitivityto dihydropyridines. The traditional view of dihydropyridine-sensitive
study was then used to screen for and clone Ca 1.2 and Ca 1.3
L-type calcium channels is that they are high-voltage–activating and
cDNAs (Biel et al. 1990; Hui et al. 1991; Koch et al. 1990;
have slow activation kinetics. These properties limit the involvement
Mikami et al. 1989; Perez-Reyes et al. 1990; Williams et al.
of L-type calcium channels to neuronal functions triggered by strong
1992). Functional analyses of cloned channels, primarily of
and sustained depolarizations. This review highlights literature, both
Ca 1.2, were generally consistent with native cardiac L-type
long-standing and recent, that points to significant functional diversity
channels and the following criteria evolved for their identifi-
among L-type calcium channels expressed in neurons and other
excitable cells. Past literature contains several reports of low-voltage–
1) Activation by strong depolarizations (high-voltage–acti-
activated neuronal L-type calcium channels that parallel the unique
vated [HVA]).
properties of recently cloned Ca 1.3 L-type channels. The fast kinet-
2) High sensitivity to dihydropyridine agonists and antago-
ics and low activation thresholds of Ca 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 encom-
3) Relatively slow activation kinetics.
passes a broad range of activation thresholds and recognizes their
4) Calcium-dependent inactivation with little voltage-de-
potential contribution to signaling cascades triggered by subthreshold
pendent inactivation (
long-lasting).
5) Large single-channel conductance.
However, a substantial body of evidence points to hetero-
geneity among neuronal L-type calcium channels that has until
L-type calcium channels regulate numerous
recently received little attention. This review highlights recent
studies of cloned channels, as well as long-standing studies of
L-type calcium channels are perhaps the best characterized
native L-type channels, that point to significant deviations from
of the voltage-gated calcium channels. They were first recog-
criteria listed above in the properties of L-type channels.
nized as essential for coupling excitation to contraction inskeletal, cardiac, and smooth muscle cells (Beam et al. 1989;
Ca 1 genes encode L-type calcium channels
Franzini-Armstrong and Protasi 1997; Reuter 1985; Schneider
and Chandler 1973; Tanabe et al. 1990). L-type calcium
Identifying the genes that encode core Ca ␣ subunits of
channels are also expressed in neurons and endocrine cells
voltage-gated calcium complexes has led to a comprehensive
where they regulate a multitude of processes including secre-
sequence-based classification scheme (Fig. 1). When se-
tion of neurohormones and transmitters, gene expression,
quences are compared, voltage-gated calcium channels fall into
mRNA stability, neuronal survival, ischemic-induced axonal
three main groups: Ca 1 (L-type), Ca 2 (P-type, N-type, and
injury, synaptic efficacy, and the activity of other ion channels
R-type), and Ca 3 (T-type) (Ertel et al. 2000; Lipscombe
(Ashcroft et al. 1994; Bading et al. 1993; Bean 1989; Charles
2002b). In general, these gene families correspond to the
et al. 1999; Christie et al. 1997; De Koninck and Cooper 1995;
subtypes of calcium channels defined by functional and phar-
Deisseroth et al. 1998; Dunlap et al. 1995; Finkbeiner and
macological criteria. Ca 1 and Ca 2 genes are more closely
Greenberg 1998; Fuchs 1996; Galli et al. 1995; Heidelberger
related to each other when compared with Ca 3 genes. Ca 3
and Matthews 1992; Kamsler and Segal 2003; Lei et al. 2003;
T-type channels possess certain functional properties that set
Marrion and Tavalin 1998; Marshall et al. 2003; Murphy et al.
them apart from other voltage-gated calcium channels (Perez-
1991; Norris et al. 1998; Ouardouz et al. 2003; Sand et al.
Reyes et al. 1998). Low-voltage–activating calcium current
2001; Schorge et al. 1999; Shinnick-Gallagher et al. 2003;
typically marks the presence of Ca 3 T-type channels. How-
Smith et al. 1993; Tachibana et al. 1993; Thaler et al. 2001;
ever, as we will discuss, this property is shared by Ca 1.3, a
Thibault et al. 2001; Weisskopf et al. 1999; Wiser et al. 1999;
member of the Ca 1 gene family, and thus should not be
Zhang and Townes-Anderson 2002). The unique pharmacolog-
considered unique to Ca 3 (Avery and Johnston 1996; Kos-
ical sensitivity of L-type calcium channels to dihydropyridine
chak et al. 2001; Lipscombe 2002a; Platzer et al. 2000; Scholze
Address for reprint requests and other correspondence: D. Lipscombe,
The costs of publication of this article were defrayed in part by the payment
Department of Neuroscience, Brown University, 190 Thayer Street, Provi-
of page charges. The article must therefore be hereby marked "
advertisement"
dence, RI 02912 (E-mail:
[email protected]).
in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
0022-3077/04 $5.00 Copyright 2004 The American Physiological Society
D. LIPSCOMBE, T. D. HELTON, AND W. XU
Schultz et al. 1993; Soldatov 1992; Takimoto et al. 1997;Welling et al. 1997). This channel opens as the membranepotential depolarizes beyond about ⫺30 mV. Ca 1.2 channels
help define the shape of the action potential in cardiac andsmooth muscle. These channels function primarily as calciumion channels and, unlike Ca 1.1 of skeletal muscle, calcium
flow through Ca 1.2 is an essential step in initiating the
signaling cascade that leads to cardiac and smooth musclecontraction (Reuter et al. 1988; Tanabe et al. 1990). In neuronsCa 1.2 channels are thought to couple membrane depolariza-
tion to regulation of gene expression (Dolmetsch et al. 2001;Weick et al. 2003).
The Ca 1.3 gene, formerly ␣ , is expressed in many of the
same cells that express Ca 1.2. In neurons, Ca 1.2 and Ca 1.3
are often found in the same general neuronal compartments,particularly dendrites, but their subcellular distributions appear
Ca ␣ subunit gene tree. Full-length amino acid sequences for all
distinct (Hell et al. 1993; Westenbroek et al. 1998). Ca 1.3 has
10 human Ca ␣ genes were aligned using a branch and bound tree search with
been found to co-localize with the small conductance calcium-
maximum parsimony (Genetic Computer Group, paupsearch and paupdisplay
activated potassium channel (Bowden et al. 2001). Ca 1.3 is
programs). Accession numbers for sequences used are: Ca 1.1, L33795;
Ca 1.2, AJ224873; Ca 1.3, M76558; Ca 1.4, AJ224874; Ca 2.1, AB035727;
also expressed in pancreatic beta cells, neuroendocrine cells,
Ca 2.2, M94173; Ca 2.3, L27745; Ca 3.1, AF190860; Ca 3.2, AF05196;
photoreceptors, amacrine cells, and hair cells of the inner ear
Ca 3.3, AF211189. Confidence values for each node were determined by
where it mediates synaptic transmission (Habermann et al.
bootstrap analysis. All unlabeled nodes represent 100% confidence. Represen-
2003; Ihara et al. 1995; Kollmar et al. 1997a,b; Liu et al. 2004;
tation was rooted using the midpoint method. Scale bar represents 1 substitu-tion per 100 amino acids. This tree is essentially the same when we exclude the
Morgans 1999; Morgans et al. 1998; Platzer et al. 2000; Russo
variable intracellular loops (I–II, II–III) and N- and C-termini (Lipscombe
et al. 2003; Safa et al. 2001; Scholze et al. 2001; Seino et al.
2002b). Our analysis indicates a stronger similarity between Ca 1.3 and
1992; Taylor and Morgans 1998). In the heart, Ca 1.3 is
Ca 1.4 genes compared with that of other published Ca gene trees (Ertel et
present in atrial tissue where it contributes to pacemaking
al. 2000). If we just align sequences of the 4 Ca 1 genes the confidence value
(Mangoni et al. 2003; Platzer et al. 2000; Takimoto et al. 1997;
at the Ca 1.3–Ca 1.4 node increases to 100%.
Zhang et al. 2002), but not in ventricular muscle that expresses
et al. 2001; Xu and Lipscombe 2001). Until recently, all
members of Ca 1 and Ca 2 gene families were considered
The Ca 1.4 gene, formerly ␣ , is expressed primarily in
high-voltage–activated. Ca 1 channels are distinguishable
retina and is linked to a rare human disorder, stationary night
from Ca 2 channels primarily by their unique pharmacology.
blindness (Bech-Hansen et al. 1998; Strom et al. 1998). Ca 1.4
Ca 1 channels are sensitive to dihydropyridine agonists and
is found at synaptic terminals of retinal bipolar cells, and RNA
antagonists, but are not blocked by -aga IVA or -conotoxin
encoding Ca 1.4 has also been PCR amplified from dorsal root
GVIA, which inhibit Ca 2.1 and Ca 2.2 channels, respectively
ganglia (Berntson et al. 2003; Murakami et al. 2001). Interest-
(Bean 1991; Cruz et al. 1987; McIntosh et al. 1999; Mintz et al.
ingly, the Ca 1.4 gene sequence is more homologous to
1992). However, as we will discuss, dihydropyridine antago-
Ca 1.3, based on comparisons among available Ca
nists do not completely inhibit all L-type channels.
Four mammalian Ca 1 genes encode L-type
L-type calcium channels are functionally diverse
calcium channels
Analyses of Ca 1 clones in various heterologous expression
Four Ca 1 genes are present in the human genome, referred
systems have provided compelling data that L-type calcium
to as Ca 1.1–1.4 (Fig. 1). The Ca 1.1 gene, formerly ␣ , is
channels are a functionally heterogeneous family.
expressed in skeletal muscle. Ca 1.1 directly links to ryano-
1) Not all L-type calcium channels require strong depolar-
dine receptors in the sarcoplasmic reticulum (Flucher and
izations for activation; Ca 1.3 and Ca 1.4 channels have low
Franzini-Armstrong 1996). Ca 1.1 primarily acts as a voltage
sensor, coupling depolarization to release of intracellular cal-
2) L-type calcium channels are not all inhibited equally well
cium by activating the ryanodine receptor. The influx of
by dihydropyridine antagonists; Ca 1.3 and Ca 1.4 L-type
calcium through the ion pore of Ca 1.1 during gating is
channels are significantly less sensitive compared with Ca 1.2.
secondary to its primary role as a voltage sensor (Schwartz et
3) Activation kinetics of L-type calcium channels vary.
al. 1985). The coupling between depolarization and channel
Ca 1.3 channels activate with fast kinetics, whereas Ca 1.1
opening is inefficient; Ca 1.1 channels open with slow kinetics
channels open slowly.
(Almers and Palade 1981; Rios and Brum 1987; Tanabe et al.
4) Certain Ca 1.4 L-type channels do not exhibit calcium-
dependent inactivation. Furthermore, in physiological solu-
The Ca 1.2 gene, formerly ␣ , is expressed in a variety of
tions that contain calcium, L-type calcium channels that do
cells including ventricular cardiac muscle, smooth muscle,
undergo calcium-dependent inactivation are not long lasting.
pancreatic cells, fibroblasts, and neurons (Diebold et al. 1992;
5) Most L-type calcium channels have relatively large sin-
Koch et al. 1990; Mori et al. 1993; Perez-Reyes et al. 1990;
gle-channel conductances when isotonic barium is the charge
J Neurophysiol • VOL 92 • NOVEMBER 2004 • www.jn.org
L-TYPE CALCIUM CHANNELS
carrier. However, analyses of single Ca 1.3 and Ca 1.4 chan-
The Ca 1.3 knockout mice, however, renewed interest and
nels are lacking.
provided compelling evidence that the Ca 1.3 gene encodes
L-type calcium channels with unusual properties (Mangoni et
Skeletal muscle Ca 1.1 and cardiac Ca 1.2 L-type channels
al. 2003; Platzer et al. 2000; Zhang 2002). At the behavioral
are functionally distinct
level, mice lacking the L-type Ca 1.3 gene experience signif-
icant sinoatrial node dysfunction characterized by sinus brady-
Although the first Ca ␣ subunit to be cloned, Ca 1.1
cardia. This unanticipated role for Ca 1.3 in pacemaking
resisted functional reconstitution in nonmuscle, heterologous
implies that these L-type calcium channels are important in
expression systems (Tanabe et al. 1987). Successful expression
mediating subthreshold depolarizations in the sinoatrial node.
of Ca 1.1 was eventually achieved using embryonic muscle
The absence of a low-threshold activating calcium current in
from dysgenic mice. In a series of classic experiments, Numa,
sinoatrial node cells of Ca 1.3 ⫺/⫺ mice confirmed this
Beam, and colleagues, rescued excitation– contraction cou-
hypothesis (Zhang 2002). Hearing loss and the absence of a
pling as well as L-type calcium channel currents in dysgenic
low-threshold activating calcium current in hair cells from
muscle by expressing Ca 1.1 cDNA in these cells (Adams et
these mice are consistent with prominent expression of Ca 1.3
al. 1990; Tanabe et al. 1988). Ca 1.1 currents were small and
in inner hair cells of the cochlea (Kollmar et al. 1997b; Platzer
activated with slow kinetics, properties consistent with native
et al. 2000). The functional properties of Ca 1.3 clones iso-
L-type currents in skeletal muscle. As in skeletal muscle,
lated from neurons and endocrine cells, more recently, confirm
contraction depended on the mobilization of intracellular cal-
that Ca 1.3 L-type channels activate at subthreshold voltages
cium stores. Ca 1.2 could also reconstitute excitation– contrac-
(Koschak et al. 2001; Platzer et al. 2000; Safa et al. 2001; Xu
tion coupling in dysgenic muscle, but the features were dis-
and Lipscombe 2001).
tinctly cardiac-like. In this case, calcium flux across the mem-brane through Ca 1.2 channels was essential to trigger muscle
Ca 1.3 L-type channels activate at relatively hyperpolarized
contraction (Tanabe et al. 1990). Ca 1.2 channels opened with
rates that were significantly faster compared with Ca 1.1.
Chimeric analyses demonstrated that sequence differences in
Figure 2 illustrates that Ca 1.2 and Ca 1.3 channels have
the II–III intracellular linker region of Ca 1.1 and Ca 1.2
very different activation thresholds. The Ca 1.2 and Ca 1.3
genes imparted the skeletal or cardiac muscle form of excita-
clones used were isolated from neuronal tissue, expressed in
tion– contraction coupling. In subsequent experiments Beam
tsA201 cells, and recorded under identical conditions. With
and colleagues demonstrated that sequence differences in the
physiological concentrations of extracellular calcium, Ca 1.3
domain IS3–IS4 linkers of Ca 1.1 and Ca 1.2 genes deter-
channels start to activate at about ⫺55 mV, a voltage that is
mined the gating phenotypes of these two L-type calcium
approximately 20 –25 mV more hyperpolarized as compared
channels (Nakai et al. 1994). The slow gating phenotype of
with Ca 1.2. Low-threshold activation is a prominent feature
Ca 1.1 channels was transferred to Ca 1.2 by swapping in the
of all Ca 1.3 clones isolated recently, independent of tissue of
IS3–IS4 linker of Ca 1.1. Variations in S3–S4 linker se-
origin and of auxiliary subunits (Koschak et al. 2001; Safa et
quences among other voltage-gated ion channel gene families
al. 2001; Scholze et al. 2001; Xu and Lipscombe 2001). Why
and their splice isoforms are similarly important in modulating
was this unique and salient feature of Ca 1.3 not highlighted in
channel gating kinetics (Lipscombe 2002b; Mathur et al. 1997;
earlier studies (Bell et al. 2001; Ihara et al. 1995; Williams et
Tang and Papazian 1997).
al. 1992)? The most likely explanation relates to the use of highconcentrations of extracellular barium and calcium in these
Ca 1.2 and Ca 1.3 L-type channels are functionally distinct
studies to compensate for low expression levels. Under these
conditions, Ca 1.3 channels would have activated at signifi-
With the exception of skeletal muscle and perhaps retina, all
cantly more depolarized voltages as a result of surface charge
excitable cells express one or both Ca 1.2 and Ca 1.3 genes.
screening (Frankenhaeuser and Hodgkin 1957; Hille 2001).
The products of these genes constitute the major fraction of
Indeed, when we studied Ca 1.3 L-type currents under similar
L-type calcium channels in mammals (Hell et al. 1993; Ludwig
conditions, 40 mM extracellular barium, the current–voltage
et al. 1997; Takimoto et al. 1997; Williams et al. 1992). Until
relationship shifted into the range of a high-voltage–activated
recently, the prevailing image of the neuronal L-type calcium
L-type calcium channel (Xu and Lipscombe 2001). Additional
channel was of a high-voltage-activated, slowly activating
factors such as interactions with other subunits, modulation by
channel with high sensitivity to dihydropyridines (Ertel et al.
second-messenger signaling cascades, and alternative splicing
2000; Hille 2001). These features have developed primarily
have the potential to influence channel properties (Birnbaumer
from biophysical analyses of heterologously expressed Ca 1.2
et al. 1998; Lipscombe 2002b; Scholze et al. 2001). However,
L-type calcium channels (Altier et al. 2001; Bourinet et al.
low-voltage activation appears to be a salient feature of
1994; Charnet et al. 1994; de Leon et al. 1995; Ivanina et al.
Ca 1.3-containing channels (Koschak et al. 2001; Michna et
2000). Although Ca 1.3 was first cloned in the early 1990s,
al. 2003; Safa et al. 2001; Scholze et al. 2001; Xu and
low expression levels in heterologous systems limited electro-
Lipscombe 2001).
physiological studies of this L-type calcium channel (Hui et al.
1991; Williams et al. 1992). Aside from a report that Ca 1.3
Ca 1.3 L-type channels are only partially inhibited
L-type channels could be reversibly inhibited by the N-type
calcium channel blocker -conotoxin GVIA (Williams et al.
1992), a result that has not been confirmed (Xu and Lipscombe
All L-type calcium channels studied to date are sensitive to
2001), Ca 1.3 channels were not considered unique.
dihydropyridine antagonists and agonists. However, Ca 1.3-
J Neurophysiol • VOL 92 • NOVEMBER 2004 • www.jn.org
D. LIPSCOMBE, T. D. HELTON, AND W. XU
currents that resemble the R-type current of many neurons(Foehring et al. 2000; Randall and Tsien 1995; Tottene et al.
1996; Yasuda et al. 2003; Zhuravleva et al. 2001).
Ca 1.3 L-type channels open with rapid kinetics
L-type calcium channels in neurons are typically thought of
as slowly activating (Mermelstein et al. 2000; Yasuda et al.
2003). If true, this property limits the involvement of L-typecalcium channels to signaling pathways triggered by moreprolonged membrane depolarizations. However, Ca 1.2 and
Ca 1.3 currents shown in Fig. 2 clearly activate with fast
kinetics (Xu and Lipscombe 2001) and, although contrary tothe pervading viewpoint, these data are consistent with certainother studies. Cloned Ca 1.2 L-type channels have been shown
to support at least as much calcium influx as Ca 2 channels, in
response to brief action potential stimuli (Liu et al. 2003) andnative L-type channels mediate spike-induced calcium influx inhippocampal dendrites (Christie et al. 1995). It is likely thatactivation kinetics of L-type calcium channels will vary de-pending on several factors including cell-type, temperature,alternative splicing, and the presence of auxiliary subunits(Birnbaumer et al. 1998; Lipscombe 2002b; Liu et al. 2003).
For example, the Ca 1.2 clone isolated from rabbit heart used
in our earlier studies (Xu and Lipscombe 2001) activates withkinetics that are slow as compared with our neuronal Ca 1.2
Ca 1.3 and Ca 1.2 L-type channels have different activation thresh-
clone (Fig. 2). However, we also suggest that pharmacological
olds. Whole cell currents measured from tsA201 cells expressing Ca 1.3 (A)
subtraction methods used frequently to isolate L-type calcium
and Ca 1.2 (B) together with Ca ␣ ␦ and Ca  . Currents were activated by
channels from other subtypes of voltage-gated calcium chan-
step depolarizations to the indicated test potentials from a holding potential of
⫺100 mV; 2 mM Ca2⫹ was the charge carrier. C: averaged current–voltage
nels in neurons, might have contributed to the notion that
relationships for Ca 1.3 () and Ca 1.2 (E) channels. Activation V
L-type calcium channels activate with slow kinetics (Fig. 3).
calculated from Boltzmann-linear fits were, ⫺40.4 ⫾ 0.9 mV and ⫺16.1 ⫾ 0.5mV for Ca 1.3 and Ca 1.2, respectively (n ⫽ 8, 11). For a more detailed
description of methods see Xu and Lipscombe (2001). All clones were isolatedin our laboratory. Ca 1.3 and Ca ␣ ␦ clones were isolated from a rat
sympathetic cDNA library, Ca 1.2 from mouse brain, and Ca  from rat
brain. Accession numbers are for Ca 1.3: AF370009; Ca 1.2: AY728090;
Ca  is identical to M88751; Ca ␣ ␦ : AF286488.
containing L-type calcium channels appear to be significantlyless sensitive to dihydropyridine antagonists (Koschak et al.
2001; Xu and Lipscombe 2001). This property complicatesidentification of Ca 1.3 currents. For example, ⬎90% of
Ca 1.2 current is inhibited by 1 M nimodipine, but this same
concentration inhibits only 50% of peak Ca 1.3 current (Xu
and Lipscombe 2001). Striessnig and colleagues obtained sim-ilar results using isradipine (Koschak et al. 2001). The lowersensitivity of Ca 1.3 channels to dihydropyridine antagonists
becomes even more significant at hyperpolarized membranepotentials. Inhibition by dihydropyridines is state-dependent:enhanced at depolarized membrane potentials that open thechannel, but reduced at hyperpolarized membrane potentials(Bean 1984; Berjukow et al. 2000). Consequently, dihydro-pyridines become particularly ineffective at inhibiting Ca 1.3
currents activated at the foot of the current–voltage curve (Xu
Dihydropyridine-sensitive component of the Ca 1.3 L-type current
appears to activate slowly. A: representative Ca 1.3 currents in the absence
and Lipscombe 2001). Interestingly, the Ca 1.3 current that
(black) and presence (gray) of 1 M nifedipine. Currents were activated by
remains in the presence of dihydropyridines takes on the profile
step depolarizations to the indicated potentials from a holding potential of
of an inactivating current with barium as the charge carrier
⫺100 mV. Currents were recorded from Xenopus oocytes expressing Ca 1.3,
(Fig. 3) (Xu and Lipscombe 2001). This is consistent with the
Ca ␣ ␦, and Ca  ; 5 mM barium is the charge carrier. Additional details can
state-dependent nature of the block by dihydropyridines (Bean
be found in Xu and Lipscombe (2001). B: subtracted Ca 1.3 currents showing
the dihydropyridine-sensitive component at the indicated test potentials. Cur-
1984; Berjukow and Hering 2001). In their presence, Ca 1.3
rents appear to activate slowing because of the time-dependent nature of
channels generate low-threshold, drug-resistant, inactivating
dihydropyridine block. Scale bars: 0.5 A, 20 ms.
J Neurophysiol • VOL 92 • NOVEMBER 2004 • www.jn.org
L-TYPE CALCIUM CHANNELS
The L-type calcium current in neurons is defined frequently as
Avery and Johnston commented that ". . the designation ‘low-
the whole cell calcium current that is inhibited by dihydropyr-
voltage-activated' should not be limited to T-type channels."
idine antagonists. Pharmacological subtraction is only appro-
These investigators ". . challenge the traditional designation
priate, however, when inhibition is complete, and independent
of L-type channels as exclusively HVA and reveal a possible
of voltage and time; conditions not met for dihydropyridine
role in subthreshold Ca2⫹ signaling" (Avery and Johnston 1996).
block of Ca 1.3 L-type currents (Figs. 2, 3). Figure 3 illustrates
Single-channel recordings also indicate differences among
that Ca 1.3 L-type currents open with rapid kinetics, and
neuronal L-type calcium channels in hippocampal pyramidal
inactivate little with barium as the charge carrier. However, in
and cerebellar granule cells (Forti and Pietrobon 1993; Kava-
the presence of nimodipine, inhibition is incomplete and the
lali and Plummer 1996; Schjott and Plummer 2000). Record-
remaining current appears to inactivate. These data are consis-
ings distinguish at least 2 gating activities of L-type calcium
tent with the state-dependent nature of nimodipine block.
channels that may represent different channel subtypes. It will
Consequently, the dihydropyridine-sensitive, subtracted cur-
be interesting to determine their molecular origins.
rent appears as slowly activating, not because L-type calciumchannels open slowly but because the block is time-dependent
and incomplete (Fig. 3).
Neuronal L-type calcium channels play established roles in
Ca 1.4 L-type channels are functionally unique
regulating gene expression, cell survival, and synaptic plastic-
ity (Christie et al. 1997; Deisseroth et al. 1998; Galli et al.
Ca 1.4 was recently cloned from human and mouse retinal
1995; Mao et al. 1999; Marshall et al. 2003; Murphy et al.
tissue and heterologously expressed in mammalian cells (Bau-
1991; Norris et al. 1998; Weisskopf et al. 1999). In select cells
mann et al. 2004; Koschak et al. 2003; McRory et al. 2004).
and synapses, L-type calcium channels can also regulate exo-
Although currents generated from cells expressing Ca 1.4
cytosis (Ashcroft et al. 1994; Fuchs 1996; Heidelberger and
clones were small, they had properties similar in several
Matthews 1992; Liu et al. 2004; Sand et al. 2001; Thaler et al.
respects to those of Ca 1.3. These include rapid activation
2001; Wiser et al. 1999). In addition, data reviewed here
kinetics, low activation threshold, and lower sensitivity to
suggest Ca 1.3 L-type calcium channels mediate subthreshold
dihydropyridine inhibition (Baumann et al. 2004; Koschak et
calcium signaling. For example, dihydropyridine antagonists
al. 2003; McRory et al. 2004). Native L-type currents in retinal
suppress spontaneous intracellular calcium oscillations and
cells, presumed to be Ca 1.4-containing (Taylor and Morgans
slow rhythmic firing in several excitable cells, including cere-
1998; Wilkinson and Barnes 1996), are likewise similar to
bellar Purkinje neurons, suprachiasmatic nucleus neurons, in-
recombinant Ca 1.3 and Ca 1.4 channels. They have lower
ferior olivary neurons, corticostriatial neurons, pituitary cells,
sensitivity to dihydropyridines and activate at negative thresh-
and GH3 cells (Charles et al. 1999; Giraldez et al. 2002;
olds. These data are consistent with the relatively high level of
Liljelund et al. 2000; Pennartz et al. 2002; Placantonakis and
sequence homology between Ca 1.3 and Ca 1.4 genes (Fig. 1)
Welsh 2001; Vergara et al. 2003). These studies and those that
(Lipscombe 2002b). Heterologously expressed Ca 1.4 L-type
show sinoatrial node dysfunction in Ca 1.3 knockout mice
calcium channels are also distinctive in lacking calcium-de-
(Mangoni et al. 2003; Platzer et al. 2000; Zhang et al. 2002)
pendent inactivation (Baumann et al. 2004; Koschak et al.
strongly implicate Ca 1.3 L-type calcium channels in driving
2003; McRory et al. 2004). The currents in these studies were
oscillatory activity. Ca 1.3 channels could also mediate sus-
all relatively small but calcium entering through a single
tained calcium entry during action potential plateaus, as calci-
channel should be sufficient to support inactivation, if compo-
um-dependent and voltage-dependent inactivation is minimal
nents of the calcium-dependent inactivation pathway are
at depolarized voltages (Figs. 2 and 3).
present (Peterson et al. 2000; Yue et al. 1990). The absence of
Many neurons express low- to mid-threshold activating,
calcium-dependent inactivation suggests that Ca 1.4 channels
inactivating, and drug-resistant calcium currents collectively
are functionally distinct from Ca 1.3 and Ca 1.2 channels
called R-type. These currents are generally attributed to Ca 2.3
(Koschak et al. 2003; McRory et al. 2004).
channels (Cloues and Sather 2003; Magistretti et al. 2000;Randall and Tsien 1995; Tottene et al. 1996; Yasuda et al.
Native neuronal L-type currents are functionally diverse
2003). The properties of Ca 1.3 channels suggest that distin-
guishing them from Ca 2.3 channels using dihydropyridine
Certain cell-types, including hair cells, amacrine cells, and
antagonists would be difficult. It is quite likely that Ca 1.3
endocrine cells express a limited number of calcium channels.
L-type calcium channels contribute a significant fraction of the
Within this background, low-threshold dihydropyridine-sensi-
R-type current in many neurons. The prevalence of significant
tive L-type calcium currents are more readily identified (Ash-
drug-resistant R-type currents in neurons of Ca 2.3 knockout
croft et al. 1994; Habermann et al. 2003; Liu et al. 2004;
mice strongly supports this proposal (Wilson et al. 2000).
Michna et al. 2003; Platzer et al. 2000; Schnee and Ricci 2003;
R-type currents contribute to presynaptic transmitter release at
Smith et al. 1993). Several groups also report low-threshold-
certain synapses (Wu et al. 1999) and to synaptic plasticity in
activating dihydropyridine-sensitive L-type currents in hip-
dendritic spines of hippocampal pyramidal neurons (Yasuda et
pocampal pyramidal, cortical striatal, suprachiasmatic, tha-
lamic, and motor neurons with properties similar to Ca 1.3
Ca 1.3 L-type channels will activate in response to physi-
channels (Avery and Johnston 1996; Cloues and Sather 2003;
ological stimuli that do not open Ca 1.2 L-type channels. This
Li and Bennett 2003; Liljelund et al. 2000; Pennartz et al.
broadens the functional importance of L-type calcium channels
2002; Sand et al. 2001; Svirskis and Hounsgaard 1997; Ver-
to included neuronal processes triggered by fast, subthreshold
gara et al. 2003; Zhuravleva et al. 2001). Notably in 1996,
depolarizations. Differences in their primary structure and
J Neurophysiol • VOL 92 • NOVEMBER 2004 • www.jn.org
D. LIPSCOMBE, T. D. HELTON, AND W. XU
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Psychiatry and Behavioral Sciences, Stanford University School of Medicine,
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La lógica de la sobredeterminación: hacia una radica- lización del análisis político Resumen. El presente artículo se plantea, desde una gramática posfundacionalista, radicalizar el análisis político proponiendo a la sobredeterminación, en tanto lógica subyacente a todo proceso de significación, como un concepto clave para poder disolver las fronteras que le ha impuesto la ciencia política canónica. Procurando avanzar en una conceptualización de la noción de sobredeterminación, este trabajo intenta mostrar la presencia de la lógica de la sobredeterminación en una serie de investigaciones empíricas realizadas desde un marco analítico posfundacionalista, en cada una de las cuales se ausenta una discusión teórica en torno a esta noción clave.
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