Kinetic properties of skeletal-muscle-like high-threshold calcium currents in a non-fusing muscle cell line

Two distinct voltage-sensing domains control voltage sensitivity and kinetics of current activation in CaV1.1 calcium channels

Alternative splicing of the skeletal muscle CaV1.1 voltage-gated calcium channel gives rise to two channel variants with very different gating properties. The currents of both channels activate slowly; however, insertion of exon 29 in the adult splice variant CaV1.1a causes an ∼30-mV right shift in the voltage dependence of activation. Existing evidence suggests that the S3-S4 linker in repeat IV (containing exon 29) regulates voltage sensitivity in this voltage-sensing domain (VSD) by modulating interactions between the adjacent transmembrane segments IVS3 and IVS4. However, activation kinetics are thought to be determined by corresponding structures in repeat I. Here, we use patch-clamp analysis of dysgenic (CaV1.1 null) myotubes reconstituted with CaV1.1 mutants and chimeras to identify the specific roles of these regions in regulating channel gating properties. Using site-directed mutagenesis, we demonstrate that the structure and/or hydrophobicity of the IVS3-S4 linker is critical for regulating voltage sensitivity in the IV VSD, but by itself cannot modulate voltage sensitivity in the I VSD. Swapping sequence domains between the I and the IV VSDs reveals that IVS4 plus the IVS3-S4 linker is sufficient to confer CaV1.1a-like voltage dependence to the I VSD and that the IS3-S4 linker plus IS4 is sufficient to transfer CaV1.1e-like voltage dependence to the IV VSD. Any mismatch of transmembrane helices S3 and S4 from the I and IV VSDs causes a right shift of voltage sensitivity, indicating that regulation of voltage sensitivity by the IVS3-S4 linker requires specific interaction of IVS4 with its corresponding IVS3 segment. In contrast, slow current kinetics are perturbed by any heterologous sequences inserted into the I VSD and cannot be transferred by moving VSD I sequences to VSD IV. Thus, CaV1.1 calcium channels are organized in a modular manner, and control of voltage sensitivity and activation kinetics is accomplished by specific molecular mechanisms within the IV and I VSDs, respectively.

Identification and functional characterization of malignant hyperthermia mutation T1354S in the outer pore of the Cavalpha1S-subunit

To identify the genetic locus responsible for malignant hyperthermia susceptibility (MHS) in an Italian family, we performed linkage analysis to recognized MHS loci. All MHS individuals showed cosegregation of informative markers close to the voltage-dependent Ca(2+) channel (Ca(V)) α(1S)-subunit gene (CACNA1S) with logarithm of odds (LOD)-score values that matched or approached the maximal possible value for this family. This is particularly interesting, because so far MHS was mapped to >178 different positions on the ryanodine receptor (RYR1) gene but only to two on CACNA1S. Sequence analysis of CACNA1S revealed a c.4060A>T transversion resulting in amino acid exchange T1354S in the IVS5-S6 extracellular pore-loop region of Ca(V)α(1S) in all MHS subjects of the family but not in 268 control subjects. To investigate the impact of mutation T1354S on the assembly and function of the excitation-contraction coupling apparatus, we expressed GFP-tagged α(1S)T1354S in dysgenic (α(1S)-null) myotubes. Whole cell patch-clamp analysis revealed that α(1S)T1354S produced significantly faster activation of L-type Ca(2+) currents upon 200-ms depolarizing test pulses compared with wild-type GFP-α(1S) (α(1S)WT). In addition, α(1S)T1354S-expressing myotubes showed a tendency to increased sensitivity for caffeine-induced Ca(2+) release and to larger action-potential-induced intracellular Ca(2+) transients under low (≤ 2 mM) caffeine concentrations compared with α(1S)WT. Thus our data suggest that an additional influx of Ca(2+) due to faster activation of the α(1S)T1354S L-type Ca(2+) current, in concert with higher caffeine sensitivity of Ca(2+) release, leads to elevated muscle contraction under pharmacological trigger, which might be sufficient to explain the MHS phenotype.

The Ca2+ Channel α2δ-1 Subunit Determines Ca2+ Current Kinetics in Skeletal Muscle but Not Targeting of α1S or Excitation-Contraction Coupling

Auxiliary channel subunits regulate membrane expression and modulate current properties of voltage-activated Ca(2+) channels and thus are involved in numerous important cell functions, including muscle contraction. Whereas the importance of the alpha(1S), beta(1a), and gamma Ca(2+) channel subunits in skeletal muscle has been determined by using null-mutant mice, the role of the alpha(2)delta-1 subunit in skeletal muscle is still elusive. We addressed this question by small interfering RNA silencing of alpha(2)delta-1 in reconstituted dysgenic (alpha(1S)-null) myotubes and in BC3H1 skeletal muscle cells. Immunofluorescence labeling of the alpha(1S) and alpha(2)delta-1 subunits and whole cell patch clamp recordings demonstrated that triad targeting and functional expression of the skeletal muscle Ca(2+) channel were not compromised by the depletion of the alpha(2)delta-1 subunit. The amplitudes and voltage dependences of L-type Ca(2+) currents and of the depolarization-induced Ca(2+) transients were identical in control and in alpha(2)delta-1-depleted muscle cells. However, alpha(2)delta-1 depletion significantly accelerated the current kinetics, most likely by the conversion of slowly activating into fast activating Ca(2+) channels. Reverse transcription-PCR analysis indicated that alpha(2)delta-1 is the exclusive isoform expressed in differentiated BC3H1 cells and that depletion of alpha(2)delta-1 was not compensated by the up-regulation of any other alpha(2)delta isoform. Thus, in skeletal muscle the Ca(2+) channel alpha(2)delta-1 subunit functions as a major determinant of the characteristic slow L-type Ca(2+) current kinetics. However, this subunit is not essential for targeting of Ca(2+) channels or for their primary physiological role in activating skeletal muscle excitation-contraction coupling.

Rundown of a transient potassium current is attributable to changes in channel voltage dependence

Many ionic currents undergo significant rundown during whole-cell recording. Although rundown is an artifact associated with the recording method, studying the mechanism of rundown may lead to understanding mechanisms regulating channel functions in physiological conditions. The mechanisms for rundown, however, remain obscure for many channels. Here we have studied the mechanism for rundown of an A-type K(+) current in mouse striatal cholinergic interneurons. The interneuron expressed a prominent component of A-type current which exhibited significant rundown during whole-cell recording. When the current was assessed with a highly hyperpolarized prepotential (-140 mV), however, the rundown was virtually fully suppressed, suggesting its being dependent on voltage. Estimation of channel voltage dependence revealed that both activation and inactivation curves shifted towards hyperpolarized potentials during rundown. The shift was suppressed by intracellular ATP, but was affected neither by phosphatase inhibitors nor by antioxidative reagents. The gradual shift of inactivation curve towards negative potentials would make the holding potential progressively inactivate the channel, resulting in apparent loss of activity of the channels. Our results thus provide a biophysical explanation for rundown of A-type current. .

Prolonged depolarization promotes fast gating kinetics of L-type Ca2+ channels in mouse skeletal myotubes

The effects of prolonged conditioning depolarizations on the activation kinetics of skeletal L-type calcium currents (L-currents) were characterized in mouse myotubes using the whole-cell patch clamp technique. The sum of two exponentials was required to adequately fit L-current activation and enabled determination of both the amplitudes (A(fast) and A(slow)) and time constants (tau(fast) and tau(slow)) of each component comprising the macroscopic current. Prepulses sufficient to activate (200 ms) or inactivate (10 s) L-channels did not alter tau(fast), tau(slow), or the fractional contribution of either the fast (A(fast)/(A(fast) + A(slow)) or slow (A(slow)/(A(fast) + A(slow))) amplitudes of subsequently activated L-currents. Prolonged depolarizations (60 s to +40 mV) resulted in the conversion of skeletal L-current to a fast gating mode following brief repriming intervals (3-10 s at -80 mV). Longer repriming intervals (30-60 s at -80 mV) restored L-channels to a predominantly slow gating mode. Accelerated L-currents originated from L-type calcium channels since they were completely blocked by a dihydropyridine antagonist (3 microM nifedipine) and exhibited a voltage dependence of activation similar to that observed in the absence of conditioning prepulses. The degree of L-current acceleration produced following prolonged depolarization was voltage dependent. For test potentials between +10 and +50 mV, the fractional contribution of Afast to the total current decreased exponentially with the test voltage (e-fold approximately 38 mV). Thus, L-current acceleration was most significant at more negative test potentials (e.g. +10 mV). Prolonged depolarization also accelerated L-currents recorded from myotubes derived from RyR1-knockout (dyspedic) mice. These results indicate that L-channel acceleration occurs even in the absence of RyR1, and is therefore likely to represent an intrinsic property of skeletal L-channels. The results describe a novel experimental protocol used to demonstrate that slowly activating mammalian skeletal muscle L-channels are capable of undergoing rapid, voltage-dependent transitions during channel activation. The transitions underlying rapid L-channel activation may reflect rapid transitions of the voltage sensor used to trigger the release of calcium from the sarcoplasmic reticulum during excitation-contraction coupling.

Functional impact of the ryanodine receptor on the skeletal muscle L-type Ca(2+) channel

L-type Ca(2+) channel (L-channel) activity of the skeletal muscle dihydropyridine receptor is markedly enhanced by the skeletal muscle isoform of the ryanodine receptor (RyR1) (Nakai, J., R.T. Dirksen, H. T. Nguyen, I.N. Pessah, K.G. Beam, and P.D. Allen. 1996. Nature. 380:72-75.). However, the dependence of the biophysical and pharmacological properties of skeletal L-current on RyR1 has yet to be fully elucidated. Thus, we have evaluated the influence of RyR1 on the properties of macroscopic L-currents and intracellular charge movements in cultured skeletal myotubes derived from normal and "RyR1-knockout" (dyspedic) mice. Compared with normal myotubes, dyspedic myotubes exhibited a 40% reduction in the amount of maximal immobilization-resistant charge movement (Q(max), 7.5 +/- 0.8 and 4.5 +/- 0.4 nC/muF for normal and dyspedic myotubes, respectively) and an approximately fivefold reduction in the ratio of maximal L-channel conductance to charge movement (G(max)/Q(max)). Thus, RyR1 enhances both the expression level and Ca(2+) conducting activity of the skeletal L-channel. For both normal and dyspedic myotubes, the sum of two exponentials was required to fit L-current activation and resulted in extraction of the amplitudes (A(fast) and A(slow)) and time constants (tau(slow) and tau(fast)) for each component of the macroscopic current. In spite of a >10-fold in difference current density, L-currents in normal and dyspedic myotubes exhibited similar relative contributions of fast and slow components (at +40 mV; A(fast)/[A(fast) + A(slow)] approximately 0.25). However, both tau(fast) and tau(slow) were significantly (P < 0.02) faster for myotubes lacking the RyR1 protein (tau(fast), 8.5 +/- 1.2 and 4.4 +/- 0.5 ms; tau(slow), 79.5 +/- 10.5 and 34.6 +/- 3.7 ms at +40 mV for normal and dyspedic myotubes, respectively). In both normal and dyspedic myotubes, (-) Bay K 8644 (5 microM) caused a hyperpolarizing shift (approximately 10 mV) in the voltage dependence of channel activation and an 80% increase in peak L-current. However, the increase in peak L-current correlated with moderate increases in both A(slow) and A(fast) in normal myotubes, but a large increase in only A(fast) in dyspedic myotubes. Equimolar substitution of Ba(2+) for extracellular Ca(2+) increased both A(fast) and A(slow) in normal myotubes. The identical substitution in dyspedic myotubes failed to significantly alter the magnitude of either A(fast) or A(slow). These results demonstrate that RyR1 influences essential properties of skeletal L-channels (expression level, activation kinetics, modulation by dihydropyridine agonist, and divalent conductance) and supports the notion that RyR1 acts as an important allosteric modulator of the skeletal L-channel, analogous to that of a Ca(2+) channel accessory subunit.

Gating of the L-type Ca channel in human skeletal myotubes: an activation defect caused by the hypokalemic periodic paralysis mutation R528H

The skeletal muscle L-type Ca channel serves a dual role as a calcium-conducting pore and as the voltage sensor coupling t-tubule depolarization to calcium release from the sarcoplasmic reticulum. Mutations in this channel cause hypokalemic periodic paralysis (HypoPP), a human autosomal dominant disorder characterized by episodic failure of muscle excitability that occurs in association with a decrease in serum potassium. The voltage-dependent gating of L-type Ca channels was characterized by recording whole-cell Ca currents in myotubes cultured from three normal individuals and from a patient carrying the HypoPP mutation R528H. We found two effects of the R528H mutation on the L-type Ca current in HypoPP myotubes: (1) a mild reduction in current density and (2) a significant slowing of the rate of activation. We also measured the voltage dependence of steady-state L-type Ca current inactivation and characterized, for the first time in a mammalian preparation, the kinetics of both entry into and recovery from inactivation over a wide range of voltages. The R528H mutation had no effect on the kinetics or voltage dependence of inactivation.

Unitary behavior of skeletal, cardiac, and chimeric L-type Ca2+ channels expressed in dysgenic myotubes

Skeletal and cardiac dihydropyridine receptors function both as voltage-dependent L-type calcium channels (L-channels) and as critical proteins that trigger calcium release from the sarcoplasmic reticulum in muscle. In spite of these similarities, skeletal L-channels exhibit a markedly slower activation rate than cardiac L-channels. We investigated the mechanisms underlying this difference by comparing the unitary behavior of L-channels in cell-attached patches of dysgenic myotubes expressing skeletal, cardiac, or chimeric dihydropyridine receptors. Our results demonstrate that ensemble averages activate rapidly for the purely cardiac dihydropyridine receptor and approximately five times more slowly for L-channels attributable to the purely skeletal dihydropyridine receptor or a chimeric dihydropyridine receptor in which only the first internal repeat and all of the putative intracellular loops are of skeletal origin. All of the constructs studied similarly exhibit a brief (2-ms) and a long (> or = 15-ms) open time in the presence of Bay K 8644, neither of which depend significantly on voltage. In the absence of Bay K 8644, the fraction of total open events is markedly shifted to the briefer open time without altering the rate of ensemble activation. Closed time analysis of L-channels with cardiac-like, rapid activation (recorded in the presence of dihydropyridine agonist) reveals both a brief (approximately 1-ms) closed time and a second, voltage-dependent, long-lasting closed time. The time until first opening after depolarization is three to six times faster for rapidly activating L-channels than for slowly activating L-channels and depends strongly on voltage for both types of channels. The results suggest that a voltage-dependent, closed-closed transition that is fast in cardiac L-channels and slow in skeletal L-channels can account for the difference in activation rate between these two channels.

Chimeric L-type Ca2+ channels expressed in Xenopus laevis oocytes reveal role of repeats III and IV in activation gating

1. Chimeric alpha 1 subunits consisting of repeat I and II from the rabbit cardiac (alpha 1C-a) and repeat III and IV from the carp skeletal muscle Ca2+ channel (alpha 1S) were constructed and expressed in Xenopus laevis oocytes without co-expressing other channel subunits. Ba2+-current kinetics of five chimeric channel constructs were studied in Xenopus oocytes using the two-microelectrode technique. 2. Exchange of repeats III and IV of alpha 1C-a with sequences of alpha 1S results in a significantly slower and biexponential activation (apparent activation time constants tau 1act = 19.8 +/- 1.8 ms and tau 2act = 214 +/- 28.7 ms, n = 7) of expressed Ca2+ channel currents; no current inactivation was observable during an 800 ms test pulse to 0 mV. 3. Activation of a chimera consisting of repeats I, II and IV from the alpha 1C-a subunit and repeat III from alpha 1S was fast and monoexponential (tau 1act = 6.33 +/- 1.7 ms, n = 5) and the current inactivated during a 350 ms test pulse to 0 mV (tau inact = 175 +/- 22 ms, n = 5). The current kinetics of this construct did not significantly differ from kinetics of a construct consisting of repeats I to IV from alpha 1C-a (tau 1act = 6.6 +/- 2.1 ms; tau inact = 198 +/- 14 ms; n = 9).(ABSTRACT TRUNCATED AT 250 WORDS)