Functional expression of the L-type calcium channel in mice skeletal muscle during prenatal myogenesis

Anaesthetic tricaine acts preferentially on neural voltage-gated sodium channels and fails to block directly evoked muscle contraction

Movements in animals arise through concerted action of neurons and skeletal muscle. General anaesthetics prevent movement and cause loss of consciousness by blocking neural function. Anaesthetics of the amino amide-class are thought to act by blockade of voltage-gated sodium channels. In fish, the commonly used anaesthetic tricaine methanesulphonate, also known as 3-aminobenzoic acid ethyl ester, metacaine or MS-222, causes loss of consciousness. However, its role in blocking action potentials in distinct excitable cells is unclear, raising the possibility that tricaine could act as a neuromuscular blocking agent directly causing paralysis. Here we use evoked electrical stimulation to show that tricaine efficiently blocks neural action potentials, but does not prevent directly evoked muscle contraction. Nifedipine-sensitive L-type Cav channels affecting movement are also primarily neural, suggesting that muscle Nav channels are relatively insensitive to tricaine. These findings show that tricaine used at standard concentrations in zebrafish larvae does not paralyse muscle, thereby diminishing concern that a direct action on muscle could mask a lack of general anaesthesia.

Fiber Types in Mammalian Skeletal Muscles

Mammalian skeletal muscle comprises different fiber types, whose identity is first established during embryonic development by intrinsic myogenic control mechanisms and is later modulated by neural and hormonal factors. The relative proportion of the different fiber types varies strikingly between species, and in humans shows significant variability between individuals. Myosin heavy chain isoforms, whose complete inventory and expression pattern are now available, provide a useful marker for fiber types, both for the four major forms present in trunk and limb muscles and the minor forms present in head and neck muscles. However, muscle fiber diversity involves all functional muscle cell compartments, including membrane excitation, excitation-contraction coupling, contractile machinery, cytoskeleton scaffold, and energy supply systems. Variations within each compartment are limited by the need of matching fiber type properties between different compartments. Nerve activity is a major control mechanism of the fiber type profile, and multiple signaling pathways are implicated in activity-dependent changes of muscle fibers. The characterization of these pathways is raising increasing interest in clinical medicine, given the potentially beneficial effects of muscle fiber type switching in the prevention and treatment of metabolic diseases.

Neural agrin controls maturation of the excitation-contraction coupling mechanism in human myotubes developing in vitro

The aim of this study was to elucidate the mechanisms responsible for the effects of innervation on the maturation of excitation-contraction coupling apparatus in human skeletal muscle. For this purpose, we compared the establishment of the excitation-contraction coupling mechanism in myotubes differentiated in four different experimental paradigms: 1) aneurally cultured, 2) cocultured with fetal rat spinal cord explants, 3) aneurally cultured in medium conditioned by cocultures, and 4) aneurally cultured in medium supplemented with purified recombinant chick neural agrin. Ca2+ imaging indicated that coculturing human muscle cells with rat spinal cord explants increased the fraction of cells showing a functional excitation-contraction coupling mechanism. The effect of spinal cord explants was mimicked by treatment with medium conditioned by cocultures or by addition of 1 nM of recombinant neural agrin to the medium. The treatment with neural agrin increased the number of human muscle cells in which functional ryanodine receptors (RyRs) and dihydropyridine-sensitive L-type Ca2+ channels were detectable. Our data are consistent with the hypothesis that agrin, released from neurons, controls the maturation of the excitation-contraction coupling mechanism and that this effect is due to modulation of both RyRs and L-type Ca2+ channels. Thus, a novel role for neural agrin in skeletal muscle maturation is proposed.

Absence of regulation of the T-type calcium current by Cav1.1, β1a and γ1 dihydropyridine receptor subunits in skeletal muscle cells

The subunit structure of low voltage activated T-type Ca2+ channels is still unknown. Co-expression of dihydropyridine receptor (DHPR) auxiliary subunits with T-type alpha1 subunits in heterologous systems has produced conflicting results. In developing foetal skeletal muscle fibres which abundantly express DHPR subunits, Cav3.2 (alpha1H) subunits are believed to underlie T-type calcium currents which disappear 2 to 3 weeks after birth. Therefore, a possible regulation of foetal skeletal muscle T-type Ca2+ channels by DHPR subunits was investigated in freshly isolated foetal skeletal muscle using knockout mice, which provide a powerful tool to address this question. The possible involvement of alpha1S (Cav1.1), beta1 and gamma1 DHPR subunits was tested using dysgenic (alpha1S-null), beta1a and gamma1 knockout mice. The results show that the absence of alpha1S, beta1 or gamma1 DHPR subunits does not significantly affect the electrophysiological properties of T-type Ca2+ currents in skeletal muscle, suggesting that (1) native Cav3.2 is not regulated by beta1 or gamma1 DHPR subunits; (2) T-type and L-type currents have distinct and not interchangeable roles.

Ca2+ channel regulation by transforming growth factor-beta 1 and bone morphogenetic protein-2 in developing mice myotubes

In skeletal muscle myogenesis, precursor cells or myoblasts fuse to form multinucleated cells (myotubes), which then further develop into functional muscle. We investigated if the inhibition of myogenesis by transforming growth factor-beta1 (TGF-beta1) and bone morphogenetic protein-2 (BMP-2) involve regulation of voltage-dependent Ca(2+) channels. Primary cultured myoblasts were kept in fusion medium (0-6 days) in either the absence (control conditions) or the presence of 40 pm TGF-beta1 or 5 nm BMP-2. Subsequently, the developing myotubes were transferred to a growth factor-free recording solution, and subjected to whole cell patch-clamp experiments. At day 0, 14% of non-fusing myoblasts exhibited T-current, whereas the L-current was practically absent. Under control conditions, however, the percentage of T- and L-channel-expressing myotubes increased sharply, from 25% at day 1 to approximately 100% at days 2-6. In addition, parallel increases were determined for Ca(2+)-currents density and cell membrane capacitance (C(m)), which is proportional to the size of myotubes. Interestingly, at days 1-2 TGF-beta1 and BMP-2 eliminated the T-current on initial 14% of T-channel-expressing myoblasts. Moreover, at day 6 the growth factors significantly reduced the maximal values of both T-current density (80%) and C(m) (60%). The effect of BMP-2 was selective on T-channels, whereas TGF-beta1 decreased also the L-current density (90%). A similar reduction in maximal conductance of the Ca(2+) channels was determined, in the absence of significant alterations in other essential properties of the channels, including the time course and voltage dependence of activation and inactivation. The results suggest these growth factors markedly reduce the number of functional T- (both TGF-beta1 and BMP-2) and L-channels (only TGF-beta1) in the surface of the plasma membrane, and contribute to explaining the associated effects on myogenesis.

Membrane cholesterol modulates dihydropyridine receptor function in mice fetal skeletal muscle cells

Caveolae and transverse (T-) tubules are membrane structures enriched in cholesterol and glycosphingolipids. They play an important role in receptor signalling and myogenesis. The T-system is also highly enriched in dihydropyridine receptors (DHPRs), which control excitation-contraction (E-C) coupling. Recent results have shown that a depletion of membrane cholesterol alters caveolae and T-tubules, yet detailed functional studies of DHPR expression are lacking. Here we studied electrophysiological and morphological effects of methyl-beta-cyclodextrin (MbetaCD), a cholesterol-sequestering drug, on freshly isolated fetal skeletal muscle cells. Exposure of fetal myofibres to 1-3 mM MbetaCD for 1 h at 37 degrees C led to a significant reduction in caveolae and T-tubule areas and to a decrease in cell membrane electrical capacitance. In whole-cell voltage-clamp experiments, the L-type Ca(2+) current amplitude was significantly reduced, and its voltage dependence was shifted approximately 15 mV towards more positive potentials. Activation and inactivation kinetics were slower in treated cells than in control cells and stimulation by a saturating concentration of Bay K 8644 was enhanced. In addition, intramembrane charge movement and Ca(2+) transients evoked by a depolarization were reduced without a shift of the midpoint, indicating a weakening of E-C coupling. In contrast, T-type Ca(2+) current was not affected by MbetaCD treatment. Most of the L-type Ca(2+) conductance reduction and E-C coupling weakening could be explained by a decrease of the number of DHPRs due to the disruption of caveolae and T-tubules. However, the effects on L-type channel gating kinetics suggest that membrane cholesterol content modulates DHPR function. Moreover, the significant shift of the voltage dependence of L-type current without any change in the voltage dependence of charge movement and Ca(2+) transients suggests that cholesterol differentially regulates the two functions of the DHPR.

Ca2+ current and charge movements in skeletal myotubes promoted by the beta-subunit of the dihydropyridine receptor in the absence of ryanodine receptor type 1

The beta-subunit of the dihydropyridine receptor (DHPR) enhances the Ca(2+) channel and voltage-sensing functions of the DHPR. In skeletal myotubes, there is additional modulation of DHPR functions imposed by the presence of ryanodine receptor type-1 (RyR1). Here, we examined the participation of the beta-subunit in the expression of L-type Ca(2+) current and charge movements in RyR1 knock-out (KO), beta1 KO, and double beta1/RyR1 KO myotubes generated by mating heterozygous beta1 KO and RyR1 KO mice. Primary myotube cultures of each genotype were transfected with various beta-isoforms and then whole-cell voltage-clamped for measurements of Ca(2+) and gating currents. Overexpression of the endogenous skeletal beta1a isoform resulted in a low-density Ca(2+) current either in RyR1 KO (36 +/- 9 pS/pF) or in beta1/RyR1 KO (34 +/- 7 pS/pF) myotubes. However, the heterologous beta2a variant with a double cysteine motif in the N-terminus (C3, C4), recovered a Ca(2+) current that was entirely wild-type in density in RyR1 KO (195 +/- 16 pS/pF) and was significantly enhanced in double beta1/RyR1 KO (115 +/- 18 pS/pF) myotubes. Other variants tested from the four beta gene families (beta1a, beta1b, beta1c, beta3, and beta4) were unable to enhance Ca(2+) current expression in RyR1 KO myotubes. In contrast, intramembrane charge movements in beta2a-expressing beta1a/RyR1 KO myotubes were significantly lower than in beta1a-expressing beta1a/RyR1 KO myotubes, and the same tendency was observed in the RyR1 KO myotube. Thus, beta2a had a preferential ability to recover Ca(2+) current, whereas beta1a had a preferential ability to rescue charge movements. Elimination of the double cysteine motif (beta2a C3,4S) eliminated the RyR1-independent Ca(2+) current expression. Furthermore, Ca(2+) current enhancement was observed with a beta2a variant lacking the double cysteine motif and fused to the surface membrane glycoprotein CD8. Thus, tethering the beta2a variant to the myotube surface activated the DHPR Ca(2+) current and bypassed the requirement for RyR1. The data suggest that the Ca(2+) current expressed by the native skeletal DHPR complex has an inherently low density due to inhibitory interactions within the DHPR and that the beta1a-subunit is critically involved in process.

L-type Ca2+ channels of the embryonic mouse heart

In the heart, where Ca(2+) influx across the sarcolemma is essential for contraction, L-type Ca(2+) channels represent the major entry pathway of Ca(2+). Mice with a homozygous deletion of the L-type Ca(v)1.2 Ca(2+) channel gene die before day 14.5 p.c. Electrophysiological and pharmacological investigations on Ca(v)1.2-/- cardiomyocytes demonstrated that contractions depended on the influx of Ca(2+) through an L-type-like Ca(2+) channel. We analyzed now the expression pattern of various L-type Ca(2+) channels. Amplification of the alternative exons 1a and 1b revealed that embryonic cardiac cells express both Ca(v)1.2a and Ca(v)1.2b subunits. Reverse transcriptase-polymerase chain reaction (RT-PCR) amplifications indicated the expression of Ca(v)1.1 and Ca(v)1.3 in about a 1:10 ratio in Ca(v)1.2-/- embryos. Two different amino termini of the Ca(v)1.3 cDNA were found in the embryonic heart, which both gave rise to functional channels. Ca(v)1.3(1a) and Ca(v)1.3(1b) channels have similar current kinetics and voltage-dependencies as described for Ca(v)1.3(8A) channels [J. Biol. Chem. 276 (2001) 22100], but the properties of Ca(v)1.3(1a) or Ca(v)1.3(1b) channels are different from that of the L-type-like current in Ca(v)1.2-/- cardiomyocytes. The I(Ba) of Ca(v)1.3(1a) was blocked by the dihydropyridine nisoldipine with an IC(50) value of 0.13 microM at a holding potential of -80 mV. In embryonic Ca(v)1.2+/+ cardiomyocytes, I(Ba) was blocked by nisoldipine with an IC(50) value of 0.1 microM. Although the expressed Ca(v)1.3 channel has a similar affinity for nisoldipine as Ca(v)1.2+/+ cardiomyocytes, the L-type-like Ca(2+) channel found in Ca(v)1.2+/+ and -/- cardiomyocytes is not identical with the new Ca(v)1.3 splice variants.

Alpha(1H) mRNA in single skeletal muscle fibres accounts for T-type calcium current transient expression during fetal development in mice

Calcium channels are essential for excitation-contraction coupling and muscle development. At the end of fetal life, two types of Ca(2+) currents can be recorded in muscle cells. Whereas L-type Ca(2+) channels have been extensively studied, T-type channels have been poorly characterized in skeletal muscle. We describe here the functional and molecular properties of T-type calcium channels in developing mouse skeletal muscle. The T-type current density increased transiently during prenatal myogenesis with a maximum at embryonic day E16 followed by a drastic decrease until birth. This current showed similar electrophysiological and pharmacological properties at all examined stages. It displayed a wide window current centred at about -35 and -55 mV in 10 and 2 mM external Ca(2+), respectively. Activation and inactivation kinetics were fast (3 and 16 ms, respectively). The current was inhibited by nickel and amiloride with an IC(50) of 5.4 and 156 microM, respectively, values similar to those described for cloned T-type alpha(1H) channels. Whole muscle tissue RT-PCR analysis revealed mRNAs corresponding to alpha(1H) and alpha(1G) subunits in the fetus but not in the adult. However, single-fibre RT-PCR demonstrated that only alpha(1H) mRNA was present in prenatal fibres, suggesting that the alpha(1G) transcript present in muscle tissue must be expressed by non-skeletal muscle cells. Altogether, these results demonstrate that the alpha(1H) subunit generates functional T-type calcium channels in developing skeletal muscle fibres and suggest that these channels are involved in the early stages of muscle differentiation.

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.