Targets for calcium channel blockers in mammalian skeletal muscle and their respective functions in excitation-contraction coupling

The voltage sensor of excitation-contraction coupling in mammals: Inactivation and interaction with Ca2

In excitation–contraction coupling, voltage-sensing modules (VSMs) of Ca_V1.1 Ca²⁺ channels simultaneously gate the associated pore and Ca²⁺ release channels in the sarcoplasmic reticulum. Ferreira Gregorio et al. find that VSMs adopt two inactivated states, and the degree of inactivation is dependent on external Ca²⁺ and the mouse strain used.

In skeletal muscle, the four-helix voltage-sensing modules (VSMs) of Ca_V1.1 calcium channels simultaneously gate two Ca²⁺ pathways: the Ca_V1.1 pore itself and the RyR1 calcium release channel in the sarcoplasmic reticulum. Here, to gain insight into the mechanism by which VSMs gate RyR1, we quantify intramembrane charge movement associated with VSM activation (sensing current) and gated Ca²⁺ release flux in single muscle cells of mice and rats. As found for most four-helix VSMs, upon sustained depolarization, rodent VSMs lose the ability to activate Ca²⁺ release channels opening; their properties change from a functionally capable mode, in which the mobile sensor charge is called charge 1, to an inactivated mode, charge 2, with a voltage dependence shifted toward more negative voltages. We find that charge 2 is promoted and Ca²⁺ release inactivated when resting, well-polarized muscle cells are exposed to low extracellular [Ca²⁺] and that the opposite occurs in high [Ca²⁺]. It follows that murine VSMs are partly inactivated at rest, which establishes the reduced availability of voltage sensing as a pathogenic mechanism in disorders of calcemia. We additionally find that the degree of resting inactivation is significantly different in two mouse strains, which underscores the variability of voltage sensor properties and their vulnerability to environmental conditions. Our studies reveal that the resting and activated states of VSMs are equally favored by extracellular Ca²⁺. Promotion by an extracellular species of two states of the VSM that differ in the conformation of the activation gate requires the existence of a second gate, inactivation, topologically extracellular and therefore accessible from outside regardless of the activation state.

DHPR alpha1S subunit controls skeletal muscle mass and morphogenesis

The alpha1S subunit has a dual function in skeletal muscle: it forms the L-type Ca(2+) channel in T-tubules and is the voltage sensor of excitation-contraction coupling at the level of triads. It has been proposed that L-type Ca(2+) channels might also be voltage-gated sensors linked to transcriptional activity controlling differentiation. By using the U7-exon skipping strategy, we have achieved long-lasting downregulation of alpha1S in adult skeletal muscle. Treated muscles underwent massive atrophy while still displaying significant amounts of alpha1S in the tubular system and being not paralysed. This atrophy implicated the autophagy pathway, which was triggered by neuronal nitric oxide synthase redistribution, activation of FoxO3A, upregulation of autophagy-related genes and autophagosome formation. Subcellular investigations showed that this atrophy was correlated with the disappearance of a minor fraction of alpha1S located throughout the sarcolemma. Our results reveal for the first time that this sarcolemmal fraction could have a role in a signalling pathway determining muscle anabolic or catabolic state and might act as a molecular sensor of muscle activity.

Intrinsic ionic conductances mediate the spontaneous electrical activity of cultured mouse myotubes

Mouse skeletal myotubes differentiated in vitro exhibited spontaneous contractions associated with electrical activity. The ionic conductances responsible for the origin and modulation of the spontaneous activity were examined using the whole-cell patch-clamp technique and measuring [Ca(2+)](i) transients with the Ca(2+) indicator, fura 2-AM. Regular spontaneous activity was characterized by single TTX-sensitive action potentials, followed by transient increases in [Ca(2+)](i). Since the bath-application of Cd(2+) (300 microM) or Ni(2+) (50 muM) abolished the cell firing, T-type (I(Ca,T)) and L-type (I(Ca,L)) Ca(2+) currents were investigated in spontaneously contracting myotubes. The low activation threshold (around -60 mV) and the high density of I(Ca,T) observed in contracting myotubes suggested that I(Ca,T) initiated action potential firing, by bringing cells to the firing threshold. The results also suggested that the activity of I(Ca,L) could sustain the [Ca(2+)](i) transients associated with the action potential, leading to the activation of apamin-sensitive SK-type Ca(2+)-activated K(+) channels and the afterhyperpolarization (AHP) following single spikes. In conclusion, an interplay between voltage-dependent inward (Na(+) and Ca(2+)) and outward (SK) conductances is proposed to mediate the spontaneous pacemaker activity in cultured muscle myotubes during the process of myogenesis.

The effect of nickel chloride on the isolated hemidiaphragm of the rat

1. NiCl2 (cumulative concentrations of 0.56-1.91 mmol 1(-1)) produced concentration-dependent depression of tension developed (Td) and the maximum rate of rise of tension (dT/dt max) of isometric contraction of the isolated rat hemidiaphragm, during direct subtetanic (DST) electrical stimulation, only. EC50 values for NiCl2-induced depression of Td and Dt/dt max were 0.88 +/- 0.06 and 0.83 +/- 0.13 mmol 1(-1), respectively. NiCl2 did not significantly change either parameter of the isometric contraction during direct single-pulse (DSP) electrical stimulation. 2. Maximal depression of Td and dT/dt max, produced by a single concentration of NiCl2 (1 mmol 1(-1)) during DST electrical stimulation was obtained 20 min after addition of the drug in the bathing medium. 3. In the normal Tyrode solution, addition of CaCl2 (final concentration of 5.86 mmol 1(-1)) almost completely antagonized the depressant effect of NiCl2 (1 mmol 1(-1)) on Td and dT/dt max during DST electrical stimulation. In the calcium-free solution, the depression both of Td and dT/dt max produced by NiCl2 (1 mmol 1(-1)) was significantly more pronounced in comparison with the effect of NiCl2 in the normal Tyrode solution. 4. L-calcium channel activator, Bay K 8644 (25 mumol 1(-1)), significantly potentiated both Td and dT/dt max during DST electrical stimulation, but NiCl2 (1 mmol 1(-1)) decreased both parameters of the isometric contraction even in the presence of this concentration of Bay K 8644. On the other hand Bay K 8644 (25 mumol 1(-1)) did not antagonize NiCl2-induced depression of Td and dT/dt max. 5. Verapamil (2.5 mumol 1(-1); 45 min of incubation) and lidocaine (0.10 mmol 1(-1); 30 min of incubation) significantly potentiated the depression of Td and dT/dt max, produced by NiCl2 (1 mmol 1(-1), during DST electrical stimulation. The addition of CaCl2 (final concentration of 7.20 mmol 1(-1)) in the bathing medium only partially antagonized the depressant synergistic action of both verapamil or lidocaine and NiCl2 on Td and dT/dt max. 6. Forskolin (cumulative concentrations of 2.60-44.20 mumol 1(-1)) fully antagonized NiCl2-induced depression of both Td and dT/dt max; propranolol (1 mumol 1(-1)) did not abolish this antagonizing action of forskolin. Also, NiCl2 (cumulative concentrations of 0.56 -1.54 mmol 1(-1)) did not change potentiating effect of forskolin (23.4 mumol 1(-1)).(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of SR33557 on intramembrane charge movement in normal and 'muscular dysgenesis' mouse skeletal muscle cells

It has been reported that the indolizinsulphone SR33557, which binds to a site on the alpha 1 subunit of the dihydropyridine receptor, blocks both L-type calcium channel activity and contraction in skeletal muscle. Moreover, we know that charge movement plays a key role in the mechanism of excitation-contraction coupling and in controlling the opening of L-type calcium channels. We demonstrate here that SR33557 reduces intramembrane charge movement in skeletal muscle from normal mice with an IC50 of approximately 10 nM. The drug does not completely inhibit charge movement since approximately 20% of total charge movement persists even in the presence of 30 microM SR33557. However, the SR33557-sensitive charge component is more important than the dihydropyridine-sensitive one. Surprisingly, SR33557 also reduces intramembrane charge movement in dysgenic myotubes which are characterized by a very strong reduction of the number of dihydropyridine binding sites. In these muscles, 10 microM SR33557 reduces approximately 40% of total charge movement. These observations suggest the presence of a new component of charge movement which is sensitive to SR33557 but insensitive to nifedipine. This component is also present in dysgenic myotubes, and it could be produced by the lower molecular weight alpha 1 subunit described by Malouf, N. N., McMahon, D. K., Hainsworth, C. N. and Kay, B. K. (1992) (Neuron, 8, 899-906).

Effects of bipyridylium compounds on calcium release from triadic vesicles isolated from rabbit skeletal muscle

1. The effects of 1,1'-diheptyl-4,4'-bipyridinium dibromide (DHBP), a viologen for electrochromic memory display agent, on calcium release and ryanodine binding were studied with triad-rich sarcoplasmic reticulum (SR) vesicles isolated from rabbit skeletal muscle. 2. DHBP inhibited the calcium release induced by 2 mM caffeine and 2 micrograms ml-1 polylysine with an IC50 value of 5 micrograms ml-1 and 4 micrograms ml-1 respectively. 3. DHBP inhibited [3H]-ryanodine binding in a dose-dependent manner with an IC50 of 2.5 micrograms ml-1 and 90-100% inhibition at 20-30 micrograms ml-1. 4. Calcium uptake by SR was inhibited in the presence of caffeine and this inhibition was antagonized by concomitant addition of DHBP. 5. The effect of DHBP on muscle twitches was studied on the mouse diaphragm. Muscle twitches elicited by direct electrical muscle stimulation and contractions induced by either 10 mM caffeine or 1 microM ryanodine were blocked by pretreatment with DHBP. 6. Data from this study provided evidence that DHBP blocked the calcium release from SR by direct interaction with the calcium release channel, also known as the ryanodine receptor. A possible use of this agent as a specific inhibitor for calcium release and as a muscle relaxant was suggested.

Some new evidence on antifatigue action of aminophylline on the isolated hemidiaphragm of the rat

1. Aminophylline (cumulative concentrations of 0.036-3.60 mmol/l) produced a concentration-dependent increase in both tension developed (Td) and the maximum rate of rise of tension (dT/dt max) of the isolated hemidiaphragm of the rat both during direct single-pulse and subtetanic stimulation. 2. The repeated series of additions of aminophylline into the bathing medium (the second and the third series) produced even further, more pronounced potentiation of both Td and dT/dt max during subtetanic stimulation only, the potentiation being the strongest after the third series of additions of the drug ("antifatigue effect"). The antifatigue effect of aminophylline was much more pronounced than the antifatigue effect of the equimolar concentrations of caffeine. 3. The presence of intact beta 1-adrenergic receptors seems to be essential for the antifatigue action of aminophylline under our experimental conditions. 4. The antifatigue effect of aminophylline was not affected by reserpine or 6-OHDA pretreatment of rats. 5. In a Ca(2+)-free medium the stimulatory effect of aminophylline on Td and dT/dt max was abolished or depressed (single-pulse and subtetanic stimulation, respectively). After returning the muscle into the medium containing Ca2+, the effect of aminophylline was significantly potentiated during both types of the stimulation. 6. The antifatigue action of aminophylline was preserved even in the presence of nicardipine or its solvent in the bathing medium. 7. In the presence of heparin (which produced a significant depression of both Td and dT/dt max by itself during direct subtetanic stimulation) the stimulatory effects of aminophylline on Td and dT/dt max (the second and third series of additions) were significantly potentiated in comparison with the effects of the first series of additions of aminophylline (with no heparin in the bathing medium). 8. The dose-response curves for the effects of aminophylline in the presence of Ni2+ on Td and dT/dt max during direct single-pulse stimulation were significantly shifted to the right. Ni2+ by itself produced significant and dose-related depression of both Td and dT/dt max during single-pulse and subtetanic stimulation, the subtetanic stimulation being much more sensitive. The antifatigue effect of aminophylline during subtetanic stimulation was preserved in the presence of Ni2+. 9. Our results indicate the important role of the extracellular calcium and the involvement of intact beta 1-adrenergic receptors in the antifatigue action of aminophylline. Also, the potentiating effect of heparin on the antifatigue action of aminophylline is presumably due to the influx of extracellular calcium through L-type Ca2+ channels during subtetanic stimulation. Our results indicate the possibility of the presence of T-type calcium channels (which can be blocked by Ni2+) in the isolated hemidiaphragm of the rat, but they do not seem to be involved in the antifatigue action of aminophylline.

Indolizinsulphones. A class of blockers with dual but discriminative effects on L-type Ca2+ channel activity and excitation-contraction coupling in skeletal muscle

The alpha 1 subunit of the L-type Ca2+ channel plays a dual role in skeletal muscle. It is essential both for L-type Ca2+ channel activity and for the functioning of the voltage-sensor structure that is situated in the triads as a key element for excitation-contraction coupling. This paper shows, with mouse muscle cells in primary culture, that indolizinsulphone SR33557 which has its binding site on the alpha 1 subunit blocks both L-type Ca2+ channel activity and contraction as the more classical 1,4-dihydropyridine blockers. However, unlike other Ca2+ channel blockers, it can pharmacologically discriminate between the two different roles of the alpha 1 subunit. SR33557 inhibition of both contractile and L-type Ca2+ channel activities is very voltage dependent and increases at depolarized potentials. Complete blockade of contraction was observed at low SR33557 concentrations (K0.5 = 20 nM) and was associated with only minor L-type Ca2+ channel blockade (30%). The remaining and major part of the L-type Ca2+ channel activity (70%) was blocked at much higher SR33557 concentrations (K0.5 = 0.6 microM). The results indicate that SR33557 has a much higher affinity for the alpha 1 subunit inserted into the voltage-sensor structure. They also suggest that the voltage-sensor structure, which probably includes most of the total T-tubule alpha 1 subunit, has intrinsic (but relatively small) Ca2+ channel activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Ca2+ channel agonists enhance thyrotropin-releasing hormone-induced inositol phosphates and prolactin secretion

The dihydropyridine Ca2+ channel activator BAY K 8644 (1 microM) stimulated basal prolactin secretion from perifused primary cultures of anterior pituitary cells and potentiated the stimulation of prolactin secretion by 1 microM thyrotropin-releasing hormone (TRH) 5-fold over 30 min. This potentiation was mimicked by other dihydropyridine agonists CGP 28392 and (+)-SDZ 202-791 and by (-)-BAY K 8644 (1 microM), but not by (+)-BAY K 8644. The Ca2+ channel antagonist nimodipine, at a concentration sufficient to block BAY K 8644-stimulated 45Ca2+ uptake in GH4C1 anterior pituitary tumor cells, decreased basal prolactin secretion and blocked the enhancement of basal and TRH-stimulated secretion by BAY K 8644. These results suggest that dihydropyridine agonists potentiate TRH-induced secretion through interaction with known stereospecific sites on Ca2+ channels. In GH4C1 cells, BAY K 8644 alone did not affect inositol polyphosphate accumulation, but potentiated TRH-stimulated accumulation of inositol 1,3,4-trisphosphate and inositol 1,3,4,5-tetrakisphosphate. Accumulation of the Ca(2+)-mobilizing isomer inositol 1,4,5-trisphosphate was not potentiated, suggesting that potentiation of TRH-stimulated hormone secretion by BAY K 8644 does not result from synergistic stimulation of phospholipase C, but may correlate with enhanced inositol trisphosphate-3-kinase activity.

Calciseptine, a peptide isolated from black mamba venom, is a specific blocker of the L-type calcium channel

The venom of the black mamba contains a 60-amino acid peptide called calciseptine. The peptide has been fully sequenced. It is a smooth muscle relaxant and an inhibitor of cardiac contractions. Its physiological action resembles that of drugs, such as the 1,4-dihydropyridines, which are important in the treatment of cardiovascular diseases. Calciseptine, like the 1,4-dihydropyridines, selectively blocks L-type Ca2+ channels and is totally inactive on other voltage-dependent Ca2+ channels such as N-type and T-type channels. To our knowledge, it is the only natural polypeptide that has been shown to be a specific inhibitor of L-type Ca2+ channels.

Fura-2 imaging of spontaneous and electrically induced oscillations of intracellular free Ca2+ in rat myotubes

Rat myotubes have a resting [Ca2+]i of about 82 nM. Myotubes 3-5 days old (quiescent myotubes) display electrically induced and spontaneous transients in the intracellular concentration of free Ca2+ ions ([Ca2+]i) uncoupled to any detectable contraction. By contrast, 1- to 2-day-old myotubes are insensitive to electrical stimuli and, after 6 days in culture, stimulated myotubes always show [Ca2+]i transients and twitch contractions. The spatial distribution of [Ca2+]i variations in quiescent myotubes is heterogeneous, local increases in [Ca2+]i being mainly observed near the periphery of the cell. The small effect of different external Ca2+ concentrations and of Cd2+ on the amplitude of the [Ca2+]i oscillation indicates that the main source of Ca2+ may be the sarcoplasmic reticulum. This conclusion is supported by the close similarity between electrically induced and caffeine-induced [Ca2+]i maps. These findings suggest that, at an early stage of myotube ontogenesis, a part of the excitation/contraction coupling, as membrane ionic channels, voltage sensors and Ca2+ release and reuptake mechanisms, is functional but, apparently, still uncoupled to the contractile machinery.

Mechanisms of the synergistic interactions between organic calcium channel antagonists and various neuromuscular blocking agents

The effects of Mn2+, neomycin and four organic Ca2(+)-channel antagonists (OCA): nicardipine, nifedipine, diltiazem and verapamil on the neuromuscular blocking activities of tubocurarine, succinylcholine (SCh), decamethonium and neomycin were studied in isolated mouse phrenic nerve-diaphragm preparations. The effective concentration of SCh for 50% inhibition (IC50) of single indirect twitch responses were reduced markedly by more than 3-fold when the preparations were pretreated with OCA at 10 microM; the latter alone did not appreciably affect the indirect twitch response or the amplitude of miniature endplate potentials. The neuromuscular blocking effect of decamethonium was also enhanced synergistically by OCA to a similar extent. On the other hand, under the comparable condition. the combined uses of OCA plus tubocurarine or neomycin, neomycin plus tubocurarine or SCh, and Mn2+ plus tubocurarine, SCh or neomycin all resulted in insignificant potentiation. These results suggest that OCA have a specific effect to enhance the agonist effect of depolarizing agents on nicotinic acetylcholine receptors. Nicardipine at 2 microM non-competitively inhibited depolarizations of endplates elicited by SCh and decamethonium and abolished them completely at 10 microM nicardipine. The IC50's in inhibiting endplate potentials and miniature endplate potentials by SCh and decamethonium were also reduced 2 to 3.5-fold by nicardipine. It is inferred that OCA are endowed with a unique capability to allosterically affect the postsynaptic nicotinic acetylcholine receptor, promoting its desensitization liability, hence synergistic interaction with depolarizing agents. Presynaptic effects of OCA are probably not involved.

Induction of normal ultrastructure by CGRP treatment in dysgenic myotubes

The calcitonin gene-related peptide (CGRP) restores an apparent normal ultrastructure in mdg/mdg muscle cells in vitro, including a normal triadic organization which is known to be essential for excitation-contraction (E-C) coupling. However, neither slow L-type Ca2+ channel activity nor E-C coupling, which are absent in mdg/mdg muscle, were re-established. These observations suggest a potential role of CGRP (and also of cAMP as the intracellular messenger) in the morphological development of the muscle fiber.

The blockade of excitation/contraction coupling by nifedipine in patch-clamped rat skeletal muscle cells in culture

The effects of the dihydropyridine derivative, nifedipine, well known as a blocker of calcium channels, were tested on cultured rat myoballs. Membrane currents and contractions were simultaneously recorded by means of the patch-clamp technique and a photoelectric transducing method. High concentrations of nifedipine (5 microM) inhibited the contractile responses and inward calcium current (ICa) elicited by long depolarizations. In the absence of ICa (1.5 mM cadmium in the bath), nifedipine inhibited both the ICa-independent contractile component and the outward current, supposed to depend on the intracellular calcium released during contraction. At low concentrations (0.5 microM) the blocking effects of nifedipine could be strongly enhanced by shifting the membrane potential towards less negative values (-60 mV) for 50 s prior to the test pulse. A blocking effect of nifedipine, at a usually ineffective concentration (0.1 microM), could also be observed when long-lasting (3 min) prepulses to 0 mV were applied from a reference membrane potential of -60 mV. This effect could be relieved by long-lasting cell hyperpolarizations (-90 mV). The blocking effects of nifedipine unrelated to ICa could be interpreted as an action on a molecule (voltage sensor) in the T-tubule membrane involved in the excitation/contraction coupling process and as a preferential binding of the dihydropyridine derivative on the inactivated form of this molecule, favored by the weak negative potentials or long-lasting depolarizations. The results provide data in favor of the existence of strong similarities between the calcium channels and voltage sensors since their operation was inhibited in a voltage-dependent manner by nifedipine.

Organic calcium channel antagonists provoke acetylcholine receptor autodesensitization on train stimulation of motor nerve

The effects of nicardipine and other organic Ca2(+)-channel antagonists on the responses induced by indirect train stimulation (3 s, 50-100 Hz) were studied in mouse phrenic nerve diaphragm preparations. Nicardipine at 1-10 microM, which alone did not affect single or tetanic contractions or the amplitude of evoked endplate potentials and spontaneous miniature endplate potentials, caused tetanic contraction to fade completely in the presence of 0.3 microM neostigmine or 50 microM diisopropylfluorophosphate. In combination with these anticholinesterases, nicardipine caused a severe run-down and shortening of endplate potentials in 1-2 s. This effect on endplate potentials was dependent on stimulus frequency and on extracellular Ca2+. The effect was accelerated by intracellular injection of Ca2+, but retarded by injection of EGTA. The amplitudes of miniature endplate potentials and the evoked endplate depolarization were also depressed during repetitive stimulation. On termination of repetitive stimulation, all postsynaptic responses, including evoked endplate potentials, miniature endplate potentials and single twitches, recovered to pre-train level in 3-10 s. These results suggest that the postsynaptic nicotinic receptors had lost the functional activity during repetitive stimulation. The time-courses of the aforementioned changes initiated by repetitive stimulation were similar to the fast phase of desensitization induced by acetylcholine. The irreversible action of alpha-bungarotoxin on acetylcholine receptor was attenuated in the presence of nicardipine and neostigmine if repetitive stimulation was applied. The same effects were observed with other organic Ca2(+)-channel antagonists (diltiazem, verapamil and nifedipine) as well as agonist (methyl-1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-trifluoromethylphenyl)-pyr idine- 5-carboxylate, BAY K8644), but not with Mn2+, theophylline or caffeine. It is inferred that organic Ca2(+)-channel antagonists interact directly with acetylcholine receptor ion channel, enhance its autodesensitization liability and thus cause extinction of endplate potentials on repetitive stimulation.

[Mechanisms of excitation-contraction coupling and calcium liberation in striated muscles of vertebrates]

In vertebrate skeletal muscle, the main part of excitation-contraction coupling occurs at the level of the triad, where membranes of T-system and of junctional SR are facing each other. From place to place, the junctional gap is bridged by "feet" structures which include the SR Ca2+ channel. Half of them are closely apposed to tubular intramembranous structures assumed to be DHP-sensitive voltage-sensors which are similar to tubular Ca2+ channels and act by controlling Ca2+ release from SR. During a twitch, the release of Ca2+ activator from SR is controlled both by voltage-sensors via the feet structures and by a tubular Na+ current via a Na+-induced Ca2+ release mechanism. During long-duration mechanical responses, additional mechanisms are involved: a Ca2+-induced Ca2+ release which can be activated by ICa; the release of Ca2+ from membrane, controlled by the operation of a Na+/Ca2+ exchanger and/or new arrangements of surface membrane charges. An IP3-mediated Ca2+ release could be involved too. All these mechanisms can be regulated by intracellular biochemical or ionic processes.

Ontogenesis and localization of Ca2+ channels in mammalian skeletal muscle in culture and role in excitation-contraction coupling

The mechanism of excitation-contraction (E-C) coupling in skeletal muscle is not yet well established. Cultured mouse skeletal muscle cells have been used to study the relationships between triad formation, Ca2+ channel activities, and contractions. The ontogenesis of voltage-dependent Ca2+ channels and their localization in relation to the ability of muscle to contract and the ultrastructural organization of sarcomeres and triads have been investigated by using an electrophysiological approach together with an electron microscope study. At an early stage of development, both fast (Ifast) and slow (Islow) types of Ca2+ channels are found at the surface membrane. At later stages of development, fast Ca2+ channels remain at the surface membrane, while slow Ca2+ channels migrate to the transverse-tubule membrane. The voltage dependence of fast Ca2+ channels compared to the voltage dependence of contraction clearly shows that these Ca2+ channels have no direct role in E-C coupling. Detubulation at all stages of development has confirmed that T tubules contain essential elements for E-C coupling. However, this work also shows that Ca2+ flowing through slow Ca2+ channels situated in the T-tubular system is not important for contraction. Myotubes lacking slow Ca2+ channels or having no slow Ca2+ channel transport activity (jumps to high membrane potentials, no external Ca2+, block of Islow by Co2+) still retain contraction.