Sodium channel distribution in normal and denervated rodent and snake skeletal muscle

Myogenic activity and serotonergic inhibition in the chromatophore network of the squids, Dosidicus gigas (Family Ommastrephidae) and Doryteuthis opalescens (Family Loliginidae)

Seemingly chaotic waves of spontaneous chromatophore activity occur in the ommastrephid squid, Dosidicus gigas, in the living state and immediately after surgical disruption of all known inputs from the central nervous system. Similar activity is apparent in the loliginid, Doryteuthis opalescens, but only after chronic denervation of chromatophores for 5-7 days. Electrically-stimulated, neurally-driven activity in intact individuals of both species is blocked by tetrodotoxin (TTX), but TTX has no effect on spontaneous wave-activity in either Dosidicus or denervated D. opalescens. Spontaneous, TTX-resistant activity of this sort is therefore likely myogenic, and such activity is eliminated in both preparations by 5-HT, a known inhibitor of chromatophore activity. Immunohistochemical techniques reveal that individual axons containing L-glutamate (L-Glu) or 5-HT (and possibly both in a minority of processes) are associated with radial muscle fibers of chromatophores in intact individuals of both species, although the area of contact between both types of axons and muscle fibers is much smaller in Dosidicus. Glutamatergic and serotonergic axons degenerate completely following denervation in D. opalescens. Spontaneous waves of chromatophore activity in both species are thus associated with reduced (or no) serotonergic input in comparison to the situation in intact D. opalescens. Such differences in the level of serotonergic inhibition are consistent with natural chromogenic behaviours in these species. Our findings also suggest that such activity might propagate via the branching distal ends of radial muscle fibers.

Effects of lactate on the voltage-gated sodium channels of rat skeletal muscle: modulating current opinion

During muscle contraction, lactate production and translocation across the membrane increase. While it has recently been shown that lactate anion acts on chloride channel, less is known regarding a potential effect on the voltage-gated sodium channel (Nav) of skeletal muscle. The electrophysiological properties of muscle Nav were studied in the absence and presence of lactate (10 mM) by using the macropatch-clamp method in dissociated fibers from rat peroneus longus (PL). Lactate in the external medium (petri dish + pipette) increases the maximal sodium current, while the voltage dependence of activation and fast inactivation are shifted toward the hyperpolarized potentials. Lactate induces a leftward shift in the relationship between the kinetic parameters and the imposed potentials, resulting in an earlier recruitment of muscle Nav. In addition, lactate significantly decreases the time constant of activation at voltages more negative than −10 mV, corresponding to an acceleration of Nav activation. The slow inactivation process is decreased by lactate, corresponding to an enhancement in the number of excitable Nav. In an additional series of experiments, lactate (10 mM) was only added to the petri dish, while the pipette remained sealed on the membrane area. With this approach, the electrophysiological properties of Nav were unaffected by lactate compared with the control condition. Altogether, these data indicate that lactate modulates muscle Nav properties by an extracellular pathway. These effects are consistent with an enhancement in excitability, providing new insights into the role of lactate in muscle physiology.

Fibrillation potentials of denervated rat skeletal muscle are associated with expression of cardiac-type voltage-gated sodium channel isoform Nav1.5

OBJECTIVE

The molecular mechanisms underlying fibrillation potentials are still unclear. We hypothesised that expression of the cardiac-type voltage-gated sodium channel isoform Nav1.5 in denervated rat skeletal muscle is associated with the generation of such potentials.

METHODS

Muscle samples were extracted and analysed biologically from surgically denervated rat extensor digitorum longus muscle after concentric needle electromyographic recording at various time points after denervation (4h to 6days).

RESULTS

Both nav1.5 messenger RNA (mRNA) signal on northern blotting and Nav1.5 protein expression on immunohistochemistry appeared on the second day after denervation, exactly when fibrillation potentials appeared. Administration of lidocaine, which has much stronger affinity for sodium channels in cardiac muscle than for those in skeletal muscle, dramatically decreased fibrillation potentials, but had no effect on contralateral compound muscle action potentials.

CONCLUSIONS

Expression of Nav1.5 participates in the generation of fibrillation potentials in denervated rat skeletal muscle.

SIGNIFICANCE

We proposed an altered expression of voltage-gated sodium channel isoforms as a novel mechanism to explain the occurrence of fibrillation potentials following skeletal muscle denervation.

Key role of mucosal primary afferents in mediating the inhibitory influence of capsaicin on vagally mediated contractions in the mouse esophagus

Transient receptor potential ion channel of the vanilloid type 1 (TRPV1)-dependent pathway, consisting of capsaicin-sensitive tachykininergic primary afferent and myenteric nitrergic neurons, was suggested to mediate the inhibitory effect of capsaicin on the vagally mediated striated muscle contractions in the rat esophagus. These primary afferent neurons upon entering into the esophagus are distributed through the myenteric plexus, terminating either in the myenteric ganglia or en route to the mucosa where they branch into a delicate net of fine varicose fibers. Therefore, this study aimed to investigate whether the mucosal primary afferents are a main mediator for the capsaicin inhibitory influence on vagally mediated contractions in the mouse esophagus. For this purpose, the vagally induced contractile activity of a thoracic esophageal segment was measured in the circular direction with a force transducer. Vagal stimulation (30 microsec, 25 V, 1-50 Hz for 1 sec) produced monophasic contractile responses, whose amplitudes were frequency-dependent. These contractions were completely abolished by d-tubocurarine (5 microM) while resistant to atropine (1 microM) and hexamethonium (100 microM). Capsaicin (30 microM) significantly inhibited the vagally induced contractions in esophagi with intact mucosa while its effect on preparations without mucosa was insignificant. Additionally, immunocytochemistry revealed the presence of TRPV1-positive nerve fibers in the tunica mucosa. Taken together, we conclude that in the mouse esophagus, capsaicin inhibits the vagally mediated striated muscle contractions mainly through its action on mucosal primary afferents, which in turn activate the presumed inhibitory local reflex arc.

Involvement of TRPV1-dependent and -independent components in the regulation of vagally induced contractions in the mouse esophagus

Transient receptor potential ion channel of the vanilloid type 1 (TRPV1)-dependent pathway, consisting of capsaicin-sensitive tachykininergic primary afferent and myenteric nitrergic neurons, has been suggested to mediate the inhibitory effect of capsaicin on vagally mediated striated muscle contractions in the rat esophagus. In a recent study, similar but also different effects of capsaicin and piperine on TRPV1 were demonstrated. Therefore, this study aimed to compare the effects of these two drugs on vagally induced contractions in the mouse esophagus. Capsaicin and piperine inhibited vagally induced contractions of a thoracic esophageal segment in a concentration-dependent manner. Ruthenium red (10 microM; a non-selective blocker of transient receptor potential cation channels) and SB-366791 (10 microM; a novel selective antagonist of TRPV1) blocked the inhibitory effect of capsaicin but not that of piperine. Piperine inhibited the vagally mediated contractions in esophagi of adult mice neonatally injected with capsaicin, while capsaicin failed to do so. Desensitization of TRPV1 in the mouse esophagus by in vitro pretreatment with capsaicin failed to affect the inhibitory effect of piperine, whereas the piperine effect was cross-desensitized by capsaicin pretreatment in rat and hamster esophagi. Additionally, a tachykinin NK(1) receptor antagonist, L-732,138 (1 microM), as well as a nitric oxide synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME 200 microM), blocked the inhibitory effect of capsaicin but not that of piperine. Taken together, the results suggest that piperine inhibits the vagally mediated striated muscle contraction in the mouse esophagus through its action on a TRPV1-dependent pathway as well as a TRPV1-independent site.

Mechanisms of adaptation in a predator-prey arms race: TTX-resistant sodium channels

Populations of the garter snake Thamnophis sirtalis have evolved geographically variable resistance to tetrodotoxin (TTX) in a coevolutionary arms race with their toxic prey, newts of the genus Taricha. Here, we identify a physiological mechanism, the expression of TTX-resistant sodium channels in skeletal muscle, responsible for adaptive diversification in whole-animal resistance. Both individual and population differences in the ability of skeletal muscle fibers to function in the presence of TTX correlate closely with whole-animal measures of TTX resistance. Demonstration of individual variation in an essential physiological function responsible for the adaptive differences among populations is a step toward linking the selective consequences of coevolutionary interactions to geographic and phylogenetic patterns of diversity.

Sodium channel mRNAs at the neuromuscular junction: distinct patterns of accumulation and effects of muscle activity

Voltage-gated sodium channels (VGSCs) are highly concentrated at the neuromuscular junction (NMJ) in mammalian skeletal muscle. Here we test the hypothesis that local upregulation of mRNA contributes to this accumulation. We designed radiolabeled antisense RNA probes, specific for the "adult" Na(V)1.4 and "fetal" Na(V)1.5 isoforms of VGSC in mammalian skeletal muscle, and used them in in situ hybridization studies of rat soleus muscles. Na(V)1.4 mRNA is present throughout normal adult muscles but is highly concentrated at the NMJ, in which the amount per myonucleus is more than eightfold greater than away from the NMJ. Na(V)1.5 mRNA is undetectable in innervated muscles but is dramatically upregulated by denervation. In muscles denervated for 1 week, both Na(V)1.4 and Na(V)1.5 mRNAs are present throughout the muscle, and both are concentrated at the NMJ. No Na(V)1.5 mRNA was detectable in denervated muscles stimulated electrically for 1 week in vivo. Neither denervation nor stimulation had any significant effect on the level or distribution of Na(V)1.4 mRNA. We conclude that factors, probably derived from the nerve, lead to the increased concentration of VGSC mRNAs at the NMJ. In addition, the expression of Na(V)1.5 mRNA is downregulated by muscle activity, both at the NMJ and away from it.

Sodium channel mRNAs at the neuromuscular junction: distinct patterns of accumulation and effects of muscle activity

Voltage-gated sodium channels (VGSCs) are highly concentrated at the neuromuscular junction (NMJ) in mammalian skeletal muscle. Here we test the hypothesis that local upregulation of mRNA contributes to this accumulation. We designed radiolabeled antisense RNA probes, specific for the "adult" Na(V)1.4 and "fetal" Na(V)1.5 isoforms of VGSC in mammalian skeletal muscle, and used them in in situ hybridization studies of rat soleus muscles. Na(V)1.4 mRNA is present throughout normal adult muscles but is highly concentrated at the NMJ, in which the amount per myonucleus is more than eightfold greater than away from the NMJ. Na(V)1.5 mRNA is undetectable in innervated muscles but is dramatically upregulated by denervation. In muscles denervated for 1 week, both Na(V)1.4 and Na(V)1.5 mRNAs are present throughout the muscle, and both are concentrated at the NMJ. No Na(V)1.5 mRNA was detectable in denervated muscles stimulated electrically for 1 week in vivo. Neither denervation nor stimulation had any significant effect on the level or distribution of Na(V)1.4 mRNA. We conclude that factors, probably derived from the nerve, lead to the increased concentration of VGSC mRNAs at the NMJ. In addition, the expression of Na(V)1.5 mRNA is downregulated by muscle activity, both at the NMJ and away from it.

Characterization of vagal input to the rat esophageal muscle

There is recent morphological evidence for an interaction of autonomic nerve fibers and extrinsic motor nerves of the rat esophagus. The aim of the present study was to investigate a possible functional role of this autonomic innervation of vagal motor fibers on rat esophageal smooth and striated muscle function in vitro. The entire esophagus with both Nn vagi, including the Nn recurrentes, was dissected and placed in an organ bath with oxygenated Krebs-Ringer buffer. Contractile activity was measured in longitudinal direction with a force transducer. Both Nn vagi were placed on a bipolar platinum electrode 2 cm apart from the esophagus. Vagal stimulation, applied for 1 s (40 V, 0.5 ms, 20 Hz) resulted in a biphasic contractile response, which was completely blocked by tetrodotoxin (10(-6) M). The first part consisted of a tetanic striated muscle contraction, which was abolished by tubocurarin (10(-5) M) but unaffected by atropine (10(-6) M) or hexamethonium (10(-4) M). In contrast, the second part was completely abolished by hexamethonium (10(-4) M) and atropine (10(-6) M), whereas tubocurarine (10(-5) M) showed no influence, suggesting a stimulation of preganglionic nerve fibers supplying esophageal smooth muscle (muscularis mucosae). In order to characterize possible autonomic transmitters of the ENS of the esophagus, the following experiments were carried out. The magnitude of the striated muscle response was unaffected by VIP (10(-7) M), 5-HT (10(-6) M) and galanin (10(-8) - 10(-7) M), whereas they caused an inhibition of the smooth muscle response (VIP: -53.8 +/- 4.2%; galanin 10(-8) M: - 18.5 +/- 2.2%; 10(-7) M: -40.4 +/- 2.9%; 5-HT: -78.2 +/- 2.1%). The inhibitory effects of VIP and galanin on smooth muscle were reversible by the antagonists VIP 10-28 and galanin 1-15. In the presence of the nitric oxide synthase (NOS) inhibitor L-NNA (10(-4) M), the smooth and striated muscle contraction were not significantly influenced. Exogenous application of the NO-donor DEA-NO (10(-4) M) reduced the smooth muscle contraction by -81.6 +/- 7.4%, but had no significant effect on the striated muscle contraction. Though immunohistochemical findings are highly suggestive of an nitrergic autonomic modulation of striated muscle contraction by enteric neurons, we could not demonstrate a NO-mediated action on striated muscle activity. Therefore, the physiological relevance of the immunohistochemical findings remain unclear.

Clustering of sodium channels at the neuromuscular junction

Voltage-gated sodium channels (NaChs) are highly concentrated in the postsynaptic region of the neuromuscular junction, especially in the depths of postsynaptic folds and in the perijunctional region. The formation of the high NaCh density occurs during synapse maturation, approximately 2 weeks after initial synaptic contact in the rodent. The concentration of NaChs and their localization in the troughs of the folds increase the safety factor for neuromuscular transmission by reducing the threshold for initiation of the action potential. There is evidence that agrin plays a role in the formation of NaCh aggregation. Molecules such as ankyrin and syntrophin that bind NaChs may be important for maintenance of the high channel density at the endplate.

Persistent expression of developmental myosin heavy chain isoforms in the tapered ends of adult pigeon pectoralis muscle fibres

We have shown previously that in addition to the adult myosin heavy chain (MyHC) isoform present throughout the length of each fast-twitch glycolytic muscle fibre within the pectoralis of the mature chicken, the neonatal isoform is retained in the tapered ends of these fibres. This work, however, has been the only published report of this phenomenon. Here, we tested the hypothesis that similar to the chicken, the ends of mature pigeon pectoralis muscle fibres contain developmental MyHC isoform(s). A histological stain was used to visualize endomysium to assist in the analysis of transverse sections of pectoralis muscle from four mature pigeons. Immunocytochemical techniques were used to localize MyHC isoform(s) characteristic of pigeon pectoralis development. We show that within mature pigeon pectoralis, the ends of both fast-twitch glycolytic and fast-twitch oxidative-glycolytic fibre types express MyHC isoform(s) characteristic of their earlier development. Thus, we extend our findings on chicken to another species and an additional muscle fibre type. Retention of developmental MyHC isoform(s) within the tapered ends of mature muscle fibres may be more widespread than is currently appreciated.

An early stage in sodium channel clustering at developing rat neuromuscular junctions

Voltage-gated sodium channels (VGSCs) are concentrated in the postsynaptic membrane at the adult rat neuromuscular junction (NMJ). We have used immunolabelling to determine the pattern of initial VGSC accumulation during development. At birth, but not 3 days before, VGSC labelling is detectable at the NMJ and in the perijunctional (periJ) membrane but not elsewhere. A much higher density cluster of VGSCs forms at the NMJ itself 1-2 weeks later. If the nerve is cut 2 days after birth, VGSC labelling persists in the periJ region for at least 4 weeks but the clustering of VGSCs at the NMJ fails to develop. Thus an early, stable accumulation of VGSCs develops near the NMJ at least a week before high density postsynaptic VGSC clusters form.

Electrophysiology of postsynaptic activation

The safety factor for neuromuscular transmission depends upon the amount of ACh released from the nerve terminal, the number of AChRs, and the concentration of Na+ channels at the end plate potential. The postsynaptic end plate membrane of the neuromuscular junctions is specialized in three ways: (1) AChRs, Na+ channels, ChE, NOS, and other membrane-associated proteins are concentrated at the end plate; (2) the end plate cytoskeleton has a different composition of proteins as compared with extrajunctional membrane; and (3) the end plate membrane is mechanically different as compared with extrajunctional membrane. A blockade of neuromuscular transmission occurs when ACh release is inadequate or the end plate response to ACh is too small to trigger an AP. A safety factor for neuromuscular transmission exists because the EPP is larger than the threshold for generating an AP. The high concentration of Na+ channels at the end plate increases the safety factor for neuromuscular transmission by reducing the threshold depolarization required to initiate an AP. In MG, the safety factor is reduced due to loss of AChRs and loss of Na+ channels. The loss of AChRs reduces the EPP and the Na+ channel loss increases the threshold for triggering an AP.

Aggregation of sodium channels induced by a postnatally upregulated isoform of agrin

Agrin is involved in signaling the formation of high concentrations of acetylcholine receptors (AChRs) at the neuromuscular junction (NMJ). There are multiple isoforms of agrin attributable to alternative splicing, and these isoforms are differentially expressed during development and between tissues. The ability to cluster AChRs varies among the agrin isoforms. Sodium channels (NaChs) are also concentrated at the NMJ. We have tested various agrin isoforms for their ability to induce formation of clusters of NaChs. We grew cocultures of dissociated adult rat muscle fibers with chinese hamster ovary (CHO) cells that had been transfected with different isoforms of agrin. Using immunocytochemical techniques, we determined that after 1 d in culture, CHO cells synthesizing the neuronally expressed isoform with an eight amino acid insert (Agrin8) were able to form NaCh clusters at sites of contact between the CHO cell and muscle cell. Clusters of NaChs could be formed anywhere along a muscle fiber, but more clusters were detected close to the endplate where the endogenous level of NaChs was higher. None of the other neuronal-specific agrin isoforms was able to cluster NaChs. Because Agrin8 is the only agrin isoform that is upregulated at birth when NaChs begin to cluster at the NMJ, we conclude that Agrin8 expression by motor neurons is a signal for NaCh clustering at the NMJ during normal development.

Aggregation of sodium channels induced by a postnatally upregulated isoform of agrin

Agrin is involved in signaling the formation of high concentrations of acetylcholine receptors (AChRs) at the neuromuscular junction (NMJ). There are multiple isoforms of agrin attributable to alternative splicing, and these isoforms are differentially expressed during development and between tissues. The ability to cluster AChRs varies among the agrin isoforms. Sodium channels (NaChs) are also concentrated at the NMJ. We have tested various agrin isoforms for their ability to induce formation of clusters of NaChs. We grew cocultures of dissociated adult rat muscle fibers with chinese hamster ovary (CHO) cells that had been transfected with different isoforms of agrin. Using immunocytochemical techniques, we determined that after 1 d in culture, CHO cells synthesizing the neuronally expressed isoform with an eight amino acid insert (Agrin8) were able to form NaCh clusters at sites of contact between the CHO cell and muscle cell. Clusters of NaChs could be formed anywhere along a muscle fiber, but more clusters were detected close to the endplate where the endogenous level of NaChs was higher. None of the other neuronal-specific agrin isoforms was able to cluster NaChs. Because Agrin8 is the only agrin isoform that is upregulated at birth when NaChs begin to cluster at the NMJ, we conclude that Agrin8 expression by motor neurons is a signal for NaCh clustering at the NMJ during normal development.

Effects of length changes on Na+ current amplitude and excitability near and far from the end-plate

Na+ current (INa), membrane capacitance (Cm), action potential (AP) properties, and cable properties were studied on the end-plate (E), the end-plate border (EB), and extrajunctional (EJ) membrane of rat fast twitch muscle fibers. INa normalized to Cm, which is proportional to the density of Na+ channels, was the same on the E and the EB and smallest on EJ membrane. The AP threshold was lower and rate of rise of the AP was larger at the EB compared with EJ membrane. On the E and the EB, Cm and INa did not change in response to changes in fiber length. On EJ membrane, INa, Cm, and membrane cable properties changed in a manner consistent with folding and unfolding of the sarcolemma during length changes. The stiffness of the E membrane may add mechanical stability of the neuromuscular junction so that the electrical properties of the end-plate do not change with fiber length. The higher density of Na+ channels near the end-plate increases the safety factor for neuromuscular transmission by lowering the AP threshold.

Myosin heavy chain expression within the tapered ends of skeletal muscle fibers

BACKGROUND

The pectoralis muscle of the chicken contains fast-twitch glycolytic fibers, which during development undergo a transformation in their myosin heavy chain (MyHC) content from embryonic to a neonatal to an adult isoform (Bandman et al., 1990). Little, however, is known of MyHC expression within the ends of these or other muscle fibers. Here we test the hypothesis that the tapered ends of mature skeletal muscle fibers contain a less mature MyHC isoform than that typically found throughout their lengths.

METHODS

We apply an ammoniacal silver histological stain for endomysium and monoclonal antibodies against neonatal and adult MyHCs of chicken pectoralis to transverse serial sections of pectoralis from five mature chickens. The "lesser fiber diameters" of populations of fibers from each bird are also measured.

RESULTS

Most (approximately 81.8%) of the small (< 12 microns) and none of the larger (> 20 microns) diameter fibers contain the neonatal MyHC. Following these smaller fibers through serial sections, we show that they are the tapered ends of the larger fibers. Whereas neonatal MyHC is restricted to the tapered fiber ends, adult MyHC is present throughout the entire lengths of all fibers. We also demonstrate acetylcholinesterase (AChE) activity at some of these fiber ends.

CONCLUSIONS

We postulate that longitudinal growth of myofibrils in adult muscle is characterized by the sequential expression of MyHC isoforms similar to that observed in rapidly growing muscle and that the presence of the neurotransmitter hydrolase AChE at the tapered fiber ends may be related to the retention of neonatal MyHC.

Na channel density in extrajunctional sarcolemma of fast and slow twitch mouse skeletal muscle fibres: functional implications and plasticity after fast motoneuron transplantation on to a slow muscle

Na channel densities were measured in fast and slow twitch mouse skeletal muscle fibres using the loose patch voltage clamp technique. It was found that Na channel density was approximately four times greater in fast twitch fibres than in slow. Computer simulations of action potential propagation in these fibres strongly suggest that the higher channel densities in fast twitch fibres are necessary to maintain action potential amplitude and fidelity of transmission across the neuromuscular junction, especially during the periods of rapid stimulation that these fibres are subjected to by their motoneurons. Transplantation of a foreign nerve containing axons which had previously innervated fast twitch fibres on to a slow twitch muscle resulted in an approximate doubling of the Na channel density in fibres innervated by the foreign nerve. These results suggest that motoneurons may exert considerable control over Na channel density in the muscle fibres they innervate.

Expression and distribution of sodium channels in short- and long-term denervated rodent skeletal muscles

1. Loose-patch voltage-clamp recordings were made from rat and mouse skeletal muscle fibres denervated for up to 6 weeks. Innervated muscles possessed a Na+ current density of 107 +/- 3.3 mA cm-2 in endplate membrane, and 6.3 +/- 0.6 mA cm-2 in extrajunctional membrane. This high concentration of Na+ channels at the endplate was gradually reduced following denervation. After 6 weeks of denervation, the endplate Na+ channel concentration was reduced by 40-50%, and the density of Na+ channels in extrajunctional membrane was increased by about 30%. 2. The tetrodotoxin (TTX)-resistant form of the Na+ channel appeared after 3 days of denervation and comprised approximately 43% of the endplate Na+ channels 5-6 days after denervation. Subsequently, TTX-resistant Na+ channels were reduced in density to approximately 25% of the postjunctional Na+ channels and remained at this level up to 6 weeks after denervation. 3. RNase protection analysis showed that mRNA encoding the TTX-resistant Na+ channel was virtually absent in innervated muscle, rose > 50-fold after 3 days of denervation, then decreased by 95% 6 weeks after denervation. The density of TTX-resistant Na+ channels correlated qualitatively with changes in mRNA levels. 4. These results suggest that the density of Na+ channels at neuromuscular junctions is maintained by two mechanisms, one influenced by the nerve terminal and the other independent of innervation.

Ionic currents during action potentials in mammalian skeletal muscle fibers analyzed with loose patch clamp

The loose patch-clamp technique was applied to analyze transmembrane currents during propagating action potentials in superficial fibers of musculi extensor digitorum longus of the mouse in vitro. Experimentally three components were identified in the transmembrane current: 1) a capacitive, 2) an inward sodium, and 3) an outward potassium current. Other components were negligible. The capacitive current was similar in shape to the first derivative of the intracellularly measured action potential. Tetrodotoxin, tetraethylammonium, and 4-aminopyridine, applied in the pipette, were used to identify the contribution in the current by sodium and potassium ions. With extracellularly applied depolarization steps only a sodium current was observed, not a potassium current. Occasionally found outward currents were artifactual. The behaviour of delayed rectifier potassium channels in muscle fiber membranes is discussed in the light of these unexpected findings. We conclude that potassium channel activity contributing to and measured during action potential generation is in some way inaccessible to loose patch extracellular voltage-clamp stimulation and that loose patch action current recording is a useful noninvasive method to analyze membrane conductances involved in action potential generation.

Toxins as tools in the study of sodium channel distribution in the muscle fibre membrane

The number of tetrodotoxin molecules bound to the membrane of the fibres of muscles in normal conditions and after detubulation produced by glycerol-induced osmotic shock pointed to a higher sodium channel density at the surface membrane than at the membrane in the transverse tubules. Study of the maximum rate of rise of the action potential at the junctional and nonjunctional regions of the muscle fibre membrane suggested that the Na+ channel density is also not the same along the muscle fibre membrane, being higher at the junctional region. Further studies on the distribution of the Na+ channel along the muscle fibre membrane were carried out with the use of (1) the loose patch voltage-clamp technique, (2) labelling the Na+ channels with fluorescently labelled scorpion toxins, (3) autoradiography of localized Na+ channels with 125I-labelled scorpion toxins, and (4) toxins that induce persistent activation of the Na+ channel. The studies referred to in (1), (2) and (3) demonstrate that the density of the Na+ channel is much higher at the junctional region than elsewhere in the membrane of the muscle fibre. On the other hand, in experiments carried out on curarized rat diaphragms several sodium channel activating toxins (crotamine, Phoneutria nigriventer venom, its toxin PhTx2, veratrine) were found to produce a much greater depolarization of the membrane at the junctional region than at nonjunctional regions. However, it was also found that some toxins (veratridine, batrachotoxin) depolarized equally well the junctional and nonjunctional regions. Two alternative hypotheses to explain the uniform depolarization of the muscle fibre membrane induced by these toxins are suggested.

muI Na+ channels expressed transiently in human embryonic kidney cells: biochemical and biophysical properties

We describe the transient expression of the rat skeletal muscle muI Na+ channel in human embryonic kidney (HEK 293) cells. Functional channels appear at a density of approximately 30 in a 10 microns 2 patch, comparable to those of native excitable cells. Unlike muI currents in oocytes, inactivation gating is predominantly (approximately 97%) fast, although clear evidence is provided for noninactivating gating modes, which have been linked to anomalous behavior in the inherited disorder hyperkalemic periodic paralysis. Sequence-specific antibodies detect a approximately 230 kd glycopeptide. The majority of molecules acquire only neutral oligosaccharides and are retained within the cell. Electrophoretic mobility on SDS gels suggests the molecules may acquire covalently attached lipid. The channel is readily phosphorylated by activation of the protein kinase A and protein kinase C second messenger pathways.

Fast and slow twitch skeletal muscle fibres differ in their distribution of Na channels near the endplate

Sodium channel distributions were measured in fast and slow twitch rodent skeletal muscle fibres using the loose patch voltage clamp technique. Large differences were found between these fibre types with respect to Na channel density in the perijunctional region. Fast twitch fibres exhibited a large increase in Na channel density near the endplate, while slow twitch fibres did not.

Effect of agrin on the distribution of acetylcholine receptors and sodium channels on adult skeletal muscle fibers in culture

We used the loose patch voltage clamp technique and rhodamine-conjugated alpha-bungarotoxin to study the regulation of Na channel (NaCh) and acetylcholine receptor (AChR) distribution on dissociated adult skeletal muscle fibers in culture. The aggregate of AChRs and NaChs normally found in the postsynaptic membrane of these cells gradually fragmented and dispersed from the synaptic region after several days in culture. This dispersal was the result of the collagenase treatment used to dissociate the cells, suggesting that a factor associated with the extracellular matrix was responsible for maintaining the high concentration of AchRs and NaChs at the neuromuscular junction. We tested whether the basal lamina protein agrin, which has been shown to induce the aggregation of AChRs on embryonic myotubes, could similarly influence the distribution of NaChs. By following identified fibers, we found that agrin accelerated both the fragmentation of the endplate AChR cluster into smaller patches as well as the appearance of new AChR clusters away from the endplate. AChR patches which were fragments of the original endplate retained a high density of NaChs, but no new NaCh hotspots were found elsewhere on the fiber, including sites of newly formed AChR clusters. The results are consistent with the hypothesis that extracellular signals regulate the distribution of AChRs and NaChs on skeletal muscle fibers. While agrin probably serves this function for the AChR, it does not appear to play a role in the regulation of the NaCh distribution.

Calcium action potentials in innervated and denervated rat muscle fibres

We studied the generation of calcium action potentials (Ca APs) in innervated and denervated fibres of the extensor digitorum longus of the rat in a tetraethylammonium (TEA) sulphate solution plus 3-4 diaminopiridine (3-4 DAP). The main results are the following: (1) more than 90% of the innervated fibres were capable of developing well-sustained Ca APs that were blocked by Cd or nifedipine; (2) the incidence of Ca APs obtained from the denervated fibres was substantially lower than in the control preparations; (3) no relation was found between the appearance of Ca APs in the denervated fibres and the resting membrane potential (Vm), stimulus duration (500-2000 ms) or holding potential (-80, -100 mV); (4) The percentage of denervated fibres that exhibited Ca APs was increased significantly with the following procedures. First, by raising the external Ca concentration to 14 mM; second, by depleting the intracellular K concentration by overnight exposure of the muscles to a free K-Cs solution; (c) and third, by incubating the muscles in 500 nM apamin, a venom that inhibits the K conductance activated by Ca. Several factors may be involved in the lower incidence of Ca APs obtained in denervated fibres: (1) a diminished Ca current due to a reduction in the driving force as a result of an increment in the intracellular Ca concentration; (2) a persistence of a shunting K conductance that is not inhibited by TEA and 3-4 DAP; (3) a shift in the voltage dependence of the activation and inactivation parameters of the Ca current or the appearance of a new type of Ca channel with a different kinetics.

Liver glucose transporter: a basolateral protein in hepatocytes and intestine and kidney cells

Pages C279–C294: B. Thorens, Z.-Q. Cheng, D. Brown, and H. F. Lodish. “Liver glucose transporter: a basolateral protein in hepatocytes and intestine and kidney cells.” B. Thorens, H. F. Lodish, and D. Brown. “Differential localization of two glucose transporter isoforms in rat kidney.” Because of the disappointing reproduction of the halftones, these articles are reprinted at the end of the December issue.

Ca sensitivity and acetylcholine receptor currents of twitch and tonic snake muscle fibers

Myofibrillar Ca sensitivity and single-channel acetylcholine receptor (AChR) currents were studied in garter snake (Thamnophis sirtalis sirtalis) costocutaneous muscle fibers. Nomarski optics were used to identify tonic and fast- and slow-twitch fibers. Measurements of tension generation were made using chemically skinned fibers. The maximum tensions of the three types of fibers were similar, and the fast- and slow-twitch fibers had similar Ca sensitivities. Compared with twitch fibers, tonic fibers had lower threshold Ca concentrations for tension generation and a larger range of Ca concentrations between threshold and maximum tension. The AChR channels were studied by enzymatically removing the nerve terminals and performing patch-clamp recordings on the exposed postsynaptic membrane. Twitch fibers had only one class of end-plate channel with a conductance of approximately 51 pS. Tonic fibers had two types of synaptic channels. One AChR channel in the tonic fibers resembled the type seen in twitch fibers. The other channel in tonic fibers had a smaller conductance of approximately 33 pS and resembled extrajunctional AChRs on denervated twitch fibers.

Localization of voltage-sensitive sodium channels on the extrasynaptic membrane surface of mouse skeletal muscle by autoradiography of scorpion toxin binding sites

Voltage-dependent sodium channels (Na+ channels) were localized by autoradiography on mouse skeletal muscle using both light and electron microscopy. 125I-scorpion toxins (ScTx) of both the alpha and beta type were used as probes. The specificity of labelling was verified by competitive inhibition with unlabelled toxin and by inhibition of alpha ScTx labelling in depolarizing conditions. Under light microscopy, the labelling of the myocyte surface appeared randomly distributed with both the alpha and beta toxins. No difference in the labelling density obtained with beta ScTx was observed between a 2 mm central segment of the fibre containing the endplate and an adjacent segment not containing the endplate. At the endplate, however, the beta ScTx binding site density was about seven fold higher at the edge of the synaptic primary clefts. This density decreased with distance from the synaptic cleft reaching the extrasynaptic value at 30-40 microns. An analysis of myocyte labelling using electron microscopy provided evidence for a specific, but very low labelling of the myocyte interior which can be attributed to the T-tubules. These results confirm a relatively high density of Na+ channels in a perijunctional zone about 50 microns in width, which could ensure the initial spread of the surface depolarization with a high safety factor, and a homogeneous distribution over the remaining surface with a low density evaluated at 5-10 per microns2. However, the very low labelling of T-tubules could be attributed mainly to a low density of tubular Na+ channels.