Na current density at and away from end plates on rat fast- and slow-twitch skeletal muscle fibers

The W' Balance Model: Mathematical and Methodological Considerations

Since its publication in 2012, the W' balance model has become an important tool in the scientific armamentarium for understanding and predicting human physiology and performance during high-intensity intermittent exercise. Indeed, publications featuring the model are accumulating, and it has been adapted for popular use both in desktop computer software and on wrist-worn devices. Despite the model's intuitive appeal, it has achieved mixed results thus far, in part due to a lack of clarity in its basis and calculation. Purpose: This review examines the theoretical basis, assumptions, calculation methods, and the strengths and limitations of the integral and differential forms of the W' balance model. In particular, the authors emphasize that the formulations are based on distinct assumptions about the depletion and reconstitution of W' during intermittent exercise; understanding the distinctions between the 2 forms will enable practitioners to correctly implement the models and interpret their results. The authors then discuss foundational issues affecting the validity and utility of the model, followed by evaluating potential modifications and suggesting avenues for further research. Conclusions: The W' balance model has served as a valuable conceptual and computational tool. Improved versions may better predict performance and further advance the physiology of high-intensity intermittent exercise.

Nature and Action of Antibodies in Myasthenia Gravis

This article discusses antibodies associated with immune-mediated myasthenia gravis and the pathologic action of these antibodies at the neuromuscular junctions of skeletal muscle. To explain how these antibodies act, we consider the physiology of neuromuscular transmission with emphasis on 4 features: the structure of the neuromuscular junction; the roles of postsynaptic acetylcholine receptors and voltage-gated Na⁺ channels and in converting the chemical signal from the nerve terminal into a propagated action potential on the muscle fiber that triggers muscle contraction; the safety factor for neuromuscular transmission; and how the safety factor is reduced in different forms of autoimmune myasthenia gravis.

PGE2 EP1 receptor inhibits vasopressin-dependent water reabsorption and sodium transport in mouse collecting duct

PGE2 regulates glomerular hemodynamics, renin secretion, and tubular transport. This study examined the contribution of PGE2 EP1 receptors to sodium and water homeostasis. Male EP1-/- mice were bred with hypertensive TTRhRen mice (Htn) to evaluate blood pressure and kidney function at 8 weeks of age in four groups: wildtype (WT), EP1-/-, Htn, HtnEP1-/-. Blood pressure and water balance were unaffected by EP1 deletion. COX1 and mPGE2 synthase were increased and COX2 was decreased in mice lacking EP1, with increases in EP3 and reductions in EP2 and EP4 mRNA throughout the nephron. Microdissected proximal tubule sglt1, NHE3, and AQP1 were increased in HtnEP1-/-, but sglt2 was increased in EP1-/- mice. Thick ascending limb NKCC2 was reduced in the cortex but increased in the medulla. Inner medullary collecting duct (IMCD) AQP1 and ENaC were increased, but AVP V2 receptors and urea transporter-1 were reduced in all mice compared to WT. In WT and Htn mice, PGE2 inhibited AVP-water transport and increased calcium in the IMCD, and inhibited sodium transport in cortical collecting ducts, but not in EP1-/- or HtnEP1-/- mice. Amiloride (ENaC) and hydrochlorothiazide (pendrin inhibitor) equally attenuated the effect of PGE2 on sodium transport. Taken together, the data suggest that EP1 regulates renal aquaporins and sodium transporters, attenuates AVP-water transport and inhibits sodium transport in the mouse collecting duct, which is mediated by both ENaC and pendrin-dependent pathways.

PGE2 receptor EP3 inhibits water reabsorption and contributes to polyuria and kidney injury in a streptozotocin-induced mouse model of diabetes

AIMS/HYPOTHESIS

The first clinical manifestation of diabetes is polyuria. The prostaglandin E2 (PGE2) receptor EP3 antagonises arginine vasopressin (AVP)-mediated water reabsorption and its expression is increased in the diabetic kidney. The purpose of this work was to study the contribution of EP3 to diabetic polyuria and renal injury.

METHODS

Male Ep 3 (-/-) (also known as Ptger3 (-/-)) mice were treated with streptozotocin (STZ) to generate a mouse model of diabetes and renal function was evaluated after 12 weeks. Isolated collecting ducts (CDs) were microperfused to study the contribution of EP3 to AVP-mediated fluid reabsorption.

RESULTS

Ep 3 (-/-)-STZ mice exhibited attenuated polyuria and increased urine osmolality compared with wild-type STZ (WT-STZ) mice, suggesting enhanced water reabsorption. Compared with WT-STZ mice, Ep 3 (-/-)-STZ mice also had increased protein expression of aquaporin-1, aquaporin-2, and urea transporter A1, and reduced urinary AVP excretion, but increased medullary V2 receptors. In vitro microperfusion studies indicated that Ep 3 (-/-) and WT-STZ CDs responded to AVP stimulation similarly to those of wild-type mice, with a 60% increase in fluid reabsorption. In WT non-injected and WT-STZ mice, EP3 activation with sulprostone (PGE2 analogue) abrogated AVP-mediated water reabsorption; this effect was absent in mice lacking EP3. A major finding of this work is that Ep 3 (-/-)-STZ mice showed blunted renal cyclooxygenase-2 protein expression, reduced renal hypertrophy, reduced hyperfiltration and reduced albuminuria, as well as diminished tubular dilation and nuclear cysts.

CONCLUSIONS/INTERPRETATION

Taken together, the data suggest that EP3 contributes to diabetic polyuria by inhibiting expression of aquaporins and that it promotes renal injury during diabetes. EP3 may prove to be a promising target for more selective management of diabetic kidney disease.

Channelopathies of skeletal muscle excitability

Familial disorders of skeletal muscle excitability were initially described early in the last century and are now known to be caused by mutations of voltage-gated ion channels. The clinical manifestations are often striking, with an inability to relax after voluntary contraction (myotonia) or transient attacks of severe weakness (periodic paralysis). An essential feature of these disorders is fluctuation of symptoms that are strongly impacted by environmental triggers such as exercise, temperature, or serum K(+) levels. These phenomena have intrigued physiologists for decades, and in the past 25 years the molecular lesions underlying these disorders have been identified and mechanistic studies are providing insights for therapeutic strategies of disease modification. These familial disorders of muscle fiber excitability are "channelopathies" caused by mutations of a chloride channel (ClC-1), sodium channel (NaV1.4), calcium channel (CaV1.1), and several potassium channels (Kir2.1, Kir2.6, and Kir3.4). This review provides a synthesis of the mechanistic connections between functional defects of mutant ion channels, their impact on muscle excitability, how these changes cause clinical phenotypes, and approaches toward therapeutics.

Postobstructive diuresis in cats with naturally occurring lower urinary tract obstruction: incidence, severity and association with laboratory parameters on admission

OBJECTIVES

The objectives of this retrospective study were to investigate the actual incidence of postobstructive diuresis after relief of urethral obstruction in cats, as well as to identify changes in blood and urine parameters that might be associated with postobstructive diuresis (POD), and to assess the impact of fluid therapy.

METHODS

The medical records of 57 male cats with urethral obstruction that were treated with an indwelling urinary catheter were retrospectively analysed. Absolute urine output in ml/kg/h every 4 h and the incidence of cats with polyuria (urine volume >2 ml/kg/h) at any time point over a 48 h period after the re-establishment of urine flow were investigated. In addition, postobstructive diuresis in relation to fluid therapy (PODFR) was defined as urine output greater than the administered amount of intravenous fluids on at least two subsequent time points. Polyuria and PODFR were investigated for their association with blood and urine laboratory parameters.

RESULTS

After 4 h, 74.1% (40/54) of the cats had polyuria, with a urine output of >2 ml/kg/h. Metabolic acidosis was present in 46.2% of the cats. Venous blood pH and bicarbonate were inversely correlated with urine output in ml/kg/h after 4 h. The overall incidence of POD within 48 h of catheterisation was 87.7%. There was a significant correlation between intravenous fluid rate at time point x and urine output at time point x + 1 at all the time points except for the fluid rate at time point 0 and the urine output after 4 h. PODFR was seen in 21/57 cats (36.8%).

CONCLUSIONS AND RELEVANCE

POD is a frequent finding in cats treated for urethral obstruction, and can be very pronounced. Further studies are required to determine whether or not a change in venous blood pH actually interferes with renal concentrating ability. The discrepancy between the frequency of cats with polyuria and PODFR (87.7% vs 36.8%) in the present study indicates that administered intravenous fluid therapy might be the driving force for the high incidence of polyuria in some cats with naturally occurring obstructive feline lower urinary tract disease.

Intramuscular determinants of the ability to recover work capacity above critical power

PURPOSE

The primary purpose of this investigation was to compare the recovery of the W' to the recovery of intramuscular substrates and metabolites using (31)P- and (1)H-magnetic resonance spectroscopy.

METHODS

Ten healthy recreationally trained subjects were tested to determine critical power (CP) and W' for single-leg-extensor exercise. They subsequently exercised in the bore of a 1.5-T MRI scanner at a supra-CP work rate. Following exhaustion, the subjects rested in place for 1, 2, 5 or 7 min, and then repeated the effort. The temporal course of W' recovery was estimated, which was then compared to the recovery of creatine phosphate [PCr], pH, carnosine content, and to the output of a novel derivation of the W' BAL model.

RESULTS

W' recovery closely correlated with the predictions of the novel model (r = 0.97, p = 0.03). [PCr] recovered faster [Formula: see text] than W' [Formula: see text] The W' available for the second exercise bout was directly correlated with the difference between [PCr] at the beginning of the work bout and [PCr] at exhaustion (r = 0.99, p = 0.005). Nonlinear regression revealed an inverse curvilinear relationship between carnosine concentration and the W' t 1/2 (r (2) = 0.55).

CONCLUSION

The kinetics of W' recovery in single-leg-extensor exercise is comparable to that observed in whole-body exercise, suggesting a conserved mechanism. The extent to which the recovery of the W' can be directly attributed to the recovery of [PCr] is unclear. The relationship of the W' to muscle carnosine content suggests novel future avenues of investigation.

Guanidine affects differentially the twitch response of diaphragm, extensor digitorum longus and soleus nerve-muscle preparations of mice

Guanidine has been used with some success to treat myasthenia gravis and myasthenic syndrome because it increases acetylcholine release at nerve terminals through K⁺, Na⁺ and Ca²⁺ channels-involving mechanisms. Currently, guanidine derivatives have been proposed for treatment of several diseases. Studies aimed at providing new insights to the drug are relevant. Experimentally, guanidine (10 mM) induces on mouse phrenic nerve-diaphragm (PND) preparations neurotransmission facilitation followed by blockade and a greatest secondary facilitation after its removal from bath. Herein, we hypothesized that this peculiar triphasic response may differ in muscles with distinct twitch/metabolic characteristics. Morphological alterations and contractile response of PND, extensor digitorum longus (EDL) and soleus (SOL) preparations incubated with guanidine (10 mM) for 15, 30, 60 min were analyzed. Guanidine concentrations of 5 mM (for PND and EDL) and 1 mM (for EDL) were also tested. Guanidine triphasic effect was only observed on PND regardless the concentration. The morphological alterations in muscle tissue varied along time but did not impede the PND post-wash facilitation. Higher doses (20-25 mM) did not increase EDL or SOL neurotransmission. The data suggest a complex mechanism likely dependent on the metabolic/contractile muscle phenotype; muscle fiber types and density/type of ion channels, sarcoplasmic reticulum and mitochondria organization may have profound impact on the levels and isoform expression pattern of Ca²⁺ regulatory membrane proteins so reflecting regulation of calcium handling and contractile response in different types of muscle.

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.

Tumor necrosis factor-α downregulates sodium current in skeletal muscle by protein kinase C activation: involvement in critical illness polyneuromyopathy

Sepsis is involved in the decrease of membrane excitability of skeletal muscle, leading to polyneuromyopathy. This effect is mediated by alterations of the properties of voltage-gated sodium channels (Na(V)), but the exact mechanism is still unknown. The aim of the present study was to check whether tumor necrosis factor (TNF-α), a cytokine released during sepsis, exerts a rapid effect on Na(V). Sodium current (I(Na)) was recorded by macropatch clamp in skeletal muscle fibers isolated from rat peroneus longus muscle, in control conditions and after TNF-α addition. Analyses of dose-effect and time-effect relationships were carried out. Effect of chelerythrine, a PKC inhibitor, was also studied to determine the way of action of TNF-α. TNF-α induced a reversible dose- and time-dependent inhibition of I(Na). A maximum inhibition of 75% of the control current was observed. A shift toward more negative potentials of activation and inactivation curves of I(Na) was also noticed. These effects were prevented by chelerythrine pretreatment. TNF-α is a cytokine released in the early stages of sepsis. Besides a possible transcriptional role, i.e., modification of the channel type and/or number, we demonstrated the existence of a rapid, posttranscriptional inhibition of Na(V) by TNF-α. The downregulation of the sodium current could be mediated by a PKC-induced phosphorylation of the sodium channel, thus leading to a significant decrease in muscle excitability.

Effects of 8 wk of voluntary unloaded wheel running on K+ tolerance and excitability of soleus muscles in rat

During intense exercise, efflux of K+ from working muscles increases extracellular K+ ([K+]o) to levels that can compromise muscle excitability and hence cause fatigue. In this context, the reduction in the exercise-induced elevation of [K+]o observed after training in humans is suggested to contribute to the increased performance after training. Although a similar effect could be obtained by an increase in the tolerance of muscle to elevated [K+]o, this possibility has not been investigated. To examine this, isolated soleus muscles from sedentary (sedentary) rats and from rats that had voluntarily covered 13.1 ± 0.7 km/day in an unloaded running wheel for 8 wk (active) were compared. In muscles from active rats, the loss of force induced by exposure to an elevated [K+]o of 9 mM was 42% lower than in muscles from sedentary rats ( P < 0.001). This apparent increase in K+ tolerance in active rats was associated with an increased excitability as evident from a 33% reduction in the electrical current needed to excite individual muscle fibers ( P < 0.0009). Moreover, muscles from active rats had lower Cl− conductance, higher maximal rate of rise of single-fiber action potentials (AP), and higher Na+/K+ pump content. When stimulated intermittently at 6.5 mM K+, muscles from active rats displayed better endurance than muscles from sedentary rats, whereas no difference was found when the muscles were stimulated continuously at 30 or 120 Hz. We conclude that voluntary running increases muscle excitability, leading to improved tolerance to elevated [K+]o.

Na v1.4 and Na v1.5 are modulated differently during muscle immobilization and contractile phenotype conversion

Muscle immobilization leads to modification in its fast/slow contractile phenotype. Since the properties of voltage-gated sodium channels (Na(v)) are different between "fast" and "slow" muscles, we studied the effects of immobilization on the contractile properties and the Na(v) of rat peroneus longus (PL). The distal tendon of PL was cut and fixed to the adjacent bone at neutral muscle length. After 4 or 8 wk of immobilization, the contractile and the Na(v) properties were studied and compared with muscles from control animals (Student's t-test). After 4 wk of immobilization, PL showed a faster phenotype with a rightward shift of the force-frequency curve and a decrease in both the Burke's index of fatigability and the tetanus-to-twitch ratio. These parameters showed opposite changes between 4 and 8 wk of immobilization. The maximal sodium current in 4-wk immobilized fibers was higher compared with that of control fibers (11.5 ± 1.2 vs. 7.8 ± 0.8 nA, P = 0.008), with partial recovery to the control values in 8-wk immobilized fibers (8.6 ± 0.7 nA, P = 0.48). In the presence of tetrodotoxin, the maximal residual sodium current decreased continuously throughout immobilization. Using the Western blot analysis, Na(v)1.4 expression showed a transient increase in 4-wk muscle, whereas Na(v)1.5 expression decreased during immobilization. Our results indicate that a muscle immobilized at optimal functional length with the preservation of neural inputs exhibits a transient fast phenotype conversion. Na(v)1.4 expression and current are related to the contractile phenotype variation.

Hypotonicity-induced reduction of aquaporin-2 transcription in mpkCCD cells is independent of the tonicity responsive element, vasopressin, and cAMP

The syndrome of inappropriate antidiuretic hormone secretion is characterized by excessive water uptake and hyponatremia. The extent of hyponatremia, however, is less than anticipated, which is ascribed to a defense mechanism, the vasopressin-escape, and is suggested to involve a tonicity-determined down-regulation of the water channel aquaporin-2 (AQP2). The underlying mechanism, however, is poorly understood. To study this, we used the mouse cortical collecting duct (mpkCCD) cell line. MpkCCD cells, transfected with an AQP2-promoter luciferase construct showed a reduced and increased AQP2 abundance and transcription following culture in hypotonic and hypertonic medium, respectively. This depended on tonicity rather than osmolality and occurred independently of the vasopressin analog dDAVP, cAMP levels, or protein kinase A activity. Although prostaglandins and nitric oxide reduced AQP2 abundance, inhibition of their synthesis did not influence tonicity-induced AQP2 transcription. Also, cells in which the cAMP or tonicity-responsive element (CRE/TonE) in the AQP2-promoter were mutated showed a similar response to hypotonicity. Instead, the tonicity-responsive elements were pin-pointed to nucleotides -283 to -252 and -157 to -126 bp. In conclusion, our data indicate that hypotonicity reduces AQP2 abundance and transcription, which occurs independently of vasopressin, cAMP, and the known TonE and CRE in the AQP2-promoter. Increased prostaglandin and nitric oxide, as found in vivo, may contribute to reduced AQP2 in vasopressin-escape, but do not mediate the effect of hypotonicity on AQP2 transcription. Our data suggest that two novel segments (-283 to -252 and -157 to -126 bp) in the AQP2-promoter mediate the hypotonicity-induced AQP2 down-regulation during vasopressin-escape.

Sodium channel Na(V)1.5 expression is enhanced in cultured adult rat skeletal muscle fibers

This study analyzes changes in the distribution, electrophysiological properties, and proteic composition of voltage-gated sodium channels (Na(V)) in cultured adult rat skeletal muscle fibers. Patch clamp and molecular biology techniques were carried out in flexor digitorum brevis (FDB) adult rat skeletal muscle fibers maintained in vitro after cell dissociation with collagenase. After 4 days of culture, an increase of the Na(V)1.5 channel type was observed. This was confirmed by an increase in TTX-resistant channels and by Western blot test. These channels exhibited increased activation time constant (tau(m)) and reduced conductance, similar to what has been observed in denervated muscles in vivo, where the density of Na(V)1.5 was increasing progressively after denervation. By real-time polymerase chain reaction, we found that the expression of beta subunits was also modified, but only after 7 days of culture: increase in beta(1) without beta(4) modifications. beta(1) subunit is known to induce a negative shift of the inactivation curve, thus reducing current amplitude and duration. At day 7, tau(h) was back to normal and tau(m) still increased, in agreement with a decrease in sodium current and conductance at day 4 and normalization at day 7. Our model is a useful tool to study the effects of denervation in adult muscle fibers in vitro and the expression of sodium channels. Our data evidenced an increase in Na(V)1.5 channels and the involvement of beta subunits in the regulation of sodium current and fiber excitability.

Differences in sodium voltage-gated channel properties according to myosin heavy chain isoform expression in single muscle fibres

The myosin heavy chain (MHC) isoform determines the characteristics and shortening velocity of muscle fibres. The functional properties of the muscle fibre are also conditioned by its membrane excitability through the electrophysiological properties of sodium voltage-gated channels. Macropatch-clamp is used to study sodium channels in fibres from peroneus longus (PL) and soleus (Sol) muscles (Wistar rats, n = 8). After patch-clamp recordings, single fibres are identified by SDS-PAGE electrophoresis according to their myosin heavy chain isoform (slow type I and the three fast types IIa, IIx, IIb). Characteristics of sodium currents are compared (Student's t test) between fibres exhibiting only one MHC isoform. Four MHC isoforms are identified in PL and only type I in Sol single fibres. In PL, maximal sodium current (I(max)), maximal sodium conductance (g(Na,max)) and time constants of activation and inactivation ((m) and (h)) increase according to the scheme I-->IIa-->IIx-->IIb (P < 0.05). (m) values related to sodium channel type and/or function, are similar in Sol I and PL IIb fibres (P = 0.97) despite different contractile properties. The voltage dependence of activation (V(a,1/2)) shows a shift towards positive potentials from Sol type I to IIa, IIx and finally IIb fibres from PL (P < 0.05). These data are consistent with the earlier recruitment of slow fibres in a fast-mixed muscle like PL, while slow fibres of postural muscle such as soleus could be recruited in the same ways as IIb fibres in a fast muscle.

Neuromuscular adaptations to respiratory muscle inactivity

Cervical spinal cord injury results in significant functional impairment. It is important to understand the neuroplasticity in response to inactivity of respiratory muscles in order to prevent any associated effects that limit functional recovery. Recent studies have examined the mechanisms involved in inactivity-induced neuroplasticity of diaphragm motor units. Both spinal hemisection at C2 (C2HS) and tetrodotoxin (TTX)-induced phrenic nerve blockade result in diaphragm paralysis and inactivity of axon terminals. However, phrenic motoneurons are inactive with C2HS but remain active after TTX. Diaphragm muscle fibers ipsilateral to C2HS display minimal changes post-injury. Neuromuscular transmission is enhanced following C2HS but impaired following TTX. Synaptic vesicle pool size at diaphragm neuromuscular junctions increases after C2HS, but decreases after TTX. Thus, inactivity-induced neuromuscular plasticity reflects specific adaptations that depend on inactivity at the motoneuron rather than at axon terminals or muscle fibers. These results indicate that neuromuscular transmission and functional properties of diaphragm fibers can be maintained after spinal cord injury, providing a substrate for functional recovery and/or specific therapeutic approaches such as phrenic pacing.

P2Y(2) receptors and water transport in the kidney

The kidneys play a critical role in the maintenance of water homeostasis. This is achieved by the inherent architecture of the nephron along with the expression of various membrane transporters and channels that are responsible for the vectorial transport of salt and water. The collecting duct has become a focus of attention by virtue of its ability to transport water independent of solutes (free-water transport), and its apparent involvement in various water balance disorders. It was originally believed that the water transport capability of the collecting duct was solely under the influence of the circulating hormone, arginine vasopressin (AVP). However, during the past decade, locally produced autocrine and/or paracrine factors have emerged as potent modulators of transport of water by the collecting duct. Recently, much attention has been focused on the purinergic regulation of renal water transport. This review focuses on the role of the P2Y(2) receptor, the predominant purinergic receptor expressed in the collecting duct, in the modulation of water transport in physiological and pathophysiological conditions, and its therapeutic potential as a drug target to treat water balance disorders in the clinic. Studies carried out by us and other investigators are unravelling potent interactions among AVP, prostanoid and purinergic systems in the medullary collecting duct, and the perturbations of these interactions in water balance disorders such as acquired nephrogenic diabetes insipidus. Future studies should address the potential therapeutic benefits of modulators of P2Y(2) receptor signalling in water balance disorders, which are extremely prevalent in hospitalised patients irrespective of the underlying pathology.

How myasthenia gravis alters the safety factor for neuromuscular transmission

Myasthenia gravis (MG), the most common of autoimmune myasthenic syndromes, is characterized by antibodies directed against the skeletal muscle acetylcholine receptors (AChRs). Endplate Na(+) channels ensure the efficiency of neuromuscular transmission by reducing the threshold depolarization needed to trigger an action potential. Postsynaptic AChRs and voltage-gated Na(+) channels are both lost from the neuromuscular junction in MG. This study examined the impact of postsynaptic voltage-gated Na(+) channel loss on the safety factor for neuromuscular transmission. In intercostal nerve-muscle preparations from MG patients, we found that endplate AChR loss decreases the size of the endplate potential, and endplate Na(+) channel loss increases the threshold depolarization needed to produce a muscle action potential. To evaluate whether AChR-specific antibody impairs the function of Na(+) channels, we tested omohyoid nerve-muscle preparations from rats injected with monoclonal myasthenogenic IgG (passive transfer model of MG [PTMG]). The AChR antibody that produces PTMG did not alter the function of Na(+) channels. We conclude that loss of endplate Na(+) channels in MG is due to complement-mediated loss of endplate membrane rather than a direct effect of myasthenogenic antibodies on endplate Na(+) channels.

Physiology and pathophysiology of the vasopressin-regulated renal water reabsorption

To prevent dehydration, terrestrial animals and humans have developed a sensitive and versatile system to maintain their water homeostasis. In states of hypernatremia or hypovolemia, the antidiuretic hormone vasopressin (AVP) is released from the pituitary and binds its type-2 receptor in renal principal cells. This triggers an intracellular cAMP signaling cascade, which phosphorylates aquaporin-2 (AQP2) and targets the channel to the apical plasma membrane. Driven by an osmotic gradient, pro-urinary water then passes the membrane through AQP2 and leaves the cell on the basolateral side via AQP3 and AQP4 water channels. When water homeostasis is restored, AVP levels decline, and AQP2 is internalized from the plasma membrane, leaving the plasma membrane watertight again. The action of AVP is counterbalanced by several hormones like prostaglandin E2, bradykinin, dopamine, endothelin-1, acetylcholine, epidermal growth factor, and purines. Moreover, AQP2 is strongly involved in the pathophysiology of disorders characterized by renal concentrating defects, as well as conditions associated with severe water retention. This review focuses on our recent increase in understanding of the molecular mechanisms underlying AVP-regulated renal water transport in both health and disease.

Skeletal myocyte plasticity: basis for improved therapeutic potential?

Skeletal muscle tissue exhibits a remarkable capacity to regenerate after injury and to adapt its properties in response to altered functional demands or environmental pressure. This potential renders skeletal myocytes especially attractive candidates to be used in therapeutic strategies. Besides the well-described adaptability of skeletal myocytes in terms of contractile function and metabolic profile, more recent research has revealed that the electrophysiological properties of myocytes are also subject to significant changes both under physiological conditions and in pathophysiological situations. A better understanding of skeletal myocyte plasticity, its regulation and its forced induction could improve existing therapeutic approaches and may pave the way for new therapeutic strategies.

Skeletal Muscle Fatigue: Cellular Mechanisms

Repeated, intense use of muscles leads to a decline in performance known as muscle fatigue. Many muscle properties change during fatigue including the action potential, extracellular and intracellular ions, and many intracellular metabolites. A range of mechanisms have been identified that contribute to the decline of performance. The traditional explanation, accumulation of intracellular lactate and hydrogen ions causing impaired function of the contractile proteins, is probably of limited importance in mammals. Alternative explanations that will be considered are the effects of ionic changes on the action potential, failure of SR Ca2+release by various mechanisms, and the effects of reactive oxygen species. Many different activities lead to fatigue, and an important challenge is to identify the various mechanisms that contribute under different circumstances. Most of the mechanistic studies of fatigue are on isolated animal tissues, and another major challenge is to use the knowledge generated in these studies to identify the mechanisms of fatigue in intact animals and particularly in human diseases.

Dysregulation of Renal Cyclooxygenase-2 in Rats with Lithium-induced Nephrogenic Diabetes Insipidus

This study aimed to examine whether the expression of major prostaglandin E₂ (PGE₂) synthesis enzyme, cyclooxygenase-2 (COX-2), is changed in the kidneys of the rats with lithium-induced nephrogenic diabetes insipidus (Li-NDI). Sprague-Dawley rats treated with lithium for 4 weeks were used as the NDI model and expression of renal COX-2 was determined by immunoblotting and immunohistochemistry. In Li-NDI where urine output was markedly increased and urine osmolality was significantly decreased, COX-2 expression in the inner medulla was decreased (28% of control), while it increased 18-fold in the cortex and outer medulla. Consistent with this, labeling intensity of COX-2 in macula densa region was increased, whereas it was decreased in the interstitial cells in the inner medulla, indicating a differential regulation of COX-2 between the cortex and inner medulla in Li-NDI. Accordingly, urinary PGE₂ excretion was significantly increased in Li-NDI. In conclusion, there is a differential regulation of COX-2 between cortex and inner medulla in Li-NDI and urinary PGE₂ excretion is increased in Li-NDI, possibly due to an increased renal production. This may suggest that increased renal production of PGE₂ could play a role in modulating water reabsorption in the renal collecting duct in Li-NDI.

Role of Microsomal Prostaglandin E Synthase 1 in the Kidney

Prostaglandin E(2) (PGE(2)) is one of the most ubiquitous prostanoids in the kidney, where it may influence a wide range of physiologic functions. PGE(2) is generated through enzymatic metabolism of prostanoid endoperoxides by specific PGE synthases (PGES). Several putative PGES have been identified and cloned, including the membrane-associated, inducible microsomal PGES1 (mPGES1), which is expressed in the kidney. To evaluate the physiologic role of mPGES1 in the kidney, mice with targeted disruption of mPges1 gene were studied, with a focus on responses where PGE(2) has been implicated, including urinary concentration, regulation of blood pressure, and response to a loop diuretic. The absence of mPGES1 was associated with a 50% decrease in basal excretion of PGE(2) in urine (P < 0.001). In female but not male mPGES1-deficient mice, there was a reciprocal increase in basal excretion of other prostanoids. Nonetheless, urinary osmolalities were similar in mPges1(+/+) and mPges1(-/-) mice at baseline and after 12 h of water deprivation. Likewise, there were no differences in blood pressure between mPGES1-deficient and wild-type mice on control or high- or low-salt diets. The furosemide-induced increase in urinary PGE(2) excretion that was seen in wild-type mice was attenuated in mPGES1-deficient mice. However, furosemide-associated diuresis was reduced only in male, not female, mPGES1-deficient mice. Stimulation of renin by furosemide was not affected by mPGES1 deficiency. These data suggest that mPGES1 contributes to basal synthesis of PGE(2), but there are other pathways that lead to renal PGE(2) synthesis. Moreover, there are significant gender differences in physiologic contributions of mPGES1 to control kidney function.

Effects of chronic sepsis on the voltage-gated sodium channel in isolated rat muscle fibers

OBJECTIVE

Physiopathology of critical illness polyneuromyopathy was investigated in several animal-based models. Electrophysiologic approach was achieved in denervated and corticosteroid-induced myopathy; other models based on sepsis or inflammatory factors (zymosan, cytokines) were also used but did not consider voltage-gated sodium channel implication in neuromuscular weakness. We have studied electrophysiologic effects of chronic sepsis on an intact neuromuscular rat model with special consideration to the subtypes of sodium channels involved.

DESIGN

Experimental animal study.

SETTING

University laboratory.

SUBJECTS

Wistar rats.

INTERVENTIONS

Chronic sepsis was achieved by a technique of cecal ligature and needle perforation. Ten days after surgery, the rats were killed. Fast-twitch flexor digitorum brevis was excised and dissociated in 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid-buffered saline supplemented with 3.0 mg/mL collagenase. Fast sodium currents were recorded by a macropatch clamp technique at room temperature (22+/-2 degrees C) in a cell-attached configuration.

MEASUREMENTS AND MAIN RESULTS

A decrease in maximal sodium current and in conductance was evidenced without modification of the sodium Nernst potential. A shift of the voltage inactivation curve toward more negative potentials could explain the observed decrease in excitability. In parallel, we observed an up-regulation of NaV 1.5-type sodium channels.

CONCLUSIONS

Chronic inflammation and sepsis induced modifications of sodium channel properties that could contribute to muscular inexcitability. This inexcitability can be elicited by a modification of properties or type of voltage-gated sodium channels. Our results lead us to explain this inexcitability by an up-regulation of NaV 1.5 sodium channel.

Safety factor for neuromuscular transmission at type-identified diaphragm fibers

The safety factor (SF) for neuromuscular transmission varies across limb muscles of different fiber-type composition. Using intracellular recordings in rat diaphragm fibers, we found that SF varies across muscle fiber types (even within a single muscle), being larger for type IIx or IIb fibers than for type I or IIa fibers. Fiber-type differences in activation history or mechanical load may contribute to differences in SF and are important determinants of neuromuscular plasticity.

Molecular architecture of the neuromuscular junction

The neuromuscular junction (NMJ) is a complex structure that serves to efficiently communicate the electrical impulse from the motor neuron to the skeletal muscle to signal contraction. Over the last 200 years, technological advances in microscopy allowed visualization of the existence of a gap between the motor neuron and skeletal muscle that necessitated the existence of a messenger, which proved to be acetylcholine. Ultrastructural analysis identified vesicles in the presynaptic nerve terminal, which provided a beautiful structural correlate for the quantal nature of neuromuscular transmission, and the imaging of synaptic folds on the muscle surface demonstrated that specializations of the underlying protein scaffold were required. Molecular analysis in the last 20 years has confirmed the preferential expression of synaptic proteins, which is guided by a precise developmental program and maintained by signals from nerve. Although often overlooked, the Schwann cell that caps the NMJ and the basal lamina is proving to be critical in maintenance of the junction. Genetic and autoimmune disorders are known that compromise neuromuscular transmission and provide further insights into the complexities of NMJ function as well as the subtle differences that exist among NMJ that may underlie the differential susceptibility of muscle groups to neuromuscular transmission diseases. In this review we summarize the synaptic physiology, architecture, and variations in synaptic structure among muscle types. The important roles of specific signaling pathways involved in NMJ development and acetylcholine receptor (AChR) clustering are reviewed. Finally, genetic and autoimmune disorders and their effects on NMJ architecture and neuromuscular transmission are examined.

Extension and magnitude of denervation in skeletal muscle from ageing mice

In this work we hypothesized that denervation in flexor digitorum brevis (FDB) muscle from ageing mice is more extensive than predicted by standard functional and structural assays used in the past. In addition, we asked whether denervation is a fully or partially developed process. Despite the reported alteration in skeletal muscle innervation, the quantification of the extension and magnitude of denervation in ageing rodents has remained elusive. To address these two questions we utilized a combination of electrophysiological and immunohistochemical assays directed to detecting the expression of tetrodotoxin (TTX)-resistant sodium channels (Na(v)1.5) in FDB muscles from young-adult and senescent mice. Sodium current density measured with the macropatch cell-attached technique did not show significant differences between FDB fibres from young and old mice. The TTX dose-response curve, using the whole cell voltage-clamp technique, showed three populations of fibres in senescent mice, one similar to fibres from young mice (TTX sensitive), another one similar to fibres from experimentally denervated muscle (TTX resistant), and a third group intermediate between these two. Partially and fully denervated fibres added up to approximately 50% of the total number of fibres tested, a number that concurs with the percentage of fibres positive for the Na(v)1.5 channel by specific immunostaining.

Rapid protein kinase C-dependent reduction of rat skeletal muscle voltage-gated sodium channels by ciliary neurotrophic factor

The ciliary neurotrophic factor (CNTF), known to exert long-term myotrophic effects, has not yet been shown to induce a rapid biological response in skeletal muscles. The present in vitro study gives rise to the possibility that CNTF could affect the sodium channel activity implied in the triggering of muscle fibre contraction. Therefore, we investigated the effects of an external CNTF application on macroscopic sodium current (I(Na)) in rat native fast-twitch skeletal muscle (flexor digitorum brevis, FDB) by using a cell-attached patch-clamp technique. The I(Na) peak amplitude measured at a depolarizing pulse from -100 to -10 mV is rapidly reduced in a time- and dose-dependent manner by CNTF (0.01-20 ng ml(-1)). The maximal decrease is 25% after 10 min incubation in 2 ng ml(-1) CNTF. There was no alteration in activation or inactivation kinetics, or in activation curves constructed from current-voltage relationships in the presence of CNTF. In contrast, the relative I(Na) inhibition induced by CNTF is accompanied by a hyperpolarizing shift in the midpoint of the inactivation curves: -6 and -10 mV for the steady-state fast and slow inactivation, respectively. Furthermore, CNTF induces a 5 mV hyperpolarization of the resting membrane potential of the fibres. The effects of CNTF are similar to those of 1-oleoyl-2-acetyl-sn-glycerol (OAG), a protein kinase C (PKC) activator, when no effect is observed in the presence of chelerythrine, a PKC inhibitor. These results suggest that, in skeletal muscle, CNTF can rapidly decrease sodium currents by altering inactivation gating, probably through an intracellular PKC-dependent mechanism that could lead to decreased membrane excitability. The present study contributes to a better understanding of the physiological role of endogenous CNTF.

Characterization of the voltage-activated currents in cultured atrial myocytes isolated from the heart of the common oysterCrassostrea gigas

Using the macro-patch clamp technique, we show that cardiac myocytes isolated from the heart of the oyster Crassostrea gigas possess several types of voltage-activated ionic currents. (1) A classical non-inactivating potassium current of the IK type that is inhibited by tetraethyl ammonium and shows an outward rectification and a slow activation. (2) A potassium current of the IA type that shows rapid activation and inactivation, and is blocked by 4-amino pyridine or preliminary depolarisation. (3) A potassium calcium-dependent current that is inhibited by charybdotoxin, activated by strong depolarisations and shows a large conductance. (4) A calcium inward current of the L-type that is inhibited by verapamil, cobalt and high concentrations of cadmium. This current is identified in most cells, but a T-type calcium current and classical fast sodium current are only identified in few cells, and only after a strong hyperpolarizing pulse. This suggests that these channels are normally inactivated in cultured cells and are not involved in the spontaneous activity of these cells. When they exist, the fast sodium channel is blocked by tetrodotoxin. The L-type calcium conductance is increased by serotonin. The identification in cultured oyster atrial cells of classical ionic currents,which are observed in most vertebrate species but only in a few species of molluscs, demonstrates that these cells are an interesting model. Moreover the viability and the electrophysiological properties of these cells are not significantly modified by freezing and thawing, thus increasing their usefulness in various bioassays.

Fiber type conversion alters inactivation of voltage-dependent sodium currents in murine C2C12 skeletal muscle cells

Each skeletal muscle of the body contains a unique composition of "fast" and "slow" muscle fibers, each of which is specialized for certain challenges. This composition is not static, and the muscle fibers are capable of adapting their molecular composition by altered gene expression (i.e., fiber type conversion). Whereas changes in the expression of contractile proteins and metabolic enzymes in the course of fiber type conversion are well described, little is known about possible adaptations in the electrophysiological properties of skeletal muscle cells. Such adaptations may involve changes in the expression and/or function of ion channels. In this study, we investigated the effects of fast-to-slow fiber type conversion on currents via voltage-gated Na+ channels in the C(2)C(12) murine skeletal muscle cell line. Prolonged treatment of cells with 25 nM of the Ca2+ ionophore A-23187 caused a significant shift in myosin heavy chain isoform expression from the fast toward the slow isoform, indicating fast-to-slow fiber type conversion. Moreover, Na+ current inactivation was significantly altered. Slow inactivation less strongly inhibited the Na+ currents of fast-to-slow fiber type-converted cells. Compared with control cells, the Na+ currents of converted cells were more resistant to block by tetrodotoxin, suggesting enhanced relative expression of the cardiac Na+ channel isoform Na(v)1.5 compared with the skeletal muscle isoform Na(v)1.4. These results imply that fast-to-slow fiber type conversion of skeletal muscle cells involves functional adaptation of their electrophysiological properties.

Detubulation experiments localise delayed rectifier currents to the surface membrane of amphibian skeletal muscle fibres

Ionic currents in intact and detubulated frog sartorius muscle fibres were compared at room temperature using a loose-patch voltage clamp configuration in four experimental groups. The test fibres (i) were detubulated by a previously established osmotic shock protocol that involved the introduction and withdrawal of extracellular glycerol followed by exposure to Ca2+/Mg2+-Ringer solution and cooling. The control fibres were spared osmotic shock and (ii) simply studied in normal Ringer solution, (iii) exposed to 30 min of steady cooling to 9-10 degrees C before electrophysiological study or (iv) exposed to and studied in glycerol-Ringer solution. The presence or absence of detubulation was confirmed for all the experimental groups through assessing for the abolition or otherwise of the delayed after-depolarisation normally associated with action potential propagation into the transverse (T) tubules. All fibre groups showed similar resting potentials (-80 to -90 mV) thus ensuring consistent baseline voltages from which the voltage clamp steps were imposed. The intact muscle fibres in the three control groups (ii)-(iv) spared osmotic shock showed both inward Na+ and delayed rectifier outward (K+) currents. In contrast, patches from detubulated muscle fibres in the test group (i) showed only delayed outward currents, consistent with contrasting contributions to Na+ and K+ currents from regions of membrane affected or spared by the detubulation procedure. Nevertheless, the voltage dependence, maximum steady state amplitudes and timecourses of the delayed outward currents were conserved through all the experimental groups. These findings suggest that the surface as opposed to the tubular membrane contributes the greater part of the delayed rectifier current in amphibian skeletal muscle.

KATP channels depress force by reducing action potential amplitude in mouse EDL and soleus muscle

Although ATP-sensitive K+ (KATP) channel openers depress force, channel blockers have no effect. Furthermore, the effects of channel openers on single action potentials are quite small. These facts raise questions as to whether 1) channel openers reduce force via an activation of KATP channels or via some nonspecific effects and 2) the reduction in force by KATP channels operates by changes in amplitude and duration of the action potential. To answer the first question we tested the hypothesis that pinacidil, a channel opener, does not affect force during fatigue in muscles of Kir6.2-/- mice that have no cell membrane KATP channel activity. When wild-type extensor digitorum longus (EDL) and soleus muscles were stimulated to fatigue with one tetanus per second, pinacidil increased the rate at which force decreased, prevented a rise in resting tension, and improved force recovery. Pinacidil had none of these effects in Kir6.2-/- muscles. To answer the second question, we tested the hypothesis that the effects of KATP channels on membrane excitability are greater during action potential trains than on single action potentials, especially during metabolic stress such as fatigue. During fatigue, M wave areas of control soleus remained constant for 90 s, suggesting no change in action potential amplitude for half of the fatigue period. In the presence of pinacidil, the decrease in M wave areas became significant within 30 s, during which time the rate of fatigue also became significantly faster compared with control muscles. It is therefore concluded that, once activated, KATP channels depress force and that this depression involves a reduction in action potential amplitude.

The role of putative phosphorylation sites in the targeting and shuttling of the aquaporin-2 water channel

In renal collecting ducts, a vasopressin-induced cAMP increase results in the phosphorylation of aquaporin-2 (AQP2) water channels at Ser-256 and its redistribution from intracellular vesicles to the apical membrane. Hormones that activate protein kinase C (PKC) proteins counteract this process. To determine the role of the putative kinase sites in the trafficking and hormonal regulation of human AQP2, three putative casein kinase II (Ser-148, Ser-229, Thr-244), one PKC (Ser-231), and one protein kinase A (Ser-256) site were altered to mimic a constitutively non-phosphorylated/phosphorylated state and were expressed in Madin-Darby canine kidney cells. Except for Ser-256 mutants, seven correctly folded AQP2 kinase mutants trafficked as wild-type AQP2 to the apical membrane via forskolin-sensitive intracellular vesicles. With or without forskolin, AQP2-Ser-256A was localized in intracellular vesicles, whereas AQP2-S256D was localized in the apical membrane. Phorbol 12-myristate 13-acetate-induced PKC activation following forskolin treatment resulted in vesicular distribution of all AQP2 kinase mutants, while all were still phosphorylated at Ser-256. Our data indicate that in collecting duct cells, AQP2 trafficking to vasopressin-sensitive vesicles is phosphorylation-independent, that phosphorylation of Ser-256 is necessary and sufficient for expression of AQP2 in the apical membrane, and that PMA-induced PKC-mediated endocytosis of AQP2 is independent of the AQP2 phosphorylation state.

Histochemical differences in the responses of predominantly fast-twitch glycolytic muscle and slow-twitch oxidative muscle to veratrine

The aim of this study was to investigate if the Na(+)-channel activating alkaloid veratrine is able to change the oxidative and m-ATPase activities of a fast-twitch glycolytic muscle (EDL, extensor digitorum longus) and slow-twitch oxidative muscle (SOL, soleus) in mice. Oxidative fibers and glycolytic fibers were more sensitive to veratrine than oxidative-glycolytic fibers 15, 30 and 60 min after the i.m. injection of veratrine (10 ng/kg) with both showing an increase in their metabolic activity in both muscles. In EDL, the m-ATPase reaction revealed a significant (p < 0.001) decrease (50%) in the number of type IIB fibers after 30 min while the number of type I fibers increased by 550%. Type I fibers decreased from 34% in control SOL to 17% (50% decrease) in veratrinized muscles, with a 10% decrease in type IIA fibers within 15 min. A third type of fiber appeared in SOL veratrinized muscle, which accounted for 28% of the fibers. Our work gives evidence that the changes in the percentage of the fiber types induced by veratrine may be the result, at least partially, from a direct effect of veratrine on muscle fibers and else from an interaction with the muscle type influencing distinctively the response of a same fiber type. Based on the results obtained in the present study the alterations in EDL may be related to the higher number of Na(+) channels present in this muscle whereas those in SOL may involve an action of veratrine on mitochondria. Although it is unlikely that the shift of enzymes activities induced by veratrine involves genotypic expression changes an alternative explanation for the findings cannot be substantiated by the present experimental approach.

Safety factor at the neuromuscular junction

Reliable transmission of activity from nerve to muscle is necessary for the normal function of the body. The term 'safety factor' refers to the ability of neuromuscular transmission to remain effective under various physiological conditions and stresses. This is a result of the amount of transmitter released per nerve impulse being greater than that required to trigger an action potential in the muscle fibre. The safety factor is a measure of this excess of released transmitter. In this review we discuss the practical difficulties involved in estimating the safety factor in vitro. We then consider the factors that influence the safety factor in vivo. While presynaptic transmitter release may be modulated on a moment to moment basis, the postsynaptic features that determine the effect of released transmitter are not so readily altered to meet changing demands. Different strategies are used by different species to ensure reliable neuromuscular transmission. Some, like frogs, rely on releasing a large amount of transmitter while others, like man, rely on elaborate postsynaptic specialisations to enhance the response to transmitter. In normal adult mammals, the safety factor is generally 3-5. Both pre- and postsynaptic components change during development and may show plasticity in response to injury or disease. Thus, both acquired autoimmune and inherited congenital diseases of the neuromuscular junction (NMJ) can significantly reduce, or even transiently increase, safety factor.

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.

Effects of training on the concentration of Na+, K+-ATPase in foal muscle

We investigated the effect of training on the Na+, K+-ATPase concentration in foal skeletal muscle by measurement of [3H]ouabain binding. From the 7th day after birth, 12 foals were divided in 3 groups: a) staying in a box stall (Box); b) staying in a box stall with a training regimen of an increasing number of sprints per day (Training); and c) staying on pasture (Pasture). Euthanasia was performed after 5 months. In semitendinosus muscle, the concentration of [3H]ouabain binding sites (pmol/g wet wt) was 181 +/- 6 in the Box, 220 +/- 15 in the Training, and 197 +/- 8 in the Pasture group (all n = 6; Box vs. Training, P<0.05). In gluteus medius, the concentration of [3H]ouabain binding sites was 168 +/- 9 in the Box, 219 +/- 12 in the Training, and 175 +/- 4 in the Pasture group (all n = 6; Box or Pasture vs. Training, P<0.02). Scatchard analysis of saturation curves showed that the difference in [3H]ouabain binding sites between the 3 groups could not be ascribed to differences in the Kd for ouabain. Both for semitendinosus and gluteus medius muscle, the concentration of [3H]ouabain binding sites increased in the order Box < Pasture < Training (a total increase of around 20%). This suggests a specific effect of the amount and intensity of exercise on the Na+, K+-ATPase concentration in horse skeletal muscle, and may lead to a better performance during exercise.

Modification by ageing of the tetrodotoxin-sensitive sodium channels in rat skeletal muscle fibres

Ageing leads to an impairment of muscle performance that may result from alteration of sarcolemma excitability. Therefore, we compare sodium channels of native fast-twitch skeletal muscle fibres of 21-26-month-old aged rats and 4-6-month-old young-adult rats, using the patch-clamp method. Extrajunctional sarcolemma of aged-rat fibres presented a higher sodium current density than that of young-rat fibres, which resulted from the presence of a higher number of available channels per membrane area. Open probability and availability voltage-dependence of sodium channels were similar in aged- and young-rat fibres, but permeation property was altered during ageing: aged-rat muscles showed a bimodal distribution of fibres with two values of sodium-channel conductance measured between -40 and 0 mV; a young phenotype with a conductance close to 18 pS overlapping that found in young-rat fibres and an aged phenotype with a lower approximately half conductance. Current-voltage curves extended to -60 and +20 mV showed that the aged-phenotype conductance level resulted from an outward rectification occurring in these aged-rat fibres. Furthermore, in these aged-rat fibres belonging to the aged phenotype, ensemble average sodium currents showed slower activation and inactivation kinetics. Sodium currents of the two phenotypes were blocked by 100 nM tetrodotoxin, therefore excluding possible denervation effect. These age-related modifications in sodium current may contribute to the alteration of muscle excitability and function observed during the ageing process.

Injury and recovery of fast and slow skeletal muscle fibers affected by ACL myotoxin isolated from Agkistrodon contortrix laticinctus (Broad-Banded copperhead) venom

The response of different types of skeletal muscle fibers to a snake venom PLA2 myotoxin was tested in vivo by injecting ACL myotoxin (ACLMT) into mice. Both the soleus (slow-twitch) and gastrocnemius (fast-twitch) were examined at different time periods (3 h, 3 and 21 d) after the injection. All animals received 5 mg/kg myotoxin into the subcutaneous lateral region of the right hind limb, near the Achilles tendon; contralateral muscles were used as controls. Cross-sections (10 microm) of frozen muscle tissue were cut from the medial region of the muscle. Alternate serial sections were stained either with toluidine blue or for acid phosphatase, myofibrillar ATPase activity after alkali (pH 10.3) or acid preincubation (pH 4.3), succinate dehydrogenase or acetylcholinesterase. Several stages of necrosis were observed 3 h after ACLMT injection, in both superficial and deep regions of both muscles. In these same regions 3 d after injection, clusters of regenerated muscle fibers were present, and some of them presented AChE activity. Twenty-one days after ACLMT injection the muscle fibers of soleus and gastrocnemius presented only chronic signs of damage such as split fibers and centralized nuclei. Using m-ATPase reactions it was possible to determine that both muscle fiber types I and II were injured in both muscles. The number of type IIC fibers was significantly increased, and the number of type II fibers significantly decreased in the gastrocnemius 21 d after ACLMT injection, suggesting a change in muscle fiber type from type II to type I, through type IIC. The increased number of type IIC fibers and the presence of AChE activity in clusters of regenerating fibers and split fibers indicate that injury by ACLMT produces axonal remodeling and muscle fiber type change.

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.

Events of the excitation-contraction-relaxation (E-C-R) cycle in fast- and slow-twitch mammalian muscle fibres relevant to muscle fatigue

The excitation-contraction-relaxation cycle (E-C-R) in the mammalian twitch muscle comprises the following major events: (1) initiation and propagation of an action potential along the sarcolemma and transverse (T)-tubular system; (2) detection of the T-system depolarization signal and signal transmission from the T-tubule to the sarcoplasmic reticulum (SR) membrane; (3) Ca2+ release from the SR; (4) transient rise of myoplasmic [Ca2+]; (5) transient activation of the Ca2+-regulatory system and of the contractile apparatus; (6) Ca2+ reuptake by the SR Ca2+ pump and Ca2+ binding to myoplasmic sites. There are many steps in the E-C-R cycle which can be seen as potential sites for muscle fatigue and this review explores how structural and functional differences between the fast- and slow-twitch fibres with respect to the E-C-R cycle events can explain to a great extent differences in their fatiguability profiles.

End-plate voltage-gated sodium channels are lost in clinical and experimental myasthenia gravis

This study examined the loss of voltage-gated Na+ channels as well as acetylcholine receptors (AChRs) from the end-plate region in patients with acquired myasthenia gravis (MG) and in rats with experimental autoimmune passively transferred MG (PTMG). Rats received a monoclonal IgG antibody directed against an extracellular epitope of the nicotinic acetylcholine receptor of muscle (AChR) to produce PTMG. At the end-plate border we examined miniature end-plate potentials (MEPPs), sodium current (INa) amplitude, and action potential (AP) properties; the latter two were also examined on the extrajunctional membrane. In the normal situation, the safety factor for neuromuscular transmission is ensured by the large INa at the end plate, which reduces the AP threshold. Among different fiber types, INa was largest for type IIb fibers and smallest for type I fibers. When end-plate border properties of fibers from 3 MG patients and 15 PTMG rats were compared with controls, INa was reduced, AP thresholds were higher, and rates of AP rise were reduced. Amplitudes of MEPPs and INa at the end plate indicated that loss of AChRs was greater than loss of Na+ channels in patients with MG and rats with PTMG; INa was reduced to about 60% of control values, whereas MEPPs were reduced to less than 30% of control values. On the extrajunctional membrane, INa and AP thresholds and rates of rise were similar for MG patients, PTMG rats, and controls. This evidence for loss of voltage-gated Na+ channels at the motor end plate in both patients with MG and in rats with PTMG reveals a hitherto unrecognized consequence of the end-plate damage initiated by the binding of complement-fixing IgG to end-plate AChRs.

The contribution of postsynaptic folds to the safety factor for neuromuscular transmission in rat fast- and slow-twitch muscles

1. At the rat neuromuscular junction, the postsynaptic folds and the voltage-gated sodium channels (VGSCs) within them are thought to amplify the effects of postsynaptic currents. In this study, the contribution of this effect to the safety factor for neuromuscular transmission, the ratio of the normal quantal content to the number of quanta required to reach threshold, has been estimated. 2. Normal quantal content was determined in isolated nerve-muscle preparations of rat soleus and extensor digitorum longus (EDL) muscles in which muscle action potentials were blocked by mu-conotoxin. The quantal content estimated from voltage recordings was 61.8 and 79.4 in soleus and EDL, respectively, and from charge measurements derived from current recordings was 46.3 (soleus) and 65.1 (EDL). 3. The threshold for action potential generation in response to nerve stimulation was determined from endplate potentials (EPPs) and endplate currents (EPCs) in preparations partially blocked with d-tubocurarine. The number of quanta required to reach threshold was estimated from voltage recordings to be 19.7 (soleus) and 23.2 (EDL) and from charge measurements derived from current recordings to be 13.3 (soleus) and 13.0 (EDL). 4. When intracellular electrodes were used to inject current into the muscle fibre, the total charge required to reach threshold was approximately twice that of the nerve-evoked threshold EPC. 5. The safety factor for nerve-evoked responses at the junction was 3.5 (soleus) and 5.0 (EDL). In the extrajunctional region the safety factor estimated from injected currents was 1.7 (soleus) and 2.5 (EDL). 6. It is concluded that the effect of the postsynaptic folds and the VGSCs within them is to double the safety factor. At normal frequencies of nerve impulse activity in vivo, this effect is likely to be crucial for ensuring effective neuromuscular transmission.