Ionic currents in mammalian fast skeletal muscle

Effects of miR-34c-5p on Sodium, Potassium, and Calcium Channel Currents in C2C12 Myotubes

The aim of this study was to investigate the effects of miR-34c-5p on the main voltage-dependent ion channels in skeletal muscle cells. This study focused on the effects of miR-34c-5p on sodium, potassium, and calcium currents in C2C12 myoblasts. The miR-34c-5p overexpression group, knockdown group, and control group were differentiated for 7 days, fused into myotubes, and used for the whole-cell patch clamp recording. Compared with the control group, the whole-cell sodium current density of the other two groups had no significant changes. In the knockdown group, the delayed rectifier potassium current density was increased (statistically significant), and the whole-cell calcium channel current density did not change. In the overexpression group, the change of rectifier potassium current density was not obvious, while the peak calcium channel current density increased (- 9.23 ± 0.95 pA/pF, n = 6 cells for the overexpression group vs. - 6.48 ± 0.64 pA/pF, n = 7 cells for the control; p < 0.05). Changes in the expression of miR-34c-5p can affect the electrophysiological characteristics of calcium and potassium voltage-gated channels in C2C12 myotubes. Overexpression of miR-34c-5p increased whole-cell L-type calcium channel current (I_Ca,L), while miR-34c-5p knockdown increased whole-cell delayed rectifier potassium current (I_Kd).

Gating behaviour of sodium currents in adult mouse muscle recorded with an improved two-electrode voltage clamp

Muscle contraction is triggered by the spread of an action potential along the fibre. The ionic current to generate the action potential is conducted through voltage-activated sodium channels, and mutations of these channels are known to cause several human muscle disorders. Mouse models have been created by introducing point mutations into the sodium channel gene. This achievement has created the need for a high-fidelity technique to record sodium currents from intact mouse muscle fibres. We have optimized a two-electrode voltage clamp, using sharp microelectrodes to preserve the myoplasmic contents. The voltage-dependent behaviour of sodium channel activation, inactivation and slow-inactivation were characterized. The voltage range for these gating behaviours was remarkably hyperpolarized, in comparison to studies in artificial expression systems. These results provide normative data for sodium channels natively expressed in mouse muscle and illustrate the need to modify model simulations of muscle excitability to account for the hyperpolarized shift.

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.

Protein kinase A modulates A-type potassium currents of larval zebrafish (Danio rerio) white muscle fibres

AIMS

Potassium (K(+)) channels are involved in regulating cell excitability and action potential shape. To our knowledge, very little is known about the modulation of A-type K(+) currents in skeletal muscle fibres. Therefore, we sought to determine whether K(+) currents of zebrafish white skeletal muscle were modulated by protein kinase A (PKA).

METHODS

Pharmacology and whole-cell patch clamp were used to examine A-type K(+) currents and action potentials associated with zebrafish white skeletal muscle fibres.

RESULTS

Activation of PKA by a combination of forskolin + 3-isobutyl-1-methylxanthine (Fsk + IBMX) decreased the peak current density by approximately 60% and altered the inactivation kinetics of A-type K(+) currents. The specific PKA inhibitor H-89 partially blocked the Fsk + IBMX-induced reduction in peak current density, but had no effect on the change in decay kinetics. Fsk + IBMX treatment did not shift the activation curve, but it significantly reduced the slope factor of activation. Activation of PKA by Fsk + IBMX resulted in a negative shift in the V(50) of inactivation. H-89 prevented all Fsk + IBMX-induced changes in the steady-state properties of K(+) currents. Application of Fsk + IBMX increased action potential amplitude, but had no significant effect on action potential threshold, half width or recovery rate, when fibres were depolarized with single pulses, paired pulses or with high-frequency stimuli.

CONCLUSION

PKA modulates the A-type K(+) current in zebrafish skeletal muscle and affects action potential properties. Our results provide new insights into the role of A-type K(+) channels in muscle physiology.

Multi-parameter Hopf-bifurcation in HHM Model Exposed to ELF Electric Field

The variation of cell trans-membrane voltage exposed to extremely low frequency (ELF) electric field is analyzed; a modified Hodgkin-Huxley model of muscle (HHM) is established by introducing a new parameter denoting the effect of external electric field, and the bifurcation caused by the new parameter as well as leakage conductance and sodium ions anti-electromotive force is studied. The algebra criterion in high dimension equations is employed to perform the analysis of multi-parameter dynamical bifurcation. The results are of biological significance and suggest that the aberration of dynamics in bio-systems may be accounted for diseases caused by electric exposure.

Development of ionic currents of zebrafish slow and fast skeletal muscle fibers

Voltage-gated Na+ and K+ channels play key roles in the excitability of skeletal muscle fibers. In this study we investigated the steady-state and kinetic properties of voltage-gated Na+ and K+ currents of slow and fast skeletal muscle fibers in zebrafish ranging in age from 1 day postfertilization (dpf) to 4-6 dpf. The inner white (fast) fibers possess an A-type inactivating K+ current that increases in peak current density and accelerates its rise and decay times during development. As the muscle matured, the V50s of activation and inactivation of the A-type current became more depolarized, and then hyperpolarized again in older animals. The activation kinetics of the delayed outward K+ current in red (slow) fibers accelerated within the first week of development. The tail currents of the outward K+ currents were too small to allow an accurate determination of the V50s of activation. Red fibers did not show any evidence of inward Na+ currents; however, white fibers expressed Na+ currents that increased their peak current density, accelerated their inactivation kinetics, and hyperpolarized their V50 of inactivation during development. The action potentials of white fibers exhibited significant changes in the threshold voltage and the half width. These findings indicate that there are significant differences in the ionic current profiles between the red and white fibers and that a number of changes occur in the steady-state and kinetic properties of Na+ and K+ currents of developing zebrafish skeletal muscle fibers, with the most dramatic changes occurring around the end of the first day following egg fertilization.

Sodium and potassium currents of larval zebrafish muscle fibres

The steady-state and kinetic properties of Na(+) and K(+) currents of inner (white) and outer (red) muscles of zebrafish larvae 4-6 days post-fertilization (d.p.f.) are described. In inner muscle, the outward currents were half-activated at -1.0 mV and half-inactivated at -30.4 mV, and completely inactivated within 100 ms of depolarization. The inward currents of inner fibres were half-activated at -7.3 mV and half-inactivated at -74.5 mV and completely inactivated within 5 ms of depolarization. Inner muscle fibres were found to support action potentials, while no action potentials could be evoked in outer muscles. In inner muscle fibres, all tested levels of depolarizing current above a threshold value evoked only one action potential. However, spiking at frequencies of up to 200 cycles s(-1) was evoked by the injection of depolarizing pulses separated by short hyperpolarizing currents. We suggest that the properties of the inward sodium and outward potassium currents permit high frequency firing in response to a pulsatile depolarizing input of the kind expected in fast swimming, whilst safeguarding against tetany during a strong depolarization.

Human myoblast fusion requires expression of functional inward rectifier Kir2.1 channels

Myoblast fusion is essential to skeletal muscle development and repair. We have demonstrated previously that human myoblasts hyperpolarize, before fusion, through the sequential expression of two K+ channels: an ether-à-go-go and an inward rectifier. This hyperpolarization is a prerequisite for fusion, as it sets the resting membrane potential in a range at which Ca2+ can enter myoblasts and thereby trigger fusion via a window current through alpha1H T channels.

Dynamics and consequences of potassium shifts in skeletal muscle and heart during exercise

Since it became clear that K(+) shifts with exercise are extensive and can cause more than a doubling of the extracellular [K(+)] ([K(+)](s)) as reviewed here, it has been suggested that these shifts may cause fatigue through the effect on muscle excitability and action potentials (AP). The cause of the K(+) shifts is a transient or long-lasting mismatch between outward repolarizing K(+) currents and K(+) influx carried by the Na(+)-K(+) pump. Several factors modify the effect of raised [K(+)](s) during exercise on membrane potential (E(m)) and force production. 1) Membrane conductance to K(+) is variable and controlled by various K(+) channels. Low relative K(+) conductance will reduce the contribution of [K(+)](s) to the E(m). In addition, high Cl(-) conductance may stabilize the E(m) during brief periods of large K(+) shifts. 2) The Na(+)-K(+) pump contributes with a hyperpolarizing current. 3) Cell swelling accompanies muscle contractions especially in fast-twitch muscle, although little in the heart. This will contribute considerably to the lowering of intracellular [K(+)] ([K(+)](c)) and will attenuate the exercise-induced rise of intracellular [Na(+)] ([Na(+)](c)). 4) The rise of [Na(+)](c) is sufficient to activate the Na(+)-K(+) pump to completely compensate increased K(+) release in the heart, yet not in skeletal muscle. In skeletal muscle there is strong evidence for control of pump activity not only through hormones, but through a hitherto unidentified mechanism. 5) Ionic shifts within the skeletal muscle t tubules and in the heart in extracellular clefts may markedly affect excitation-contraction coupling. 6) Age and state of training together with nutritional state modify muscle K(+) content and the abundance of Na(+)-K(+) pumps. We conclude that despite modifying factors coming into play during muscle activity, the K(+) shifts with high-intensity exercise may contribute substantially to fatigue in skeletal muscle, whereas in the heart, except during ischemia, the K(+) balance is controlled much more effectively.

Activation of Ca2+-activated K+ channels by an increase in intracellular Ca2+ induced by depolarization of mouse skeletal muscle fibres

1. Ionic currents were simultaneously recorded at macroscopic and unitary level using the whole-cell and cell-attached patch-clamp procedures together on the same portion of isolated mouse skeletal muscle fibres. 2. In the presence of Tyrode solution in the patch pipette and Tyrode-TTX solution in the bath, macroscopic and unitary currents through delayed rectifier K+ channels were simultaneously recorded in response to depolarizing pulses of 1 s duration. 3. In five fibres, successive long-lasting incremental depolarizing levels induced, at -40 mV or -30 mV, the opening of a high conductance channel carrying an outward current superimposed on delayed rectifier K+ channel activity. Opening of this high conductance channel was not observed when the depolarization steps were applied in the patch pipette. 4. Using the same depolarizing protocol, activation of a high conductance channel was also observed in two fibres in the presence of a K+-rich solution in the pipette (145 mM K+) . 5. With either Tyrode or K+-rich solution in the pipette, unitary current amplitudes of the high conductance channel matched well with the values obtained for Ca2+-activated K+ (KCa) channels in inside-out patches under similar ionic conditions. 6. Indo-1 fluorescence measurements showed that the stimulation protocol that led to KCa channel opening induced stepwise increases in intracellular [Ca2+] in the submicromolar range. 7. Our results provide evidence that activation of sarcolemmal KCa channels can be induced by a rise in intracellular [Ca2+] following voltage-activated sarcoplasmic reticulum Ca2+ release.

Loss of electrical excitability in an animal model of acute quadriplegic myopathy

In rats treated with high-dose corticosteroids, skeletal muscle that is denervated in vivo (steroid-denervated [S-D]) develops electrical inexcitability similar to that seen in patients with acute quadriplegic myopathy. In studies of affected muscles in vitro, the majority of S-D fibers failed to generate action potentials in response to intracellular stimulation although the average resting potential of these fibers was no different from that of control denervated muscle. The downregulation of membrane chloride conductance (G[Cl]) seen in normal muscle after denervation did not occur in S-D muscle. Although block of chloride channels in S-D muscle produced high specific membrane resistance, comparable to similarly treated control denervated muscle, and partially restored excitability in many fibers, action potential amplitude was still reduced in S-D fibers, suggesting a concomitant reduction in sodium current. 3H-saxitoxin binding measurements revealed a reduction in the density of the adult muscle sodium channel isoform in S-D muscle, suggesting that a decrease in the number of sodium channels present may play a role in the reduction of sodium current, although altered properties of channels may also contribute. The weakness seen in S-D muscle may involve the interaction of a number of factors that modify membrane excitability, including membrane depolarization, persistence of G(Cl), and reduced voltage-gated sodium currents.

A gap isolation method to investigate electrical and mechanical properties of fully contracting skeletal muscle fibers

We describe here a single-gap isolation method that allows the simultaneous measurement of electrical activity and tension output from fully contracting segments of frog skeletal muscle fibers. By using single pulses and pulse trains of varying frequency (5-100 Hz), records obtained for both electrical and mechanical fiber response demonstrate that the physiological properties of the fiber segments have been preserved. Action potentials could be recorded free of movement artifacts, even while segments were in fused tetani and developing maximum tensions of more than 600 kN/m2. Single current pulses evoked action potentials that averaged 144 +/- 16 mV (mean +/- SD, n = 8) in amplitude and twitches that averaged 285 +/- 66 kN/m2 and 55 +/- 5 ms (mean +/- SD, n = 20) in magnitude and time to peak, respectively. Trains of action potentials elicited patterns of tension development that exhibited summation, unfused tetani, and fused tetani in a frequency-dependent manner. The AC and DC electrical properties of the single grease gap were modeled with a simple Thévenin equivalent circuit, which satisfactorily predicted the experimental results. Our methodology is easily implemented and potentially applicable to any muscle preparation in which fiber segments with an intact end attached to a piece of tendon can be dissected.

A patch-clamp study of delayed rectifier currents in skeletal muscle of control and mdx mice

1. Potassium currents were measured in the extensor digitorum longus muscle of normal and mdx mice, which lack the protein dystrophin, using the cell-attached and inside-out patch clamp techniques, in the presence of asymmetrical K+ concentrations (3 mM in the pipette, 160 mM in the bath). 2. In cell-attached patches, the delayed rectifier was the most commonly found potassium channel, with a density of roughly 8 channels microns-2. Outward macroscopic currents were activated in macropatches depolarized to potentials positive to -60 mV. The probability of opening reached half-maximal values around -40 mV for control patches and -31 mV for patches from mdx mice. 3. Tail currents were linear in the range between -60 and +20 mV, reversing close to -100 mV. The single channel current at 0 mV, estimated from non-stationary analysis of variance, was used in conjunction with the slope of the linear part of the tail current to calculate the single channel conductance, yielding a value of 19 +/- 1 pS. 4. At 0 mV, the delayed rectifier inactivated with two time constants, of 70 +/- 20 ms and 600 +/- 200 ms. Prepulses of 500 ms duration to different potentials produced incomplete inactivation with inactivation reaching 50% of its maximum at -50 mV. 5. Single channel activity was recorded using small pipettes. Both single channel conductance and kinetic behaviour were in agreement with the macroscopic current data. 6. In excised patches, the delayed rectifier current ran down, unmasking other K+ channels. A Ca(2+)-dependent K+ channel of 186 pS (BK-like channel) was found frequently in patches bathed in solutions containing appropriate concentrations of calcium, especially at stronger depolarizations. A K+ channel of 63 pS was unmasked in control excised patches bathed in solutions devoid of ATP. This channel was not observed in patches excised from mdx fibers.

Action potential generation in rat slow- and fast-twitch muscles

1. In skeletal muscle fibres, voltage-gated sodium channels are concentrated at the neuromuscular junction. The effect of this accumulation of sodium channels on action potential generation was investigated in rat slow- and fast-twitch muscle fibres. 2. Intracellular microelectrodes were used to generate and record action potentials, from an imposed membrane potential of -75 and -90 mV, in junctional and extrajunctional regions of the muscle fibre. To identify junctional regions, preparations were incubated with 5 x 10(-7) M d-tubocurarine (dTC) to block muscle contraction in response to nerve stimulation whilst allowing endplate potentials (EPPs) to be recorded. Injection of rectangular depolarizing current pulses initiated action potentials at the endplate with threshold values several millivolts lower than those generated elsewhere in the fibre. In addition, the maximum rate of rise of the action potential was greater at the endplate than in extrajunctional regions. 3. In other muscles, neuromuscular transmission was partially blocked with dTC (2 x 10(-7) M), such that repetitive nerve stimulation evoked action potentials and EPPs in the same fibre. The threshold of these nerve-evoked action potentials was approximately 50% lower than values derived from action potentials generated by current injection. 4. It is concluded that the threshold for action potential generation is significantly lower at the neuromuscular junction than in extrajunctional regions of skeletal muscle fibres. Furthermore, nerve-evoked current is more effective at generating an action potential than is injected current.

Expression of a voltage-dependent potassium current precedes fusion of human muscle satellite cells (myoblasts)

Using the whole-cell recording patch clamp technique in clonal cultures of human muscle satellite cells (SC), we studied a voltage-gated potassium current analogous to the delayed rectifier current (IKdr) described in adult human skeletal muscle. This current was absent in proliferating SC cultured in a growth medium containing 15% serum, except when the SC approached the end of their replicative life (between 77 and 124 days in culture); at that time, approximately 50% of the SC possessed IKdr. In contrast, IKdr was expressed within less than 4 days in approximately 70% of the SC cultured in a serum-free medium (SFM) and within 24 h in differentiating medium. We believe that IKdr may be a characteristic feature of fusion-component SC and that it may be involved in the fusion process for the following reasons: 1) after the transfer in differentiating medium, cultures of SC in which the expression of IKdr was previously promoted by exposure to SFM were found to fuse immediately, without the initial 24 h lag time observed in control sister cultures; 2) in the latter "naive" SC, IKdr was expressed during the first day in differentiating medium, before SC began to fuse; 3) most of the SC that did not fuse even after weeks of exposure to differentiating medium did not express IKdr; 4) TEA, at a concentration of 3 mM, reduces the amplitude of IKdr by 55% and the fusion index by 55-67%.

Effects of veratridine on Na and Ca currents in frog skeletal muscle

1. Voltage-clamp experiments were performed to determine the effects of veratridine on Na and Ca currents in frog skeletal muscle fibres. 2. Veratridine (1 microM) did not affect the kinetics of the fast Na current but it did induce a slowly inactivating tetrodotoxin-sensitive inward current that was apparent after Na current inactivation. This slow current had a peak amplitude of 6.7 +/- 0.7 microA/cm2 at -20 mV and decayed monoexponentially with a time constant of 606 +/- 77 ms. 3. The slow current had a voltage-dependence for activation that was similar to that of the fast Na current. Single depolarizing prepulses that induced complete inactivation of the fast Na channels, prevented development of the slow current. Trains of brief depolarizations at increasing frequencies increased the amplitude of the slow current. These results suggest that the slow current may be mediated by veratridine modified Na channels that must be in the open position. 4. The low concentration of veratridine (1 microM) did not affect the Ca current, while 100 microM veratridine reversibly suppressed the Ca current and shifted its peak current-voltage relation towards more negative potentials. Thus, veratridine appears not to be a selective fast Na channel modifier as it may also alter Ca channel gating properties in skeletal muscle fibres.

Sodium and potassium currents in freshly isolated and in proliferating human muscle satellite cells

1. Human muscle satellite cells (SC) were studied either immediately after dissociation of muscle biopsies or later, as they proliferated in culture. A purification procedure combined with clonal cultures ensured that electrophysiological recordings were done in myogenic cells. Hoechst staining for the DNA attested that cells were mononucleated. 2. The goals of this study were to examine (i) whether the electrophysiological properties of freshly isolated SC resembled those of SC that proliferated in culture for several weeks, (ii) whether freezing and thawing affected these properties, and (iii) whether SC constituted a homogeneous population. 3. We found that there were only subtle differences between the electrophysiological results obtained in freshly isolated SC and in proliferating SC with or without previous freezing and thawing. Most SC expressed two voltage-gated currents, a TTX-resistant Na+ current and a calcium-activated potassium current (IK, Ca). 4. The level of expression of the Na+ current and of IK, Ca was affected in a different way by cellular proliferation; the normalized Na+ conductance (pS pF-1) of proliferating cells resembled that of freshly isolated SC, whereas the IK, Ca conductance increased 10 times. The analysis of the amplitude distributions of the Na+ current and of IK, Ca in the various SC preparations suggested that there was only one class of SC.

Potassium channels from normal and denervated mouse skeletal muscle fibers

The properties of singles K+ channels in normal and denervated muscles were compared using the "patch-clamp" technique. Single channels were recorded from vesicles obtained by stretching bundles of normal and denervated extensor digitorium longus (EDL) muscles. The most frequently observed channel in normal muscles was a high conductance (266 pS) Ca++ activated K+ channel. Although channel density, as estimated by patch recording, showed a significant decrease in denervated muscles, no differences were found in conductance and gating properties. Another voltage-dependent K+ channel (81 pS) was only recorded from normal muscles, but never from denervated ones. In addition, a 35 pS conductance was recorded from both normal and denervated fibers. This channel displayed neither voltage dependence nor sensitivity to tetraethylammonium (TEA). In contrast, another TEA-insensitive (16 pS) channel was recorded only from denervated muscles. We conclude that denervation induces significant changes in the distribution and expression of K+ channels in mammalian skeletal muscles.

Studies on the inhibition by chlorpromazine of myotonia induced by ion channel modulators in mouse skeletal muscle

The myotonic activity of mouse soleus and extensor digitorum longus muscles induced by either a combination of K+ channel blockers (4-aminopyridine) and a Cl- channel blocker (9-anthracene carboxylic acid) or a Cl- channel blocker in low Ca2+ (0.25 mM) Krebs or a Na+ channel activator (veratridine) was characterized in this paper. Myotonic activity was characterized by an increase in both the contraction amplitude and contraction duration accompanied by stimulus-related repeated action potentials. The slow soleus and fast extensor digitorum longus muscles appeared to differ in their responses to these ion channel modifiers. Nevertheless, chlorpromazine at a low concentration of 1 microM significantly inhibited all kinds of myotonic activity; it reduced the prolonged contraction duration and attenuated the stimulus-related repeated action potential firing. This depressant action of chlorpromazine was apparently not correlated with inhibition of either calmodulin or phospholipase A2 activity, since the myotonic depressant action of calmodulin inhibitors, such as dibucaine, flunarizine, chlorpromazine, trifluoperazine and diltiazem, was unrelated to their potency in inhibiting the activity of calmodulin or phospholipase A2. However, phosphatidylcholine was found to inhibit the myotonic depressant action of chlorpromazine. It is therefore, tentatively concluded that chlorpromazine interacted with membrane phospholipids, thereby changing membrane ion channel activity and depressing myotonic activity. These findings indicate that chlorpromazine might be useful in the management of clinical myotonia.

Poneratoxin, a novel peptide neurotoxin from the venom of the ant, Paraponera clavata

1. At concentrations varying from 10(-8) to 10(-6) M synthetic poneratoxin (PoTX) is a strong, but very slowly acting agonist for smooth muscles and its blocks synaptic transmission in the insect CNS in a concentration-dependent manner and depolarizes giant interneurons. 2. However, in isolated dorsal unpaired median cells 10(-6) M PoTX causes only a reversible hyperpolarization of about 5 mV. 3. At concentrations from 10(-8) to 10(-6) M PoTX affects the electrical activity of isolated cockroach axons, as well as isolated frog and rat skeletal muscle fibres. 4. PoTX prolongs action potentials and induces slow automatic activity, due to a slow Na(+)-current activation at very negative values of potential and due to slow deactivation.

Regulation of muscle sodium channel transcripts during development and in response to denervation

We have recently described the cloning and functional expression of a new sodium channel subtype, microI, isolated from a denervated rat skeletal muscle cDNA library. In studies described here, we have used RNase protection and Northern blot analyses to examine the expression of microI mRNA in different tissues and in neonatal, adult, and adult denervated muscle. We found that microI transcripts were not expressed in brain or heart, or in the myogenic cell line L6, even after differentiation to myotubes. Transcripts for microI were present at low levels in neonatal skeletal muscle and increased to maximum levels in adult tissue, paralleling the expression of tetrodotoxin (TTX)-sensitive sodium currents. Surprisingly, denervation of adult muscle was also followed by a rise in microI mRNA, at a time when TTX-insensitive currents reappear. These results show that expression of this channel subtype is regulated by tissue type, development, and innervation.

A tetrodotoxin- and Mn2(+)-insensitive Na+ current in Duchenne muscular dystrophy

Muscle myotube cultures were obtained from normal and Duchenne muscular dystrophy (DMD) biopsies by using an explant technique. The current-voltage (I/V) curve of the whole sodium (Na+) current (INa) in normal myotubes was similar to that obtained from DMD myotubes. However, the inactivation curve of the whole INa was different in normal myotubes when compared to that obtained from DMD myotubes. Addition of 10(-4) M tetrodotoxin (TTX, a fast INa blocker) decreased the whole INa in both preparations. The inorganic calcium (Ca2+) blocker manganese (Mn2+) completely blocked the remaining TTX-resistant INa of normal myotubes and decreased this current in DMD myotubes leaving behind a TTX- and Mn2(+)-insensitive INa that was insensitive to the Ca2+ blocker desmetoxyverapamil ((-)D888). The slow inward barium current (IBa) of both normal and DMD myotubes was blocked by Mn2+ and (-)D888. However the kinetics of the slow channel in normal myotubes was different from that of DMD myotubes. This study demonstrates the presence of a TTX- and Mn2(+)-insensitive INa in DMD myotubes. This channel may contribute to the increase of intracellular Na+ [( Na]i) in DMD and allow Ca2+ to enter the cells through the Na(+)-Ca2+ exchanger, thus contributing to calcium loading.

State-dependent inactivation of K+ currents in rat type II alveolar epithelial cells

Inactivation of K+ channels responsible for delayed rectification in rat type II alveolar epithelial cells was studied in Ringer, 160 mM K-Ringer, and 20 mM Ca-Ringer. Inactivation is slower and less complete when the extracellular K+ concentration is increased from 4.5 to 160 mM. Inactivation is faster and more complete when the extracellular Ca2+ concentration is increased from 2 to 20 mM. Several observations suggest that inactivation is state-dependent. In each of these solutions depolarization to potentials near threshold results in slow and partial inactivation, whereas depolarization to potentials at which the K+ conductance, gK, is fully activated results in maximal inactivation, suggesting that open channels inactivate more readily than closed channels. The time constant of current inactivation during depolarizing pulses is clearly voltage-dependent only at potentials where activation is incomplete, a result consistent with coupling of inactivation to activation. Additional evidence for state-dependent inactivation includes cumulative inactivation and nonmonotonic from inactivation. A model like that proposed by C.M. Armstrong (1969. J. Gen. Physiol. 54: 553-575) for K+ channel block by internal quaternary ammonium ions accounts for most of these properties. The fundamental assumptions are: (a) inactivation is strictly coupled to activation (channels must open before inactivating, and recovery from inactivation requires passage through the open state); (b) the rate of inactivation is voltage-independent. Experimental data support this coupled model over models in which inactivation of closed channels is more rapid than that of open channels (e.g., Aldrich, R.W. 1981. Biophys. J. 36:519-532). No inactivation results from repeated depolarizing pulses that are too brief to open K+ channels. Inactivation is proportional to the total time that channels are open during both a depolarizing pulse and the tail current upon repolarization; repolarizing to more negative potentials at which the tail current decays faster results in less inactivation. Implications of the coupled model are discussed, as well as additional states needed to explain some details of inactivation kinetics.

Changes in Na channel properties of frog and rat skeletal muscles induced by the AaH II toxin from the scorpion Androctonus australis

The effects of the mammal toxin II isolated from the venom of the scorpion Androctonus australis Hector (AaH II) were studied under current and voltage clamp conditions in frog (semitendinosus) and rat (fast e.d.l. and slow soleus) skeletal twitch muscle fibres. In both species, AaH II induced a dose-dependent prolongation of the action potential (AP) leading at saturating concentration to APs with long plateaus of about 1.5 s in frog and 5 s in rat e.d.l. and soleus fibres. The concentrations to induce 50% of the maximal effect (K0.5) were 9.1 x 10(-9) M in the frog and 1.4 x 10(-9) M in the rat. AaH II increased the time constants of inactivation of the peak Na current and induced a maintained Na current that was greater in rat e.d.l. and soleus (31.6% of peak current amplitude at -30 mV; K0.5 = 0.8 x 10(-9) M) than in frog (16.5%; K0.5 = 15.5 x 10(-9) M) muscles. Peak and maintained Na currents were TTX-sensitive and had identical threshold and reversal potentials. The half-maximum maintained permeability occurred at a potential 20 mV more positive than the peak permeability. Recovery from inactivation and steady-state inactivation of the inactivating Na current remained unchanged. The maintained current deactivated with normal fast kinetics. The action of the toxin reversed poorly on washout but could be largely removed by conditioning depolarizations more positive than the reversal potential of the Na current. Our results suggest that, in vertebrate skeletal muscle fibres, AaH II affects all the Na channels and are consistent with the hypothesis that the maintained current originates from a reopening of previously inactivated Na channels.

Slow sodium channel inactivation in mammalian muscle: a possible role in regulating excitability

Sodium currents were recorded in rat fast and slow twitch muscle fibers. Changes in the membrane potential around the resting potential produced slow changes in the sodium current amplitude due to alterations of the slow inactivation process that was increased by steady depolarization and removed by prolonged hyperpolarization. In contrast, classical fast inactivation was not operative around the resting potential, and depolarizations of greater than 20 mV were required to close half of the channels by fast inactivation. Because slow inactivation is operative around the resting potential of mammalian muscle fibers, it may partially explain why small depolarizations, such as those that occur in some patients with periodic paralysis, can reduce excitability.

Tension activation and relaxation in frog atrial fibres. Evidence for direct effects of divalent cations (Ca2+, Sr2+, Ba2+) on contractile proteins and Na-Ca exchange

The effect of alkali-earth cations (Ca2+, Sr2+, Ba2+) on the excitation-contraction coupling events of the frog atrial fibres were studied using a double mannitol gap voltage clamp technique coupled with a mechano-electric transducer. Photoremoval of the suppressive effect of nifedipine on the calcium channels allowed to obtain rapid transient Ca2+, Sr2+ or Ba2+ ions current jumps. The effect on the amplitude of the associated contraction was proportional to the current jumps. These results together with the correlation established between the estimated increase in the internal concentration of divalent cations and the amplitude of the phasic tension suggest that the essential source of divalent cations for activation of contraction is the extracellular space. Also Ba2+ ions reduced the tonic tension and strongly slowed the relaxation of the phasic component whereas Sr2+ exhibited smaller effects. Sr2+ ions could be more efficient than Ba2+ ions in substituting for Ca2+ ions in the Na+-Ca2+ exchange mechanism known to regulate these two mechanical events. The conclusions are that the order of effectiveness of these ions (Ca2+ greater than Sr2+ greater than Ba2+) is the same with regard to transarcolemmal exchange for Na+ ions, presumed uptake by a "second relaxing system", activation of contraction, and inactivation of the slow inward current.

Cultivation, morphology, and electrophysiology of contractile rat myoballs

Myoballs were cultured from neonatal rat skeletal muscle without the use of antimitotic drugs. Electron microscopic investigation showed that 7-day-old myoballs are multinucleated syncytia in a state of differentiation where filaments are abundant and already in hexagonal arrays. The resting potential of 142 myoballs kept at 20 degrees C was not correlated with the cell size. Its mean value was -64 mV. Cells with a high resting potential were capable of generating action potentials with a threshold of -51 mV, an overshoot of +31 mV, and a rate of rise of 100 V/s. The steady-state current-voltage relation showed inward rectification on hyperpolarization and outward rectification on depolarization. The dynamic sodium and potassium currents were investigated at 37 degrees C with the whole-cell-recording technique. The sodium current had its maximum at -20 mV. The potassium current showed delayed activation and a very slow and incomplete inactivation. The electrophysiological results from these cultured cells are very similar to those obtained from adult cells.

Membrane Ca2+ interactions and contraction in denervated rat soleus muscle

Under voltage clamp conditions contractile responses and ionic currents of single fibres isolated from rat soleus, denervated for more than 20 days, were recorded in Na-free TEA containing solutions. The relationship between membrane potential and contraction has been analysed under various conditions. The addition of trivalent cations (La3+, Gd3+) resulted in a dose dependent reduction of the contractile response and similar effects were produced by polymyxin B (0.05-0.5 mM). By contrast in the presence of phospholipase D (1-5 U/ml) contractions were significantly increased for all values of depolarization. The time course of the change of tension amplitude after the application of Ca-free medium, was dependent on the amplitude, the duration and the frequency of the depolarization. Upon depolarization glycerol-treated fibres generated contractile responses which were similar to those recorded in normal muscle and were also dependent on [Ca]o. It is proposed that in denervated soleus muscle the negatively charged phospholipids at the outside of the membrane were involved in the depolarization-contraction coupling by means of their Ca binding properties. The quantity of Ca binding sites would be dependent on [Ca]o and membrane potential and their binding properties modified during and/or following variation in membrane potential.

Slow sodium channel inactivation in rat fast-twitch muscle

1. Voltage-clamp Na+ currents (INa) were measured in rat fast-twitch fibres using the loose-patch-clamp technique. Changes in the conditioning membrane potential produced slow changes in the peak INa elicited by short test depolarizations, due to a slow inactivation process. 2. Inactivation was increased by application of steady depolarizing potentials and was reversed by steady hyperpolarizations. These changes in peak INa could be well fitted by single-exponential functions with time constants in the range of 1-4 min. 3. The steady-state values of the maximum peak INa at any potential could be well fitted by a function identical to the one describing the fast inactivation process. This gave a potential of -108 mV at which 50% of the channels were closed due to slow inactivation. 4. The maximum peak current densities obtained with the slow inactivation fully removed were as large as 20 mA cm-2.

Effects of external cations and calcium-channel blockers on depolarization-contraction coupling in denervated rat twitch skeletal muscles

In the double mannitol gap arrangement the contraction was estimated in single fibres isolated from rat extensor digitorum longus (e.d.l.) muscles that had been denervated for 2-48 days. Denervation induced large changes in the characteristics of the action potential and of the twitch. Up to 15-20 days after denervation the contraction-depolarization curve was sigmoidal and the maximum amplitude of the contraction was not modified by variation of [Ca]o or [Na]o. After 15-20 days of denervation a bell-shaped curve described the relation between contraction and potential. The maximum amplitude was dependent upon the [Ca]o. In Ca-free solution no contractile response was obtained. In Na-free, Ca-containing solution the relationship between contraction and potential was not modified by the addition of divalent cations Co2+, Cd2+, Mn2+, Mg2+, Ni2+, or Ba2+. The contraction, which appeared in Ca-free solution, was restored by adding Sr2+. D600, verapamil and bepridil failed to change the amplitude of the contraction while a marked reduction was found with dihydropyridines. The reduction was overcome by increasing [Ca]o. The present results suggest that the slow calcium current is not involved in the generation of the contractile responses developed by denervated muscles in Na-free (TEA) solution.

Comparison between slow sodium channel inactivation in rat slow- and fast-twitch muscle

1. Voltage-clamp Na+ currents (INa) were studied in rat soleus slow-twitch muscle fibres at about 18 degrees C using the loose-patch-clamp technique. The maximum inward current density was produced by depolarizations to about -19 mV. 2. Fast inactivation was studied utilizing 20 ms conditioning potentials. INa was reduced by 50% with conditioning potentials to about -70 mV. 3. Changes in the conditioning membrane potential produced slow changes in the peak INa due to a slow inactivation process. INa was reduced by 50% at about -86 mV due to slow inactivation. 4. The mean maximum inward INa when slow inactivation was fully removed was 6.83 mA cm-2. 5. Due to the slow inactivation process, slow-twitch fibres were less susceptible to reduction in INa than fast-twitch fibres.

Ionic channels: II. Voltage- and agonist-gated and agonist-modified channel properties and structure

This article reviews the different forms of ionic channels: voltage-gated, agonist-gated, and agonist- and second messenger-modified channels. The recent advances in our knowledge of the amino acid sequence of the sodium channel and the nicotinic acetylcholine receptor and the relationship of the primary structure to the channels' quarternary structure and function are discussed.

Ionic channels: I. The biophysical basis for ion passage and channel gating

This article reviews the biophysics of ion passage through membrane pores, as well as the physical factors that control the ion selectivity, gating, and conductance of an ionic channel. Different voltage clamp techniques are discussed in detail. The biophysical properties of sodium channels are reviewed.

Asymmetric charge movement in contracting muscle fibres in the rabbit

The Vaseline-gap technique was used to record asymmetric charge movement in small segments of muscle fibres from the white sternomastoid or the soleus muscle of the rabbit. At 22 degrees C, non-linear ionic currents (Na+, K+, Cl-, Ca2+) were virtually eliminated for potential steps to 0 mV or below by specific blocking agents or ion substitution. A Boltzmann fit of charge movement (Q) vs. potential (V) produced the mean values Qmax = 15.2 nC/microF, V = -26.8 mV and k = 15.3 mV for twenty-three sternomastoid fibres, and 4.8 nC/microF, -32 mV and 13.7 mV for seven soleus fibres. Qmax for the sternomastoid fibres was similar to that for other fast-twitch fibres when normalized by surface area rather than capacitance. Using a 55 ms step, the mean threshold potential (Vth) for contraction in twenty-eight fibres was -25.9 (+/- 2.9) mV (+/- S.E. of mean), and the mean amount of charge moved (qth) at the threshold potential was 8.5 (+/- 0.4) nC/microF. In some contracting fibres, a component of charge movement was observed which was analogous to q gamma in amphibian muscle in its time course and potential dependence. Addition of 80 mM-sucrose to the external solution increased the speed of both the asymmetric charge movement and the charging of the linear capacitance of each fibre. The effect was reversible. A clear relation between the time course of these two parameters was established, and this strongly indicated that the majority of the asymmetric charge was located in the transverse tubular system or beyond. Moreover, it was shown that at 22 degrees C nearly all asymmetric charge moved in less than 0.5 ms after depolarization of the T-system. Sucrose in the external solution affected the Q vs. V relation, steepening the curve and shifting it to more negative potentials, as well as slightly increasing Qmax. The actions of sucrose strongly suggest that it effectively dilates and/or shortens the transverse tubular system, probably by osmotic effects.

Voltage-dependent channels of human muscle cultures

Cultures were grown from satellite cells obtained from biopsies of normal children and of boys having Duchenne muscular dystrophy (DMD). Patch-clamp recordings were obtained from mononucleated cells and from young myotubes containing up to five nuclei. Four current types were distinguished. Na currents had a maximum amplitude near -10 mV and a half inactivation point near -60 mV. Single channel currents observed in isolated patches had a main unit size of 1.8 pA at -30 mV. Voltage dependent outward K currents were blocked by applying tetraethylammonium to the bath solution. In some cells, outward currents had a rather slow activation and did not inactivate. In other cells, activation was faster, and the currents inactivated. At large positive potentials, another K current was activated. The corresponding channels displayed large unit steps in isolated patches. Hyperpolarizing voltage pulses elicited in about one third of the cells inward rectifier currents. All currents types were found with similar characteristics in normal and DMD cultures. Whole cell results were very variable. Cells displayed various combinations of the four kinds of currents. To understand the origin of this diversity, clonal cultures were investigated. Clones displayed more homogeneous electrical properties than standard cultures, suggesting the presence of various types of stem cells in the non-clonal cultures.

Changes in the ionic currents sensitivity to inhibitors in twitch rat skeletal muscles following denervation

Under voltage clamp conditions, using the double mannitol gap technique, ionic currents developed by fast (e.d.l.) and slow (soleus) twitch muscle fibers of the rat were analysed at different times following denervation and the results compared with those obtained in normal cells. In slow fibers, denervation caused the appearance of a new population of TTX-resistant Na+ channels (dissociation constant K2 = 2,800 nM) compared with the normal TTX-sensitive Na+ channels (K1 = 9 nM). This new population of Na channels appeared in 5 days and contributed about 32% of the total Na conductance. Denervated fast fibres developed a slow component in the delayed outward current which was found to be typical of slow innervated muscles. This component appeared 5 to 20 days after nerve section. These changes are associated with modifications of potassium channels' sensitivity for specific inhibitors (TEA and 4-AP). After denervation, the delayed outward current in the two types of muscles becomes resistant to 4-AP whereas TEA, which blocks the total delayed outward current in innervated fibers (dissociation constant of 21.4 mM) becomes more effective in blocking the fast component (dissociation constant of 0.61 mM) and less effective in blocking the slow component in denervated cells. The analysis of the characteristics of the TEA sensitive and TEA insensitive components of the outward current leads to the proposal that these components were related to the fast and to the slow components previously described in fast and slow twitch mammalian skeletal muscles.

Ionic currents and charge movements in organ-cultured rat skeletal muscle

The middle of the fibre voltage-clamp technique was used to measure ionic currents and non-linear charge movements in intact, organ-cultured (in vitro denervated) mammalian fast-twitch (rat extensor digitorum longus) muscle fibres. Muscle fibres organ cultured for 4 days can be used as electrophysiological and morphological models for muscles in vivo denervated for the same length of time. Sodium currents in organ-cultured muscle fibres are similar to innervated fibres except that in the temperature range 0-20 degrees C (a) in the steady state, the voltage distribution of inactivation in cultured fibres is shifted negatively some 20 mV; (b) at the same temperature and membrane potential, the time constant of inactivation in cultured fibres is about twice that of innervated fibres. Potassium currents in innervated and cultured fibres at 15 degrees C can be fitted with the Hodgkin-Huxley n variable raised to the second power. Despite the large range we would estimate that the maximum value of the steady-state potassium conductance of cultured fibres is about one-half that of innervated fibres. The estimated maximum amount of charge moved in cultured fibre is about one-third that in innervated fibres. Compared to innervated fibres, culturing doubles the kinetics of the decay phase of charge movement. The possibility of a negative shift of the voltage distribution of charge movements in cultured fibres is discussed.

The kinetics of recovery and development of potassium channel inactivation in perfused squid (Loligo pealei) giant axons

K+ currents were studied at a normal (-69 mV) and at a depolarized (-49 mV) membrane potential in voltage-clamped squid giant axons perfused with 350 mM-K+ and bathed in K+-free artificial sea water containing tetrodotoxin to block the Na+ channels. Steady-state and instantaneous K+ currents were reduced by over 50% at corresponding voltages at the depolarized membrane potential. Instantaneous chord conductance-voltage curves showed that the depolarized membrane potential caused a uniform reduction of K+ conductance across the voltage range under study. The driving force for K+ ions was comparable at both membrane potentials when a short (2 ms) pre-pulse was used to open the K+ channels. When a longer (7.5 ms) pre-pulse was used, the driving force was actually larger at the depolarized membrane potential. The depolarized membrane potential did drive some K+ ions into the periaxonal space. The amount of K+ ions driven into the periaxonal space was estimated by two independent methods, with similar results. The resulting increase of K+ ions in the periaxonal space (10 mM) was about 40 times too small to account for the large reduction in currents in terms of a reduced driving force for K+ ions. The kinetics of recovery and development of inactivation were monitored by repeatedly applying a 7.5 ms test pulse followed by a long conditioning potential. Both recovery and development of inactivation, from the depolarized membrane potential, were described by the sum of two exponential terms plus a constant. The time constant-voltage curves for both phases of inactivation peaked at about -54 mV at 10 degrees C. The time constant of the slow phase of inactivation at -54 mV was about 12.4 s, while the corresponding time constant for the fast phase was about 2.3 s. The slow relaxation had an apparent plateau of about 11 s at more depolarized membrane potentials. Recovery from inactivation was rapid at hyperpolarized membrane potentials. The steady-state inactivation curve of the K+ channel was incomplete in the depolarizing region; and apparent plateau was reached with about 75% of the K+ current inactivated. The temperature sensitivity of both phases of inactivation corresponded to a Q10 of about 3. Elevated external concentrations of K+ ions did not block either phase of the inactivation process, although the kinetics of recovery from inactivation were slightly faster under these conditions.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of apamin on the outward potassium current of isolated frog skeletal muscle fibres

The effect of apamin, a polypeptidic toxin from bee venom which is a specific blocker of certain Ca2+-dependent K+ channels, has been tested (50-100nM) on voltage clamped single skeletal muscle fibres of the frog. The results have shown the existence of an inhibitory effect of the toxin on the slow outward K+ current which suggests the existence of a Ca2+-sensitive component of the slow K+ permeability in the plasma membrane of the frog muscle fibre.

Increased sodium conductance in the synaptic region of rat skeletal muscle fibres

Differences in sodium conductance between end-plate and extrajunctional regions of rat lumbrical muscle fibres were measured by comparing action potential maximum rate of rise (Vmax) in the two regions and by using a vibrating micro-electrode to record steady inward current produced by application of veratridine. In normal Krebs solution, action potential Vmax was significantly greater (by 43%) in the end-plate region than in extrajunctional regions of the fibres. When chloride conductance was greatly reduced by bathing muscles in solutions with low chloride concentration, Vmax was still significantly higher (by 28%) in the end-plate region than in extrajunctional regions. The increased Vmax could be recorded only within a distance of about 150-200 microns of the end-plate. Steady inward current was recorded with a vibrating micro-electrode at the end-plate in response to veratridine; the current persisted when veratridine was introduced in low-chloride Krebs solution, and it was rapidly reversed by tetrodotoxin. The current reflected a 5 mV difference in membrane potential between the end-plate region and extrajunctional regions. The results suggest that sodium conductance is increased in the synaptic region relative to extrajunctional regions of the fibres.

Membrane currents in human intercostal muscle at varied extracellular potassium

Hyperpolarizing and depolarizing square steps were imposed on the membrane potential of excised human intercostal muscle fibers by means of a 3-microelectrode voltage clamp. The steady-state amplitudes of the membrane currents inducing such steps were investigated as a function of the membrane potential, while the muscle was bathed in solutions varying in potassium content (Ke = 1, 3.5, 7, 20, and 60 mM). At all potassium concentrations, the membrane acted as a rectifier, both in the inward- and outward-going directions. Inward currents were much reduced when Ke was lowered from 3.5 to 1 mM, and were increased when Ke was raised beyond 3.5 mM. The delayed outward current was reduced when Ke was increased from 3.5 mM to 7 mM and higher potassium concentration. The results were qualitatively similar to those reported for rat skeletal muscle.

Voltage clamp of rat and human skeletal muscle: measurements with an improved loose-patch technique

Intact fibres of human intercostal and rat omohyoid muscles were studied at 23 degree C with a loose-patch voltage-clamp technique that employed two concentric micropipettes to electrically isolate small-diameter (10-15 microns) patches of sarcolemma. This method allows investigation of membrane excitability under highly physiological conditions. Step depolarizations to 0 mV elicited sodium inward currents that reached peak values of up to 20 mA/cm2 within 250 microseconds, and then declined. In human muscle, the reversal potential (ENa) was approximately 40 mV, and maximal conductances (GNa) ranged from 44 to 360 mS/cm2. In rat muscle, ENa was 42 mV and GNa ranged from 100 to 250 mS/cm2. Sodium channels in rat and human muscle were indistinguishable in most aspects of their kinetic behaviour and voltage dependence. Outward potassium currents were small by comparison (usually less than 2 mA/cm2) and saturated at positive potentials. The maximum potassium conductance (GK) ranged from 0 to 19 mS/cm2 (human) and from 4 to 12 mS/cm2 (rat muscle).

A quantitative study of potassium channel kinetics in rat skeletal muscle from 1 to 37 degrees C

Potassium currents were measured using the three-microelectrode voltage-clamp technique in rat omohyoid muscle at temperatures from 1 to 37 degrees C. The currents were fitted according to the Hodgkin-Huxley equations as modified for K currents in frog skeletal muscle (Adrian et al., 1970a). The equations provided an approximate description of the time course of activation, the voltage dependence of the time constant of activation (tau n), and the voltage dependence of gK infinity. At higher temperatures the relationship between gK infinity and voltage was shifted in the hyperpolarizing direction. The effect of temperature on tau n was much greater in the cold than in the warm: tau n had a Q10 of nearly 6 at temperatures below 10 degrees C, but a Q10 of only approximately 2 over the range of 30-38 degrees C. The decreasing dependence of tau n on temperature was gradual and the Arrhenius plot of tau n revealed no obvious break-points. In addition to its quantitative effect on activation kinetics, temperature also had a qualitative effect. Near physiological temperatures (above approximately 25 degrees C), the current was well described by n4 kinetics. At intermediate temperatures (approximately 15-25 degrees C), the current was well described by n4 kinetics, but only if the n4 curve was translated rightward along the time axis (i.e., the current had a greater delay than could be accounted for by simple n4 kinetics). At low temperatures (below approximately 15 degrees C), n4 kinetics provided only an approximate fit whether or not the theoretical curve was translated along the time axis. In particular, currents in the cold displayed an initial rapid phase of activation followed by a much slower one. Thus, low temperatures appear to reveal steps in the gating process which are kinetically "hidden" at higher temperatures. Taken together, the effects of temperature on potassium currents in rat skeletal muscle demonstrate that the behavior of potassium channels at physiological temperatures cannot be extrapolated, either quantitatively or qualitatively, from experiments carried out in the cold.

Slow components of potassium tail currents in rat skeletal muscle

The kinetics of potassium tail currents have been studied in the omohyoid muscle of the rat using the three-microelectrode voltage-clamp technique. The currents were elicited by a two-pulse protocol in which a conditioning pulse to open channels was followed by a test step to varying levels. The tail currents reversed at a single well-defined potential (VK). At hyperpolarized test potentials (-100 mV and below), tail currents were inward and exhibited two clearly distinguishable phases of decay, a fast tail with a time constant of 2-3 ms and a slow tail with a time constant of approximately 150 ms. At depolarized potentials (-60 mV and above), tail currents were outward and did not show two such easily separable phases of decay, although a slow kinetic component was present. The slow kinetic phase of outward tail currents appeared to be functionally distinct from the slow inward tail since the channels responsible for the latter did not allow significant outward current. Substitution of Rb for extracellular K abolished current through the anomalous (inward-going) rectifier and at the same time eliminated the slow inward tail, which suggests that the slow inward tail current flows through anomalous rectifier channels. The amplitude of the slow inward tail was increased and VK was shifted in the depolarizing direction by longer conditioning pulses. The shift in VK implies that during outward currents potassium accumulates in a restricted extracellular space, and it is suggested that this excess K causes the slow inward tail by increasing the inward current through the anomalous rectifier. By this hypothesis, the tail current slowly decays as K diffuses from the restricted space. Consistent with such a hypothesis, the decay of the slow inward tail was not strongly affected by changing temperature. It is concluded that a single delayed K channel is present in the omohyoid. Substitution of Rb for K has little effect on the magnitude or time course of outward current tails, but reduces the magnitude and slows the decay of the fast component of inward tails. Both effects are consistent with a mechanism proposed for squid giant axon (Swenson and Armstrong, 1981): that (a) the delayed potassium channel cannot close while Rb is inside it, and (b) that Rb remains in the channel longer than K.

Effect of glucocorticoid treatment on the excitability of rat skeletal muscle

Dexamethasone treatment in the rat produced depolarization of extensor digitorum longus (EDL) muscle fibers but not soleus (SOL) fibers studied in vitro at 23 degrees C. The depolarization of EDL fibers was most prominent after 1 day of treatment (treated -77.5 +/- 1.1 mV, control -87.2 +/- 0.8 mV; mean +/- S.E.), and was associated with elevation of the action potential threshold and reduction of the action potential overshoot. In vivo, or in vitro in chloride-free solution, the resting potential and action potential threshold and overshoot of EDL fibers from glucocorticoid-treated and control rats were similar. Sodium currents were studied with a patch voltage clamp. Glucocorticoid treatment did not alter the voltage dependence of sodium channel activation or inactivation in fast twitch muscle fibers. Maximal inward currents occurred at about -29 mV and half-maximal inward currents at about -50 mV. Sodium channels were half inactivated at about -71 mV. Glucocorticoid treatment did not alter the sarcolemmal resistance or capacitance. We conclude that glucocorticoid treatment does not produce muscle weakness or atrophy by altering the excitability of muscle fibers.