Contractile activation in skeletal muscle

How the Brain May Have Shaped Muscle Anatomy and Physiology: A Preliminary Study

Skeletal muscle fibers are often used to evaluate functional differences in locomotion. However, because there are energetic differences among muscle fiber cells, muscle fiber composition could be used to address evolutionary questions about energetics. Skeletal muscle is composed of two main types of fibers: Type I and II. The difference between the two can be reduced to how these muscle cells use oxygen and glucose. Type I fibers convert glucose to ATP using oxygen, while Type II fibers rely primarily on anaerobic metabolic processes. The expensive tissue hypothesis (ETH) proposes that the energetic demands imposed on the body by the brain result in a reduction in other expensive tissues (e.g., gastrointestinal tract). The original ETH dismisses the energetic demands of skeletal muscle, despite skeletal muscle being (1) an expensive tissue when active and (2) in direct competition for glucose with the brain. Based on these observations we hypothesize that larger brained primates will have relatively less muscle mass and a decrease in Type I fibers. As part of a larger study to test this hypothesis, we present data from 10 species of primates. We collected body mass, muscle mass, and biopsied four muscles from each specimen for histological procedures. We collected endocranial volumes from the literature. Using immunohistochemistry, a muscle fiber composition profile was created for each species sampled. Results show that larger brained primates have less muscle and fewer Type I fibers than primates with smaller brains. Results clarify the relationship between muscle mass and brain mass and illustrate how muscle mass could be used to address energetic questions. Anat Rec, 301:528-537, 2018. © 2018 Wiley Periodicals, Inc.

Emerging roles of ROS/RNS in muscle function and fatigue

Reactive oxygen and nitrogen species (ROS/RNS) are involved in numerous aspects of cellular signaling. Classically ROS/RNS have been associated with cellular dysfunction and disease, but it is now clear that they are also of integral importance under normal conditions. In this review, we discuss ROS/RNS effects in skeletal muscle, with special focus on changes in contractile function. The review deals with the tentative roles of ROS/RNS for acute changes that can occur during strenuous exercise resulting in muscle fatigue, for the recovery from fatigue, and for the effects of training/overtraining. We also discuss two groups of inherited diseases; muscle dystrophies, where recent data suggest that ROS/RNS may be of unexpectedly large importance, and mitochondrial myopathies, where the role of ROS seems more limited than originally thought.

Reciprocal dihydropyridine and ryanodine receptor interactions in skeletal muscle activation

Dihydropyridine (DHPR) and ryanodine receptors (RyRs) are central to transduction of transverse (T) tubular membrane depolarisation initiated by surface action potentials into release of sarcoplasmic reticular (SR) Ca2+ in skeletal muscle excitation-contraction coupling. Electronmicroscopic methods demonstrate an orderly positioning of such tubular DHPRs relative to RyRs in the SR at triad junctions where their membranes come into close proximity. Biochemical and genetic studies associated expression of specific, DHPR and RyR, isoforms with the particular excitation-contraction coupling processes and related elementary Ca2+ release events found respectively in skeletal and cardiac muscle. Physiological studies of intramembrane charge movements potentially related to voltage triggering of Ca2+ release demonstrated a particular qγ charging species identifiable with DHPRs through its T-tubular localization, pharmacological properties, and steep voltage-dependence paralleling Ca2+ release. Its nonlinear kinetics implicated highly co-operative conformational events in its transitions in response to voltage change. The effects of DHPR and RyR agonists and antagonists upon this intramembrane charge in turn implicated reciprocal rather than merely unidirectional DHPR-RyR interactions in these complex reactions. Thus, following membrane potential depolarization, an orthograde qγ-DHPR-RyR signaling likely initiates conformational alterations in the RyR with which it makes contact. The latter changes could then retrogradely promote further qγ-DHPR transitions through reciprocal co-operative allosteric interactions between receptors. These would relieve the resting constraints on both further, delayed, nonlinear qγ-DHPR charge transfers and on RyR-mediated Ca2+ release. They would also explain the more rapid charging and recovery qγ transients following larger depolarizations and membrane potential repolarization to the resting level.

Sarcolemmal-restricted localization of functional ClC-1 channels in mouse skeletal muscle

Skeletal muscle fibers exhibit a high resting chloride conductance primarily determined by ClC-1 chloride channels that stabilize the resting membrane potential during repetitive stimulation. Although the importance of ClC-1 channel activity in maintaining normal muscle excitability is well appreciated, the subcellular location of this conductance remains highly controversial. Using a three-pronged multidisciplinary approach, we determined the location of functional ClC-1 channels in adult mouse skeletal muscle. First, formamide-induced detubulation of single flexor digitorum brevis (FDB) muscle fibers from 15-16-day-old mice did not significantly alter macroscopic ClC-1 current magnitude (at -140 mV; -39.0 +/- 4.5 and -42.3 +/- 5.0 nA, respectively), deactivation kinetics, or voltage dependence of channel activation (V(1/2) was -61.0 +/- 1.7 and -64.5 +/- 2.8 mV; k was 20.5 ± 0.8 and 22.8 +/- 1.2 mV, respectively), despite a 33% reduction in cell capacitance (from 465 +/- 36 to 312 +/- 23 pF). In paired whole cell voltage clamp experiments, where ClC-1 activity was measured before and after detubulation in the same fiber, no reduction in ClC-1 activity was observed, despite an approximately 40 and 60% reduction in membrane capacitance in FDB fibers from 15-16-day-old and adult mice, respectively. Second, using immunofluorescence and confocal microscopy, native ClC-1 channels in adult mouse FDB fibers were localized within the sarcolemma, 90 degrees out of phase with double rows of dihydropyridine receptor immunostaining of the T-tubule system. Third, adenoviral-mediated expression of green fluorescent protein-tagged ClC-1 channels in adult skeletal muscle of a mouse model of myotonic dystrophy type 1 resulted in a significant reduction in myotonia and localization of channels to the sarcolemma. Collectively, these results demonstrate that the majority of functional ClC-1 channels localize to the sarcolemma and provide essential insight into the basis of myofiber excitability in normal and diseased skeletal muscle.

IP(3)-induced tension and IP(3)-receptor expression in rat soleus muscle during postnatal development

The present study was designed to examine whether changes in Ca(2+) release by inositol-1,4,5-trisphosphate (IP(3)) in 8-, 15-, and 30-day-old rat skeletal muscles could be associated with the expression of IP(3) receptors. Experiments were conducted in slow-twitch muscle in which both IP(3)-induced Ca(2+) release and IP(3)-receptor (IP(3)R) expression have been shown to be larger than in fast-twitch muscle. In saponin-skinned fibers, IP(3) induced transient contractile responses in which the amplitude was dependent on the Ca(2+)-loading period with the maximal IP(3) contracture being at 20 min of loading. The IP(3) tension decreased during postnatal development, was partially inhibited by ryanodine (100 microM), and was blocked by heparin (20-400 microg/ml). Amplification of the DNA sequence encoding for IP(3)R isoforms (using the RT-PCR technique) showed that in slow-twitch muscle, the type 2 isoform is mainly expressed, and its level decreases during postnatal development in parallel with changes in IP(3) responses in immature fibers. IP(3)-induced Ca(2+) release would then have greater participation in excitation-contraction coupling in developing fibers than in mature muscle.

Mechanism of action of sodium cyanide on rat diaphragm muscle

The effects of sodium cyanide (NaCN) were investigated on the contractile and electrophysiological properties of rat diaphragm muscles in vitro. Sodium cyanide (0.1-1.0 mM) produced an initial potentiation of directly elicited twitch tensions, followed by a slow progressive depression. The potentiation and depression were both dependent on the NaCN concentration and stimulation frequency. Muscles exposed to NaCN exhibited marked reductions of creatine phosphate concentration, but ATP levels were not significantly lowered. Sodium cyanide had no effect on the resting potential, input resistance or action potential, indicating that the toxicity of the metabolic inhibitor is not mediated by alterations of membrane excitability or passive electrical properties. Sodium cyanide reduced the amplitude of contractures elicited by 70 mM K(2)SO(4), suggesting that the actions of NaCN cannot be explained by a failure of action potentials to propagate across the muscle surface or within t-tubular membranes. Sodium cyanide suppressed the first phase of the caffeine contracture, an observation consistent with an impaired release of, or reduced sensitivity to, sarcoplasmic reticular Ca(2+), but did not alter the amplitude of the second phase, which represents rigor following ATP depletion. These results, in conjunction with those of previous studies, suggest that the depression in muscle tension following exposure to NaCN may result from alterations in Ca(2+) homeostasis, intracellular acidosis or from accumulation of one or more products of phosphocreatine breakdown.

Negative inotropic effect of heparin on tension development in rat skinned skeletal muscle fibres

Heparin inhibits inositol trisphosphate receptors, particularly in smooth muscle, but its effect on skeletal muscle is controversial. Our study showed that heparin induced a decrease in the amplitude of 10 mM caffeine-induced contracture in slow and fast saponin-skinned fibres. Moreover, measurements on Triton X-100-skinned fibres in soleus muscle showed that heparin alone decreased maximal Ca2(+)-activated tension and Ca2+ sensitivity of contractile proteins, whereas no significant effect was observed in extensor digitorum longus muscle. However, in the presence of caffeine, heparin decreased maximal Ca2(+)-activated tension in both muscles. It would appear that the heparin-induced decrease in the amplitude of caffeine contracture in rat skeletal muscle was not related to a direct inhibition of Ca2+ release from sarcoplasmic reticulum but to a desensitising effect of heparin and caffeine on myofilaments.

Cardiac-type excitation-contraction coupling in dysgenic skeletal muscle injected with cardiac dihydropyridine receptor cDNA

There are dihydropyridine (DHP)-sensitive calcium currents in both skeletal and cardiac muscle cells, although the properties of these currents are very different in the two cell types (for simplicity, we refer to currents in both tissues as L-type). The mechanisms of depolarization-contraction coupling also differ. As the predominant voltage-dependent calcium current of cardiac cells, the L-type current represents a major pathway for entry of extracellular calcium. This entry triggers the subsequent large release of calcium from the sarcoplasmic reticulum (SR). In contrast, depolarization of skeletal muscle releases calcium from the SR without the requirement for entry of extracellular calcium through L-type calcium channels. To investigate the molecular basis for these differences in calcium currents and in excitation-contraction (E-C) coupling, we expressed complementary DNAs for the DHP receptors from skeletal and cardiac muscle in dysgenic skeletal muscle. We compared the properties of the L-type channels produced and showed that expression of a cardiac calcium channel in skeletal muscle cells results in E-C coupling resembling that of cardiac muscle.

Activation and conductance properties of ryanodine-sensitive calcium channels from brain microsomal membranes incorporated into planar lipid bilayers

Rat brain microsomal membranes were found to contain high-affinity binding sites for the alkaloid ryanodine (kd 3 nM, Bmax 0.6 pmol per mg protein). Exposure of planar lipid bilayers to microsomal membrane vesicles resulted in the incorporation, apparently by bilayer-vesicle fusion, of at least two types of ion channel. These were selective for Cl- and Ca2+, respectively. The reconstituted Ca2+ channels were functionally modified by 1 microM ryanodine, which induced a nearly permanently open subconductance state. Unmodified Ca2+ channels had a slope conductance of almost 100 pS in 54 mM CaHEPES and a Ca2+/TRIS+ permeability ratio of 11.0. They also conducted other divalent cations (Ba2+ greater than Ca2+ greater than Sr2+ greater than Mg2+) and were markedly activated by ATP and its nonhydrolysable derivative AMP-PCP (1 mM). Inositol 1,4,5-trisphosphate (1-10 microM) partially activated the same channels by increasing their opening rate. Brain microsomes therefore contain ryanodine-sensitive Ca2+ channels, sharing some of the characteristics of Ca2+ channels from striated but not smooth muscle sarcoplasmic reticulum. Evidence is presented to suggest they were incorporated into bilayers following the fusion of endoplasmic reticulum membrane vesicles, and their sensitivity to inositol trisphosphate may be consistent with a role in Ca2+ release from internal membrane stores.

Excitation of skinned muscle fibers by imposed ion gradients. III. Distribution of permeant ions in unstimulated and stimulated fibers

Ion gradients imposed across an internal membrane system stimulate skinned muscle fibers; to evaluate the sarcoplasmic reticulum (SR) as the primary target site, SR polarization under resting and stimulatory conditions was assessed from fiber uptake of permeant probe ions. Solvent spaces were estimated from simultaneous [14C]urea (U) or [3H]deoxyglucose (DOG) uptake in segments of fibers from bullfrog semitendinosus muscle, skinned by microdissection. The distribution spaces, i.e., virtual solvent volumes at bath concentrations (Vu and VDOG), of these uncharged probes correlated well with the protein content of the same segments, which validated the tracer methodology for volume normalization. The membrane-bounded volume fraction (Vm), derived from the difference between total solvent volume (Vs) and the non-membrane-bounded solvent volume (Vc), was sufficient to detect appreciable SR ion accumulation. The Vm estimated from the difference between VU and VDOG assayed simultaneously with 2 or 5-6 min exposures was 10-11%, which is consistent with the morphometric volume fraction (mostly SR) in frog fibers; however, the change in this difference after membrane permeabilization corresponded to Vm only 5%. The change in permeant ion distribution space caused by member permeabilization was used to assess SR membrane polarization, assuming the free ions distribute across the intact membrane according to the Nernst ratio. Resting polarization (SR lumen positive) was assessed from [14C]SCN- or [14C]propionate- distribution spaces in unstimulated fibers, expressed relative to VDOG (assayed simultaneously). The ratios for (a) [14C]SCN- space (carrier 2 mM) and (b) [14C]propionate- space (carrier 120 mM) were not decreased by membrane permeabilization. This indicated that anion distribution was independent of membrane integrity and did not reflect an SR transmembrane potential, although a was more and b was less than 1. Polarization under stimulatory conditions (lumen negative) was assessed from 86Rb+ distribution, before and after an imposed ion gradient (choline Cl replacement of K methanesulfonate (KMes) at constant [K+] [Cl-]) that theoretically could generate a 48-fold transmembrane cation ratio; Ca release was minimized by EGTA. The ratio of 86Rb+ space to VU, greater than 1 in KMes (120 mM K, the effective carrier), was higher in choline Cl (2.5 mM K) but not decreased by membrane permeabilization; this indicated that 86Rb+ distribution did not reflect an SR transmembrane potential. Similar results in the presence of valinomycin ruled out the possibility of inadequate 86Rb+ equilibration.(ABSTRACT TRUNCATED AT 400 WORDS)

Restoration of excitation-contraction coupling and slow calcium current in dysgenic muscle by dihydropyridine receptor complementary DNA

Microinjection of an expression plasmid that carries complementary DNA encoding the receptor for dihydropyridine calcium channel blockers of skeletal muscle restores both excitation-contraction coupling and slow calcium current in cultured skeletal muscle cells from mice with muscular dysgenesis. This suggests that the dihydropyridine receptor in the transverse tubule membrane of skeletal muscle functions both as the voltage sensor for excitation-contraction coupling and as the slow calcium channel.

Voltage dependence of inositol 1,4,5-trisphosphate-induced Ca2+ release in peeled skeletal muscle fibers

Excitation-contraction coupling in skeletal muscle is known to be under absolute control of plasmalemma voltage, but the steps from transverse (T)-tubule depolarization to Ca2+ release from the sarcoplasmic reticulum have not been elucidated. The effect of changing T-tubule membrane potential on inositol 1,4,5-trisphosphate (InsP3) stimulation of Ca2+ release from the sarcoplasmic reticulum was studied to explore a possible role for InsP3 as a chemical signal in excitation-contraction coupling. InsP3 was microinjected into peeled rabbit skeletal muscle fibers at a pipette concentration of 0.5 microM; Ca2+ release from the sarcoplasmic reticulum was monitored as an isometric tension transient. The response to 0.5 microM InsP3 was significantly larger when T-tubules were in a depolarized state than when they were in a polarized state, and this difference in response was independent of the ionic composition of the bathing solutions or the method for depolarizing the T-tubules. Thus, T-tubule depolarization may sensitize the sarcoplasmic reticulum to a preexisting low concentration of InsP3 and greatly reduce the need for InsP3 production. Plasmalemma voltage control of the stimulatory effects of InsP3 may have relevance for mechanisms in excitable nonmuscle cells.

Effect of postnatal development on calcium currents and slow charge movement in mammalian skeletal muscle

Single- (whole-cell patch) and two-electrode voltage-clamp techniques were used to measure transient (Ifast) and sustained (Islow) calcium currents, linear capacitance, and slow, voltage-dependent charge movements in freshly dissociated fibers of the flexor digitorum brevis (FDB) muscle of rats of various postnatal ages. Peak Ifast was largest in FDB fibers of neonatal (1-5 d) rats, having a magnitude in 10 mM external Ca of 1.4 +/- 0.9 pA/pF (mean +/- SD; current normalized by linear fiber capacitance). Peak Ifast was smaller in FDB fibers of older animals, and by approximately 3 wk postnatal, it was so small as to be unmeasurable. By contrast, the magnitudes of Islow and charge movement increased substantially during postnatal development. Peak Islow was 3.6 +/- 2.5 pA/pF in FDB fibers of 1-5-d rats and increased to 16.4 +/- 6.5 pA/pF in 45-50-d-old rats; for these same two age groups, Qmax, the total mobile charge measurable as charge movement, was 6.0 +/- 1.7 and 23.8 +/- 4.0 nC/microF, respectively. As both Islow and charge movement are thought to arise in the transverse-tubular system, linear capacitance normalized by the area of fiber surface was determined as an indirect measure of the membrane area of the t-system relative to that of the fiber surface. This parameter increased from 1.5 +/- 0.2 microF/cm2 in 2-d fibers to 2.9 +/- 0.4 microF/cm2 in 44-d fibers. The increases in peak Islow, Qmax, and normalized linear capacitance all had similar time courses. Although the function of Islow is unknown, the substantial postnatal increase in its magnitude suggests that it plays an important role in the physiology of skeletal muscle.

Lipid composition of purified transverse tubule membranes isolated from amphibian skeletal muscle

The level and proportion of lipids and their fatty acid composition were analyzed in highly purified transverse tubule membranes of amphibian skeletal muscle. Tubule membranes show (a) a higher content of lipids, (b) a higher phospholipid/cholesterol ratio and (c) a different phospholipid composition from other subcellular fractions, such as the light and heavy membranes from sarcoplasmic reticulum, which are similar in lipid profile. Transverse tubule membranes are characterized by a high percentage of phosphatidylserine and sphingomyelin and a low proportion of phosphatidylcholine compared with the other membranes. All three show a high proportion of ethanolamine plasmalogens (50% of the total ethanolamine glycerophospholipid). Transverse tubule membrane lipids contain a high proportion of 20- and 22-carbon polyunsaturated fatty acids, predominantly 20:4, 20:5, 22:5 and 22:6. Arachidonate predominates in phosphatidylinositol, eicosapentaenoate and docosahexaenoate in ethanolamine and serine glycerophospholipids.

A lethal mutation in mice eliminates the slow calcium current in skeletal muscle cells

Contraction of a vertebrate skeletal muscle fibre is triggered by electrical depolarization of sarcolemmal infoldings termed transverse-tubules (t-tubules), which in turn causes the release of calcium from an internal store, the sarcoplasmic reticulum (SR). The mechanism that links t-tubular depolarization to SR calcium release remains poorly understood. In principle, this link might be provided by the prominent slow calcium current that has been described in skeletal muscle cells of adult frogs and rats. However, blocking this current does not abolish the depolarization-induced contractile responses of frog muscle, and the function of this slow calcium current is unknown. Here we describe measurements of calcium currents in developing skeletal muscle cells of normal rats and mice, and of mice with muscular dysgenesis, a mutation that causes excitation-contraction (E-C) coupling to fail. We find that a slow calcium current is present in skeletal muscle cells of normal animals but absent from skeletal muscle cells of mutant animals. The effect of the mutation is specific to the slow calcium current of skeletal muscle; a fast calcium current is present in developing skeletal muscle cells of both normal and mutant animals, and slow calcium currents are present in cardiac and sensory neurones of mutant animals. We believe this to be the first report of a mutation affecting calcium currents in a multicellular organism. The effects of the mutation raise important questions about the relationship between the slow calcium current and skeletal muscle E-C coupling.

Effect of halothane on Ca2+-induced Ca2+ release from sarcoplasmic reticulum vesicles isolated from rat skeletal muscle

Halothane induces the release of Ca2+ from a subpopulation of sarcoplasmic reticulum vesicles that are derived from the terminal cisternae of rat skeletal muscle. Halothane-induced Ca2+ release appears to be an enhancement of Ca2+-induced Ca2+ release. The low-density sarcoplasmic reticulum vesicles which are believed to be derived from nonjunctional sarcoplasmic reticulum lack the capability of both Ca2+-induced and halothane-induced Ca2+ release. Ca2+ release from terminal cisternae vesicles induced by halothane is inhibited by Ruthenium red and Mg2+, and require ATP (or an ATP analogue), KCl (or similar salt) and extravesicular Ca2+. Ca2+-induced Ca2+ release has similar characteristics.

Force and membrane potential in acetylcholine and potassium contractures of denervated mouse muscles

Depolarization and contracture force (P) provoked by acetylcholine (ACh) and by K ions were studied in bundles dissected from mouse soleus muscles that had been denervated for 4-7 days. Cl-free solutions were used. The muscle fibres were depolarized by solutions containing 150 mM K or 10 microM ACh to nearly zero mV resulting in maximum P (Pmax). Threshold P was produced when the membrane was depolarized to more than about -60 mV by both agents. 50% Pmax was produced by [K] causing the membrane to depolarize to -42 mV, whereas a potential more positive than -20 mV was required for 50% Pmax to be produced when ACh was used. The rate of depolarization was always higher for ACh than for K. Pretreatment by 0.05 microM ACh (about threshold for P) did not affect the P-[K] relation appreciably showing that ACh did not "stabilize" the membrane. Nearly equal P was provoked by successive applications of just suprathreshold agent concentrations when the order of application was ACh----K but not with the reverse order. Hypertonicity (by addition of 300 mM sucrose to all solutions) caused PACh to decrease and PK to increase. It was concluded that the ACh receptors are located in the surface membrane of the muscle fibres, not in the T-system membranes.

Extracellular ions and excitation-contraction coupling in frog twitch muscle fibres

Intracellular calcium transients were recorded from voltage-clamped frog twitch muscle fibres using Arsenazo III. The possible role of extracellular ions in excitation-contraction (e.-c.) coupling was examined using ion substitutions and blocking drugs in the bathing medium. Parameters measured included the Arsenazo response size to a standard depolarizing pulse (5 ms, 0 mV) and the strength-duration curve for threshold Arsenazo signal. Addition of tetrodotoxin (TTX) decreased the response size to small (-30 mV, 5 ms), but not large (+30 mV, 10 ms) depolarizations, probably because of poor voltage clamp of the tubular membrane in the absence of TTX. Clamping TTX-treated fibres with the wave form of a recorded action potential gave an Arsenazo response similar to that elicited by the normal action potential (at 10 degrees C). Complete substitution of sodium (by choline, lithium or Tris) or chloride (by methyl sulphate or maleate) in the bathing solution gave no appreciable changes in the size of the Arsenazo response. Reduction of extracellular free [Ca2+] to low levels using EGTA caused a slight reduction in the calcium signal elicited by the standard depolarization (to 74% after a few hours, and to 62% after 2 days; temperature 5-10 degrees C). The strength-duration curve was unchanged. Arsenazo responses about 75% of the control size could be elicited in high potassium solution (42 mM-K2SO4) by strong (+80 mV, 20 ms) depolarizations, after re-polarizing the fibres to -90 mV for a few minutes. The voltage dependence of activation was shifted to more positive potentials in this solution. Tetraethylammonium (TEA) bromide at a concentration of 20 mM did not alter the Arsenazo signal, whilst 120 mM-TEA reduced the response by 25%. 3,4-diaminopyridine (DAP) reduced the size of the Arsenazo signal at a concentration of 5 mM, and caused spontaneous release of calcium from the sarcoplasmic reticulum (s.r.) in the absence of membrane potential changes. The Arsenazo signal elicited by an action potential was enhanced by 1 mM-DAP, because of prolongation of the action potential, but was depressed by higher concentrations. We conclude that e.-c. coupling does not involve the influx of any external ions into the muscle fibre. If a current flow between the T-tubules and the s.r. is involved in e.-c. coupling, then this is probably carried by an efflux of potassium ions.

Model of calcium movements during activation in the sarcomere of frog skeletal muscle

A model of calcium movement during activation of frog skeletal muscle is described. The model was based on the half sarcomere of a myofibril and included compartments representing the terminal cisternae, the longitudinal sarcoplasmic reticulum, the extramyofibrillar space, and the myofibrillar space. The calcium-binding proteins troponin, parvalbumin, and calsequestrin were present in appropriate locations and with realistic binding kinetics. During activation a time-dependent permeability in the terminal cisternal wall led to calcium release into the myoplasm and its diffusion through the myoplasm longitudinally and radially was computed. After adjustment of three parameters, the model produced a myoplasmic free-calcium concentration that was very similar to those recorded experimentally with calcium indicators. The model has been used to demonstrate the importance of parvalbumin in the relaxation of skeletal muscle, to describe the time course and magnitude of calcium gradients associated with diffusion across the sarcomere, and to estimate the errors associated with the use of aequorin as an intracellular calcium indicator in muscle.

Tityus gamma toxin, a high affinity effector of the Na+ channel in muscle, with a selectivity for channels in the surface membrane

Toxin gamma from the venom of Tityus serrulatus scorpion produces a partial block of the surface Na+ channel in frog muscle. This block occurs with no change in the voltage-dependence or in the kinetics of the remaining surface Na+ current. The partial blockade of Na+ channel activity occurs with no change in tubular Na+ currents nor in twitch tension. The maximum effect of the toxin is attained at concentrations as low as 3 X 10(-10) M. Hyperpolarization to potentials more negative than the resting potential (E = -90 mV) reduces or abolishes the effect of the toxin. Radioiodinated toxin gamma binds to frog muscle membranes with a very high affinity corresponding to a dissociation constant of about 1 X 10(-11) M. Data obtained with both rabbit and frog muscle indicate that toxin gamma is specific for Na+ channels in surface membranes. Toxin gamma does not seem to bind to Na+ channels in T-tubule membranes. The biochemical data are in good agreement with electrophysiological studies and data on contraction. There is one Tityus gamma toxin binding site per tetrodotoxin binding site in surface membranes. Competition experiments have confirmed that Tityus gamma toxin binds to a new toxin receptor site on the Na+ channel structure. This site is the same that the toxin II from Centruroides suffusus binding site, but this toxin has 100 times less affinity for the Na+ channel than Tityus gamma toxin.

Evoked responses in normal and diseased muscle with particular reference to twitch potentiation

The compound muscle action potential and isometric twitch tension evoked by single and repetitive electrical stimuli are indicators of the number of motor units activated and of the contractile properties of the muscle. The action potentials and mechanical responses were recorded in proximal and distal muscles in patients with myasthenia gravis and myopathy and compared with findings in normal subjects. In normal muscle, at low rates of stimulation (2-3 s-1) the decrement was at most 5% in the action potential and 15-24% in the twitch tension. Tetanic stimuli (50 s-1) were unsuitable for diagnostic purposes because of movement artefact. In patients with myasthenia gravis, the incidence and size of the decrement of evoked responses were greater in the platysma than in the elbow flexors and the adductor pollicis (ADP) muscles. The 2-3 times greater post-tetanic facilitation (PTF) of the action potential in the platysma than in extremity muscles also indicates a more severe functional block in facial muscle. The PTF is an indicator of recruitment of blocked fibres. The maximal decrement was grossly related to the titre of antibodies against the acetylcholine receptor. To reveal failure of neuromuscular transmission in patients with myasthenia gravis without a decrement, a small dose of d-tubocurarine (0.2 mg in 30 ml of saline) was injected i.v. in the upper arm in a regional curare test. The sensitivity was greater in patients with myasthenia gravis than in controls and in patients with myopathy. Potentiation of twitch tension reflects contractile properties. In normal muscle twitch potentiation in the staircase (1-3 s-1, 100 s in duration) and after tetanus (50 s-1, 1.5 s in duration) was 2-3 times greater in the platysma than in the elbow flexors and ADP, presumably related to the greater proportion of fast-twitch fibers in facial muscle. The amplitude of the action potential and the twitch tension varied proportionally with the number of fibers activated and the difference in the decrements of the action potential and the twitch during the staircase in some patients with myasthenia gravis showed that the staircase phenomenon was diminished suggesting abnormalities in the excitation-contraction coupling. The diminution of the staircase and post-tetanic potentiation (PTP) in myopathy also indicates impairment of the excitation-contraction coupling. In rats with severe chronic myasthenia gravis, the staircase and PTP were decreased even when the failing neuromuscular transmission was circumvented by applying direct stimuli to the extensor digitorum longus muscle (EDL).

Ion conductances of the surface and transverse tubular membranes of skeletal muscle

A combination voltage clamp and admittance analysis of single skeletal muscle fibers showed that moderate depolarizations activated a steady-state negative sodium conductance in both the surface and transverse tubular membranes. The density of the voltage-dependent channels was similar for the surface and tubular conductances. The relaxation times associated with the negative conductance were in the millisecond range and markedly potential dependent. The negative tubular conductance has the consequence of increasing the apparent steady-state radial space constant to large values. This occurs because the positive conductance is counterbalanced by the maintained inward-going sodium current. The enhancement of the space constant by a negative conductance provides a means for the nearly simultaneous activation of excitation-contraction coupling.

Calcium transients studied under voltage-clamp control in frog twitch muscle fibres

1. Intracellular calcium transients were recorded from frog twitch muscle fibres in response to voltage-clamped depolarizing pulses, using arsenazo III as an intracellular calcium monitor. The object was to investigate the time- and voltage-dependent characteristics of the coupling process between membrane depolarization and calcium release from the sarcoplasmic reticulum (s.r.)2. To examine the extent to which the T-tubule membrane potential was controlled during clamp pulses, the dye NK 2367 was used as an optical probe of tubular potential. This indicated that the tubular time constant is about 0.6 msec.3. Strength-duration curves were obtained for depolarizing pulses required to give both threshold mechanical contraction and calcium signal. Curves measured in these two ways were closely similar.4. Changes in holding potential altered the strength-duration curve for calcium release so that at more positive holding potentials a shorter pulse was needed to obtain a response for any given pulse amplitude.5. A latency of a few milliseconds was observed between the onset of depolarization and the initial rise of the calcium signal. This became shorter with stronger depolarizations, but approached a minimum at potentials above about +25 mV.6. Subthreshold depolarizations applied before a test pulse increased the size and decreased the latency of the calcium signal. Conditioning hyperpolarizations had opposite effects.7. The rate of build-up of potentiation or depression of response size seen with subthreshold de- and hyperpolarizing conditioning pulses was examined using conditioning pulses of different durations. For both pulses this process showed a time constant of about 3 msec (at 10 degrees C).8. The rate of decay of potentiation or depression was similarly measured, using a gap of variable duration between conditioning and test pulses. For both de- and hyperpolarizing pulses this showed a time constant of about 5 msec (10 degrees C).9. The relationship between conditioning pulse potential, and the size of calcium signal elicited by a following test pulse was non-linear.10. Subthreshold pulses immediately following a brief test pulse affected the size of the calcium signal in a similar way to preceding conditioning pulses.11. The relationship between potential and size of the calcium signal was examined using pulses of 3 and 20 msec duration. With the long pulse the relation was roughly sigmoid, but with the short pulse continued to rise even at strongly positive potentials.12. The results are discussed in terms of a model in which the exponential build-up of a hypothetical coupler in the excitation-contraction (e.-c.) coupling process is presumed to lead to calcium release when a threshold level is exceeded.

Differences in the properties of Na+ channels in muscle surface and T-tubular membranes revealed by tetrodotoxin derivatives

The effect of ethylenediamine derivatives of tetrodotoxin (enTTXI and enTTXII) on frog skeletal muscle was studied both electrophysiologically and biochemically. Electrophysiological experiments with one of these molecules (enTTXI) showed that the concentrations needed to block the early phase of the inward sodium current (K0.5 = 7 nM) are much lower than those needed to block the late phase of inward current or muscle contraction (K0.5 = 40 nM). Conversely, tubular Na+ channels are more sensitive to enTTXII than are surface Na+ channels. Toxin binding to isolated muscle membranes was studied using 3H-enTTXI and 3H-enTTXII. The first derivative (3H-enTTXI) has a higher affinity Kd = 8 nM) for Na+ channels in the surface membrane than for Na+ channels in the T-tubular membrane (Kd greater than 20 nM). In contrast 3H-enTTXII has a higher affinity for the tubular Na+ channel (Kd = 0.2 nM) than for the receptor in surface membranes (Kd = 4nM). We conclude that Na+ channels in muscle surface and T-tubular membranes have different toxin-binding properties, which must reflect a difference in molecular structure.

Calcium efflux during the cold-induced contraction of mammalian striated muscle fibres

The efflux of 45Ca from mammalian slow twitch muscle fibres has been studied to provide a measure of the concentration of free Ca2+ in the sarcoplasm. The kinetically complex early phases of washout of the isotope are succeeded by a prolonged slower phase which exhibits first-order kinetics. This later phase is accelerated by caffeine, by preventing oxidative phosphorylation and also during an isometric contraction, whether this contraction is produced by lowering the temperature or by electrical stimulation. The local anaesthetic tetracaine abolishes the contraction caused by cold and in this case the rate constant for efflux is progressively lowered as the temperature is reduced (Q10 value of 2.3). The removal of external Na+ and Ca2+ reduces the efflux rate constant. Caffeine, sodium removal and the inhibition of oxidative phosphorylation, all potentiate the cold contraction and the associated extra 45Ca efflux. Ca removal causes the cold contraction to become phasic. It appears that caffeine, sodium removal, the inhibition of oxidative phosphorylation and a decrease in temperature to below 10 degrees C are all treatments which, like electrical stimulation, increase the sarcoplasmic free calcium concentration to varying degrees.

Calcium uptake into acini from rat pancreas: evidence for intracellular ATP-dependent calcium sequestration

Intracellular ATP-dependent Ca2+-sequestration mechanisms were studied in isolated dispersed rat pancreatic acini following treatment with saponin or digitonin to disrupt their plasma membranes. In the presence of 45Ca2+ concentrations less than 10(-6) mol/liter, addition of 5 mmol/liter ATP caused a rapid increase in 45Ca2+ uptake exceeding the control by fivefold. ADP mimicked the ATP effect by 50 to 60%, whereas other nucleotides such as AMP-PNP, AMP-PCP, CTP, UTP, ITP, GTP, cAMP and cGMP did not. Maximal ATP-promoted Ca2+ uptake was obtained at 10(-5) mol/liter Ca2+. Inhibition of Ca2+ uptake by mitochondrial inhibitors was dependent on the Ca2+ concentration, indicating the presence of different Ca2+ storage systems. Whereas the apparent half-saturation constant found for mitochondrial Ca2+ uptake was approximately 4.5 X 10(-7) mol/liter, in the presence of antimycin and oligomycin (nonmitochondrial uptake) it was approximately 1.4 X 10(-8) mol/liter. In the absence of Mg2+ both ATP- and ADP-promoted Ca2+ uptake was nearly abolished. The Ca2+ ionophore and mersalyl blocked Ca2+ uptake, Electron microscopy showed electron-dense precipitates in the rough endoplasmic reticulum of saponin-treated cells in the presence of Ca2+, oxalate and ATP, which were absent in intact cells and in saponin-cells without ATP or pretreated with A23187. The data suggest the presence of mitochondrial and nonmitochondrial ATP-dependent C2+ storage systems in pancreatic acini. The latter is likely to be located in the rough endoplasmic reticulum.

Identification of a constituent of the junctional feet linking terminal cisternae to transverse tubules in skeletal muscle

This study describes the biochemical composition of junctional feet in skeletal muscle utilizing a fraction of isolated triad junctions. [3H]Ouabain entrapment was employed as a specific marker for T-tubules. The integrity of the triad junction was assayed by the isopycnic density of [3H]ouabain activity (24-30% sucrose for free T-tubules, 38-42% sucrose for intact triads). Trypsin, chymotrypsin, and pronase all caused separation of T-tubules from terminal cisternae, indicating that the junction is composed as least in part of protein. Trypsin and chymotrypsin hydrolyzed four proteins: the Ca2+ pump, a doublet 325,000, 300,000, and an 80,000 Mr protein. T-tubules which had been labeled covalently with 125I were joined to unlabeled terminal cisternae by treatment with K cacodylate. The reformed triads were separated from free T-tubules and then severed by passage through a French press. When terminal cisternae were separated from T-tubules, some 125I label was transferred from the labeled T-tubules to the unlabeled terminal cisternae. Gel electrophoresis showed that, although T-tubules were originally labeled in a large number of different proteins, only a single protein doublet was significantly labeled in the originally unlabeled terminal cisternae. This protein pair had molecular weights of 325,000 and 300,000 daltons. Transfer of label did not occur to a substantial degree without K cacodylate treatment. We propose that the transfer of 125I label from T-tubules to terminal cisternae during reformation and breakage of the triad junction is a property of the protein which spans the gap between T-tubules and terminal cisternae.

Calcium transients evoked by action potentials in frog twitch muscle fibres

1. Intracellular Ca(2+) transients were recorded from frog twitch muscle fibres in response to action potentials and repetitive stimulation, using ionophoretically injected arsenazo III as a Ca(2+) monitor. A dual wave-length optical system was used to measure absorbance changes of the injected dye from small areas of single fibres within the cutaneous pectoris muscle.2. The absorbance spectrum of the injected arsenazo III in a resting fibre was consistent with an intracellular free Mg(2+) level of a few hundred micromolar, assuming an intracellular pH of 7.1. The resting free Ca(2+) concentration was below the limit of resolution.3. The wave-length dependence of the arsenazo light absorbance signal during twitches followed that expected for Ca(2+) binding to the dye. Recordings made at wave-lengths where the dye is maximally sensitive to pH or Mg(2+) concentration changes indicated that interference from these sources is minimal at the usual wave-length pair (650-700 nm) used for Ca(2+) recordings.4. Over a wide range of intracellular dye concentrations, the size of the arsenazo response to an action potential increased linearly with dye concentration (100-1000 muM), although there were deviations from this relationship at low and high concentrations.5. An approximate estimate of 8 muM was obtained for the peak free Ca(2+) concentration change following a single action potential. Changes in temperature (6-25 degrees C) did not significantly affect the size of the free Ca(2+) transient. During maximal tetanic stimulation the signal rose to about three times higher than the twitch response. An approximate minimum estimate of the increase in total cytoplasmic Ca(2+) concentration during a twitch gave a value of 220 muM.6. A latency of about 1.5 ms (at 10 degrees C) was observed between the foot of an action potential and the onset of the arsenazo response. Recordings made using a narrow measuring light slit, placed either at the edge or the centre of a fibre, suggested that only a small part of this latency could be due to inward conduction of the action potential along the T-tubules.7. The decay phase of the arsenazo response to an action potential followed an exponential time course, with a time constant of 71 ms at 10 degrees C. This time constant was strongly temperature-dependent, with a Q(10) of about 2.4. An Arrhenius plot of the decay time constant gave a straight line.8. During repetitive stimulation, the arsenazo responses evoked by successive impulses showed two changes: a progressive decrease in amplitude and a slowing of the decay. The extent to which successive responses summated during a tetanus depended upon the balance between these two effects.

Calcium transients in normal and denervated slow muscle fibres of the frog

1. Intracellular changes in free Ca2+ concentration were recorded from slow muscle fibres in the pyriformis muscle of Rana temporaria, using the dye arsenazo III. Fibres were voltage clamped, and arsenazo signals were recorded in response to depolarizing pulses. 2. The size of the arsenazo response to depolarizing pulses of 100 msec duration was a sigmoid function of membrane potential over the range -45 to 0 mV, and remained constant with further depolarizations up to +100 mV. 3. The peak size of the arsenazo signal to supramaximal depolarizations increased with increasing pulse length. The initial rising phase during a pulse was much slower than in twitch fibres, and this phase was followed by an even slower rise. Following short pulses the decay of the response was exponential, with a time constant of about 1.4 sec, while after long pulses the decline became much slower. 4. Decreasing free Ca2+ concentration in the bathing medium to very low levels, using EGTA , did not affect the responses to short (100 msec) depolarizations. 5. Slow fibres bathed in Ringer's solution containing 12 mM-Ca2+ showed a well maintained arsenazo response to supramaximal depolarizations lasting over 1 min. Reduction of external Ca2+ to 1.8 and (nominally) 0 mM caused the response to become progressively more transient. 6. After denervation, slow fibres developed action potentials, but non of the parameters of the arsenazo response was significantly changed. During the early phase of reinnervation by a mixed nerve, when fast conduction axons begin to innervate slow fibres, the ability to give a maintained response during long depolarizations was reduced. 7. It is concluded that intracellular Ca2+ transients in slow muscle fibres are probably generated by a similar mechanism as in twitch fibres and entry of external Ca2+ is not an appreciable factor. The slow time course of the transients may be important in determining the time courses of tension development and relaxation.

Calcium release and ionic changes in the sarcoplasmic reticulum of tetanized muscle: an electron-probe study

Approximately 60-70% of the total fiber calcium was localized in the terminal cisternae (TC) in resting frog muscle as determined by electron-probe analysis of ultrathin cryosections. During a 1.2 s tetanus, 59% (69 mmol/kg dry TC) of the calcium content of the TC was released, enough to raise total cytoplasmic calcium concentration by approximately 1 mM. This is equivalent to the concentration of binding sites on the calcium-binding proteins (troponin and parvalbumin) in frog muscle. Calcium release was associated with a significant uptake of magnesium and potassium into the TC, but the amount of calcium released exceeded the total measured cation accumulation by 62 mEq/kg dry weight. It is suggested that most of the charge deficit is apparent, and charge compensation is achieved by movement of protons into the sarcoplasmic reticulum (SR) and/or by the movement of organic co- or counterions not measured by energy dispersive electron-probe analysis. There was no significant change in the sodium or chlorine content of the TC during tetanus. The unchanged distribution of a permeant anion, chloride, argues against the existence of a large and sustained transSR potential during tetanus, if the chloride permeability of the in situ SR is as high as suggested by measurements on fractionated SR. The calcium content of the longitudinal SR (LSR) during tetanus did not show the LSR to be a major site of calcium storage and delayed return to the TC. The potassium concentration in the LSR was not significantly different from the adjacent cytoplasmic concentration. Analysis of small areas of I-band and large areas, including several sarcomeres, suggested that chloride is anisotropically distributed, with some of it probably bound to myosin. In contrast, the distribution of potassium in the fiber cytoplasm followed the water distribution. The mitochondrial concentration of calcium was low and did not change significantly during a tetanus. The TC of both tetanized and resting freeze-substituted muscles contained electron-lucent circular areas. The appearance of the TC showed no evidence of major volume changes during tetanus, in agreement with the estimates of unchanged (approximately 72%) water content of the TC obtained with electron-probe analysis.

The effect of dantrolene on the enhancement and diminution of tension evoked by staircase and by tetanus in rat muscle

1. The effect of Dantrolene on the potentiation of isometric twitch tension was examined during and after the staircase (5/sec, 250 stimuli) and after the tetanus (167/sec, 250 stimuli) in the extensor digitorum longus muscle of adult Lewis rats at 37-38 degrees C.2. The study confirmed that Dantrolene decreased the twitch tension much more than the tetanic tension. The drug shortened the contraction time of the twitch. The rate of force development of the twitch was diminished by only half that of the twitch tension. The findings may suggest that the drug both shortened and diminished the activation of the muscle during the twitch.3. Dantrolene decreased the potentiation produced by a given number of stimuli early in the staircase. At the 250th stimulus the staircase was about 25% larger after than before application of the drug. Dantrolene increased the potentiation 2 sec after the tetanus by about 60%.4. After application of Dantrolene, the decay of potentiation after the staircase indicated that the process that diminished the twitch during the staircase, present before application of the drug, was absent. The size of the process causing potentiation was the same with and without the drug. Both events of potentiation after the tetanus were increased by Dantrolene. Both after the staircase and after the tetanus, Dantrolene increased the slow phase of decay by 60-70%. The fast rate of decay after the tetanus was unchanged by the drug.5. The contraction time was prolonged less in the potentiated twitch after than before application of Dantrolene. This was presumably due to a greater relative increase in activation rather than to a prolongation of the time during which the muscle was activated.6. A model is proposed where the delay in potentiation during the staircase and the increase in potentiation after the tetanus are due to the proportion of sites in the excitation-contraction coupling occupied by Dantrolene being reduced by repetitive depolarizations of the transverse tubules during trains of repetitive stimuli.

Temperature dependence of enhancement and diminution of tension evoked by staircase and by tetanus in rat muscle

1. The effect of temperature (20-37.5 degrees C) on the potentiation of twitch tension was examined during and after the staircase (250 stimuli, 5/sec) and after the tetanus (188 stimuli, 125/sec) in the extensor digitorum longus muscle of adult Lewis rats.2. During the staircase at 20 degrees C the twitch tension decreased (negative staircase) by 10-20%. At 25-30 degrees C the staircase was initially negative and later positive. At 37.5 degrees C the staircase was positive throughout the train. Both at the end of the staircase and 2 sec after the tetanus the potentiation increased linearly with increasing temperature.3. After the staircase and the tetanus at 20-30 degrees C the twitch tension increased initially rapidly and later after the staircase at a slower rate. Maximal potentiation at 20 degrees C was attained 3 min after the staircase (+ 30 +/- 3%, n = 10, s.e. of mean) and 1 min after the tetanus (+ 16 +/- 1%, n = 10, s.e. of mean). At 37.5 degrees C the potentiation decayed rapidly after the staircase and the tetanus.4. During the staircase the time course of the twitch was shortened twice as much at 20 as at 37.5 degrees C. At the end of the staircase and 2 sec after the tetanus the contraction time was the more prolonged the greater the potentiation. At maximal potentiation the contraction time was prolonged three times as much at 20 degrees C (+ 19 +/- 3%, n = 10, s.e. of mean) as at 37.5 degrees C (P < 0.005). The half-relaxation time at the end of the staircase was prolonged 10 times more at 20 than at 37.5 degrees C (P < 0.02).5. When extrapolated to time zero after the staircase and the tetanus the potentiation at 20 degrees C was still marked (20-50%). The rate of decay of potentiation (time constant, 20 degrees C, 561.2 +/- 37.4 sec, n = 20, s.e. of mean) increased with increasing temperature (Q(10) = 2.6). The event of potentiation with a fast rate of decay, present after the tetanus but not after the staircase at 37.5 degrees C, was abolished below 30 degrees C.6. The increase in twitch tension after the staircase and the tetanus at 20-30 degrees C was taken to indicate the recovery of events that diminished the twitch, occurring simultaneously with potentiation.7. (i) One process of diminution, present after the staircase but not after the tetanus, increased on cooling and was assumed to be due to fatigue. The rate of recovery of the process (time constant, 20 degrees C, 79.6 +/- 7.4 sec, n = 10, s.e. of mean) increased with increasing temperature (Q(10) = 1.9). The half-relaxation time of the last twitch in the staircase was the more prolonged the greater the process. (ii) Another process causing diminution was present after the staircase and the tetanus at 20-30 degrees C. It recovered at 20 degrees C with a time constant of 14.9 +/- 2.2 sec (n = 10, s.e. of mean). This process, possibly responsible for the initially negative staircase, was not thought to be due to fatigue. It may reflect a diminished depolarization of the transverse tubules by repetitive stimuli.

Mechanical activation in slow and twitch skeletal muscle fibres of the frog

1. Slow and twitch muscle fibres of the frog were studied with a two-micro-electrode point voltage-clamp method. Slow fibres were identified in pyriformis and cruralis muscles by their appearance in the light microscope, electrical characteristics, and rate of sarcomere shortening or of tension development. 2. The relation between the amplitude and duration of threshold depolarizing pulses was determined in sartorius twitch and pyriformis slow fibres. Strength-duration relations for contractile activation are very similar in the two fibre types. 3. The effect of a brief subthreshold pulse on the threshold voltage level decays with a half-time of 1-2 msec at 9 degrees C in both slow and twitch fibres. This fast decay, thought to reflect voltage-dependent deactivation of Ca2+ release following repolarization, is followed by a slower decay of greatly different rates in the two fibre types. the slower components of decay might reflect the rate of background Ca2+ removal by the sarcoplasmic reticulum. 4. Reducing external Ca2+ levels to about about 0.1 microM with 2.5 mM-EGTA has no effect on the shapes of strength-duration curves for both slow and twitch fibres, suggesting that activator Ca2+ in both fibre types originates entirely from intracellular stores. 5. "Tonic' contractions were studied using voltage-clamped short cruralis slow fibres at 20 degrees C. Reducing external Ca2+ to about 0.1 microM had no effect on the steepness of the steady-state tension-voltage relation or on the ability of slow fibres to maintain maximal tension during long (200 sec) depolarizations to membrane potentials of up to +50 mV. 6. Functional similarities in activation kinetics of slow and twitch fibres are discussed in relation to the sensing of tubular membrane potential by the sarcoplasmic reticulum, to Ca2+ release from it, and to possible mechanisms involved in these processes. Processes leading to the rapid turning on and off of Ca2+ release in response to changes in tubular membrane potential are probably similar in slow and twitch fibres. However, the apparent lack of voltage-and time-dependent inactivation of Ca2+ release in slow fibre points to a major difference in the two types of muscle.

Optical probe responses on sarcoplasmic reticulum: oxacarbocyanines as probes of membrane potential

The relationship between Ca2+ fluxes and the ion diffusion potential was analyzed on sarcoplasmic reticulum membranes using oxacarbocyanine dyes as optical probes for membrane potential. 3.3'-Diethyloxodicarbocyanine responds to ATP-induced Ca2+ uptake by isolated sarcoplasmic reticulum vesicles with a decrease in absorbance at 600 nm. The optical change is reversed during Ca2+ release from sarcoplasmic reticulum induced by KCl or by ADP and inorganic phosphate. The absorbance changes are largely attributable to the binding of accumulated Ca2+ to the membrane. There is no indication that sustained changes in membrane diffusion potential would accompany pump-mediated Ca2+ fluxes. A large change in the absorbance of 3,3'-diethyloxodicarbocyanine was observed on sarcoplasmic reticulum vesicles under the influence of membrane potential generated by valinomycin in the presence of a K+ gradient or by ionophore A23187 in the presence of a Ca2+ gradient. The maximum of the potential-dependent absorbance change is at 575--580 nm. The potentials generated by valinomycin or ionophore A23187 are short-lived due to the high permeability of sarcoplasmic reticulum membranes for cations and anions. There is no correlation between the direction and magnitude of the artifically imposed membrane potential and the rate of Ca2+ uptake or release by isolated sarcoplasmic reticulum vesicles.

Delayed rectification in the transverse tubules: origin of the late after-potential in frog skeletal muscle

Tetanic stimulation of skeletal muscle fibers elicits a train of spikes followed by a long-lasting depolarization called the late after-potential (LAP). We have conducted experiments to determine the origin of the LAP. Isolated single muscle fibers were treated with a high potassium solution (5 mM or 10 mM K) followed by a sudden reduction of potassium concentration to 2.5 mM. This procedure produced a slow repolarization (K repolarization), which reflects a diffusional outflow of potassium from inside the lumen of the transverse tubular system (T system). Tetanic stimulation was then applied to the same fiber and the LAP was recorded. The time courses of K repolarization and LAP decay were compared and found to be roughly the same. This approximate equality held under various conditions that changed the time courses of both events over a wide range. Both K repolarization and the LAP became slower as fiber radius increased. These results suggest that LAP decay and K repolarization represent the same process. Thus, we conclude that the LAP is caused by potassium accumulation in the T system. A consequence of this conclusion is that delayed rectification channels exist in the T system. A rough estimation suggests that the density of delayed rectification channels is less in the T system than in the surface membrane.

Electrical properties of frog skeletal muscle fibers interpreted with a mesh model of the tubular system

This paper presents the construction, derivation, and test of a mesh model for the electrical properties of the transverse tubular system (T-system) in skeletal muscle. We model the irregular system of tubules as a random network of miniature transmission lines, using differential equations to describe the potential between the nodes and difference equations to describe the potential at the nodes. The solution to the equations can be accurately represented in several approximate forms with simple physical and graphical interpretations. All the parameters of the solution are specified by impedance and morphometric measurements. The effect of wide circumferential spacing between T-system openings is analyzed and the resulting restricted mesh model is shown to be approximated by a mesh with an access resistance. The continuous limit of the mesh model is shown to have the same form as the disk model of the T-system, but with a different expression for the tortuosity factor. The physical meaning of the tortuosity factor is examined, and a short derivation of the disk model is presented that gives results identical to the continuous limit of the mesh model. Both the mesh and restricted mesh models are compared with experimental data on the impedance of muscle fibers of the frog sartorius. The derived value for the resistivity of the lumen of the tubules is not too different from that of the bathing solution, the difference probably arising from the sensitivity of this value to errors in the morphometric measurements.