Ca2+ and activation mechanisms in skeletal muscle

Voltage-Dependent Ca2+ Release Is Impaired in Hypokalemic Periodic Paralysis Caused by CaV1.1-R528H but not by NaV1.4-R669H

Hypokalemic periodic paralysis (HypoPP) is a channelopathy of skeletal muscle caused by missense mutations in the voltage sensor domains (usually at an arginine of the S4 segment) of the Ca_V1.1 calcium channel or of the Na_V1.4 sodium channel. The primary clinical manifestation is recurrent attacks of weakness, resulting from impaired excitability of anomalously depolarized fibers containing leaky mutant channels. While the ictal loss of fiber excitability is sufficient to explain the acute episodes of weakness, a deleterious change in voltage sensor function for Ca_V1.1 mutant channels may also compromise excitation-contraction coupling (EC-coupling). We used the low-affinity Ca²⁺ indicator OGN-5 to assess voltage-dependent Ca²⁺-release as a measure of EC-coupling for our knock-in mutant mouse models of HypoPP. The peak in fibers isolated from Ca_V1.1-R528H mice was about two-thirds of the amplitude observed in WT mice; whereas in HypoPP fibers from Na_V1.4-R669H mice the was indistinguishable from WT. No difference in the voltage dependence of from WT was observed for fibers from either HypoPP mouse model. Because late-onset permanent muscle weakness is more severe for Ca_V1.1-associated HypoPP than for Na_V1.4, we propose the reduced Ca²⁺-release for Ca_V1.1-R528H mutant channels may increase the susceptibility to fixed myopathic weakness. In contrast the episodes of transient weakness are similar for Ca_V1.1- and Na_V1.4-associated HypoPP, consistent with the notion that acute attacks of weakness are primarily caused by leaky channels and are not a consequence of reduced Ca²⁺-release.

Ingestion of a Nitric Oxide Enhancing Supplement Improves Resistance Exercise Performance

Mosher, SL, Sparks, SA, Williams, EL, Bentley, DJ, and Mc Naughton, LR. Ingestion of a nitric oxide enhancing supplement improves resistance exercise performance. J Strength Cond Res 30 (12): 3520-3524, 2016-Studies have established that supplementation of nitrate increases nitric oxide which in turn improves exercise performance. The current study aimed to investigate the effects of nitrate ingestion on performance of bench press resistance exercise until failure. Twelve recreationally active (age, 21 ± 2 years, height, 177.2 ± 4.0 cm, weight, 82.49 ± 9.78 kg) resistance-trained men participated in the study. The study used a double-blind, randomized cross-over design, where participants ingested either 70 ml of "BEET It Sport" nitrate shot containing 6.4 millimoles (mmol·L) or 400 mg of nitrate or a blackcurrant placebo drink. Participants completed a resistance exercise session, consisting of bench press exercise at an intensity of 60% of their established 1 repetition maximum (1RM), for 3 sets until failure with 2 minutes rest interval between sets. The repetitions completed, total weight lifted, local and general rate of perceived exertion (RPE), and blood lactate were all measured. The results showed a significant difference in repetitions to failure (p ≤ 0.001) and total weight lifted (p ≤ 0.001). However, there were no significant difference between blood lactate over the 2 trials (p = 0.238), and no difference in Local (p = 0.807) or general (p = 0.420) indicators of fatigue as measured by RPE. This study demonstrates that nitrate supplementation has the potential to improve resistance training performance and work output compared with a placebo.

Mechanical signaling via nonlinear wavefront propagation in a mechanically excitable medium

Models that invoke nonlinear wavefront propagation in a chemically excitable medium are rife in the biological literature. Indeed, the idea that wavefront propagation can serve as a signaling mechanism has often been invoked to explain synchronization of developmental processes. In this paper we suggest a kind of signaling based not on diffusion of a chemical species but on the propagation of mechanical stress. We construct a theoretical approach to describe mechanical signaling as a nonlinear wavefront propagation problem and study its dependence on key variables such as the effective elasticity and damping of the medium.

A coupled electromechanical model for the excitation-dependent contraction of skeletal muscle

This work deals with the development and implementation of an electromechanical skeletal muscle model. To this end, a recently published hyperelastic constitutive muscle model with transversely isotropic characteristics, see Ehret et al. (2011), has been weakly coupled with Ohm's law describing the electric current. In contrast to the traditional way of active muscle modelling, this model is rooted on a non-additive decomposition of the active and passive components. The performance of the proposed modelling approach is demonstrated by the use of three-dimensional illustrative boundary-value problems that include electromechanical analysis on tissue strips. Further, simulations on the biceps brachii muscle document the applicability of the model to realistic muscle geometries.

Force relaxation and thin filament protein phosphorylation during acute myocardial ischemia

Ischemia impairs myocardial function and may contribute to the progression of heart failure. In this study, rats subjected to acute ischemia demonstrated reduced Ca(2+) -activated force as well as a decrease in myosin-binding protein-C, titin, and Ser23/24 phosphorylation of troponin I (TnI). All three proteins have been demonstrated to be downstream targets of β-adrenergic receptor activation (β-AR), leading to the hypothesis that decreased β-AR signaling during ischemia leads to reduced protein phosphorylation and reduced rate constants of force relaxation. To test this hypothesis, force relaxation transients were recorded from permeabilized perfused and ischemic rat heart fibers following photolysis of the caged chelator diazo-2. Relaxation transients were best fit by double exponential functions whereby the majority (>70%) of the force decline was described by the fast rate constant, which was ∼5 times faster than the slow rate constant. However, rate constants of relaxation between perfused and ischemic fibers were not different, despite significant decreases in sarcomeric protein phosphorylation in ischemic fibers. Treatment of perfused fibers with a cAMP analog increased Ser23/24 phosphorylation of TnI, yet the rate constants of relaxation remained unchanged. Interestingly, similar treatment of ischemic fibers did not impact TnI phosphorylation or force relaxation transients. Therefore, acute ischemia does not influence the rate constants of relaxation of permeabilized fibers. These results also suggest that the physiological level of sarcomeric protein phosphorylation is unlikely to be the primary driver of relaxation kinetics in permeabilized cardiac muscle fibers.

Mutations in Troponin that cause HCM, DCM AND RCM: what can we learn about thin filament function?

Troponin (Tn) is a critical regulator of muscle contraction in cardiac muscle. Mutations in Tn subunits are associated with hypertrophic, dilated and restrictive cardiomyopathies. Improved diagnosis of cardiomyopathies as well as intensive investigation of new mouse cardiomyopathy models has significantly enhanced this field of research. Recent investigations have showed that the physiological effects of Tn mutations associated with hypertrophic, dilated and restrictive cardiomyopathies are different. Impaired relaxation is a universal finding of most transgenic models of HCM, predicted directly from the significant changes in Ca(2+) sensitivity of force production. Mutations associated with HCM and RCM show increased Ca(2+) sensitivity of force production while mutations associated with DCM demonstrate decreased Ca(2+) sensitivity of force production. This review spotlights recent advances in our understanding on the role of Tn mutations on ATPase activity, maximal force development and heart function as well as the correlation between the locations of these Tn mutations within the thin filament and myofilament function.

Perturbing plasma membrane hemichannels attenuates calcium signalling in cardiac cells and HeLa cells expressing connexins

Many cell signalling pathways are driven by changes in cytosolic calcium. We studied the effects of a range of inhibitors of connexin channels on calcium signalling in cardiac cells and HeLa cells expressing connexins. Gap 26 and 27, peptides that mimic short sequences in each of the extracellular loops of connexin 43, and anti-peptide antibodies generated to extracellular loop sequences of connexins, inhibited calcium oscillations in neonatal cardiac myocytes, as well as calcium transients induced by ATP in HL-1 cells originating from cardiac atrium and HeLa cells expressing connexin 43 or 26. Comparison of single with confluent cells showed that intracellular calcium responses were suppressed by interaction of connexin mimetic peptides and antibodies with hemichannels present on unapposed regions of the plasma membrane. To investigate how inhibition of hemichannels in the plasma membrane by the applied reagents was communicated to calcium store operation in the endoplasmic reticulum, we studied the effect of Gap 26 on calcium entry into cells and on intracellular IP3 release; both were inhibited by Gap 26. Calcium transients in both connexin 43- and connexin 26-expressing HeLa cells were inhibited by the peptides suggesting that the extended cytoplasmic carboxyl tail domain of larger connexins and their interactions with intracellular scaffolding/auxiliary proteins were unlikely to feature in transmitting peptide-induced perturbations at hemichannels in the plasma membrane to IP3 receptor channel central to calcium signalling. The results suggest that calcium levels in a microenvironment functionally connecting plasma membrane connexin hemichannels to downstream IP3-dependent calcium release channels in the endoplasmic reticulum were disrupted by the connexin mimetic peptide, although implication of other candidate hemichannels cannot be entirely discounted. Since calcium signalling is fundamental to the maintenance of cellular homeostasis, connexin hemichannels emerge as therapeutic targets open to manipulation by reagents interacting with external regions of these channels.

Adenosine reduces the reverse mode of the Na+/Ca(2+) exchanger in ferret cardiac fibres

The aim of this study was to investigate the effects of adenosine on reverse mode Na+/Ca(2+) exchange. In intact ferret cardiac trabeculae, Na+-free contractures were investigated after treating preparations with ryanodine, a sarcoplasmic reticulum Ca(2+) -channel inhibitor, and thapsigargin, a sarcoplasmic reticulum Ca(2+) -pump inhibitor added to suppress the sarcoplasmic reticulum function. The effects of adenosine (50-100 nmol/L), adenosine deaminase (ADA, 0.1-0.5 U/L), the A1 and A2A receptor agonists CCPA (3-100 nmol/L) and CGS 21680 (25-100 nmol/L), and the A1 and A2A receptor antagonists DPCPX (25 nmol/L) and ZM 241385 (25 nmol/L) were tested on Na+-free contractures. The application of adenosine (50-100 nmol/L) had no significant effect on the characteristics of the Na+-free contractures. However, the results show that treatment with ADA (0.3 U/L), adenosine (> or =50 nmol/L) and CCPA, a specific A1 receptor agonist (3-100 nmol/L), all reduced the Na+-free contracture amplitude. In the presence of ADA, the effects of adenosine and CCPA were also reduced by a specific antagonist of A1 receptors (DPCPX, 25 nmol/L). Furthermore, adenosine, ADA, and CCPA did not affect the properties of the contractile apparatus in Triton-skinned fibres. It is therefore proposed that endogenous adenosine reduced the reverse mode of the Na+/Ca(2+) exchanger by acting on A1 receptors present in the sarcolemmal membrane.

Skeletal Muscle Fatigue: Cellular Mechanisms

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

Muscle fibers from senescent mice retain excitation-contraction coupling properties in culture

In the present study, we test the hypothesis that mouse skeletal muscle in culture retains the fundamental properties of excitation-sarcoplasmic reticulum Ca(2+) release coupling reported for young-adult (3-4 mo) and senescent (22-23) mice. Dissociated flexor digitorum brevis (FDB) muscles from young-adult and senescent mice were cultured for 7 d in a serum-free medium. During this period, the overall morphology of cultured fibers resembled that exhibited by acutely dissociated cells. In addition, survival analysis revealed that more than 70% of the fibers from both young and old mice remained suitable for electrophysiological studies during this same culture period. Charge movement and intracellular Ca(2+) recordings in FDB fibers, voltage clamped in the whole cell configuration of the patch-clamp technique, reproduced the maximal values, and voltage dependence similarly displayed by acutely dissociated cells for both parameters in young-adult and senescent mice. The analysis of the dihydropyridine receptor by immunoblots confirmed, in the culture system, the age-dependent decrease in the expression of this protein. In conclusion, FDB fibers from young-adult and old mice retain the excitation-contraction coupling phenotype during the course of a week in serum-free medium culture.

Regulation of contraction kinetics in skinned skeletal muscle fibers by calcium and troponin C

The influences of [Ca(2+)] and Ca(2+) dissociation rate from troponin C (TnC) on the kinetics of contraction (k(Ca)) activated by photolysis of a caged Ca(2+) compound in skinned fast-twitch psoas and slow-twitch soleus fibers from rabbits were investigated at 15 degrees C. Increasing the amount of Ca(2+) released increased the amount of force in psoas and soleus fibers and increased k(Ca) in a curvilinear manner in psoas fibers approximately 5-fold but did not alter k(Ca) in soleus fibers. Reconstituting psoas fibers with mutants of TnC that in solution exhibited increased Ca(2+) affinity and approximately 2- to 5-fold decreased Ca(2+) dissociation rate (M82Q TnC) or decreased Ca(2+) affinity and approximately 2-fold increased Ca(2+) dissociation rate (NHdel TnC) did not affect maximal k(Ca). Thus the influence of [Ca(2+)] on k(Ca) is fiber type dependent and the maximum k(Ca) in psoas fibers is dominated by kinetics of cross-bridge cycling over kinetics of Ca(2+) exchange with TnC.

Progressive disorganization of the excitation-contraction coupling apparatus in aging human skeletal muscle as revealed by electron microscopy: a possible role in the decline of muscle performance

An impairment of the mechanisms controlling the release of calcium from internal stores (excitation-contraction [EC] coupling) has been proposed to contribute to the age-related decline of muscle performance that accompanies aging (EC uncoupling theory). EC coupling in muscle fibers occurs at the junctions between sarcoplasmic reticulum and transverse tubules, in structures called calcium release units (CRUs). We studied the frequency, cellular localization, and ultrastructure of CRUs in human muscle biopsies from male and female participants with ages ranging from 28 to 83 years. Our results show significant alterations in the CRUs' morphology and cellular disposition, and a significant decrease in their frequency between control and aged samples: 24.4/100 microm(2) (n = 2) versus 11.6/100 microm(2) (n = 7). These data indicate that in aging humans the EC coupling apparatus undergoes a partial disarrangement and a spatial reorganization that could interfere with an efficient delivery of Ca(2+) ions to the contractile proteins.

Functional effects of the DCM mutant Gly159Asp Troponin C in skinned muscle fibres

We recently reported a dilated cardiomyopathy (DCM) causing mutation in a novel disease gene, TNNC1, which encodes cardiac troponin C (TnC). We have determined how this mutation, Gly159Asp, affects contractile regulation when incorporated into muscle fibres. Endogenous troponin in rabbit skinned psoas fibres was partially replaced by recombinant human cardiac troponin containing either wild-type or Gly159Asp TnC. We measured both the force-pCa relationship of these fibres and the activation rate using the caged-Ca(2+) compound nitrophenyl-EGTA. Gly159Asp TnC had no significant effect on either the Ca(2+) sensitivity or cooperativity of force generation when compared to wild type. However, the mutation caused a highly significant (ca. 50%) decrease in the rate of activation. This study shows that whilst not affecting the force-pCa relationship, the mutation Gly159Asp causes a significant decrease in the rate of force production and a change in the relationship between the rate of force production and generated force. In vivo, this mutation may cause both a slowing of force generation and reduction in total systolic force. This represents a novel mechanism by which a cardiomyopathy-causing mutation can affect contractility.

Eugenol-induced contractions of saponin-skinned fibers are inhibited by heparin or by a ryanodine receptor blocker

The effects of eugenol on the sarcoplasmic reticulum (SR) and contractile apparatus of chemically skinned skeletal muscle fibers of the frog Rana catesbeiana were investigated. In saponin-skinned fibers, eugenol (5 mmol/L) induced muscle contractions, probably by releasing Ca(2+) from the SR. The Ca(2+)-induced Ca(2+) release blocker ruthenium red (10 micromol/L) inhibited both caffeine- and eugenol-induced muscle contractions. Ryanodine (200 micromol/L), a specific ryanodine receptor/Ca(2+) release channel blocker, promoted complete inhibition of the contractions induced by caffeine, but only partially blocked the contractions induced by eugenol. Heparin (2.5 mg/mL), an inositol 1,4,5-trisphosphate (InsP3) receptor blocker, strongly inhibited the contractions induced by eugenol but had only a small effect on the caffeine-induced contractions. Eugenol neither altered the Ca(2+) sensitivity nor the maximal force in Triton X-100 skinned muscle fibers. These data suggest that muscle contraction induced by eugenol involves at least 2 mechanisms of Ca(2+) release from the SR: one related to the activation of the ryanodine receptors and another through a heparin-sensitive pathway.

Physiological consequences of thin filament cooperativity for vertebrate striated muscle contraction: a theoretical study

Bindings of both myosin and Ca(2+) to the thin filament of vertebrate striated muscle are known to be strongly cooperative. Here the relation between these two sources of cooperativity and their consequences for physiological properties are assessed by comparing two models, with and without Monod-type myosin-binding cooperativity. In both models a thin filament regulatory unit (RU) is in either 'off' or 'on' state, and the equilibrium between them (K (on)) is [Ca(2+)]-dependent. The calculations predict the following: (1) In both models, myosin binding stabilizes the RU in the 'on' state, causing troponin to trap Ca(2+). This stabilization in turn increases the Ca(2+)-binding cooperativity, ensuring efficient regulation to occur in a narrow [Ca(2+)] range. (2) In the cooperative model, the RU is stabilized with a relatively low myosin affinity for actin (K approximately approximately 1), while the non-cooperative model requires a much higher affinity (K approximately approximately 10) to produce the same effect. (3) The cooperative model reproduces the known effects of [Ca(2+)] on the rate of force development and shortening velocity with a low K, but again the non-cooperative model requires a higher value. (4) Because of the finite value of K (on), the thin filaments can never be fully activated by increasing [Ca(2+)], indicating that contracting muscles are under strong influence of thin-filament cooperativity even at saturating [Ca(2+)]. Interpretation of data on muscle mechanics without considering these cooperative effects could therefore lead to a substantial (10-fold) overestimate of cross-bridge binding properties.

Sarcomeric determinants of striated muscle relaxation kinetics

Ca2+ is the primary regulator of force generation by cross-bridges in striated muscle activation and relaxation. Relaxation is as necessary as contraction and, while the kinetics of Ca2+-induced force development have been investigated extensively, those of force relaxation have been both studied and understood less well. Knowledge of the molecular mechanisms underlying relaxation kinetics is of special importance for understanding diastolic function and dysfunction of the heart. A number of experimental models, from whole muscle organs and intact muscle fibres down to single myofibrils, have been used to explore the cascade of kinetic events leading to mechanical relaxation. By using isolated myofibrils and fast solution switching techniques we can distinguish the sarcomeric mechanisms of relaxation from those of myoplasmic Ca2+ removal. There is strong evidence that cross-bridge mechanics and kinetics are major determinants of the time course of striated muscle relaxation whilst thin filament inactivation kinetics and cooperative activation of thin filament by cycling, force-generating cross-bridges do not significantly limit the relaxation rate. Results in myofibrils can be explained well by a simple two-state model of the cross-bridge cycle in which the apparent rate of the force generating transition is modulated by fast, Ca2+-dependent equilibration between off- and on-states of actin. Inter-sarcomere dynamics during the final rapid phase of full force relaxation are responsible for deviations from this simple model.

Activation kinetics of skinned cardiac muscle by laser photolysis of nitrophenyl-EGTA

The kinetics of Ca(2+)-induced contractions of chemically skinned guinea pig trabeculae was studied using laser photolysis of NP-EGTA. The amount of free Ca(2+) released was altered by varying the output from a frequency-doubled ruby laser focused on the trabeculae, while maintaining constant total [NP-EGTA] and [Ca(2+)]. The time courses of the rise in stiffness and tension were biexponential at 23 degrees C, pH 7.1, and 200 mM ionic strength. At full activation (pCa < 5.0), the rates of the rapid phase of the stiffness and tension rise were 56 +/- 7 s(-1) (n = 7) and 48 +/- 6 s(-1) (n = 11) while the amplitudes were 21 +/- 2 and 23 +/- 3%, respectively. These rates had similar dependencies on final [Ca(2+)] achieved by photolysis: 43 and 50 s(-1) per pCa unit, respectively, over a range of [Ca(2+)] producing from 15% to 90% of maximal isometric tension. At all [Ca(2+)], the rise in stiffness initially was faster than that of tension. The maximal rates for the slower components of the rise in stiffness and tension were 4.1 +/- 0.8 and 6.2 +/- 1.0 s(-1). The rate of this slower phase exhibited significantly less Ca(2+) sensitivity, 1 and 4 s(-1) per pCa unit for stiffness and tension, respectively. These data, along with previous studies indicating that the force-generating step in the cross-bridge cycle of cardiac muscle is marginally sensitive to [Ca(2+)], suggest a mechanism of regulation in which Ca(2+) controls the attachment step in the cross-bridge cycle via a rapid equilibrium with the thin filament activation state. Myosin kinetics sets the time course for the rise in stiffness and force generation with the biexponential nature of the mechanical responses to steps in [Ca(2+)] arising from a shift to slower cross-bridge kinetics as the number of strongly bound cross-bridges increases.

Biomimetic model of skeletal muscle isometric contraction: II. A phenomenological model of the skeletal muscle excitation–contraction coupling process

This paper describes a new macroscopic, phenomenological model of the skeletal muscle excitation-contraction coupling process, as represented by four principal and consecutive compartments (biophysical, biochemical, and biomechanical phases) characteristic of isometric excitation-contraction coupling in mammalian skeletal muscle, and coupled by a system of simultaneous, first-order linear ordinary differential equations. The model is based upon biological compartmental transport kinetics and irreversible thermodynamic energy transformation, and represents a distinct improvement over other biomimetic models. The model was derived using physiological parameter data published in the literature, and validated using MATLAB R12.

Effects of glycine and proline on the calcium activation properties of skinned muscle fibre segments from crayfish and rat

The effects of the polar amino acid glycine (20 mmol l(-1)) and the non-polar amino acid proline (20 mmol l(-1)) on Ca(2+)-activated contraction have been examined in four types of striated muscle fibres. Single fibres dissected from the claw muscle of a crustacean (long- and short-sarcomere) and the hindlimb muscles of the rat (slow-twitch from soleus and fast-twitch from extensor digitorum longus) were activated in matched solutions that either contained the amino acid ('test') or not ('control'). The steady-state force produced in these solutions was used to determine the relation between force production and pCa (-log10[Ca2+]). The results show that in the concentrations used, glycine and proline had only small effects on the maximum Ca(2+)-activated force, pCa corresponding to 10, 50 and 90% maximum force (pCa10, pCa50, pCa90, respectively) or on the slope of the force-pCa curves in the four different fibre types. The relative lack of effects of glycine and proline on contractile activation would confer a distinct physiological advantage to force production of muscle of Cherax, where the concentrations of glycine and proline vary considerably. Finally, the results show that glycine and proline may be useful to balance control solutions when the effects of other amino acids or zwitterions on contractile activation are examined.

Insulin-like growth factor-1 prevents age-related decrease in specific force and intracellular Ca2+ in single intact muscle fibres from transgenic mice

In the present work we test the hypothesis that sustained transgenic overexpression of insulin-like growth factor-1 (IGF-1) in skeletal muscle prevents age-related decreases in myoplasmic Ca2+ concentration and consequently in specific force in single intact fibres from the flexor digitorum brevis (FDB) muscle from the mouse. Measurements of IGF-1 concentration in FDB muscle showed higher levels in transgenic than in wild-type mice at all ages. The specific tetanic force decreased significantly in single muscle fibres from old (286 +/- 22 kPa) compared to young wild-type (455 +/- 28 kPa), young transgenic (423 +/- 43 kPa), and old transgenic mice (386 +/- 15 kPa) (P < 0.05). These results are consistent with measurements in whole FDB muscles. The peak Ca2+ concentration values in response to prolonged stimulation were: 1.47 +/- 0.15, 1.70 +/- 0.29, 0.97 +/- 0.13 and 1.7 +/- 0.22 microM, in fibres from young wild-type, young transgenic, old wild-type and old transgenic mice, respectively. The effects of caffeine on FDB fibres support the conclusion that the age-related decline in peak myoplasmic Ca2+ and specific force is not explained by sarcoplasmic reticulum Ca2+ depletion. Immunohistochemistry in muscle cross-sections was performed to determine whether age and/or IGF-1 overexpression induce changes in fibre type composition. The relative percentages of type IIa, IIx and I myosin heavy chain (MHC) isoforms did not change significantly with age or genotype. Therefore, IGF-1 prevents age-related decline in peak intracellular Ca2+ and specific force in a muscle that does not exhibit changes in fibre type composition with senescence.

Slowing effects of Mg2+ on contractile kinetics of skinned preparations of rat hearts depending on myosin heavy chain isoform content

The effects of changes in Mg2+ concentration on the kinetics of stretch activation were investigated on skinned rat heart preparations under maximal Ca2+ activation. Muscle strips of hyper- and hypothyroid rat hearts were investigated at 0.5 and 1 mM free Mg2+; the total ATP concentration was 8 mM which resulted in saturating MgATP2- concentrations above 5 mM. Preparations containing exclusively the cardiac alpha-myosin heavy chain (hyper- and hypothyroid atria, hyperthyroid ventricles) showed an acceleration of the kinetics of stretch activation by a factor of about 1.5 (P<0.01, paired t-test) when free Mg2+ was decreased from 1 to 0.5 mM. Conversely, preparations containing exclusively the beta-myosin heavy chain isoform showed only a small acceleration by a factor of 1.05 (P<0.05, paired t-test) under the same conditions. The fact that the Mg2+ sensitivity was dependent on the myosin heavy chain isoform excludes the possibility that Mg2+ exhibits only unspecific effects on contractile proteins. Several hypotheses for explaining the observed Mg2+ effects are discussed. The conditions used in our experiments might be close to the physiological situation and, thus, changes of Mg2+ concentration must be considered as possible factors modulating the contractile kinetics especially of atrial muscle tissue.

Location of ryanodine and dihydropyridine receptors in frog myocardium

Frog myocardium depends almost entirely on calcium entry from extracellular spaces for its beat-to-beat activation. Atrial myocardium additionally shows internal calcium release under certain conditions, but internal release in the ventricle is absent or very low. We have examined the content and distribution of the sarcoplasmic reticulum (SR) calcium release channels (ryanodine receptors, RyRs) and the surface membrane calcium channels (dihydropyridine receptors, DHPRs) in myocardium from the two atria and the ventricle of the frog heart using binding of radioactive ryanodine, immunolabeling of RyR and DHPR, and thin section and freeze-fracture electron microscopy. In cells from both types of chambers, the SR forms peripheral couplings and in both chambers peripheral couplings colocalize with clusters of DHPRs. However, although a low level of high affinity binding of ryanodine is detectable and RyRs are present in peripheral couplings of the atrium, the ventricle shows essentially no ryanodine binding and RyRs are not detectable either by electron microscopy or immunolabeling. The results are consistent with the lack of internal calcium release in the ventricle, and raise questions regarding the significance of DHPR at peripheral couplings in the absence of RyR. Interestingly, the free SR membrane in both heart chambers shows a low but equal density of intramembrane particles representing the Ca(2+) ATPase.

Relaxation kinetics following sudden Ca(2+) reduction in single myofibrils from skeletal muscle

To investigate the roles of cross-bridge dissociation and cross-bridge-induced thin filament activation in the time course of muscle relaxation, we initiated force relaxation in single myofibrils from skeletal muscles by rapidly (approximately 10 ms) switching from high to low [Ca(2+)] solutions. Full force decay from maximal activation occurs in two phases: a slow one followed by a rapid one. The latter is initiated by sarcomere "give" and dominated by inter-sarcomere dynamics (see the companion paper, Stehle, R., M. Krueger, and G. Pfitzer. 2002. Biophys. J. 83:2152-2161), while the former occurs under nearly isometric conditions and is sensitive to mechanical perturbations. Decreasing the Ca(2+)-activated force preceding the start of relaxation does not increase the rate of the slow isometric phase, suggesting that cycling force-generating cross-bridges do not significantly sustain activation during relaxation. This conclusion is strengthened by the finding that the rate of isometric relaxation from maximum force to any given Ca(2+)-activated force level is similar to that of Ca(2+)-activation from rest to that given force. It is likely, therefore, that the slow rate of force decay in full relaxation simply reflects the rate at which cross-bridges leave force-generating states. Because increasing [P(i)] accelerates relaxation while increasing [MgADP] slows relaxation, both forward and backward transitions of cross-bridges from force-generating to non-force-generating states contribute to muscle relaxation.

Voltage-clamp analysis of membrane currents and excitation-contraction coupling in a crustacean muscle

A single fibre preparation from the extensor muscle of a marine isopod crustacean is described which allows the analysis of membrane currents and simultaneously recorded contractions under two-electrode voltage-clamp conditions. We show that there are three main depolarisation-gated currents, two are outward and carried by K+, the third is an inward Ca2+ current, I(Ca). Normally, the K+ currents which can be isolated by using K+ channel blockers, mask I(Ca). I(Ca) activates at potentials more positive than -40 mV, is maximal around 0 mV, and shows strong inactivation at higher depolarisation. Inactivation depends on current rather than voltage. Ba2+, Sr2+ and Mg2+ can substitute for Ca2+. Ba2+ currents are about 80% larger than Ca2+ currents and inactivate little. The properties of I(Ca) characterise it as a high threshold L-type current. The outward current consists primarily of a fast, transient A current, I(K(A)) and a maintained, delayed rectifier current, I(K(V)). In some fibres, a small Ca2+-dependent K+ current is also present. I(K(A)) activates fast at depolarisation above -45 mV, shows pronounced inactivation and is almost completely inactivated at holding potentials more positive than -40 mV. I(K(A)) is half-maximally blocked by 70 microM 4-aminopyridine (4-AP), and 70 mM tetraethylammonium (TEA). I(K(V)) activates more slowly, at about -30 mV, and shows no inactivation. It is half-maximally blocked by 2 mM TEA but rather insensitive to 4-AP. Physiologically, the two K+ currents prevent all-or-nothing action potentials and determine the graded amplitude of active electrical responses and associated contractions. Tension development depends on and is correlated with depolarisation-induced Ca2+ influx mediated by I(Ca). The voltage dependence of peak tension corresponds directly to the voltage dependence of the integrated I(Ca). The threshold potential for contraction is at about -38 mV. Peak tension increases with increasing voltage steps, reaches maximum at around 0 mV, and declines with further depolarisation.

The off rate of Ca(2+) from troponin C is regulated by force-generating cross bridges in skeletal muscle

The effects of dissociation of force-generating cross bridges on intracellular Ca(2+), pCa-force, and pCa-ATPase relationships were investigated in mouse skeletal muscle. Mechanical length perturbations were used to dissociate force-generating cross bridges in either intact or skinned fibers. In intact muscle, an impulse stretch or release, a continuous length vibration, a nonoverlap stretch, or an unloaded shortening during a twitch caused a transient increase in intracellular Ca(2+) compared with that in isometric controls and resulted in deactivation of the muscle. In skinned fibers, sinusoidal length vibrations shifted pCa-force and pCa-actomyosin ATPase rate relationships to higher Ca(2+) concentrations and caused actomyosin ATPase rate to decrease at submaximal Ca(2+) and increase at maximal Ca(2+) activation. These results suggest that dissociation of force-generating cross bridges during a twitch causes the off rate of Ca(2+) from troponin C to increase (a decrease in the Ca(2+) affinity of troponin C), thus decreasing the Ca(2+) sensitivity and resulting in the deactivation of the muscle. The results also suggest that the Fenn effect only exists at maximal but not submaximal force-activating Ca(2+) concentrations.

States of thin filament regulatory proteins as revealed by combined cross-linking/X-ray diffraction techniques

The regulatory protein system in the skeletal muscle thin filaments is known to exhibit three discrete states, called "off" or "blocked" (no Ca2+), "on" or "closed" (with Ca2+ alone) and "potentiated" or "open" (with strongly bound myosin head) states. Biochemical studies have shown that only weak interactions with myosin are allowed in the second state. Characterization of each state is often difficult, because the equilibria among these states are readily shifted by experimental conditions. To overcome this problem, we chemically cross-linked the skeletal muscle thin filament in the three states with the zero-length cross-linker 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC), in overstretched muscle fibers. The state of the regulatory proteins was monitored by measuring the intensity of the second actin layer-line (2nd LL) reflection in X-ray diffraction patterns. Structurally, the thin filaments cross-linked in the three states exhibited three corresponding discrete levels of 2nd LL intensities, which were not Ca2+-sensitive any more. Functionally, the thin filament cross-linked in the "off-blocked" state inhibited strong interaction with myosin head (subgfragment-1 or S1). The thin filament cross-linked in the "potentiated-open" state allowed strong interaction and full ATPase activity of S1 as described previously. The thin filament cross-linked in the "on-closed" state allowed strong interactions with S1 and actin-activated ATPase without enhancing the 2nd LL to the level of "potentiated-open" state, contrary to the expectations from the biochemical studies. The results demonstrate the potential of EDC as a tool for studying the states of calcium regulation, and the apparent uncoupling between the 2nd LL intensity and the function provides a new insight into the mechanism of thin filament regulation.

Sustained overexpression of IGF-1 prevents age-dependent decrease in charge movement and intracellular Ca(2+) in mouse skeletal muscle

In this work we tested the hypothesis that transgenic sustained overexpression of IGF-1 prevents age-dependent decreases in charge movement and intracellular Ca(2+) in skeletal muscle fibers. To this end, short flexor digitorum brevis (FDB) muscle fibers from 5-7- and 21-24-month-old FVB (wild-type) and S1S2 (IGF-1 transgenic) mice were studied. Fibers were voltage-clamped in the whole-cell configuration of the patch-clamp technique according to described procedures (Wang, Z. M., M. L. Messi, and O. Delbono. 1999. Biophys. J. 77:2709-2716). Charge movement and intracellular Ca(2+) concentration were recorded simultaneously. The maximum charge movement (Q(max)) recorded in young wild-type and transgenic mice was (mean +/- SEM, in nC microF(-1)): 52 +/- 2.1 (n = 46) and 54 +/- 1.9 (n = 38) (non-significant, ns), respectively, whereas in old wild-type and old transgenic mice the values were 36 +/- 2.1 (n = 32) and 49 +/- 2.3 (n = 35), respectively (p < 0.01). The peak intracellular calcium [Ca(2+)](i) recorded in young wild-type and transgenic mice was (in muM): 14.5 +/- 0.9 and 16 +/- 2.1 (ns), whereas in old wild-type and transgenic mice the values were 9.9 +/- 0.1 and 14 +/- 1.1 (p < 0.01), respectively. No significant changes in the voltage distribution or steepness of the Q-V or [Ca(2+)]-V relationship were found. These data support the concept that overexpression of IGF-1 in skeletal muscle prevents age-dependent reduction in charge movement and peak [Ca(2+)](i).

Spectroscopic monitoring of local conformational changes during the intramolecular domain-domain interaction of the ryanodine receptor

The amino (N)-terminal and central regions of the ryanodine receptor (RyR) containing most mutation sites of malignant hyperthermia (MH) and central core disease (CCD) seem to be involved in the Ca(2+) channel regulation. Our recent peptide probe study (Yamamoto, T., El-Hayek, R., and Ikemoto, N. (2000) J. Biol. Chem. 275, 11618-11625) suggested the hypothesis that a close contact between the N-terminal and central domains (zipping) stabilizes the closed-state of the channel, while removal of the contact (unzipping) deblocks the channel, causing channel-activation effects. We here report the results of our recent effort to monitor local conformational changes in the putative domain-domain interaction site to test this hypothesis. The conformation-sensitive fluorescence probe, methyl coumarin acetamide (MCA), was incorporated into RyR in a protein- and site-specific manner by using DP4 (the peptide corresponding to the Leu(2442)-Pro(2477) region of the central domain) as a site-directing carrier. The site of MCA labeling was localized in the 150 kDa N-terminal region of RyR, indicating that DP4 and its in vivo counterpart (a portion of the central domain) interact with the N-terminal region. RyR-activating domain peptides, DP4 and DP1 (corresponding to the Leu(590)-Cys(609) region of the N-terminal domain), and depolarization of the T-tubule moiety of the triad (physiologic stimulation) induced a rapid decrease in the fluorescence intensity of the protein-bound MCA and Ca(2+) release at a somewhat slower rate. The accessibility of the protein-bound MCA to the fluorescence quencher was increased in the presence of DP4. These results are all consistent with the above hypothesis.

Ca2+-dependent dual functions of peptide C. The peptide corresponding to the Glu724-Pro760 region (the so-called determinant of excitation-contraction coupling) of the dihydropyridine receptor alpha 1 subunit II-III loop

Both in vivo and in vitro studies suggest that the Glu(724)-Pro(760) (peptide C) region of the dihydropyridine receptor alpha1 II-III loop is important for excitation-contraction coupling, although its actual function has not yet been elucidated. According to our recent studies, peptide C inhibits Ca(2+) release induced by T-tubule depolarization or peptide A. Here we report that peptide C has Ca(2+)-dependent dual functions on the skeletal muscle ryanodine receptor. Thus, at above-threshold [Ca(2+)]s (> or =0.1 microm) peptide C blocked peptide A-induced activation of the ryanodine receptor (ryanodine binding and Ca(2+) release); peptide C also blocked T-tubule depolarization-induced Ca(2+) release. However, at sub-threshold [Ca(2+)]s (<0.1 microm), peptide C enhanced ryanodine binding and induced Ca(2+) release. If peptide A was present, together with peptide C, both peptides produced additive activation effects. Neither peptide A nor peptide C produced any appreciable effect on the cardiac muscle ryanodine receptor at both high (1.0 microm) and low (0.01 microm) Ca(2+) concentrations. These results suggest the possibility that the in vivo counterpart of peptide C retains both activating and blocking functions of the skeletal muscle-type excitation-contraction coupling.

Variability of calcium binding to EF-hand motifs probed by electrospray ionization mass spectrometry

The modulation of calcium binding by the EF-hand motifs present in a calmodulin (CAM) homologue, a calcium binding protein (CaBP) from Entamoeba histolytica by three external parameters-pH, ligand coordinator EGTA, and fragmentor voltage was investigated by mass spectrometry. Calcium binding follows expected patterns at highly acidic and alkaline pH with the preponderance of the apo and the completely saturated forms, respectively. Surprisingly, additional nonspecific binding is observed near neutral pH. Studies on EGTA chelation and effects of fragmentor voltage showed cooperativity in calcium removal in at least one of the domains. Similar studies on a smaller construct containing the two high affinity carboxy terminal sites revealed interesting differences and provided an estimate of the specificity and tolerance of the EF-hand motifs to calcium binding and removal.

Temporal aspects of excitation-contraction coupling in airway smooth muscle

In airway smooth muscle (ASM), ACh induces propagating intracellular Ca2+ concentration ([Ca2+]i) oscillations (5-30 Hz). We hypothesized that, in ASM, coupling of elevations and reductions in [Ca2+]i to force generation and relaxation (excitation-contraction coupling) is slower than ACh-induced [Ca2+]i oscillations, leading to stable force generation. When we used real-time confocal imaging, the delay between elevated [Ca2+]i and contraction in intact porcine ASM cells was found to be approximately 450 ms. In beta-escin-permeabilized ASM strips, photolytic release of caged Ca2+ resulted in force generation after approximately 800 ms. When calmodulin (CaM) was added, this delay was shortened to approximately 500 ms. In the presence of exogenous CaM and 100 microM Ca2+, photolytic release of caged ATP led to force generation after approximately 80 ms. These results indicated significant delays due to CaM mobilization and Ca2+-CaM activation of myosin light chain kinase but much shorter delays introduced by myosin light chain kinase-induced phosphorylation of the regulatory myosin light chain MLC20 and cross-bridge recruitment. This was confirmed by prior thiophosphorylation of MLC20, in which force generation occurred approximately 50 ms after photolytic release of caged ATP, approximating the delay introduced by cross-bridge recruitment alone. The time required to reach maximum steady-state force was >15 s. Rapid chelation of [Ca2+]i after photolytic release of caged diazo-2 resulted in relaxation after a delay of approximately 1.2 s and 50% reduction in force after approximately 57 s. We conclude that in ASM cells agonist-induced [Ca2+]i oscillations are temporally and spatially integrated during excitation-contraction coupling, resulting in stable force production.

The power of single channel recording and analysis: its application to ryanodine receptors in lipid bilayers

1. Since the inception of the patch-clamp technique, single-channel recording has made an enormous impact on our understanding of ion channel function and its role in membrane transport and cell physiology. 2. However, the impact of single-channel recording methods on our understanding of intracellular Ca2+ regulation by internal stores is not as broadly recognized. There are several possible reasons for this. 3. First, ion channels in the membranes of intracellular organelles are not directly accessible to patch pipettes, requiring other methods that are not as widely known as the patch-clamp techniques. 4. Second, bulk assays for channel activity have proved successful in advancing our knowledge of Ca2+ handling by intracellular stores. These assays include Ca2+ imaging, ryanodine binding assays and measurements of muscle tension and Ca2+ release and uptake by vesicles that have been isolated from internal stores. 5. The present review describes methods used for single- channel recording and analysis, as applied to the calcium release channels in striated muscle, and details some of the unique contributions that single-channel recording and analysis have made to our current understanding of the release of Ca2+ from the internal stores of muscle. 6. With this in mind, the review focuses on three aspects of channel function and shows how single-channel investigations have led to an improved understanding of physiological processes in muscle. 7. Finally, the review describes some of the latest improvements in membrane technology that will underpin future advances in single-channel recording.

Conformation and structural transitions in the EF-hands of calmodulin

Calcium plays a key role in cellular signal transduction. Calmodulin, a protein binding four calcium ions, is found in all eukaryotic cells and is believed to activate such processes. The calcium binding loop found in this protein, the canonical EF-hand, is also found in a large number of other proteins such as troponins, parvalbumins, calbindins etc. Earlier analysis of the amino acid sequences of these proteins with a view of understanding evolution of protein families and signaling mechanisms have provided extensive evidence for a characteristic double gene duplication event in this family of proteins. These analyses have been extended here to the three dimensional structures and the biophysical properties of the sequence segments of calmodulin EF-hands. The clear evolutionary history that shows up in sequences is not reflected as clearly in the conformation of individual EF-hands, which may be a consequence of the much higher conservation pressure on the structure. Some evidence for the proposed gene duplication is implicit in the apo-holo structural transitions of the EF-hands. The profile of amino acid properties that might be significant for calcium binding, however, clearly reflects the gene duplication. These profiles might also provide insightful information on the calcium affinity of the EF-hand motifs and the nature of amino acid residues that constitute them.

Regulation of force development studied by photolysis of caged ADP in rabbit skinned psoas fibers

The present study examined the effects of Ca(2+) and strongly bound cross-bridges on tension development induced by changes in the concentration of MgADP. Addition of MgADP to the bath increased isometric tension over a wide range of [Ca(2+)] in skinned fibers from rabbit psoas muscle. Tension-pCa (pCa is -log [Ca(2+)]) relationships and stiffness measurements indicated that MgADP increased mean force per cross-bridge at maximal Ca(2+) and increased recruitment of cross-bridges at submaximal Ca(2+). Photolysis of caged ADP to cause a 0.5 mM MgADP jump initiated an increase in isometric tension under all conditions examined, even at pCa 6.4 where there was no active tension before ADP release. Tension increased monophasically with an observed rate constant, k(ADP), which was similar in rate and Ca(2+) sensitivity to the rate constant of tension re-development, k(tr), measured in the same fibers by a release-re-stretch protocol. The amplitude of the caged ADP tension transient had a bell-shaped dependence on Ca(2+), reaching a maximum at intermediate Ca(2+) (pCa 6). The role of strong binding cross-bridges in the ADP response was tested by treatment of fibers with a strong binding derivative of myosin subfragment 1 (NEM-S1). In the presence of NEM-S1, the rate and amplitude of the caged ADP response were no longer sensitive to variations in the level of activator Ca(2+). The results are consistent with a model in which ADP-bound cross-bridges cooperatively activate the thin filament regulatory system at submaximal Ca(2+). This cooperative interaction influences both the magnitude and kinetics of force generation in skeletal muscle.

The effect of Mg2+ on cardiac muscle function: Is CaATP the substrate for priming myofibril cross-bridge formation and Ca2+ reuptake by the sarcoplasmic reticulum?

Kinetics are established for the activation of the myofibril from the relaxed state [Smith, Dixon, Kirschenlohr, Grace, Metcalfe and Vandenberg (2000) Biochem. J. 346, 393-402]. These require two troponin Ca2+-binding sites, one for each myosin head, to act as a single unit in initial cross-bridge formation. This defines the first, or activating, ATPase reaction, as distinct from the further activity of the enzyme that continues when a cross-bridge to actin is already established. The pairing of myosin heads to act as one unit suggests a possible alternating mechanism for muscle action. A large positive inotropic (contraction-intensifying) effect of loading the Mg2+ chelator citrate, via its acetoxymethyl ester, into the heart has confirmed the competitive inhibition of the Ca2+ activation by Mg2+, previously seen in vitro. In the absence of a recognized second Ca2+ binding site on the myofibril, with appropriate binding properties, the bound ATP is proposed as the second activating Ca2+-binding site. As ATP, free or bound to protein, can bind either Mg2+ or Ca2+, this leads to competitive inhibition by Mg2+. Published physico-chemical studies on skeletal muscle have shown that CaATP is potentially a more effective substrate than MgATP for cross-bridge formation. The above considerations allow calculation of the observed variation of fractional activation by Ca2+ as a function of [Mg2+] and in turn reveal simple Michaelis-Menten kinetics for the activation of the ATPase by sub-millimolar [Mg2+]. Furthermore the ability of bound ATP to bind either cation, and the much better promotion of cross-bridge formation by CaATP binding, give rise to the observed variation of the Hill coefficient for Ca2+ activation with altered [Mg2+]. The inclusion of CaADP within the initiating cross-bridge and replacement by MgADP during the second cycle is consistent with the observed fall in the rate of the myofibril ATPase that occurs after two phosphates are released. The similarity of the kinetics of the cardiac sarcoplasmic reticulum ATPase to those of the myofibril, in particular the positive co-operativity of both Mg2+ inhibition and Ca2+ activation, leads to the conclusion that this ATPase also has an initiation step that utilizes CaATP. The first-order activation by sub-millimolar [Mg2+], similar to that of the myofibril, may be explained by Mg2+ involvement in the phosphate-release step of the ATPase. The inhibition of both the myofibril and sarcoplasmic reticulum Ca2+ transporting ATPases by Mg2+ offers an explanation for the specific requirement for phosphocreatine (PCr) for full activity of both enzymes in situ and its effect on their apparent affinities for ATP. This explanation is based on the slow diffusion of Mg2+ within the myofibril and on the contrast of PCr with both ATP and phosphoenolpyruvate, in that PCr does not bind Mg2+ under physiological conditions, whereas both the other two bind it more tightly than the products of their hydrolysis do. The switch to supply of energy by diffusion of MgATP into the myofibril when depletion of PCr raises [ATP]/[PCr] greatly, e.g. during anoxia, results in a local [Mg2+] increase, which inhibits the ATPase. It is possible that mechanisms similar to those described above occur in skeletal muscle but the Ca2+ co-operativity involved would be masked by the presence of two Ca2+ binding sites on each troponin.

Physiology of nitric oxide in skeletal muscle

In the past five years, skeletal muscle has emerged as a paradigm of "nitric oxide" (NO) function and redox-related signaling in biology. All major nitric oxide synthase (NOS) isoforms, including a muscle-specific splice variant of neuronal-type (n) NOS, are expressed in skeletal muscles of all mammals. Expression and localization of NOS isoforms are dependent on age and developmental stage, innervation and activity, history of exposure to cytokines and growth factors, and muscle fiber type and species. nNOS in particular may show a fast-twitch muscle predominance. Muscle NOS localization and activity are regulated by a number of protein-protein interactions and co- and/or posttranslational modifications. Subcellular compartmentalization of the NOSs enables distinct functions that are mediated by increases in cGMP and by S-nitrosylation of proteins such as the ryanodine receptor-calcium release channel. Skeletal muscle functions regulated by NO or related molecules include force production (excitation-contraction coupling), autoregulation of blood flow, myocyte differentiation, respiration, and glucose homeostasis. These studies provide new insights into fundamental aspects of muscle physiology, cell biology, ion channel physiology, calcium homeostasis, signal transduction, and the biochemistry of redox-related systems.

Effects of halothane on the membrane potential in skeletal muscle of the frog

Halothane has many effects on the resting membrane potential (V(m)) of excitable cells and exerts numerous effects on skeletal muscle one of which is the enhancement of Ca(2+) release by the sarcoplasmic reticulum (SR) resulting in a sustained contracture. The aim of this study was to analyse the effects of clinical doses of halothane on V(m), recorded using intracellular microelectrodes on cleaned and non stimulated sartorius muscle which was freshly isolated from the leg of the frog Rana esculenta. We assessed the mechanism of effects of superfused halothane on V(m) by the administration of selective antagonists of membrane bound Na(+), K(+) and Cl(-) channels and by inhibition of SR Ca(2+) release. Halothane (3%) induced an early and transient depolarization (4.5 mV within 7 min) and a delayed and sustained hyperpolarization (about 11 mV within 15 min) of V(m). The halothane-induced transient depolarization was sensitive to ryanodine (10 microM) and to 4-acetamido-4'-isothiocyanatostilbene 2,2' disulphonic acid (SITS, 1 mM). The hyperpolarization of V(m) induced by halothane (0.1 - 3%) was dose-dependent and reversible. It was insensitive to SITS (1 mM), tetrodotoxin (0.6 microM), and tetraethylammonium (10 mM) but was blocked and/or prevented by ryanodine (10 microM), charybdotoxin (CTX, 1 microM), and glibenclamide (10 nM). Our observations revealed that the effects of halothane on V(m) may be related to the increase in intracellular Ca(2+) concentration produced by the ryanodine-sensitive Ca(2+) release from the SR induced by the anaesthetic. The depolarization may be attributed to the activation of Ca(2+)-dependent Cl(-) (blocked by SITS) channels and the hyperpolarization to the activation of large conductance Ca(2+)-dependent K(+) channels, blocked by CTX, and to the opening of ATP-sensitive K(+) channels, inhibited by glibenclamide.

Postulated role of interdomain interaction within the ryanodine receptor in Ca(2+) channel regulation

Localized distribution of malignant hyperthermia (MH) and central core disease (CCD) mutations in N-terminal and central domains of the ryanodine receptor suggests that the interaction between these domains may be involved in Ca(2+) channel regulation. To test this hypothesis, we investigated the effects of a new synthetic domain peptide DP4 corresponding to the Leu(2442)-Pro(2477) region of the central domain. DP4 enhanced ryanodine binding and induced a rapid Ca(2+) release. The concentration for half-maximal activation by agonists was considerably reduced in the presence of DP4. These effects of DP4 are analogous to the functional modifications of the ryanodine receptor caused by MH/CCD mutations (viz. hyperactivation of the channel and hypersensitization of the channel to agonists). Replacement of Arg of DP4 with Cys, mimicking the in vivo Arg(2458)-to-Cys(2458) mutation, abolished the activating effects of DP4. An N-terminal domain peptide DP1 (El-Hayek, R., Saiki, Y., Yamamoto, T., and Ikemoto, N. (1999) J. Biol. Chem. 274, 33341-33347) shows similar activation/sensitization effects. The addition of both DP4 and DP1 produced mutual interference of their activating functions. We tentatively propose that contact between the two (N-terminal and central) domains closes the channel, whereas removal of the contact by these domain peptides or by MH/CCD mutations de-blocks the channel, resulting in hyperactivation/hyper-sensitization effects.

Regulation of contraction in striated muscle

Ca(2+) regulation of contraction in vertebrate striated muscle is exerted primarily through effects on the thin filament, which regulate strong cross-bridge binding to actin. Structural and biochemical studies suggest that the position of tropomyosin (Tm) and troponin (Tn) on the thin filament determines the interaction of myosin with the binding sites on actin. These binding sites can be characterized as blocked (unable to bind to cross bridges), closed (able to weakly bind cross bridges), or open (able to bind cross bridges so that they subsequently isomerize to become strongly bound and release ATP hydrolysis products). Flexibility of the Tm may allow variability in actin (A) affinity for myosin along the thin filament other than through a single 7 actin:1 tropomyosin:1 troponin (A(7)TmTn) regulatory unit. Tm position on the actin filament is regulated by the occupancy of NH-terminal Ca(2+) binding sites on TnC, conformational changes resulting from Ca(2+) binding, and changes in the interactions among Tn, Tm, and actin and as well as by strong S1 binding to actin. Ca(2+) binding to TnC enhances TnC-TnI interaction, weakens TnI attachment to its binding sites on 1-2 actins of the regulatory unit, increases Tm movement over the actin surface, and exposes myosin-binding sites on actin previously blocked by Tm. Adjacent Tm are coupled in their overlap regions where Tm movement is also controlled by interactions with TnT. TnT also interacts with TnC-TnI in a Ca(2+)-dependent manner. All these interactions may vary with the different protein isoforms. The movement of Tm over the actin surface increases the "open" probability of myosin binding sites on actins so that some are in the open configuration available for myosin binding and cross-bridge isomerization to strong binding, force-producing states. In skeletal muscle, strong binding of cycling cross bridges promotes additional Tm movement. This movement effectively stabilizes Tm in the open position and allows cooperative activation of additional actins in that and possibly neighboring A(7)TmTn regulatory units. The structural and biochemical findings support the physiological observations of steady-state and transient mechanical behavior. Physiological studies suggest the following. 1) Ca(2+) binding to Tn/Tm exposes sites on actin to which myosin can bind. 2) Ca(2+) regulates the strong binding of M.ADP.P(i) to actin, which precedes the production of force (and/or shortening) and release of hydrolysis products. 3) The initial rate of force development depends mostly on the extent of Ca(2+) activation of the thin filament and myosin kinetic properties but depends little on the initial force level. 4) A small number of strongly attached cross bridges within an A(7)TmTn regulatory unit can activate the actins in one unit and perhaps those in neighboring units. This results in additional myosin binding and isomerization to strongly bound states and force production. 5) The rates of the product release steps per se (as indicated by the unloaded shortening velocity) early in shortening are largely independent of the extent of thin filament activation ([Ca(2+)]) beyond a given baseline level. However, with a greater extent of shortening, the rates depend on the activation level. 6) The cooperativity between neighboring regulatory units contributes to the activation by strong cross bridges of steady-state force but does not affect the rate of force development. 7) Strongly attached, cycling cross bridges can delay relaxation in skeletal muscle in a cooperative manner. 8) Strongly attached and cycling cross bridges can enhance Ca(2+) binding to cardiac TnC, but influence skeletal TnC to a lesser extent. 9) Different Tn subunit isoforms can modulate the cross-bridge detachment rate as shown by studies with mutant regulatory proteins in myotubes and in in vitro motility assays. (ABSTRACT TRUNCATED)

L-Type Ca(2+) channel charge movement and intracellular Ca(2+) in skeletal muscle fibers from aging mice

In this work we tested the hypothesis that skeletal muscle fibers from aging mice exhibit a significant decline in myoplasmic Ca(2+) concentration resulting from a reduction in L-type Ca(2+) channel (dihydropyridine receptor, DHPR) charge movement. Skeletal muscle fibers from the flexor digitorum brevis (FDB) muscle were obtained from 5-7-, 14-18-, or 21-24-month-old FVB mice and voltage-clamped in the whole-cell configuration of the patch-clamp technique according to described procedures (Wang, Z.-M., M. L. Messi, and O. Delbono. 1999. Biophys. J. 77:2709-2716). Total charge movement or the DHPR charge movement was measured simultaneously with intracellular Ca(2+) concentration. The maximum charge movement (Q(max)) recorded (mean +/- SEM, in nC microF(-1)) was 53 +/- 3.2 (n = 47), 51 +/- 3.2 (n = 35) (non-significant, ns), and 33 +/- 1.9 (n = 32) (p < 0.01), for the three age groups, respectively. Q(max) corresponding to the DHPR was 43 +/- 3.3, 38 +/- 4.1 (ns), and 25 +/- 3.4 (p < 0.01) for the three age groups, respectively. The peak intracellular [Ca(2+)] recorded at 40 mV (in microM) was 15.7 +/- 0. 12, 16.7 +/- 0.18 (ns), and 8.2 +/- 0.07 (p < 0.01) for the three age groups, respectively. No significant changes in the voltage distribution or steepness of the Q-V or [Ca(2+)]-V relationship were found. These data support the concept that the reduction in the peak intracellular [Ca(2+)] results from a larger number of ryanodine receptors uncoupled to DHPRs in skeletal muscle fibers from aging mammals.

Patch-clamp recording of charge movement, Ca2+ current, and Ca2+ transients in adult skeletal muscle fibers

Intramembrane charge movement (Q), Ca(2+) conductance (G(m)) through the dihydropyridine-sensitive L-type Ca(2+) channel (DHPR) and intracellular Ca(2+) fluorescence (F) have been recorded simultaneously in flexor digitorum brevis muscle fibers of adult mice, using the whole-cell configuration of the patch-clamp technique. The voltage distribution of Q was fitted to a Boltzmann equation; the Q(max), V(1/2Q), and effective valence (z(Q)) values were 41 +/- 3.1 nC/microF, -17.6 +/- 0.7 mV, and 2.0 +/- 0.12, respectively. V(1/2G) and z(G) values were -0.3 +/- 0.06 mV and 5.6 +/- 0.34, respectively. Peak Ca(2+) transients did not change significantly after 30 min of recording. F was fit to a Boltzmann equation, and the values for V(F1/2) and z(F) were 6.2 +/- 0.04 mV and 2.4, respectively. F was adequately fit to the fourth power of Q. These results demonstrate that the patch-clamp technique is appropriate for recording Q, G(m), and intracellular [Ca(2+)] simultaneously in mature skeletal muscle fibers and that the voltage distribution of the changes in intracellular Ca(2+) can be predicted by a Hodgkin-Huxley model.

Involvement of the Glu724-Pro760 region of the dihydropyridine receptor II-III loop in skeletal muscle-type excitation-contraction coupling

Our previous study (El-Hayek, R., Antoniu, B., Wang, J. P., Hamilton, S. L., and Ikemoto, N. (1995) J. Biol. Chem. 270, 22116-22118) suggested the hypothesis that skeletal muscle-type excitation-contraction coupling is regulated by two domains (activating and blocking) of the II-III loop of the dihydropyridine receptor alpha1 subunit. We investigated this hypothesis by examining conformational changes in the ryanodine receptor induced by synthetic peptides and by transverse tubular system (T-tubule) depolarization. Peptide A, corresponding to the Thr671-Leu690 region, rapidly changed the ryanodine receptor conformation from a blocked state (low fluorescence of the conformational probe, methyl coumarin acetamide, attached specifically to the ryanodine receptor) to an activated state (high methyl coumarin acetamide fluorescence) as T-tubule depolarization did. Peptide C, corresponding to the Glu724-Pro760 region, blocked both conformational changes induced by peptide A and T-tubule depolarization. Its ability to block peptide A-induced and depolarization-induced activation was considerably impaired by replacing the portion of peptide C corresponding to the Phe725-Pro742 region of the loop with cardiac muscle-type sequence. These results are consistent with the model that depolarization-induced activation of excitation-contraction coupling and blocking/repriming are mediated by the peptide A region and the peptide C region (containing the critical Phe725-Pro742 sequence) of the II-III loop, respectively.

Roles for the troponin tail domain in thin filament assembly and regulation. A deletional study of cardiac troponin T

Striated muscle contraction is regulated by Ca2+ binding to troponin, which has a globular domain and an elongated tail attributable to the NH2-terminal portion of the bovine cardiac troponin T (TnT) subunit. Truncation of the bovine cardiac troponin tail was investigated using recombinant TnT fragments and subunits TnI and TnC. Progressive truncation of the troponin tail caused progressively weaker binding of troponin-tropomyosin to actin and of troponin to actin-tropomyosin. A sharp drop-off in affinity occurred with NH2-terminal deletion of 119 rather than 94 residues. Deletion of 94 residues had no effect on Ca2+-activation of the myosin subfragment 1-thin filament MgATPase rate and did not eliminate cooperative effects of Ca2+ binding. Troponin tail peptide TnT1-153 strongly promoted tropomyosin binding to actin in the absence of TnI or TnC. The results show that the anchoring function of the troponin tail involves interactions with actin as well as with tropomyosin and has comparable importance in the presence or absence of Ca2+. Residues 95-153 are particularly important for anchoring, and residues 95-119 are crucial for function or local folding. Because striated muscle regulation involves switching among the conformational states of the thin filament, regulatory significance for the troponin tail may arise from its prominent contribution to the protein-protein interactions within these conformations.

Mechanisms involved in the cellular calcium homeostasis in vascular smooth muscle: calcium pumps

The regulation of cytosolic Ca2+ homeostasis is essential for cells, and particularly for vascular smooth muscle cells. In this regulation, there is a participation of different factors and mechanisms situated at different levels in the cell, among them Ca2+ pumps play an important role. Thus, Ca2+ pump, to extrude Ca2+; Na+/Ca2+ exchanger; and different Ca2+ channels for Ca2+ entry are placed in the plasma membrane. In addition, the inner and outer surfaces of the plasmalemma possess the ability to bind Ca2+ that can be released by different agonists. The sarcoplasmic reticulum has an active role in this Ca2+ regulation; its membrane has a Ca2+ pump that facilitates luminal Ca2+ accumulation, thus reducing the cytosolic free Ca2+ concentration. This pump can be inhibited by different agents. Physiologically, its activity is regulated by the protein phospholamban; thus, when it is in its unphosphorylated state such a Ca2+ pump is inhibited. The sarcoplasmic reticulum membrane also possesses receptors for 1,4,5-inositol trisphosphate and ryanodine, which upon activation facilitates Ca2+ release from this store. The sarcoplasmic reticulum and the plasmalemma form the superficial buffer barrier that is considered as an effective barrier for Ca2+ influx. The cytosol possesses different proteins and several inorganic compounds with a Ca2+ buffering capacity. The hypothesis of capacitative Ca2+ entry into smooth muscle across the plasma membrane after intracellular store depletion and its mechanisms of inhibition and activation is also commented.

Coordination between Ca2+ release and subsequent re-uptake in the sarcoplasmic reticulum

We here report the results of our recent effort to produce, in the isolated sarcoplasmic reticulum (SR), a biphasic Ca2+ release and Ca2+ re-uptake transient and to resolve the kinetic relationship between Ca2+ release and re-uptake of the released Ca2+. Ca2+ release from the SR was induced by polylysine (the ryanodine receptor-specific Ca2+ release trigger) at various levels of calcium loading, or at various doses of the trigger. The changes in the Ca2+ concentration in the reaction solution and in the lumenal Ca2+ concentration were determined by stopped-flow spectroscopy using fluo-3 and mag-fura-2AM, respectively. At higher levels of calcium loading (>150 nmol/mg), polylysine induced monophasic Ca2+ release curves (without an appreciable re-uptake phase) as reported in most studies in the literature. However, lowering the calcium loading level to an intermediate range (100-150 nmol/mg) produced the desired biphasic transient curves consisting of Ca2+ release and Ca2+ re-uptake phases. Under these conditions, the increase in the polylysine concentration resulted in the increase of both the rate of Ca2+ release and that of re-uptake of the released Ca2+. The maximal rate of Ca2+ release and that of re-uptake showed a parallel relationship in the polylysine concentration range of 0-10 microM. This indicates that Ca2+ release from the SR and re-uptake of the released Ca2+ via the SR Ca2+ pump are well-coordinated processes. The changes in the lumenal Ca2+ concentration during the release and re-uptake reaction were monitored at an optimum level of calcium loading while clamping the extravesicular Ca2+ concentration at a constant value. There was again a tight correlation between Ca2+ release (decrease of the lumenal Ca2+ concentration) and re-uptake (increase of the lumenal Ca2+ concentration), indicating that acceleration of the re-uptake is controlled by the rate of decrease of the lumenal Ca2+ concentration. We propose that one of the mechanisms, by which the mode of coordination between the two components of the biphasic Ca2+ transient (viz. Ca2+ release via the ryanodine receptor and Ca2+ re-uptake via the SR Ca2+ pump) is controlled, is the change in the Ca2+ concentration gradient across the SR membrane.