Neither changes in phosphorus metabolite levels nor myosin isoforms can explain the weakness in aged mouse muscle

Creatine Kinase Equilibration and ΔGATP over an Extended Range of Physiological Conditions: Implications for Cellular Energetics, Signaling, and Muscle Performance

In this report, we establish a straightforward method for estimating the equilibrium constant for the creatine kinase reaction (CK Keq″) over wide but physiologically and experimentally relevant ranges of pH, Mg2+ and temperature. Our empirical formula for CK Keq″ is based on experimental measurements. It can be used to estimate [ADP] when [ADP] is below the resolution of experimental measurements, a typical situation because [ADP] is on the order of micromolar concentrations in living cells and may be much lower in many in vitro experiments. Accurate prediction of [ADP] is essential for in vivo studies of cellular energetics and metabolism and for in vitro studies of ATP-dependent enzyme function under near-physiological conditions. With [ADP], we were able to obtain improved estimates of ΔGATP, necessitating the reinvestigation of previously reported ADP- and ΔGATP-dependent processes. Application to actomyosin force generation in muscle provides support for the hypothesis that, when [Pi] varies and pH is not altered, the maximum Ca2+-activated isometric force depends on ΔGATP in both living and permeabilized muscle preparations. Further analysis of the pH studies introduces a novel hypothesis around the role of submicromolar ADP in force generation.

Improved spectral resolution and high reliability of in vivo (1) H MRS at 7 T allow the characterization of the effect of acute exercise on carnosine in skeletal muscle

The aims of this study were to observe the behavior of carnosine peaks in human soleus (SOL) and gastrocnemius (GM) muscles following acute exercise, to determine the relaxation times and to assess the repeatability of carnosine quantification by (1) H MRS at 7 T. Relaxation constants in GM and SOL were measured by a stimulated echo acquisition mode (STEAM) localization sequence. For T1 measurement, an inversion recovery sequence was used. The repeatability of the measurement and the absolute quantification of carnosine were determined in both muscles in five healthy volunteers. For absolute quantification, an internal water reference signal was used. The effect of acute exercise on carnosine levels and resonance lines was tested in eight recreational runners/cyclists. The defined carnosine measurement protocol was applied three times - before and twice after (approximately 20 and 40 min) a 1-h submaximal street run and additional toe-hopping. The measured T1 relaxation times for the C2-H carnosine peak at 7 T were 2002 ± 94 and 1997 ± 259 ms for GM and SOL, respectively, and the T2 times were 95.8 ± 9.4 and 81.0 ± 21.8 ms for GM and SOL, respectively. The coefficient of variation of the carnosine quantification measurement was 9.1% for GM and 6.3% for SOL, showing high repeatability, and the intraclass correlation coefficients (ICCs) of 0.93 for GM and 0.98 for SOL indicate the high reliability of the measurement. Acute exercise did not change the concentration of carnosine in the muscle, but affected the shape of the resonance lines, in terms of the shifting and splitting into doublets. Carnosine measurement by (1) H MRS at 7 T in skeletal muscle exhibits high repeatability and reliability. The observed effects of acute exercise were more prominent in GM, probably as a result of the larger portion of glycolytic fibers in this muscle and the more pronounced exercise-induced change in pH. Our results support the application of the MRS-based assessment of carnosine for pH measurement in muscle compartments.

Age affects the contraction-induced mitochondrial redox response in skeletal muscle

Compromised mitochondrial respiratory function is associated with advancing age. Damage due to an increase in reactive oxygen species (ROS) with age is thought to contribute to the mitochondrial deficits. The coenzyme nicotinamide adenine dinucleotide in its reduced (NADH) and oxidized (NAD⁺) forms plays an essential role in the cyclic sequence of reactions that result in the regeneration of ATP by oxidative phosphorylation in mitochondria. Monitoring mitochondrial NADH/NAD⁺ redox status during recovery from an episode of high energy demand thus allows assessment of mitochondrial function. NADH fluoresces when excited with ultraviolet light in the UV-A band and NAD⁺ does not, allowing NADH/NAD⁺ to be monitored in real time using fluorescence microscopy. Our goal was to assess mitochondrial function by monitoring the NADH fluorescence response following a brief period of high energy demand in muscle from adult and old wild-type mice. This was accomplished by isolating whole lumbrical muscles from the hind paws of 7- and 28-month-old mice and making simultaneous measurements of force and NADH fluorescence responses during and after a 5 s maximum isometric contraction. All muscles exhibited fluorescence oscillations that were qualitatively similar and consisted of a brief transient increase followed by a longer transient period of reduced fluorescence and, finally, an increase that included an overshoot before recovering to resting level. Compared with the adult mice, muscles from the 28 mo mice exhibited a delayed peak during the first fluorescence transient and an attenuated recovery following the second transient. These findings indicate an impaired mitochondrial capacity to maintain NADH/NAD⁺ redox homeostasis during contractile activity in skeletal muscles of old mice.

Aging related changes in determinants of muscle force generating capacity: a comparison of muscle aging in men and male rodents

Human aging is associated with a progressive decline in skeletal muscle mass and force generating capacity, however the exact mechanisms underlying these changes are not fully understood. Rodents models have often been used to enhance our understanding of mechanisms of age-related changes in human skeletal muscle. However, to what extent age-related alterations in determinants of muscle force generating capacity observed in rodents resemble those in humans has not been considered thoroughly. This review compares the effect of aging on muscle force generating determinants (muscle mass, fiber size, fiber number, fiber type distribution and muscle specific tension), in men and male rodents at similar relative age. It appears that muscle aging in male F344*BN rat resembles that in men most; 32-35-month-old rats exhibit similar signs of muscle weakness to those of 70-80-yr-old men, and the decline in 36-38-month-old rats is similar to that in men aged over 80 yrs. For male C57BL/6 mice, age-related decline in muscle force generating capacity seems to occur only at higher relative age than in men. We conclude that the effects on determinants of muscle force differ between species as well as within species, but qualitatively show the same pattern as that observed in men.

Effects of aging on the lateral transmission of force in rat skeletal muscle

The age-related reduction in muscle force cannot be fully explained by the loss of muscle fiber mass or degeneration of myofibers. Our previous study showed that changes in lateral transmission of force could affect the total force transmitted to the tendon. The extracellular matrix (ECM) of skeletal muscle plays an important role in lateral transmission of force. The objective of this study was to define the effects of aging on lateral transmission of force in skeletal muscles, and explore possible underlying mechanisms. In vitro contractile tests were performed on extensor digitorum longus (EDL) muscle of young and old rats with series of tenotomy and myotomy. We concluded that lateral transmission of force was impaired in the old rats, and this deficit could be partly due to increased thickness of the ECM induced by aging.

Effects of ageing and gender on contractile properties in human skeletal muscle and single fibres

AIM

The objective of this study is to improve our understanding of the mechanisms underlying the ageing- and gender-related muscle weakness.

METHODS

Ageing- and gender-related differences in regulation of muscle contraction have been studied in knee-extensor muscles at the whole muscle and single muscle fibre levels in young and old sedentary men and women. In vivo knee-extensor muscle function was measured at slow (30 degrees s(-1)) and faster (180 degrees s(-1)) speeds of movement. Maximum velocity of unloaded shortening (V(0)) and maximum force normalized to cross-sectional area (CSA) [specific tension (ST)] were measured in single 'skinned' skeletal muscle fibre segments.

RESULTS

Significant ageing- and gender-related differences were observed in muscle torque. A 33-55% ageing-related decline (P < 0.001) in maximum torque was observed irrespective of gender. At the single muscle fibre level, the ageing-related decline in knee-extensor muscle function was accompanied by a 20-28% decline in ST in muscle fibres expressing the type I MyHC isoform in both men and women, and a 29% decline in type IIa muscle fibre CSA, but the decreased fast-twitch fibre size was restricted to the men. Furthermore, in both men and women, V(0) decreased in muscle cells expressing the type I and IIa MyHC isoforms.

CONCLUSION

The present results provide evidence of specific ageing- and gender-related differences in regulation of muscle contraction at the cellular level. It is suggested that these cellular changes have a significant impact on muscle function and the ageing-related motor handicap.

Soleus and EDL muscle contractility across the lifespan of female C57BL/6 mice

All previous aging research on the contractility of rodent skeletal muscle has been conducted on male rodents. Because males and females age differently, we undertook this study to determine if and when age-related decrements in skeletal muscle contractility occur in female mice. Soleus and extensor digitorum longus (EDL) muscles from female C57BL/6 mice aged approximately 4, 8, 16, 24 and 28 mo were assessed in vitro for contractility and subsequently contractile protein content. EDL muscle was resistant to age-related changes in force generation but displayed characteristics of becoming more slow-twitch like. Maximal isometric tetanic force (Po) generated by soleus muscle declined with age. Soleus muscle size and contractile protein contents were not affected by age and thus could not explain the age-related force decrements. Soleus muscle specific Po declined with age being approximately 26% lower in muscles of 16-28 mo-old mice indicating that a deterioration in soleus muscle quality of female mice occurred beginning around the age of ovarian failure. Thus this study provides essential, comprehensive baseline data for future studies on age-related muscle dysfunction in the female mouse.

Myofibrillar myosin ATPase activity in hindlimb muscles from young and aged rats

We tested the hypothesis that Ca(2+)-activated myosin ATPase activity is lower in muscles of aged rats relative to muscles of young rats, independent of changes in myosin isoform expression. Myofibrils were prepared from permeabilized fibers of soleus, plantaris, and semimembranosus muscles of young (8-12 months) and aged (32-38 months) F344 x BN rats and assayed for resting myosin ATPase, Ca(2+)-activated myosin ATPase, and myosin heavy chain (MHC) and myosin light chain (MLC) isoform compositions. Resting myosin ATPases were not affected by age in any muscle (P > or = 0.42). Ca(2+)-activated myosin ATPases of soleus and plantaris myofibrils were not affected by age (P > or = 0.31) but were 16% lower in semimembranosus myofibrils from aged rats (0.448 +/- 0.019 micromol P(i)/min/mg) compared to young rats (0.533 +/- 0.031 micromol P(i)/min/mg; P = 0.03). Correspondingly, maximal unloaded shortening velocity of single semimembranosus fibers from aged rats was slow (4.6 +/- 0.2 fiber lengths/s) compared with fibers from young rats (5.8 +/- 0.3 fiber lengths/s; P < 0.01). No age-related changes in MHC or regulatory MLC isoforms were detected in any muscle (P > or = 0.08) but changes in the essential MLC occurred in plantaris and semimembranosus muscles. The data indicate that Ca(2+)-activated myosin ATPase activity is reduced with age in semimembranosus muscle, independent of age-related changes in MHC isoform expression, and is one mechanism contributing to age-related slowing of contraction in that muscle.

Excitation-contraction coupling and sarcoplasmic reticulum function in mechanically skinned fibres from fast skeletal muscles of aged mice

Ageing is generally associated with a decline in skeletal muscle mass and strength, and a slowing of muscle contraction, factors that impact upon the quality of life for the elderly. Alterations in Ca2+ handling are thought to contribute to these age-related changes in muscle contractility, yet the effects of ageing on sarcoplasmic reticulum (SR) Ca2+ handling and the Ca2+ transport system remain unresolved. We used mechanically skinned single fibres from the fast twitch extensor digitorum longus (EDL) muscles from young (4-month-old) and old (27- to 28-month-old) mice to test the hypothesis that the age-related changes in skeletal muscle contractility, especially the slower rate of contraction, are due to changes intrinsic to the muscle fibres. There were no age-related differences in the peak height of depolarization-induced contractile response (DICR) or the number of DICRs elicited before rundown (DICR < 50 % of initial). The time taken to reach peak DICR (TPDICR) was approximately12 % slower in single muscle fibres from old compared with young mice (P < 0.05). The rate of relaxation following DICR was not different in young and old mice. Examination of SR function demonstrated that SR Ca2+ reloading in Ca2+ -depleted skinned fibres was not different in young and old mice, nor was there any age-related difference in Ca2+ leak from the SR. However, low [caffeine] contracture in fibres from old mice was only half of that observed in fibres from young mice (P < 0.05), indicating a lower sensitivity of the SR Ca2+ release channel (CRC) to caffeine. We found no difference in maximum Ca2+ -activated force (P(o)) or specific force (sP(o); P(o) corrected for cross-sectional area) in EDL muscle fibres from young and old mice. Impaired excitation-contraction (E-C) coupling and a decrease in SR CRC function are mechanisms which are likely to contribute to the overall slowing of muscle contraction with age.

Velocity of actomyosin sliding in vitro is reduced in dystrophic mouse diaphragm

It has recently been suggested that dystrophin deficiency in mdx diaphragm muscle is associated with quantitative changes in the myosin molecular motor. In vitro motility assays were used to study the kinetics of actomyosin interactions between purified actin filaments and myosin molecules. Monomeric myosin was obtained from the diaphragm and limb (semitendinosus) muscles of 9-mo-old male mdx (mdx) and age-matched control mice. The sliding velocity (vo, microm/s) of fluorescent-labeled actin filaments moving over a myosin-coated surface (40 microg/ml) was measured. In diaphragm, vo was significantly slower in mdx than in control mice (1.2 +/- 0.1 microm s(-1) versus 1.9 +/- 0.1 microm s(-1), p < 0.001). Conversely, there was no significant difference in vo between control and mdx semitendinous muscles (2.4 +/- 0.1 microm s(-1) versus 2.5 +/- 0.1 micro(-1)). As compared with control mice, mdx diaphragm exhibited a shift from IIX-MHC to IIA-MHC (p < 0.001) and a reduction in IIB-MHC (p < 0.01). Semitendinous muscle from control and mdx mice contained almost exclusively type IIB MHC. Our results are in good agreement with the proposal that myosin is altered in dystrophic mouse diaphragm.

Rigor force responses of permeabilized fibres from fast and slow skeletal muscles of aged rats

1. Ageing is generally associated with a decline in skeletal muscle mass and strength and a slowing of muscle contraction, factors that impact upon the quality of life for the elderly. The mechanisms underlying this age-related muscle weakness have not been fully resolved. The purpose of the present study was to determine whether the decrease in muscle force as a consequence of age could be attributed partly to a decrease in the number of cross-bridges participating during contraction. 2. Given that the rigor force is proportional to the approximate total number of interacting sites between the actin and myosin filaments, we tested the null hypothesis that the rigor force of permeabilized muscle fibres from young and old rats would not be different. 3. Permeabilized fibres from the extensor digitorum longus (fast-twitch; EDL) and soleus (predominantly slow-twitch) muscles of young (6 months of age) and old (27 months of age) male F344 rats were activated in Ca2+-buffered solutions to determine force-pCa characteristics (where pCa = -log(10)[Ca2+]) and then in solutions lacking ATP and Ca2+ to determine rigor force levels. 4. The rigor forces for EDL and soleus muscle fibres were not different between young and old rats, indicating that the approximate total number of cross-bridges that can be formed between filaments did not decline with age. We conclude that the age-related decrease in force output is more likely attributed to a decrease in the force per cross-bridge and/or decreases in the efficiency of excitation-contraction coupling.

Specific force deficit in skeletal muscles of old rats is partially explained by the existence of denervated muscle fibers

We tested the hypothesis that denervated muscle fibers account for part of the specific force (sF(o)) deficit observed in muscles from old adult (OA) mammals. Whole muscle force (F(o)) was quantified for extensor digitorum longus (EDL) muscles of OA and young adult (YA) rats. EDL muscle sF(o) was calculated by dividing F(o) by either total muscle fiber cross-sectional area (CSA) or by innervated fiber CSA. Innervated fiber CSA was estimated from EDL muscle cross sections labeled for neural cell adhesion molecules, whose presence is a marker for muscle fiber denervation. EDL muscles from OA rats contained significantly more denervated fibers than muscles from YA rats (5.6% vs 1.1% of total CSA). When compared with YA muscle, OA muscle demonstrated deficits of 34.1% for F(o), 28.3% for sF(o), and 24.9% for sF(o) calculated by using innervated CSA as the denominator. Denervated muscle fibers accounted for 11.3% of the specific force difference between normal YA and OA skeletal muscle. Other mechanisms in addition to denervation account for the majority of the sF(o) deficit with aging.

Electron paramagnetic resonance reveals age-related myosin structural changes in rat skeletal muscle fibers

We tested the hypothesis that low specific tension (force/cross-sectional area) in skeletal muscle from aged animals results from structural changes in myosin that occur with aging. Permeabilized semimembranosus fibers from young adult and aged rats were spin labeled site specifically at myosin SH1 (Cys-707). Electron paramagnetic resonance (EPR) was then used to resolve and quantify the structural states of the myosin head to determine the fraction of myosin heads in the strong-binding (force generating) structural state during maximal isometric contraction. Fibers from aged rats generated 27 +/- 0.8% less specific tension than fibers from younger rats (P < 0.001). EPR spectral analyses showed that, during contraction, 31.6 +/- 2.1% of myosin heads were in the strong-binding structural state in fibers from young adult animals but only 22.1 +/- 1.3% of myosin heads in fibers from aged animals were in that state (P = 0.004). Biochemical assays indicated that the age-related change in myosin structure could be due to protein oxidation, as indicated by a decrease in the number of free cysteine residues. We conclude that myosin structural changes can provide a molecular explanation for age-related decline in skeletal muscle force generation.

Human skeletal muscle fibres: molecular and functional diversity

Contractile and energetic properties of human skeletal muscle have been studied for many years in vivo in the body. It has been, however, difficult to identify the specific role of muscle fibres in modulating muscle performance. Recently it has become possible to dissect short segments of single human muscle fibres from biopsy samples and make them work in nearly physiologic conditions in vitro. At the same time, the development of molecular biology has provided a wealth of information on muscle proteins and their genes and new techniques have allowed analysis of the protein isoform composition of the same fibre segments used for functional studies. In this way the histological identification of three main human muscle fibre types (I, IIA and IIX, previously called IIB) has been followed by a precise description of molecular composition and functional and biochemical properties. It has become apparent that the expression of different protein isoforms and therefore the existence of distinct muscle fibre phenotypes is one of the main determinants of the muscle performance in vivo. The present review will first describe the mechanisms through which molecular diversity is generated and how fibre types can be identified on the basis of structural and functional characteristics. Then the molecular and functional diversity will be examined with regard to (1) the myofibrillar apparatus; (2) the sarcolemma and the sarcoplasmic reticulum; and (3) the metabolic systems devoted to producing ATP. The last section of the review will discuss the advantage that fibre diversity can offer in optimizing muscle contractile performance.

Excitability and contractility of skeletal muscle engineered from primary cultures and cell lines

The purpose of this study was to compare the excitability and contractility of three-dimensional skeletal muscle constructs, termed myooids, engineered from C2C12 myoblast and 10T1/2 fibroblast cell lines, primary muscle cultures from adult C3H mice, and neonatal and adult Sprague-Dawley rats. Myooids were 12 mm long, with diameters of 0.1-1 mm, were excitable by transverse electrical stimulation, and contracted to produce force. After approximately 30 days in culture, myooid cross-sectional area, rheobase, chronaxie, resting baseline force, twitch force, time to peak tension, one-half relaxation time, and peak isometric force were measured. Specific force was calculated by dividing peak isometric force by cross-sectional area. The specific force generated by the myooids was 2-8% of that generated by skeletal muscles of control adult rodents. Myooids engineered from C2C12-10T1/2 cells exhibited greater rheobase, time to peak tension, and one-half relaxation time than myooids engineered from adult rodent cultures, and myooids from C2C12-10T1/2 and neonatal rat cells had greater resting baseline forces than myooids from adult rodent cultures.

Age-associated changes in the innervation of muscle fibers and changes in the mechanical properties of motor units

In both humans and animals there is a progressive loss of muscle strength with age. Tests of handgrip and knee extension in men show that some decline in strength is evident by the age of 55 years and is pronounced by the age of 65, compared with the 25- to 35-year period when strength is at a maximum. A comparable age-related decline in peak force development has also been shown in hind-limb muscles of aged rats. Motoneurons and consequently motor units are lost with age, and this is apparent in man after the age of 60. Again, a comparable decline has been demonstrated in the motoneuron population of hindlimb muscles of rats aged 20-24 months. Loss of motoneurons in young adults (through either injury or disease) results in the remaining intact motoneurons sprouting to innervate the denervated fibers. This capacity for sprouting has been shown to be seriously impaired in the hindlimb muscles of aged rats. Furthermore, the well-established relationship between motor unit size and fatigability (smaller units tend to be more fatigue resistant) also tends to break down, with large units just as likely to be fatigable as fatigue resistant. The normally large, fatigable motor units also appear to be reduced in size in the aged muscles. The age-related loss of motoneurons and associated loss of muscle fibers accounts in part for the reduced functional capacity of muscle with age. The reason for the impairment of the aged motoneuron remains to be investigated, but it may relate to the integrity of the oxidative metabolic pathways within the cell, given that mitochondrial respiratory chain function is known to be reduced with age.

A study of force and cross-sectional area of adductor pollicis muscle in female hip fracture patients

OBJECTIVES

To determine the extent of muscle weakness in older female hip fracture patients compared with healthy older and young women; to determine the extent to which this weakness is caused by a decline of the force produced per unit area of muscle rather than by a decline in muscle bulk; and to investigate the mechanism of the decline in force per unit area.

DESIGN

This was an open study of three groups of subjects, two age matched older groups and one young group.

SETTING

University College London, Royal Free Hospital, and St. Thomas's Hospital, London.

PARTICIPANTS

Twenty-nine older female hip fracture patients (mean age 85.6 +/- 0.9 SEM), 18 healthy older women (mean age 84.7 +/- 1.2 SEM), and 43 young women (mean age 28.9 +/- 1.2 SEM).

MEASUREMENTS

Adductor pollicis muscle maximum voluntary force (MVF) during isometric and pliometric contractions and cross-sectional area (CSA), body weight, height, and demi-span.

RESULTS

Isometric MVF was lowest in the hip fracture group. In both older groups, isometric MVF and CSA were lower than in the young women. Only part of this weakness in the older groups could be explained by the smaller CSAs. The isometric force per unit area (MVF/CSA) was also lower in both older groups, the hip fracture patients again having the lowest values. Analysis of variance showed a significant difference between groups. The age-related declines in pliometric force were much less than the declines in isometric force. This resulted in an increase in the pliometric/isometric force ratio both for the hip fracture patients and for the healthy older women compared with that for young women.

CONCLUSION

In comparison with the results from young women, the adductor pollicis muscles of female hip fracture patients were even weaker than those of healthy older women when normalized for muscle size. This decline in isometric MVF/CSA accounted for at least half of the overall weakness in the hip fracture patients. Inasmuch as pliometric force is maintained in situations where weakness is caused by a decline in the force produced per muscle cross-bridge, this is the likely mechanism of the declines in isometric MVF/CSA observed in this study.

The effect of age and temperature on mdx muscle fatigue

We have studied the in vitro contractile and fatigue characteristics of extensor digitorum longus (EDL) muscles from 8- and 62-week-old dystrophin-deficient (mdx) and control mice at 20 degrees C and 35 degrees C. There were no differences in fatigability at 20 degrees C, but at 35 degrees C the dystrophin-deficient muscles demonstrated increased fatigability compared to controls, with the older mice exhibiting the greatest fatigue. These results suggest a temperature-related mechanism of myofibrillar fatigue in dystrophin-deficient EDL muscles.

The magnitude of the initial injury induced by stretches of maximally activated muscle fibres of mice and rats increases in old age

1. Our purpose was to compare the susceptibilities of muscles in animals of different ages to the injuries induced by stretching the contracting muscle. Single stretches provide an effective method for studying the factors that contribute to the initiation of contraction-induced injury. We hypothesized that, for maximally activated muscles in old compared with young or adult mice, the work input during a single stretch of any given strain is not different, but for a given work input the magnitude of the injury is greater. 2. The force deficit resulting from each single stretch was calculated as the decrease in the maximum isometric force expressed as a percentage of the maximum force prior to the stretch. Force deficits were compared 1 min after single stretches of in situ extensor digitorum longus (EDL) muscles of young, adult and old mice. In addition, measurements of force deficits immediately following single stretches of single permeabilized fibre segments from EDL muscles of young and old rats permitted investigation of the initial injury at the level of the contractile apparatus. 3. For maximally activated EDL muscles in young, adult and old mice, no differences were observed for the work input during stretches of any given strain. Furthermore, the relationships between the work and the resultant force deficit were not different for muscles in young and adult mice. In contrast, compared with the work-force deficit relationships for muscles in either young or adult mice, the relationship was significantly steeper for muscles in old mice. For single permeabilized fibres from muscles of old rats, the force deficits immediately after single stretches were greater than those observed for fibres from muscles of young rats. We conclude that the increased susceptibility of muscles in old animals to contraction-induced injury resides at least in part within the myofibrils.

Changes in muscle strength, relaxation rate and fatiguability during the human menstrual cycle

1. The effect of the different phases of the menstrual cycle on skeletal muscle strength, contractile properties and fatiguability was investigated in ten young, healthy females. Results were compared with a similar group on the combined (non-phasic) oral contraceptive pill (OC). Cycle phases were divided into the early and mid-follicular, mid-cycle (ovulatory) and mid- and late luteal. Cycle phases were estimated from the first day of the menstrual bleed. 2. Subjects were studied weekly through two complete cycles. Measurements included quadriceps and handgrip maximum voluntary isometric force and the relaxation times, force-frequency relationship and fatigue index of the quadriceps during percutaneous stimulation at a range of frequencies from 1 to 100 Hz. 3. In the women not taking the OC there was a significant increase of about 11% in quadriceps and handgrip strength at mid-cycle compared with both the follicular and luteal phases. Accompanying the increases in strength there was a significant slowing of relaxation and increase in fatiguability at mid-cycle. No changes in any parameter were found in the women taking the OC. 4. The changes in muscle function at mid-cycle may be due to the increase in oestrogen that occurs prior to ovulation.

Excitation-calcium release uncoupling in aged single human skeletal muscle fibers

The biological mechanisms underlying decline in muscle power and fatigue with age are not completely understood. The contribution of alterations in the excitation-calcium release coupling in single muscle fibers was explored in this work. Single muscle fibers were voltage-clamped using the double Vaseline gap technique. The samples were obtained by needle biopsy of the vastus lateralis (quadriceps) from 9 young (25-35 years; 25.9 +/- 9.1; 5 female and 4 male) and 11 old subjects (65-75 years; 70.5 +/- 2.3; 6 f, 5 m). Data were obtained from 36 and 39 fibers from young and old subjects, respectively. Subjects included in this study had similar physical activity. Denervated and slow-twitch muscle fibers were excluded from this study. A significant reduction of maximum charge movement (Qmax) and DHP-sensitive Ca current were recorded in muscle fibers from the 65-75 group. Qmax values were 7.6 +/- 0.9 and 3.2 +/- 0.3 nC/muF for young and old muscle fibers, respectively (P < 0.01). No evidences of charge inactivation or interconversion (charge 1 to charge 2) were found. The peak Ca current was (-)4.7 +/- 0.08 and (-)2.15 +/- 0.11 muA/muF for young and old fibers, respectively (P < 0.01). The peak calcium transient studied with mag-fura-2 (400 microM) was 6.3 +/- 0.4 microM and 4.2 +/- 0.3 microM for young and old muscle fibers, respectively. Caffeine (0.5 mM) induced potentiation of the peak calcium transient in both groups. The decrease in the voltage-/Ca-dependent Ca release ratio in old fibers (0.18 +/- 0.02) compared to young fibers (0.47 +/- 0.03) (P < 0.01), was recorded in the absence of sarcoplasmic reticulum calcium depletion. These data support a significant reduction of the amount of Ca available for triggering mechanical responses in aged skeletal muscle and, the reduction of Ca release is due to DHPR-ryanodine receptor uncoupling in fast-twitch fibers. These alterations can account, at least partially for the skeletal muscle function impairment associated with aging.

Concentric and eccentric knee extension strength in older and younger men and women

Decrements in isometric and concentric (Conc) muscular strength with increased age are well documented. However, little information is available on the effects of aging on eccentric (Ecc) strength, even though Ecc or lengthening muscle actions are used in most physical activities. This study examined Conc and Ecc peak torques (PT) during knee extension at 90 degrees/sec in healthy older (62 to 89 yrs) and younger (20 to 29 yrs) men and women. Conc PT decreased more with age than did Ecc (p < 0.01), for both men and women. PT values for the older men and women as a percentage of those of the younger ones were 58.3 and 46.6% for Conc, and 75.1 and 61.7% for Ecc, respectively. These age differences need to be considered during isokinetic evaluations of Conc versus Ecc strength. Other muscle groups should be investigated, and mechanisms remain to be determined.

Effects of alpha-cyano-4-hydroxycinnamic acid on fatigue and recovery of isolated mouse muscle

Fatigue and recovery of mouse soleus and extensor digitorum longus muscles were investigated in standard saline and in saline containing the lactate + hydrogen ion transport blocker, alpha-cyano-4-hydroxycinnamic acid (cinnamate). The fatigue protocol was a series of brief isometric tetani which reduced isometric force by about 25%. Recovery was monitored by test tetani during recovery. Both muscles recovered completely in standard saline. Soleus muscle also recovered completely in the presence of cinnamate, whereas extensor digitorum longus hardly recovered at all. Force during fatigue and recovery can be described in a mathematical simulation in which force depends on intracellular inorganic phosphate and pH, and the only effect of cinnamate is to block lactate + hydrogen ion transport. The results of the simulation suggest that during the fatiguing series of tetani pH changes are small and have a negligible effect on force, but pH is a major determinant of the timecourse of recovery in extensor digitorum longus.

Electrically evoked torque-velocity characteristics and isomyosin composition of the triceps surae in young and elderly men

The electrically evoked isokinetic torque-velocity relationship of the triceps surae of eight elderly and four non-trained young men was examined in relation to the isomyosin composition of the soleus and the gastrocnemius muscles, determined under non-denaturing conditions using pyrophosphate gel electrophoresis. The angle specific torque-velocity properties of the triceps surae were measured using maximal percutaneous electrical stimulation at 50 Hz and a release technique. The elderly subjects generated significantly (P < 0.05) less absolute torque at all angular velocities when compared with the young subjects. When the isokinetic data were normalized to the isometric torque, the lower normalized torques generated by the elderly subjects were not statistically different from the young. The total fast isomyosin (FM) content of the soleus and gastrocnemius in the elderly subjects was 22 +/- 13 and 35+/- 18%, respectively. This compared with 29 +/- 8 (n.s) and 44 +/- 8% (n.s.) in the young subjects. When the gastrocnemius and soleus muscles were given an equal weighting and considered together to represent the whole triceps surae, the normalized torque at the fixed angular velocity of 5 rads s-1 was significantly associated with %FM (r = 0.90, P < 0.01), and the isomyosin bands %FM1 (r = 0.90, P < 0.01) and %FM2 (r = 0.93, P < 0.001) when only the elderly subjects were considered. No relationships were observed between contractile characteristics and contractile protein profile when only the young subjects were considered. This was despite the inclusion of a further two sprint and three endurance trained athletes to increase the range of contractile characteristics and differences in muscle composition.

Creatine kinase equilibration follows solution thermodynamics in skeletal muscle. 31P NMR studies using creatine analogs

The hypothesis tested was whether creatine kinase (CK) equilibrates with its substrates and products in the cytosol as if in solution. We used the creatine analogs cyclocreatine (cCr) or beta-guanidopropionate (beta GPA) to test if mass action ratios (gamma) for CK in muscle could be predicted from combined equilibrium constants (Kcomb) measured in solutions mimicking the intracellular environment. Mice were fed cCr or beta GPA and their muscles assayed for substrates and products of the CK reaction by 31P NMR spectroscopy and high performance liquid chromatography. After three weeks of feeding, gamma was indistinguishable from Kcomb in cCr-treated muscles demonstrating both PCr/Cr and phospho-analog/analog must have equilibrated with a constant and uniform cellular ATP/ADP ratio. In beta GPA-treated muscles, gamma was smaller than Kcomb due to a higher content of muscle beta GPA. Feeding beta GPA for 9-12 weeks resulted in a closer agreement between Kcomb and gamma, suggesting ATP/ADP ratios are not uniform within the muscle perhaps due to transient metabolic stress in some cells. From this analysis it follows that calculation of free ADP from the CK equilibrium for a heterogeneous population of cells with respect to total Cr and ATP content is correct only if chemical potentials of these cells are uniform.

Isometric, shortening, and lengthening contractions of muscle fiber segments from adult and old mice

In old animals, skeletal muscle force decreases during both isometric and shortening contractions. In contrast, force during lengthening appears to be unaffected by aging. We hypothesized that with aging single permeabilized muscle fibers would demonstrate the same impairments in force as are observed for whole muscles. For single permeabilized fibers from extensor digitorum longus muscles of adult and old mice, forces were measured during isometric, shortening, and lengthening contractions performed at 15 degrees C. Maximum isometric forces normalized for fiber area were not different for fibers from adult and old mice. During submaximal isometric contractions a decreased calcium sensitivity resulted in lower forces for fibers from old compared with adult mice. In contrast to a lack of difference in forces developed by fibers from old and adult mice during shortening contractions, during lengthening contractions fibers from old mice developed forces approximately 30% higher than those of adult mice. We conclude that the impairments in force of whole muscles with aging are not the result of impairments in intrinsic force-generating capacity of cross bridges, but changes do occur in single permeabilized muscle fibers of old mice that result in higher forces during stretch.