Administration of a creatine analogue induces isomyosin transitions in muscle

Β-GPA administration activates slow oxidative muscle signaling pathways and protects soleus muscle against the increased fatigue under 7-days of rat hindlimb suspension

Unloading of slow-twitch muscles results in increased muscle fatigue and the mechanisms of this effect are poorly studied. We aimed to analyze the role of high-energy phosphates accumulation during the first week of rat hindlimb suspension plays in a fiber-type phenotype shift towards fast-type fatigable muscle fibers. Male Wistar rats were divided into 3 groups (n = 8): C - vivarium control; 7HS - 7-day hindlimb suspension; 7HB - 7-day hindlimb suspension with intraperitoneal injection of beta-guanidine propionic acid (β-GPA, 400 mg/kg b w). β-GPA is a competitive inhibitor of creatine kinase and it reduces concentrations of ATP and phosphocreatine. In the 7HB group, β-GPA treatment protected a slow-type signaling network in an unloaded soleus muscle, including MOTS-C, AMPK, PGC1 α and micro-RNA-499. These signaling effects resulted in a preserved soleus muscle fatigue resistance, slow-type muscle fibers percentage and mitochondrial DNA copy number under muscle unloading.

The Contribution of Neuromuscular Stimulation in Elucidating Muscle Plasticity Revisited

Studies carried out during the past 45 years on the effects of chronic low-frequency stimulation on skeletal muscle have revealed a multiplicity of adaptive changes of muscle fibres in response to increased activity. As reflected by induced changes in the metabolic properties, protein profiles of the contractile machinery and elements of the Ca²⁺-regulatory system, all essential components of the muscle fibre undergo pronounced changes in their properties that ultimately lead to their reversible transformation from fast-to-slow phenotype. The chronic low-frequency stimulation experiment thus allows exploring many aspects of the plasticity of mammalian skeletal muscle. Moreover it offers the possibility of elucidating molecular mechanisms that remodel phenotypic properties of a differentiated post-mitotic cell during adaptation to altered functional demands. The understanding of the adaptive potential of muscle can be taken advantage of for repairing muscle damage in various muscle diseases. In addition it can be used to prevent muscle wasting during inactivity and aging. Indeed, pioneering studies are still the sound grounds for the many current applications of Functional Electrical Stimulation and for the related research activities that are still proposed and funded.

Responses of skeletal muscles to gravitational unloading and/or reloading

Adaptation of morphological, metabolic, and contractile properties of skeletal muscles to inhibition of antigravity activities by exposure to a microgravity environment or by simulation models, such as chronic bedrest in humans or hindlimb suspension in rodents, has been well reported. Such physiological adaptations are generally detrimental in daily life on earth. Since the development of suitable countermeasure(s) is essential to prevent or inhibit these adaptations, effects of neural, mechanical, and metabolic factors on these properties in both humans and animals were reviewed. Special attention was paid to the roles of the motoneurons (both efferent and afferent neurograms) and electromyogram activities as the neural factors, force development, and/or length of sarcomeres as the mechanical factors and mitochondrial bioenergetics as the metabolic factors.

Use of dietary supplementation with β-guanidinopropionic acid to alter the muscle phosphagen system, postmortem metabolism, and pork quality

Rate and extent of postmortem metabolism control pork quality development. Our objective was to evaluate the role of the phosphagen system (phosphocreatine, PCr; and creatine, Cr) on metabolism and pork quality. Muscle PCr and Cr were manipulated by feeding pigs the creatine analogue, β-guanidinopropionic acid (β-GPA). In experiment 1, pigs received standard (control) diet or β-GPA supplemented (2%) diet (1 wk or 2 wk). Supplementation with β-GPA (2 wk) decreased total Cr (PCr+Cr; P=0.02) and improved pork color (decreased reflectance, P=0.003); however, β-GPA supplementation reduced growth performance (P=0.007). To separate effects of phosphagen system and growth, a second experiment was conducted with control, pair-fed, and 2 wk β-GPA (1%) supplementation; pigs were also offered a control or β-GPA supplemented flavored beverage. Neither treatment influenced pork quality. Immediately postmortem, ATP/ADP was higher in control compared to pair-fed (P<0.05); subsequently, ATP/ADP was similar among all groups. Loss of the phosphagen system may lead to adaptive changes that promote conservation of cellular ATP.

The effect of the creatine analogue beta-guanidinopropionic acid on energy metabolism: a systematic review

Background

Creatine kinase plays a key role in cellular energy transport. The enzyme transfers high-energy phosphoryl groups from mitochondria to subcellular sites of ATP hydrolysis, where it buffers ADP concentration by catalyzing the reversible transfer of the high-energy phosphate moiety (P) between creatine and ADP. Cellular creatine uptake is competitively inhibited by beta-guanidinopropionic acid. This substance is marked as safe for human use, but the effects are unclear. Therefore, we systematically reviewed the effect of beta-guanidinopropionic acid on energy metabolism and function of tissues with high energy demands.

Methods

We performed a systematic review and searched the electronic databases Pubmed, EMBASE, the Cochrane Library, and LILACS from their inception through March 2011. Furthermore, we searched the internet and explored references from textbooks and reviews.

Results

After applying the inclusion criteria, we retrieved 131 publications, mainly considering the effect of chronic oral administration of beta-guanidinopropionic acid (0.5 to 3.5%) on skeletal muscle, the cardiovascular system, and brain tissue in animals. Beta-guanidinopropionic acid decreased intracellular creatine and phosphocreatine in all tissues studied. In skeletal muscle, this effect induced a shift from glycolytic to oxidative metabolism, increased cellular glucose uptake and increased fatigue tolerance. In heart tissue this shift to mitochondrial metabolism was less pronounced. Myocardial contractility was modestly reduced, including a decreased ventricular developed pressure, albeit with unchanged cardiac output. In brain tissue adaptations in energy metabolism resulted in enhanced ATP stability and survival during hypoxia.

Conclusion

Chronic beta-guanidinopropionic acid increases fatigue tolerance of skeletal muscle and survival during ischaemia in animal studies, with modestly reduced myocardial contractility. Because it is marked as safe for human use, there is a need for human data.

Disturbed energy metabolism and muscular dystrophy caused by pure creatine deficiency are reversible by creatine intake

Creatine (Cr) plays an important role in muscle energy homeostasis by its participation in the ATP-phosphocreatine phosphoryl exchange reaction mediated by creatine kinase. Given that the consequences of Cr depletion are incompletely understood, we assessed the morphological, metabolic and functional consequences of systemic depletion on skeletal muscle in a mouse model with deficiency of l-arginine:glycine amidinotransferase (AGAT(-/-)), which catalyses the first step of Cr biosynthesis. In vivo magnetic resonance spectroscopy showed a near-complete absence of Cr and phosphocreatine in resting hindlimb muscle of AGAT(-/-) mice. Compared with wild-type, the inorganic phosphate/β-ATP ratio was increased fourfold, while ATP levels were reduced by nearly half. Activities of proton-pumping respiratory chain enzymes were reduced, whereas F(1)F(0)-ATPase activity and overall mitochondrial content were increased. The Cr-deficient AGAT(-/-) mice had a reduced grip strength and suffered from severe muscle atrophy. Electron microscopy revealed increased amounts of intramyocellular lipid droplets and crystal formation within mitochondria of AGAT(-/-) muscle fibres. Ischaemia resulted in exacerbation of the decrease of pH and increased glycolytic ATP synthesis. Oral Cr administration led to rapid accumulation in skeletal muscle (faster than in brain) and reversed all the muscle abnormalities, revealing that the condition of the AGAT(-/-) mice can be switched between Cr deficient and normal simply by dietary manipulation. Systemic creatine depletion results in mitochondrial dysfunction and intracellular energy deficiency, as well as structural and physiological abnormalities. The consequences of AGAT deficiency are more pronounced than those of muscle-specific creatine kinase deficiency, which suggests a multifaceted involvement of creatine in muscle energy homeostasis in addition to its role in the phosphocreatine-creatine kinase system.

Plasticity of microvascular oxygenation control in rat fast-twitch muscle: effects of experimental creatine depletion

Aging, heart failure and diabetes each compromise the matching of O2 delivery (Q˙O2)-to-metabolic requirements (O2 uptake, V˙O2) in skeletal muscle such that the O2 pressure driving blood-myocyte O2 flux (microvascular PO2, PmvO2) is reduced and contractile function impaired. In contrast, β-guanidinopropionic acid (β-GPA) treatment improves muscle contractile function, primarily in fast-twitch muscle (Moerland and Kushmerick, 1994). We tested the hypothesis that β-GPA (2% wt/BW in rat chow, 8 weeks; n=14) would improve Q˙O2-to-V˙O2 matching (elevated PmvO2) during contractions (4.5V @ 1Hz) in mixed (MG) and white (WG) portions of the gastrocnemius, both predominantly fast-twitch). Compared with control (CON), during contractions PmvO2 fell less following β-GPA (MG -54%, WG -26%, P<0.05), elevating steady-state PmvO2 (CON, MG: 10±2, WG: 9±1; β-GPA, MG 16±2, WG 18±2 mmHg, P<0.05). This reflected an increased Q˙O2/V˙O2 ratio due primarily to a reduced V˙O2 in β-GPA muscles. It is likely that this adaptation helps facilitate the β-GPA-induced enhancement of contractile function in fast-twitch muscles.

High efficiency in human muscle: an anomaly and an opportunity?

Can human muscle be highly efficient in vivo? Animal muscles typically show contraction-coupling efficiencies <50% in vitro but a recent study reports that the human first dorsal interosseous (FDI) muscle of the hand has an efficiency value in vivo of 68%. We examine two key factors that could account for this apparently high efficiency value: (1) transfer of cross-bridge work into mechanical work and (2) the use of elastic energy to do external work. Our analysis supports a high contractile efficiency reflective of nearly complete transfer of muscular to mechanical work with no contribution by recycling of elastic energy to mechanical work. Our survey of reported contraction-coupling efficiency values puts the FDI value higher than typical values found in small animals in vitro but within the range of values for human muscle in vivo. These high efficiency values support recent studies that suggest lower Ca(2+) cycling costs in working contractions and a decline in cost during repeated contractions. In the end, our analysis indicates that the FDI muscle may be exceptional in having an efficiency value on the higher end of that reported for human muscle. Thus, the FDI muscle may be an exception both in contraction-coupling efficiency and in Ca(2+) cycling costs, which makes it an ideal muscle model system offering prime conditions for studying the energetics of muscle contraction in vivo.

Increased resistance to fatigue in creatine kinase deficient muscle is not due to improved contractile economy

There has been speculation on the origin of the increased endurance of skeletal muscles in creatine kinase (CK)-deficient mice. Important factors that have been raised include the documented increased mitochondrial capacity and alterations in myosin heavy chain (MyHC) isoform composition in CK-deficient muscle. More recently, the absence of inorganic phosphate release from phosphocreatine hydrolysis in exercising CK-deficient muscle has been postulated to contribute to the lower fatigueability in skeletal muscle. In this study, we tested the hypothesis that the reported shift in MyHC composition to slower isoforms in CK-deficient muscle leads to a decrease in oxygen cost of twitch performance. To that aim, extensor digitorum longus (EDL) and soleus (SOL) muscles were isolated from wild-type (WT) and knock-out mice deficient in the cytoplasmic muscle-type and sarcomeric mitochondrial isoenzymes of CK, and oxygen consumption per twitch time-tension-integral (TTI) was measured. The results show that the adaptive response to loss of CK function does not involve any major change to contractile economy of skeletal muscle.

Creatine kinase is an alpha myosin heavy chain 3'UTR mRNA binding protein

Altered cardiac workload regulates the translation and localization of the alpha myosin heavy chain (alphaMyHC) messenger RNA through the 3' untranslated region (UTR) by protein-RNA interactions. We used the alphaMyHC 3'UTR from neonatal rat heart tissue in a gel shift analysis to find RNA binding proteins. One was identified by microsequencing as creatine kinase, brain form B (CKBB). The affinity of its binding interaction was evaluated using sense and antisense alphaMyHC 3'UTR and 3'UTR probes from myosin isoforms of 2B and 2X skeletal muscle. Removal of calcium by the chelating agent EGTA had a potentiating effect on the formation of the CKBB/alphaMyHC 3'UTR complex in vitro . Varying the concentration of ATP (0.1-1 mM) also enhanced this interaction, suggesting that autophosphorylation of CKBB is taking place. Our novel finding that CKBB, an energy transduction enzyme, binds to the RNA of the 3'UTR of the faster ATP consuming alphaMyHC suggests a possible regulatory linkage between the metabolic state of the cell and myosin isoform expression.

Metabolic modulation of muscle fiber properties unrelated to mechanical stimuli

The effects of chronically increasing (creatine-fed) or decreasing (beta-guanidinopropionic acid [beta-GPA]-fed) high-energy phosphates for up to 8 weeks on daily voluntary activity levels, swimming endurance capacity, electromyogram (EMG) activity, and the morphological and metabolic properties of single fibers in the soleus and extensor digitorum longus (EDL) muscles in young rats were determined. High-energy phosphate, voluntary activity, and soleus-integrated EMG levels were lower in beta-GPA-fed rats than in control rats. Endurance capacity was higher at a relatively low intensity of swimming and lower at a relatively high intensity in beta-GPA-fed rats than in control rats. Muscle mass and fiber size were smaller, and the percentage of slow fibers was higher in the soleus and EDL of beta-GPA-fed rats than in control rats. Succinate dehydrogenase activity was higher in both the fast and slow fibers of the EDL of beta-GPA-fed rats than in control rats. Thus, a reduction in high-energy phosphates transformed some fast fibers toward a slow phenotype. Creatine supplementation had minimal effects: The only significant change was an increase in alpha-glycerophosphate dehydrogenase activity in the fast fibers of the EDL. These results indicate that the metabolic environment of a muscle fiber can influence the prominence of a given muscle fiber independent of the activity level of muscle.

Adaptations in metabolic capacity of rat soleus after paralysis

To determine whether long-term reductions in neuromuscular activity result in alterations in metabolic capacity, the activities of oxidative, i.e., succinate dehydrogenase (SDH) and citrate synthase (CS), and glycolytic, i.e., α-glycerophosphate dehydrogenase (GPD), enzyme markers were quantified in rat soleus muscles 1, 3, and 6 mo after a complete spinal cord transection (ST). In addition, the proportional content of lactate dehydrogenase (LDH) isozymes was used as a marker for oxidative and glycolytic capacities. The myosin heavy chain (MHC) isoform content of a fiber served as a marker of phenotype. In general, MHC isoforms shifted from MHC1 toward MHC2, particularly MHC2x, after ST. Mean SDH and CS activities were higher in ST than control at all time points. The elevated SDH and CS activities were indicative of an enhanced oxidative capacity. GPD activities were higher in ST than control rats at all time points. The increase in activity of SDH was larger than GPD. Thus the GPD-to-SDH (glycolytic-to-oxidative) ratio was decreased after ST. Compared with controls, total LDH activity increased transiently, and the LDH isozyme profile shifted from LDH-1 toward LDH-5, indicative of an enhanced glycolytic capacity. Combined, these results indicate that 1) the metabolic capacities of soleus fibers were not compromised, but the interrelationships among oxidative and glycolytic capacity and MHC content were apparently dissociated after ST; 2) enhancements in oxidative and glycolytic enzyme activities are not mutually exclusive; and 3) chronic reductions in skeletal muscle activity do not necessarily result in a reduced oxidative capacity.

Adenylate kinase 1 deficiency induces molecular and structural adaptations to support muscle energy metabolism

Genetic ablation of adenylate kinase 1 (AK1), a member of the AK family of phosphotransfer enzymes, disturbs muscle energetic economy and decreases tolerance to metabolic stress, despite rearrangements in alternative high energy phosphoryl transfer pathways. To define the mechanisms of this adaptive response, soleus and gastrocnemius muscles from AK1(-/-) mice were characterized by cDNA array profiling, Western blot and ultrastructural analysis. We demonstrate that AK1 deficiency induces fiber-type specific variation in groups of transcripts involved in glycolysis and mitochondrial metabolism and in gene products defining structural and myogenic events. This was associated with increased phosphotransfer capacities of the glycolytic enzymes pyruvate kinase and 3-phosphoglycerate kinase. Moreover, in AK1(-/-) mice, fast-twitch gastrocnemius, but not slow-twitch soleus, had an increase in adenine nucleotide translocator (ANT) and mitochondrial creatine kinase protein, along with a doubling of the intermyofibrillar mitochondrial volume. These results provide molecular evidence for wide-scale remodeling in AK1-deficient muscles aimed at preservation of efficient energetic communication between ATP producing and utilizing cellular sites.

AMP-activated protein kinase activates transcription of the UCP3 and HKII genes in rat skeletal muscle

AMP-activated protein kinase (AMPK) has recently emerged as a key signaling protein in skeletal muscle, coordinating the activation of both glucose and fatty acid metabolism in response to increased cellular energy demand. To determine whether AMPK signaling may also regulate gene transcription in muscle, rats were given a single subcutaneous injection (1 mg/g) of the AMP analog 5-aminoimidazole-4-carboxamide-1-beta-d-ribonucleoside (AICAR). AICAR injection activated (P < 0.05) AMPK-alpha 2 ( approximately 2.5-fold) and transcription of the uncoupling protein-3 (UCP3, approximately 4-fold) and hexokinase II (HKII, approximately 10-fold) genes in both red and white skeletal muscle. However, AICAR injection also elicited (P < 0.05) an acute drop (60%) in blood glucose and a sustained (2-h) increase in blood lactate, prompting concern regarding the specificity of AICAR on transcription. To maximize AMPK activation in muscle while minimizing potential systemic counterregulatory responses, a single-leg arterial infusion technique was employed in fully conscious rats. Relative to saline-infused controls, single-leg arterial infusion of AICAR (0.125, 0.5, and 2.5 micro g. g(-1). min(-1) for 60 min) induced a dose-dependent increase (2- to 4-fold, P < 0.05) in UCP3 and HKII transcription in both red and white skeletal muscle. Importantly, AICAR infusion activated transcription only in muscle from the infused leg and had no effect on blood glucose or lactate levels. These data provide evidence that AMPK signaling is linked to the transcriptional regulation of select metabolic genes in skeletal muscle.

Effects of creatine loading and depletion on rat skeletal muscle contraction

1. In humans, the effects of dietary creatine supplementation are controversial, with some studies showing increased muscle force and fatigue resistance and others reporting no effect on exercise performance. Little is known about the effects of creatine on muscle contractile properties. 2. Rats were fed a standard diet, creatine for 10 days or beta-guanidinopropionate, which depletes muscle creatine, for 7 days. Contractile properties were measured in isolated extensor digitorum longus and sternohyoid muscle as representative limb and upper airway dilator muscles, respectively. 3. Creatine had no effect on specific twitch and tetanic tension, contractile kinetics, twitch/tetanus tension ratio, the tension-frequency relationship or fatigue in both muscles. beta-Guanidinopropionate had no effect on the twitch and tetanic tension, contractile kinetics, twitch/tetanus tension ratio or tension-frequency relationship, but significantly increased (P < 0.05, anova) fatigue in both muscles. 4. Therefore, although creatine depletion increases fatigue, creatine loading has no effects on extensor digitorum longus and sternohyoid muscle contractile properties.

Effects of endurance training on activity and expression of AMP-activated protein kinase isoforms in rat muscles

The effects of endurance training on the response of muscle AMP-activated protein kinase (AMPK) and acetyl-CoA carboxylase (ACC) to moderate treadmill exercise were examined. In red quadriceps, there was a large activation of alpha 2-AMPK and inactivation of ACC in response to exercise. This response was greatly reduced after training, probably because of reduced metabolic stress. In white quadriceps, there were no effects of exercise on AMPK or ACC, but alpha 2-activity was higher after training because of increased phosphorylation of Thr(172). In soleus, there were small increases in alpha 2-activity during exercise that were not affected by training. The expression of all seven AMPK subunit isoforms was also examined. The beta 2- and gamma 2-isoforms were most highly expressed in white quadriceps, and gamma 3 was expressed in red quadriceps and soleus. There was a threefold increase in expression of gamma 3 after training in red quadriceps only. Our results suggest that gamma 3 might have a special role in the adaptation to endurance exercise in muscles utilizing oxidative metabolism.

Changes in mRNA expression profile underlie phenotypic adaptations in creatine kinase-deficient muscles

We have studied the mechanisms that regulate the remodeling of the glycolytic, mitochondrial and structural network of muscles of creatine kinase M (M-CK)/sarcomeric mitochondrial creatine kinase (ScCKmit) knockout mice by comparison of wild-type and mutant mRNA profiles on cDNA arrays. The magnitudes of changes in mRNA levels were most prominent in M-CK/ScCKmit (CK(-/-)) double mutants but did never exceed those of previously observed changes in protein level for any protein examined. In gastrocnemius of CK(-/-) mice we measured a 2.5-fold increase in mRNA level for mitochondrial encoded cytochrome c oxidase (COX)-III which corresponds to the increase in protein content. The level of the nuclear encoded mRNAs for COX-IV, H(+)-ATP synthase-C, adenine nucleotide translocator-1 and insulin-regulatable glucose transporter-4 showed a 1.5-fold increase, also in agreement with protein data. In contrast, no concomitant up-regulation in mRNA and protein content was detected for the mitochondrial inorganic phosphate-carrier, voltage-dependent anion channel and certain glycolytic enzymes. Our results reveal that regulation of transcript level plays an important role, but it is not the only principle involved in the remodeling of mitochondrial and cytosolic design of CK(-/-) muscles.

Creatine and creatinine metabolism

The goal of this review is to present a comprehensive survey of the many intriguing facets of creatine (Cr) and creatinine metabolism, encompassing the pathways and regulation of Cr biosynthesis and degradation, species and tissue distribution of the enzymes and metabolites involved, and of the inherent implications for physiology and human pathology. Very recently, a series of new discoveries have been made that are bound to have distinguished implications for bioenergetics, physiology, human pathology, and clinical diagnosis and that suggest that deregulation of the creatine kinase (CK) system is associated with a variety of diseases. Disturbances of the CK system have been observed in muscle, brain, cardiac, and renal diseases as well as in cancer. On the other hand, Cr and Cr analogs such as cyclocreatine were found to have antitumor, antiviral, and antidiabetic effects and to protect tissues from hypoxic, ischemic, neurodegenerative, or muscle damage. Oral Cr ingestion is used in sports as an ergogenic aid, and some data suggest that Cr and creatinine may be precursors of food mutagens and uremic toxins. These findings are discussed in depth, the interrelationships are outlined, and all is put into a broader context to provide a more detailed understanding of the biological functions of Cr and of the CK system.

Muscle creatine phosphate in gyrate atrophy of the choroid and retina with hyperornithinaemia--clues to pathogenesis

BACKGROUND

In gyrate atrophy of the choroid and retina with hyperornithinaemia (GA), inherited deficiency of ornithine-o-aminotransferase leads to progressive fundus destruction and atrophy of type II skeletal muscle fibres. Because high ornithine concentrations inhibit creatine biosynthesis, the ensuing deficiency of high-energy creatine phosphate may mediate the pathogenesis.

MATERIALS AND METHODS

Relative concentrations of inorganic phosphate (Pi), creatine phosphate (PCr) and ATP in resting calf muscle were recorded in 23 GA patients and 33 control subjects using 31P-magnetic resonance spectroscopy (MRS). Eight patients with autosomal recessive retinitis pigmentosa with matched control subjects constituted an additional reference group.

RESULTS

The PCr/Pi and PCr/ATP ratios (means +/- SD) were lower for the GA patients than for healthy control subjects [4.66 +/- 0.37 vs. 9.75 +/- 2.17 (P < 0.0001) and 2.85 +/- 0.37 vs. 3.70 +/- 0.50 (P < 0.05) respectively]. In retinitis pigmentosa the respective values were 9.12 +/- 2.57 and 4.25 +/- 0.45. Age and stage of the disease had no effect.

CONCLUSION

Muscle 31P-MRS spectra were markedly abnormal in all GA patients.

Shortening of muscle relaxation time after creatine loading

The effect of creatine (Cr) supplementation on muscle isometric torque generation and relaxation was investigated in healthy male volunteers. Maximal torque (Tmax), contraction time (CT) from 0.25 to 0.75 of Tmax, and relaxation time (RT) from 0.75 to 0.25 of Tmax were measured during 12 maximal isometric 3-s elbow flexions interspersed by 10-s rest intervals. Between the pretest and the posttest, subjects ingested Cr monohydrate (4 x 5 g/day; n = 8) or placebo (n = 8) for 5 days. Pretest Tmax, CT, and RT were similar in Cr and placebo groups. Also in the posttest, Tmax and CT were similar between groups. However, posttest RT was decreased consistently by approximately 20% (P < 0.05) in the Cr group from the first to the last of the 12 contractions. In addition, the mean decrease in RT after Cr loading was positively correlated with pretest RT (r = 0.82). It is concluded that Cr loading facilitates the rate of muscle relaxation during brief isometric muscle contractions without affecting torque production.

Alterations in AMP deaminase activity and kinetics in skeletal muscle of creatine kinase-deficient mice

Alterations in the competency of the creatine kinase system elicit numerous structural and metabolic compensations, including changes in purine nucleotide metabolism. We evaluated molecular and kinetic changes in AMP deaminase from skeletal muscles of mice deficient in either cytosolic creatine kinase alone (M-CK-/-) or also deficient in mitochondrial creatine kinase (CK-/-) compared with wild type. We found that predominantly fast-twitch muscle, but not slow-twitch muscle, from both M-CK-/- and CK-/- mice had much lower AMP deaminase; the quantity of AMP deaminase detected by Western blot was correspondingly lower, whereas AMP deaminase-1 (AMPD1) gene expression was unchanged. Kinetic analysis of AMP deaminase from mixed muscle revealed negative cooperativity that was significantly greater in creatine kinase deficiencies. Treatment of AMP deaminase with acid phosphatase abolished negative cooperative behavior, indicating that a phosphorylation-dephosphorylation cycle may be important in the regulation of AMP deaminase.

Molecular and kinetic alterations of muscle AMP deaminase during chronic creatine depletion

We examined a possible mechanism to account for the maintenance of peak AMP deamination rate in fast-twitch muscle of rats fed the creatine analog beta-guanidinopropionic acid (beta-GPA), in spite of reduced abundance of the enzyme AMP deaminase (AMPD). AMPD enzymatic capacity (determined at saturating AMP concentration) and AMPD protein abundance (Western blot) were coordinately reduced approximately 80% in fast-twitch white gastrocnemius muscle by beta-GPA feeding over 7 wk. Kinetic analysis of AMPD in the soluble cell fraction demonstrated a single Michaelis-Menten constant (Km; approximately 1.5 mM) in control muscle extracts. An additional high-affinity Km (approximately 0.03 mM) was revealed at low AMP concentrations in extracts of beta-GPA-treated muscle. The kinetic alteration in AMPD reflects increased molecular activity at low AMP concentrations; this could account for high rates of deamination in beta-GPA-treated muscle in situ, despite the loss of AMPD enzyme protein. The elimination of this kinetic effect by treatment of beta-GPA-treated muscle extracts with acid phosphatase in vitro suggests that phosphorylation is involved in the kinetic control of skeletal muscle AMPD in vivo.

Crystalline mitochondrial inclusion bodies isolated from creatine depleted rat soleus muscle

Rats were fed a 2% guanidino propionic acid diet for up to 18 weeks to induce cellular creatine depletion by inhibition of creatine uptake by this creatine analogue. Ultrastructural analysis of creatine depleted tissues showed that mitochondrial intermembrane inclusion bodies appeared in all skeletal muscles analysed, after 11 weeks of feeding. Heart had relatively few even after 18 weeks of analogue feeding and none were evident in kidney, brain or liver. These structures were strongly immuno-positive for sarcomeric mitochondrial creatine kinase and upon removal from mitochondria, the inclusion bodies were shown to diffract to a resolution of 2.5 nm. Two-dimensional image analysis and three-dimensional reconstruction revealed arrays of creatine kinase octamers with additional components between the octameric structures. The same mitochondria had a 3-fold higher extractable specific creatine kinase activity than controls. Molecular mass gel filtration of inclusion body containing mitochondrial extracts from analogue fed rat solei revealed mitochondrial creatine kinase eluting as an aggregate of an apparent molecular mass &gt; or = 2,000 kDa. Mitochondrial creatine kinase of control soleus mitochondrial extract eluted as an octamer, with a molecular mass of 340 kDa. Respiration measurements of control solei mitochondria displayed creatine mediated stimulation of oxidative phosphorylation that was absent in analogue-fed rat solei mitochondria. The latter also had 19% and 14% slower rates of state 4 and maximal state 3 respiration, respectively, than control mitochondria. These results indicate that mitochondrial creatine kinase co-crystallises with another component within the inter membrane space of select mitochondria in creatine depleted skeletal muscle, and is inactive in situ.

Improved fatigue resistance not associated with maximum oxygen consumption in creatine-depleted rats

Effects of feeding of either creatine or its analog beta-guanidinopropionic acid (beta-GPA) on endurance work capacity and oxygen consumption were studied in rats. Resting high-energy phosphate contents in hindlimb muscles were lower in the beta-GPA group and higher in the creatine group than in controls. The glycogen contents in resting hindlimb muscles of rats fed beta-GPA were significantly higher than those in controls. The endurance run and swimming times to exhaustion were significantly greater (32-70%) in the beta-GPA group than in the control and creatine groups. However, there were no beneficial effects on the maximum oxygen consumption (VO2max) and oxygen transport capacity of blood by the feeding of beta-GPA. None of these parameters were significantly influenced by creatine supply. Both maximum exercise time and VO2max in the beta-GPA group were not changed by normalization of glycogen levels. The activities of mitochondrial enzymes in skeletal muscles were higher in the beta-GPA group than in the controls. Thus endurance capacity is improved if the respiratory capacity of muscles is increased, even when the contents of high-energy phosphates in muscles are lower. Increased endurance capacity was not directly associated with the elevated levels of muscle glycogen, oxygen transport capacity of blood, or VO2max.

Combined myofibrillar and mitochondrial creatine kinase deficiency impairs mouse diaphragm isotonic function

Creatine kinase (CK) is an enzyme central to cellular high-energy phosphate metabolism in muscle. To characterize the physiological role of CK in respiratory muscle during dynamic contractions, we compared the force-velocity relationships, power, and work output characteristics of the diaphragm (Dia) from mice with combined myofibrillar and sarcomeric mitochondrial CK deficiency (CK[-/-]) with CK-sufficient controls (Ctl). Maximum velocity of shortening was significantly lower in CK[-/-] Dia (14.1 +/- 0.9 Lo/s, where Lo is optimal fiber length) compared with Ctl Dia (17.5 +/- 1.1 Lo/s) (P < 0.01). Maximum power was obtained at 0.4-0.5 tetanic force in both groups; absolute maximum power (2,293 +/- 138 W/m2) and work (201 +/- 9 J/m2) were lower in CK[-/-] Dia compared with Ctl Dia (2,744 +/- 146 W/m2 and 284 +/- 26 J/m2, respectively) (P < 0.05). The ability of CK[-/-] Dia to sustain shortening during repetitive isotonic activation (75 Hz, 330-ms duration repeated each second at 0.4 tetanic force load) was markedly impaired, with CK[-/-] Dia power and work declining to zero by 37 +/- 4 s, compared with 61 +/- 5 s in Ctl Dia. We conclude that combined myofibrillar and sarcomeric mitochondrial CK deficiency profoundly impairs Dia power and work output, underscoring the functional importance of CK during dynamic contractions in skeletal muscle.

Mammalian skeletal muscle fiber type transitions

Mammalian skeletal muscle is an extremely heterogeneous tissue, composed of a large variety of fiber types. These fibers, however, are not fixed units but represent highly versatile entities capable of responding to altered functional demands and a variety of signals by changing their phenotypic profiles. This adaptive responsiveness is the basis of fiber type transitions. The fiber population of a given muscle is in a dynamic state, constantly adjusting to the current conditions. The full range of adaptive ability spans fast to slow characteristics. However, it is now clear that fiber type transitions do not proceed in immediate jumps from one extreme to the other, but occur in a graded and orderly sequential manner. At the molecular level, the best examples of these stepwise transitions are myofibrillar protein isoform exchanges. For the myosin heavy chain, this entails a sequence going from the fastest (MHCIIb) to the slowest (MHCI) isoform, and vice-versa. Depending on the basal protein isoform profile and hence the position within the fast-slow spectrum, the adaptive ranges of different fibers vary. A simple transition scheme has emerged from the multitude of data collected on fiber type conversions under a variety of conditions.

Morphological changes in the masseter muscle and its motoneurons during postnatal development

BACKGROUND

It has been suggested that the morphological properties of the masseter muscle are changed by the masticatory activity pattern. In the rat, the activity pattern of the muscle alters from sucking to biting around 3 weeks after birth. The working hypothesis in this study is that the unique alteration in masticatory activity has an important influence on the development of the masseter muscle and its motoneurons.

METHODS

We examined the morphological changes in the muscle fibers of the superficial masseter muscle and its motoneurons from 2 days to 280 days after birth in the rat. The change in masseter muscle activity sucking and biting was confirmed by electromyography. To label motoneurons innervating the muscle, horseradish peroxidase was injected into the muscle. The muscle and lower brain stem were sliced and processed histochemically to measure the diameters of muscle fibers and its motoneurons in the trigeminal motor nucleus. In addition, composition of myosin heavy chain (MHC) isoforms of the muscles were analyzed using gradient sodium dodecyl sulphate-polyacrylamide gel electrophoresis.

RESULTS

There was a rapid growth in both types of muscle fibers (fast-twitch oxidative glycolytic muscle fibers and fast-twitch glycolytic fibers) for 42 days after birth, and then a gradual growth lasting until 280 days after birth. Particularly, rapid growth of the muscle fibers was seen between 21 days and 42 days after birth. A large amount of neonatal type MHC disappeared between 21 days and 42 days after birth. In the motoneuron, there was a rapid growth of motoneurons by 42 days after birth but no significant growth was seen thereafter.

CONCLUSIONS

These results suggest that the alteration of mastication activity from sucking to biting has a significant influence on morphological development of both types of muscle fibers, but not on that of motoneurons innervating the masseter muscle.

Relationships between fiber composition and NMR measurements in human skeletal muscle

The purpose of this study was to examine the relationships between the relative contents of phosphocreatine (PCr), inorganic phosphate (Pi), beta-adenosine triphosphate (ATP), and transverse relaxation time (T2) with fiber composition, which determined histochemically in the human skeletal muscle. The vastus lateralis muscles of 28 volunteers were subjected to phosphorus nuclear magnetic resonance (31P NMR) spectroscopy, magnetic resonance imaging (MRI) and muscle biopsy. Muscle fibers were divided into type I and type II fibers using myosin ATPase stain. A wide range of fiber composition levels were observed in the subjects (27.3-74.6% type I fibers). The PCr/ATP, Pi/ATP and (PCr + Pi)/ATP ratios were positively related to the percentage of type II fibers (r = 0.695, p < 0.001, r = 0.429, p < 0.05 and r = 0.773, p < 0.001, respectively). There was no correlation between fiber composition and the PCr/Pi ratio (r 0.127, n.s.) or intracellular pH (r = 0.305, n.s.). Moreover, no correlation was found between T2 and fiber type (r = 0.144, n.s.). These results suggest that 31P NMR can detect the differences in relative content of phosphates between type I and type II fibers, thereby noninvasively evaluating fiber composition in human skeletal muscle.

Muscle creatine kinase-deficient mice. I. Alterations in myofibrillar function

The regulation of contractile activity in mice bearing a null mutation of the M-isoform of creatine kinase gene, has been investigated in tissue extracts and Triton X-100-treated preparations of ventricular, soleus, and gastrocnemius muscles of control and transgenic mice. Skinned fiber experiments did not evidence any statistical difference in the maximal force or the calcium sensitivity of either muscle type. Rigor tension development at a low MgATP concentration was greatly influenced by phosphocreatine in control but not in transgenic mice as should be expected. In calcium-activated ventricular preparations, although the force developed by each cross-bridge was the same in control and transgenic animals, the rate constant of tension changes appeared to be markedly slowed in transgenic animals. As the ventricular isomyosin pattern was not altered, we suggested that, in transgenic animals, cross-bridge cycling was hindered by a local decrease in the MgATP to MgADP ratio, due to lack of a local MgATP regenerating system. Myokinase activity was not significantly changed while activities of pyruvate kinase or glyceraldehyde-3-phosphate dehydrogenase were found to be increased in transgenic animals. These results show that no fundamental remodelling occurs in myofibrils of transgenic animals but that important adaptations modify the bioenergetic pathways including glycolytic metabolism.

Responses of mouse fast and slow skeletal muscle to streptozotocin diabetes: myosin isoenzymes and phosphorous metabolites

A condition similar to insulin-dependent diabetes mellitus (IDDM) was induced in male CD-1 mice by injection of streptozotocin (STZ). Five weeks after treatment, the fast-twitch extensor digitorum longus (EDL) and slow-twitch soleus (SOL) muscles were isolated for analysis. Phosphorous metabolites were quantified by 31P-NMR and HPLC, native myosin was characterized electrophoretically, and activities of metabolic enzymes were measured spectrophotometrically. Relative to control animals, STZ-diabetes resulted in a significant 32% decrease in the FM1 isoform of myosin in EDL and a 24% decrease in IM myosin of SOL. Mass-specific activities of phosphofructokinase, citrate synthase, and cytochrome oxidase were significantly lower in SOL from STZ-diabetic mice than in controls by 23, 18, and 36%, respectively. Intracellular ATP was significantly lower in SOL from STZ-diabetic mice than in controls (3.44 +/- 0.20 mumol g-1 wet weight vs. 4.61 +/- 0.20 mumol g-1, respectively), as was creatine phosphate (11.98 +/- 0.80 mumol g-1 wet weight vs. 14.22 +/- 0.44 mumol g-1). In contrast to results from SOL, there were no significant changes in phosphorus metabolites or enzyme activity in EDL. These results show that the effects of IDDM on levels of phosphorus containing metabolites and maximal activities of key regulatory enzymes in muscle are markedly fiber-type specific. It is suggested that the muscle type-specific effects of STZ-diabetes may be a consequence of differential accumulation of intracellular fatty acids.

Effect of I1-imidazoline receptor activation on responses of hypoglossal and phrenic nerve to chemical stimulation

Sedation elicited by some centrally acting antihypertensive agents may interfere with respiratory control, and by selectively inhibiting upper airway dilating muscle activity it may facilitate obstructive sleep apnea. Autoradiographic studies with [125I]p-iodoclonidine in the presence of 10 microM epinephrine to block alpha 2-adrenergic sites or 100 nM moxonidine to mask I1-imidazoline sites show that both I1- as well as alpha 2-sites are localized in putative chemosensory areas of the rostral ventrolateral medulla in the cat. We sought to determine the effect of activating I1 and alpha 2-receptors on central chemosensitivity by using moxonidine as a selective I1 agonist, clonidine as a mixed I1/alpha 2 agonist, SK&F-86466 as a specific alpha 2-antagonist, and efaroxan as a mixed I1/alpha 2 antagonist. We recorded responses of phrenic, hypoglossal, and cervical sympathetic nerve activities to progressive hypercapnia after hyperventilation to apnea. Moxonidine (3-100 micrograms/kg i.v.) caused dose-dependent decreases in tonic cervical sympathetic nerve activity and blood pressure, but had no effect on the CO2 threshold (after 30 or 100 micrograms/kg moxonidine, phrenic nerve activity reappeared at 5.8 +/- 0.2% CO2 versus 5.6 +/- 0.3% CO2 in control). Following moxonidine, the slope of the steep portion of the CO2 response tended to increase (10.3 +/- 1.8 versus 7.3 +/- 0.9). Peak phrenic nerve activity was comparable to control at 7.5% CO2 (20 +/- 2 U in control) and at 9.5% CO2 (30 +/- 3 versus 27. +/- 2 U). Similarly, the response of hypoglossal and inspiratory phasic cervical sympathetic nerve activity to a progressive CO2 rise was not affected by moxonidine. By contrast, clonidine in the same doses decreased CO2 sensitivity, because the CO2 threshold was elevated from 5.3 +/- 0.5% to 6.7 +/- 0.4% (p < 0.001). The slope of the CO2 response was decreased from 9.7 +/- 1.9 to 7.4 +/- 1.3 (p = 0.05). Peak phrenic nerve activity was reduced at 7.5% CO2 (11 +/- 5 versus 25 +/- 2 U; p < 0.05) and at 9.5% CO2 (21 +/- 4 versus 33 +/- 2 U; p = 0.06). Clonidine selectively inhibited the response of hypoglossal nerve activity to CO2. The depressive effects of clonidine were reversed by alpha 2-blockade with SK&F-86466 (0.5 or 1 mg/kg). Inspiratory phasic cervical sympathetic nerve activity increased after SK&F-86466 in parallel with phrenic and hypoglossal nerve activity, but the tonic component of cervical sympathetic nerve activity and blood pressure increased only transiently.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of beta-guanidinopropionic acid-feeding on the patterns of myosin isoforms in rat fast-twitch muscle

Administration of beta-guanidinopropionic acid (beta-GPA) to rats as 1% of their diet for 6 weeks led to an accumulation of beta-GPA and beta-GPA-phosphate and to a depletion of creatine and phosphocreatine in the fast-twitch plantaris muscle. Adenosine triphosphate concentration was also decreased. Electrophoretic analyses were performed to investigate the effects of beta-GPA on the patterns of fast (FM) and slow (SM) isomyosins, myosin heavy chain (HC) isoforms and myosin light chain (LC) isoforms. The relative concentrations of fast isomyosins FM1 and FM2 decreased, whereas slow isomyosin SM increased. The increase in slow isomyosin corresponded to an increase in the relative concentration of the slow myosin HCI. The changes of the myosin light chain pattern consisted of increases in the relative concentrations of the two slow isoforms, LC1sb and LC2s, and decreases in the fast isoforms LC2f and LC3f. These results demonstrate that beta-GPA administration, leading to a depletion in energy-rich phosphates and a reduced phosphorylation potential, has an impact on myosin isoform expression in rat fast-twitch skeletal muscle.

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.

Energetic status and mitochondrial oxidative capacity of rat skeletal muscle in response to creatine analogue ingestion

A creatine analogue, beta-guanidinopropionic acid (beta-GPA), was administered in the food (1% w/w) of 8 male rats for 6 weeks, while 8 control rats received a standard diet. Mitochondrial oxidative capacity and cytosolic modulators of mitochondrial oxidative phosphorylation (free ADP, ATP-to-free ADP ratio) were evaluated in the soleus and extensor digitorum longus (EDL) muscles. Mitochondrial adaptation to the diet was significantly different between muscles. Citrate synthase activity and mitochondrial ATP synthesis rate were 35 and 45% higher in EDL muscle, respectively, whereas they were virtually unchanged in the soleus muscle. In both muscles, 3-hydroxyacyl-CoA dehydrogenase activity remained unaffected. Regardless of muscle type, creatine, phosphocreatine and ATP concentrations, as well as the total adenine nucleotide content (ATP + ADP + AMP), were significantly lower in beta-GPA fed rats. Whereas free ADP concentration remained unchanged, a significantly greater decrease in ATP-to-free ADP ratio was observed in EDL than in the soleus muscle. It is suggested that regulation of mitochondrial oxidative phosphorylation, through changes in metabolite concentrations, could be an important factor to consider for mitochondrial adaptation induced by beta-GPA feeding.

Bioenergetics and muscle cell types

Potential complexities in biochemical and bioenergetic interpretation due fiber type heterogeneity are not significant for human muscle. Paradigms for understanding muscle bioenergetics then can be understood from a set of basic premises of biochemical energy balance 1) ATP provides the energy for all forms of muscle work; 2) chemical energy is stored in cells as phosphocreatine, a biochemical capacitor; 3) the sum of the coupled ATPases sets the demand side of the balance and defines energetic states; and 4) this demand is supplied by aerobic metabolism and the products of the coupled ATPases provide control signals for regulation of energy balance. We speculate that cytoplasmic signals at work in energy balance may also control muscle plasticity.

Effects of the creatine analogue beta-guanidinopropionic acid on skeletal muscles of mice deficient in muscle creatine kinase

To evaluate the effects of phosphocreatine (PCr) and creatine (Cr) depletion on skeletal muscles of mice deficient in muscle creatine kinase (M-CK), we have fed mutant mice a diet containing the creatine analogue beta-guanidinopropionic acid (beta GPA). After 8-10 weeks of feeding, accumulation of the creatine analogue in M-CK-deficient muscles was comparable to that observed in muscles of wild-type mice. Strikingly, and unlike wild types, mutants did not accumulate phosphorylated beta GPA, indicating that MM-CK is the only muscle CK isoform which can phosphorylate beta GPA. In M-CK-deficient muscles there was respective depletion of PCr, Cr and ATP levels to 31, 41 and 83% of normal. The average cross-sectional area of type 2B fibres in gastrocnemius muscles was very much reduced and was similar to type 1 and type 2A fibres which maintained their normal size. The maximal isometric twitch force developed by gastrocnemius-plantaris-soleus (GPS) muscle complexes of beta GPA-treated mutants was reduced by about 30%, but these muscles showed an increased fatigue resistance during 1 and 5 Hz contraction. Mitochondrial enzyme activities in the upper hind limb musculature of null mutants were 20-35% increased by the beta GPA diet. Altogether, these results provide evidence that certain functions of the creatine kinase/phosphocreatine (CK/PCr) system are not eliminated solely by the loss of M-CK.

Creatine metabolism and the consequences of creatine depletion in muscle

Currently, considerable research activities are focussing on biochemical, physiological and pathological aspects of the creatine kinase (CK)-phosphorylcreatine (PCr)-creatine (Cr) system (for reviews see [1,2]), but only little effort is directed towards a thorough investigation of Cr metabolism as a whole. However, a detailed knowledge of Cr metabolism is essential for a deeper understanding of bioenergetics in general and, for example, of the effects of muscular dystrophies, atrophies, CK deficiencies (e.g. in transgenic animals) or Cr analogues on the energy metabolism of the tissues involved. Therefore, the present article provides a short overview on the reactions and enzymes involved in Cr biosynthesis and degradation, on the organization and regulation of Cr metabolism within the body, as well as on the metabolic consequences of 3-guanidinopropionate (GPA) feeding which is known to induce a Cr deficiency in muscle. In addition, the phenotype of muscles depleted of Cr and PCr by GPA feeding is put into context with recent investigations on the muscle phenotype of 'gene knockout' mice deficient in the cytosolic muscle-type M-CK.

The acute effects of the creatine analogue, beta-guanidinopropionic acid, on cardiac energy metabolism and function

1. Perfusion of isolated rat hearts with 150 mM beta GPA led to the linear accumulation of intracellular P beta GPA (approx. 150 nmol/min per g (dry wt.)) after an initial lag period of 20 min. 2. This accumulation of intracellular P beta GPA was accompanied by a decrease in PCr (30%) and an increase in total phosphagen content (20%). These results show that PCr was not equally replaced by P beta GPA, but was degraded at the expense of beta GPA phosphorylation to produce a net increase in cardiac phosphagen content. Correspondingly, total phosphate (the sum of PCr, P beta GPA, Pi and ATP) was increased, indicating that there was no cellular necrosis and that the sarcolemma remained intact throughout the perfusion. 3. An increase in Pi and decrease in ATP also occurred concomitantly with P beta GPA accumulation, indicating that ATP synthesis was not keeping up with demand. This may be due to the gradual replacement of PCr by the less efficient phosphagen, P beta GPA, resulting in inadequate transduction of energy and hence an imbalance between energy demand and supply. However, the increased hyperosmolarity of the perfusate may be partly responsible for these effects on cardiac energy metabolism, as perfusion with 150 mM mannitol produced a similar decrease in ATP, but a smaller rise in Pi. 4. Perfusion with either 150 mM beta GPA or mannitol led to a significant intracellular alkalosis (max. pHi 7.3), which was reversed on returning to normal perfusate. In addition, both hyperosmolar perfusions led to a significant reduction in cardiac frequency (40 and 15%, respectively). However, only beta GPA caused significant negative inotropism. The time-courses for the changes in cardiac frequency and pHi did parallel the increase in P beta GPA. This suggests that both hyperosmolarity and the production of P beta GPA during beta GPA perfusions determine the degree of negative chronotropism, but that hyperosmolarity alone causes alkalosis and beta GPA phosphorylation, a decrease in developed tension. 5. When hearts, acutely loaded with P beta GPA were perfused with control medium, the levels of ATP, PCr and P beta GPA stabilised to produce a new steady state. There was no decrease in P beta GPA concentration during this procedure, implying that beta GPA efflux was negligible.

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

1. The contractile force, phosphorus metabolite levels, intracellular pH and myosin isoforms were compared in isolated soleus and extensor digitorum longus (EDL) muscles from young (6 month old) and aged (28 month old) mice, at 23 degrees C. 2. The isometric force per unit cross-sectional area was significantly lower by 21 +/- 5% in soleus muscles from aged mice compared to those from young mice (mean +/- S.E.M., n = 11 and 9 respectively). 3. The EDL muscle contained twice as much total creatine and phosphocreatine as the soleus, 1.7 times as much ATP, and 0.4 times the inorganic phosphate (Pi) per unit weight. The intracellular pH and free ADP levels were not significantly different between these muscle types. 4. There was no significant difference in resting metabolite levels between young and old EDL or soleus despite the difference in mechanical strength. 5. Examination of the expression of myosin isoforms by non-denaturing gel electrophoresis has shown that the percentage of each isoform does not change with respect to age; thus, if there is an atrophic process occurring, it is not fibre type specific. 6. We have determined that neither the Pi levels nor the intracellular pH can explain the differences seen in muscle strength with age. There is also no correlation between muscle weakness and any of the other metabolites responsible for energy transduction (phosphocreatine, ATP or ADP).

Two classes of mammalian skeletal muscle fibers distinguished by metabolite content

Phosphorus NMR spectroscopy and HPLC analyses were made on isolated rat and mouse muscles selected for different volume fractions of the major known fiber types. We tested the hypothesis that muscle cell types at rest have intrinsically different contents of PCr, ATP and Pi. The Pi content was low and the PCr and ATP contents were high in muscles with large contents of type 2b and 2a fibers, and vice versa in muscles with large volume fraction of types 1 and 2x fibers. From the profile of these metabolites we could distinguish only two classes of fibers in the murine muscles and predict well the composition of cat muscles. For the first class, types 2a and 2b fibers, the intracellular concentrations were: ATP 8 mM; total Cr 39 mM; PCr 32 mM; Pi 0.8 mM; ADP 8 microM. For the second class, type 1 and 2x fibers, these quantities are: ATP 5 mM; TCr 23 mM; PCr 16 mM; Pi 6 mM; ADP 11 microM. Thus our results establish a new and apparently general criterion upon which to distinguish skeletal muscle cells, one based on the resting content of bioenergetically important metabolites.

Mammalian skeletal muscle fibers distinguished by contents of phosphocreatine, ATP, and Pi

We tested the proposition that muscle cell types have different contents of phosphocreatine (PCr), ATP, and Pi by 31P NMR spectroscopy and HPLC analyses of adult rat and mouse muscles containing various volume fractions of different fiber types. There was a 2-fold difference in the PCr content between muscles with a high volume fraction of fiber types 1 and 2x versus those with fast-twitch (types 2a and 2b) fiber types. Pi content was low, and PCr and ATP contents were high in muscles with large contents of type 2b and 2a fibers; the reverse was true in muscles with a large volume fraction of type 1 and 2x fibers. There is a large range in the Pi/PCr ratios in normal resting muscles, from less than 0.05 in type 2 to 0.51 in type 1 fibers, depending upon the distribution of their component fiber types. In all muscles, the peak area resulting from the beta phosphate of ATP constituted approximately 13% of the sum of all peak areas observable in the 31P spectrum. Fiber types 2a and 2b were not distinguishable, and the content of type 2x fibers was similar to type 1 fibers. From the profile of these metabolites, we could distinguish only two classes of fibers. For type 2a and 2b fibers, the intracellular concentrations were 8 mM ATP, 39 mM total creatine, 32 mM PCr, 0.8 mM Pi, and 8 microM ADP. For type 1 and 2x fibers, these quantities were 5 mM ATP, 23 mM total creatine, 16 mM PCr, 6 mM Pi, and 11 microM ADP. Thus our results establish an additional criterion upon which to distinguish skeletal muscle cells, one based on the resting content of bioenergetically important metabolites. These results also provide the basis for estimating skeletal muscle fiber-type composition from noninvasive NMR spectroscopic data.

High-performance liquid chromatographic assays for free and phosphorylated derivatives of the creatine analogues beta-guanidopropionic acid and 1-carboxy-methyl-2-iminoimidazolidine (cyclocreatine)

Creatine and phosphocreatine are substrates for creatine kinase which is a key enzyme involved in energy transfer within the cell. Analogues of creatine have been fed to animals to determine the role this enzyme plays in energy metabolism, but progress in interpretation has been hampered by the lack of quantitative techniques to determine tissue content of these compounds. We describe the separation and quantitation of substituted guanidino compounds and their phosphorylated forms by high-performance liquid chromatography. First, a cation-exchange column is used to assay free creatine and its unphosphorylated analogues, and then phosphocreatine and its phosphorylated analogues as well as adenylate content (AMP, ADP, ATP) are assayed on an anion-exchange column. These methods have proven successful in measuring the chemical contents of these compounds in neutralized perchloric acid extracts of mammalian skeletal muscles. The sensitivity of this method ranges from 50 to 200 pmol, which is adequate to provide information from tissue extracts of 5- to 10-mg samples.

Activities of creatine kinase isoenzymes in single skeletal muscle fibres of trained and untrained rats

Biochemical changes in the creatine kinase isoenzyme compositions in single muscle fibres of different types in rats were induced by endurance running training. Single muscle fibres were dissected from the soleus and extensor digitorum longus muscles of Wistar-strain male rats trained on a motor-driven treadmill for 16 weeks. Each fibre was typed histochemically (SO, slow-twitch oxidative; FOG, fast-twitch oxidative glycolytic; FG, fast-twitch glycolytic), and the activities of total creatine kinase and its four isoenzymes (CK-MM, -MB, -BB, and mitochondrial creatine kinase) were measured. The endurance training did not affect the total creatine kinase activity, but resulted in significantly increased activities of CK-MB and CK-BB in SO and FOG fibres, and the mitochondrial enzyme activity in FOG and FG fibres. Endurance training induced biochemical changes in the isoenzyme compositions, specifically in FOG fibres. These results suggest that changes in creatine kinase isoenzymes with endurance training reflect changes in the energy metabolism in the different muscle fibres, supporting the hypothesis that the different isoenzymes play different roles in energy transduction.

Phosphocreatine-dependent protein phosphorylation in rat skeletal muscle

Phosphocreatine (PCr) was found to alter the phosphorylation state of two proteins of apparent molecular masses 18 and 29 kDa in dialysed cell-free extracts of rat skeletal muscle in the presence of [gamma-32P]ATP. The 29 kDa protein was identified as phosphoglycerate mutase (PGM), phosphorylated at the active-site histidine residue by 2,3-bisphosphoglycerate (2,3-biPG). 2,3-biPG labelling from [gamma-32P]ATP occurred through the concerted action of phosphoglycerate kinase and 2,3-bisphosphoglycerate mutase. PCr-dependent labelling, which required creatine kinase, resulted from a shift in the phosphoglycerate kinase equilibrium towards 1,3-bisphosphoglycerate (1,3-biPG) synthesis, ultimately resulting in an increase in available [2-32P]2,3-biPG. The maximal catalytic activity of PGM was unaffected by PCr. The 18 kDa protein was transiently phosphorylated at a histidine residue, probably by 1,3-biPG. No proteins of this monomeric molecular mass are known to bind 1,3-biPG, suggesting that the 18 kDa protein is an undescribed phosphoenzyme intermediate. Previous observations of 2- and 3-phosphoglycerate-dependent protein phosphorylation in cytosolic extracts [Ueda & Plagens (1987) Proc. Natl. Acad. Sci. U.S.A. 84, 1229-1233; Pek, Usami, Bilir, Fischer-Bovenkerk & Ueda (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 4294-4298], attributed to the action of novel kinases, are likely to represent phosphoenzyme intermediates labelled by bisphosphorylated metabolites in a similar manner.

Creatine kinase activity is required for mineral deposition and matrix synthesis in endochondral growth cartilage

In earlier studies, we have drawn attention to the unique changes in energy metabolism that accompany the maturation of epiphyseal growth plate chondrocytes. The objective of this investigation was to examine the importance of the ATP generating enzyme creatine kinase (CK), in the development and mineralization of the growth plate. We inhibited CK function by administering beta-guanidinopropionic acid (beta-GPA) to rats in vivo and to cultured chick chondrocytes in vitro. We found that this agent inhibited normal development of cartilage. Disorganization of chondrocytes in the proliferative and hypertrophic zones, poor vascular invasion, and retention of calcified cartilage occurred in the long bones of beta-GPA-fed rats. beta-GPA caused a change in the electrophoretic mobility of type II and type X collagens. Inhibition of apatite formation in the bones of shell-less chick embryos was accompanied by a CK isoenzyme shift from a bone-specific phenotype to a CK isozyme profile similar to that of cartilage. The results of these studies indicate that CK activity is required for normal development of the growth plate and that interference with creatine phosphate metabolism results in profound changes in the synthesis of cartilage and the maturational activities of chondrocytes.