The amount and compartmentalization of adenosine diphosphate in muscle

AMPK is a direct adenylate charge-regulated protein kinase

The adenosine monophosphate (AMP)-activated protein kinase (AMPK) regulates whole-body and cellular energy balance in response to energy demand and supply. AMPK is an αβγ heterotrimer activated by decreasing concentrations of adenosine triphosphate (ATP) and increasing AMP concentrations. AMPK activation depends on phosphorylation of the α catalytic subunit on threonine-172 (Thr(172)) by kinases LKB1 or CaMKKβ, and this is promoted by AMP binding to the γ subunit. AMP sustains activity by inhibiting dephosphorylation of α-Thr(172), whereas ATP promotes dephosphorylation. Adenosine diphosphate (ADP), like AMP, bound to γ sites 1 and 3 and stimulated α-Thr(172) phosphorylation. However, in contrast to AMP, ADP did not directly activate phosphorylated AMPK. In this way, both ADP/ATP and AMP/ATP ratios contribute to AMPK regulation.

Spectrophotometric studies on intact muscle. II. Recovery from contractile activity

The kinetics of the mitochondrial respiratory chain of intact muscle and the concomitant changes of the intercellular pH were investigated. Addition of lactate and pyruvate under resting conditions produces reductions of DPN and cytochrome b, and, occasionally, of cytochrome c and flavoprotein. Succinate gives similar but smaller changes. In recently excised muscles moderate contractile activity produces a reduction of cytochrome c and oxidations of DPNH, cytochrome b, and sometimes of the flavoproteins. Tetanic contractions and larger numbers of twitches produce reductions of DPN and of cytochromes b and c. In sartorii of the tropical toad, stored for approximately 2 days at 0-3 degrees C, contractile activity always gives rise to long lasting oxidations of DPNH and cytochrome b. Addition of pyruvate or lactate shortens these oxidation cycles with a concomitant reduction of cytochrome c. These responses to contractions agree with those of mitochondria isolated from leg muscles of the toad upon the addition of ADP. Apparently the mitochondria in resting, excised muscles are not supplied with an excess of substrate. Measurements on the intercellular pH showed that even limited activity ( < 5 twitches) initiates glycolysis. The primary control of respiration resides, nevertheless, in the ADP concentration, rather than in the levels of substrate or inorganic phosphate. The results are quantitatively consistent with the view that ATP is the primary energy donor for muscular contraction.

MgADP promotes a catch-like state developed through force-calcium hysteresis in tonic smooth muscle

Tonic rabbit femoral artery and phasic rabbit ileum smooth muscles permeabilized with Triton X-100 were activated either by increasing [Ca2+] from pCa > 8.0 to pCa 6.0 (calcium-ascending protocol) or contracted at pCa 6.0 before lowering [Ca2+] (calcium-descending protocol). The effects of, respectively, high [MgATP]/low [MgADP] [10 mM MgATP + creatine phosphate (CP) + creatine kinase (CK)] or low [MgATP]/[MgADP] (2 mM MgATP, 0 CP, 0 CK) on the "force-[Ca]" relationships were determined. In femoral artery at low, but not at high, [MgATP]/[MgADP] the force and the ratio of stiffness/force at pCa 7.2 were significantly higher under the calcium-descending than calcium-ascending protocols (54% vs. 3% of Po, the force at pCa 6.0) (force hysteresis); the levels of regulatory myosin light chain (MLC20) phosphorylation (9 +/- 2% vs. 10 +/- 2%) and the velocities of unloaded shortening V0 (0.02 +/- 0.004 l/s with both protocols) were not significantly different. No significant force hysteresis was detected in rabbit ileum under either of these experimental conditions. [MgADP], measured in extracts of permeabilized femoral artery strips by two methods, was 130-140 microM during maintained force under the calcium-descending protocol. Exogenous CP (10 mM) applied during the descending protocol reduced endogenous [MgADP] to 46 +/- 10 microM and abolished force hysteresis: residual force at low [Ca2+] was 17 +/- 5% of maximal force. We conclude that the proportion of force-generating nonphosphorylated (AMdp) relative to phosphorylated cross-bridges is higher on the Ca2+-descending than on the Ca2+-ascending force curve in tonic smooth muscle, that this population of positively strained dephosphorylated cross-bridges has a high affinity for MgADP, and that the dephosphorylated AMdp . MgADP state makes a significant contribution to force maintenance at low levels of MLC20 phosphorylation.

Determination of free creatine and phosphocreatine concentrations in the isolated perfused rat heart by 1H- and 31P-NMR

To measure free creatine in the isolated perfused rat heart, the concentration of phosphocreatine, and phosphocreatine plus creatine (sigma Cr) were measured by 31P- and 1H-NMR, respectively. Quantification was performed in the presence and absence of an intraventricular balloon filled with a known amount of PCr, which acted as an external standard. Total (free plus bound) phosphocreatine and creatine were measured by HPLC analysis of extracts from the same hearts, freeze-clamped at the end of the perfusions. A greater concentration of creatine (mumol/g dry wt.) in the perfused rat heart was measured by HPLC analysis (40.3 +/- 2.38 (11)) as compared to NMR (34.6 +/- 1.95 (11)), whilst no significant difference was observed in the measurement of phosphocreatine between the two assay methods. Consequently, a greater sigma Cr was measured by HPLC. This work suggests that the majority of Cr in the heart is NMR visible and unbound, so available to interact with creatine kinase. The lower free ADP concentration calculated from NMR measurements (53.3 +/- 3.80 microM (9)) was not significantly different from that determined by HPLC analysis (56.9 +/- 5.90 microM (9)). This suggests that the concentration of free ADP in the heart is higher than values where it can regulate oxidative phosphorylation most effectively.

In vivo regulation of mitochondrial respiration in cardiomyocytes: specific restrictions for intracellular diffusion of ADP

Relative diffusivities of ADP and creatine in cardiomyocytes were studied. The isolated rat cardiomyocytes were lysed with saponin (40 micrograms/ml) to perforate or completely disrupt sarcolemma that was evidenced by leakage of 80-100% lactate dehydrogenase. In these cardiomyocytes mitochondria were used as 'enzymatic probes' to determine the average local concentration of substrates exerting acceptor control of respiration--ADP or creatine (the latter activates respiration via mitochondrial creatine kinase reaction)--when their concentrations in the surrounding medium were changed. The kinetic parameters for ADP and creatine in control of respiration of saponin-treated cardiomyocytes were compared with those determined in isolated mitochondria and skinned cardiac fibers. The apparent Km for creatine (at 0.2 mM ATP) was very close and in a range of 6.0-6.9 mM in all systems studied, showing the absence of diffusion difficulties for this substrate. On the contrary, the apparent Km for ADP increased from 18 +/- 1 microM for isolated mitochondria to 250 +/- 59 microM for cardiomyocytes with the lysed sarcolemma and to 264 +/- 57 microM for skinned fibers. This elevation of Km was not eliminated by inhibition of myokinase with diadenosine pentaphosphate. When 25 mM creatine was present, the apparent Km for ADP decreased to 36 +/- 6 microM. These data are taken to indicate specific restrictions of diffusion of ADP most probably due to its interaction with intermediate binding sites in cardiomyocytes. The important role of phosphocreatine-creatine kinase system of energy transport is to overcome the restrictions in regulation of energy fluxes due to decreased diffusivity of ADP.

Differential extractability of creatine phosphate and ATP from cardiac muscle with ethanol and perchloric acid solution

To compare the extractability of creatine phosphate with that of ATP by alcohol extraction, both compounds were extracted from normal perfused rat heart tissues by using various stepwise concentrations of ethanol and 0.4 M HClO4. Powdered samples (6-15 mg wet wt) from the freeze-clamped tissues were homogenized in 2 ml of the ethanol solutions. After centrifugation, the supernatant was removed; each centrifuged sediment was rehomogenized with 2 ml of 0.4 M HClO4 and centrifuged. The supernatant was neutralized with 0.4 m KHCO3. The same powdered samples were directly homogenized with 2 ml of 0.4 M HClO4 and treated in the same manner. Only a small amount of ATP in the tissues was extracted by an 85% or higher concentration of ethanol. Further, about 13% of the tissue ATP was not extractable by the subsequent perchloric acid extraction. In contrast to ATP, creatine phosphate in the tissues was partially extracted by 95% ethanol and nearly all of the tissue creatine phosphate was extracted by 70% ethanol. The total creatine phosphate obtained by 70% ethanol and by subsequent perchloric acid extraction was significantly higher than that obtained by direct perchloric acid extraction. From these results, it was concluded that the extractability of creatine phosphate in the tissue by alcohol extraction is clearly different from that of ATP. Additionally, the stepwise extraction is recommended as a useful method for the extraction of energy metabolites in perfused rat heart tissue.

Cellular concentrations of enzymes and their substrates

The activity of crude and pure enzyme preparations as well as the molecular weight of these enzymes were obtained from the literature for several organisms. From these data enzyme concentrations were calculated and compared to the concentration(s) of their substrates in the same organism. The data are expressed as molar ratios of metabolite concentration to enzyme site concentration. Of the 140 ratios calculated, 88% were one or greater, indicating that in general substrates exceed their cognate enzyme concentrations. Of the 17 cases where enzyme exceeds metabolite concentration, 16 were in glycolysis. The data in general justify the use of enzyme kinetic mechanisms determined in vitro in the construction of dynamic models which simulate in vivo metabolism.

Role of phosphocreatine in energy transport in skeletal muscle of bullfrog studied by 31P-NMR

To evaluate the energy-shuttle hypothesis of the phosphocreatine/creatine kinase system, diffusion rates for ATP, phosphocreatine and flux through the creatine kinase reaction were determined by 31P-NMR in resting bullfrog biceps muscle. The diffusion coefficient of phosphocreatine measured by 31P-pulsed gradient NMR was 1.4-times larger than ATP in the muscle, indicating the advantage of phosphocreatine molecules for the intracellular energy transport. The flux of the creatine kinase reaction measured by 31P-saturation transfer NMR was 3.6 mmol/kg wet wt. per s in the resting muscle. The flux is equal to the turnover rate of ATP, ADP, phosphocreatine and creatine molecules, therefore, the life-times of these substrates and the average distance traversed after the life-times by the diffusing molecules were calculated using the diffusion coefficients obtained by 31P-NMR. The mean square length of one-dimensional diffusion was 22 microns in ATP molecules and the minimum diffusion length was 1.8 microns in ADP molecules. The latter was calculated using free ADP concentration, 30 mumol/kg wet wt., obtained from the equilibrium constant of the creatine kinase reaction and the diffusion coefficient assumed to be the same of ATP in muscle. Similar diffusion lengths of ADP were calculated using the reported values for the flux of the creatine kinase reaction in heart and smooth-muscle. The diffusion lengths of all substrates involved in the creatine kinase reaction were larger than the radii of myofibrils. Therefore, in the muscles with an alternating arrangement of mitochondria and myofibrils, such as heart and certain skeletal muscles, ATP and ADP molecules can move freely between myofibrils and mitochondria without the aid of the creatine kinase reaction; thus, we conclude that the energy-shuttle hypothesis is not obligatory for energy transport between the mitochondria and the myofibrils.

Extraction of adenine nucleotides from cultured endothelial cells

The goal of this work was to find a method suitable for the extraction of adenine nucleotides from cultured vascular endothelial cells. Extraction of cell monolayers with 80% methanol in water yielded extracts with a higher content of ATP than did extraction of cells with perchloric acid, trichloroacetic acid, or boiling water. The optimal extraction solution was 80% methanol with 0.5 mM ethylene glycol bis(beta-aminoethyl ether) N,N'-tetraacetic acid (EGTA) or EDTA, heated to 70 degrees C immediately before use. Extraction of nucleotides by this solution was rapid and the recovery of exogenous ATP added during the extraction process was generally greater than 90%. An aqueous methanol or ethanol solution may be applicable for the extraction of nucleotides and other metabolites from cultured animal cells, dispersed cells, and frozen, powdered tissues.

Compartmentation of high-energy phosphates in resting and beating heart cells

The subcellular distribution of ATP, ADP, creatine phosphate and creatine has been analyzed by fast detergent fractionation of isolated frog heart cells. Digitonin fractionation (0.5 mg/ml, 10 s at 2 degrees C in 20 mM 4-morpholinepropanesulfonic acid/3 mM EDTA/230 mM mannitol medium) was used to separate mitochondria and myofilaments from cytosol. To separate myofilaments from the other cellular compartments. Triton X-100 was used (2%, 15 s in the same medium as digitonin). For either resting or beating cells the total cellular contents of ATP, ADP, creatine phosphate and creatine was similar, nevertheless the O2 consumption was 6-times higher. The compartmentation of these metabolites was also identical. Myofilaments contain 1.1 nmol ADP per mg total cellular proteins. In the cytosolic compartment the metabolite concentrations, all measured in nmol per mg total cellular proteins, were: ATP, 13; ADP, 0.25-0.05; creatine phosphate, 18.5 and creatine, 14. This indicated that the reaction catalyzed by creatine kinase was in a state of (or near) equilibrium.

Cytosolic adenylates and adenosine release in perfused working heart. Comparison of whole tissue with cytosolic non-aqueous fractionation analyses

Free cytosolic adenylates were examined in relation to adenosine plus inosine released from perfused working guinea-pig hearts. Whole-tissue adenylate data from freeze-clamped hearts were quantitatively compared with corresponding values obtained by subcellular fractionation of homogenized myocardium in non-aqueous media. Adenosine and inosine in venous cardiac effluents were measured by high-performance liquid chromatography. Hearts, perfused at their natural flows, were subjected to various workloads, substrates and catecholamines to alter myocardial energy metabolism and respiration over a wide physiological range. Non-aqueous cytosolic ATP and creatine phosphate (CrP) accounted for more than 80% of the respective total myocardium content. The cytosolic CrP/Pi ratio was in near-quantitative agreement with the overall tissue CrP/Pi ratio when the latter parameter was corrected for extracellular Pi. This was conclusive evidence that ATP, CrP and Pi were predominantly located in the cytosol of the well-oxygenated cardiomyocyte. Measured myocardial oxygen uptake (MVO2) was reciprocally related to the phosphorylation state of CrP [( CrP]/[Cr] X [Pi]) and hence that of ATP [( ATP]/[ADP] X [Pi]) assuming the creatine kinase at near-equilibrium at a near-constant pH of 7.2. On the other hand, calculated mean free cytosolic ADP concentrations increased essentially linearly up to threefold with increasing MVO2 in the presence of virtually unchanged or only slightly decreased ATP levels; this was found both according to the whole tissue and the special subcellular fractionation data. Employing the myokinase mass-action ratio and substituting total cardiac ADP by the mean free cytosolic ADP concentrations, the mean free cytosolic AMP concentrations proved to be in the nanomolar range, i.e. up to three orders of magnitude lower than the overall tissue AMP content. We propose, therefore, that in the normoxic heart, AMP is located predominantly in the mitochondrial compartment. Nevertheless, both free cytosolic AMP concentration and release of adenosine plus inosine were apparently square or even higher-power functions of the rate of cardiac respiration. On the other hand, the mean purine nucleoside release seemed linearly correlated (r = 0.920) with the calculated free cytosolic AMP concentration. Our observations seem to suggest that the concentrations of free ADP and AMP in the cytosol are major determinants of the production of inosine and coronary vasodilator adenosine.(ABSTRACT TRUNCATED AT 400 WORDS)

Energetics of human muscle: exercise-induced ATP depletion

The energetics of human muscle have been investigated in vivo during and after fatiguing aerobic, dynamic exercise. Changes in cytoplasmic pH and concentrations of phosphocreatine, ATP and Pi were followed using 31P nuclear magnetic resonance spectroscopy. ATP was significantly depleted in 6 out of 12 experiments and in these 6 experiments decreased to 55 +/- 5% of the pre-exercise concentration. Depleted muscle had a lower phosphocreatine concentration (17 +/- 5% of resting value) and lower pH (6.12 +/- 0.04) than fatigued muscle in which ATP loss was not observed (26 +/- 5% for phosphocreatine and 6.37 +/- 0.09 for pH). The free energy of hydrolysis of ATP was not significantly different in the two groups and was also similar in exhausted and nonexhausted muscle. Loss of ATP was associated with altered recovery of the muscle: [phosphocreatine], [Pi], and pH returned more slowly to their pre-exercise values and the initial rate of oxidative phosphorylation was diminished. The restitution of [ATP] to its pre-exercise value was much slower than that of the other metabolites.

Features of glycogen phosphorylase from the body wall musculature of the lugworm Arenicola marina and the mode of activation during anoxia

The activities of glycogen phosphorylases a and b from the body wall musculature of the marine worm Arenicola marina (Annelida, Polychaeta) were determined after various periods of anoxia. Already under normoxic conditions one third of the total activity was produced from the a form. During anoxia the ratio of both forms as well as the total activity did not change. The activity of soluble phosphorylase kinase was comparatively low in this tissue 4.3 +/- 1.2 nmol . min-1 . (g wet wt.)-1; the fast twitching tail muscle of shrimps, e.g., had a 10-fold higher phosphorylase kinase activity, whereas phosphorylase activities in both tissues were about the same 2.3 +/- 0.5 mumol . min-1 . (g wet wt.)-1. Glycogen phosphorylase b was purified from the body wall tissue of the marine worm in one step by 5'-AMP-Sepharose resulting in a single protein band in SDS-PAGE. This preparation was accepted as substrate by the phosphorylase kinase from rabbit muscle but a complete phosphorylation could not be achieved. The molecular mass of native phosphorylase was approximately 216 kDa, that of subunits 95 kDa indicating that the enzyme exists as a dimer. There were no isozymes in this preparation, the RF-value (0.17) of the single band in PAGE ranged between those of the isozymes from mice hearts. The activities of phosphorylases b and a were similarly dependent on pH and temperature but differed drastically in the affinities to phosphate and AMP. In presence of 1 mM AMP the app. Km of phosphorylase a for phosphate was 16 mM, that of phosphorylase b above 100 mM.(ABSTRACT TRUNCATED AT 250 WORDS)

Mitochondrial respiratory control in the myocardium

The heart muscle has proved to be a practical model for studying respiratory control in intact tissues. It also demonstrates that control at the level of the respiratory chain is augmented by metabolic control at the substrate level as exemplified by the very narrow range of changes in the redox state of the mitochondrial NADH/NAD couple even during extensive changes in ATP and oxygen consumption. The behaviour of mitochondria when isolated can largely be duplicated in the intact myocardium. Moreover, the high intracellular concentrations of enzymes, coenzymes and adenine nucleotides create conditions of high reaction rates, enabling the formation of a near equilibrium network of certain main pathways. This equilibrium network in connection with metabolic regulation of the hydrogen pressure upon the matrix NADH/NAD pool is a prerequisite for the regulation of cellular respiration at a high efficiency of energy transfer. Experimentation on the intact myocardium also seems to be capable of resolving some of the uncertainties about prevailing mechanisms for the regulation of cellular respiration.

Phosphagens and intracellular pH in intact rabbit smooth muscle studied by 31P-NMR

Phosphorus-31 nuclear magnetic resonance (NMR) spectra (103.2 MHz) were obtained from rabbit portal vein, urinary bladder, and taenia coli smooth muscle. The muscles were mounted isometrically as strips in a horizontal probe and superfused with oxygenated medium at 23 degrees C. Under these conditions the preparations could remain in a stable metabolic state for at least 24 h. The resonances observed represented phosphocreatine (PC), adenosine triphosphate (ATP), and phosphomonoester. The PC-ATP ratio in relaxed preparations was approximately 1.9 (bladder) and 1.4 (taenia coli), approximately consistent with chemical analysis of tissue extracts by isotachophoresis. However, the levels of adenosine diphosphate (ADP) and inorganic phosphate (Pi) were considerably lower as estimated by NMR, suggesting intracellular compartmentalization. Contraction for 30 min in high-K+ medium caused a reversible 10-30% decline of PC but no change in ATP. Intracellular pH was 7.0 +/- 0.1 and was unchanged by contraction or inhibition of tissue respiration by cyanide. Treatment with 2-deoxyglucose resulted in accumulation of 2-deoxyglucose 6-phosphate, verifying the pH assignment. However, the phosphomonoester resonance of normal spectra is not glucose 6-phosphate. Treatment with Mg2+-free, high-Mg2+ (10 mM), or hyperosmotic media did not alter the Mg2+ saturation of ATP. The results obtained by the nontissue-destructive 31P-NMR measurements are consistent with the view that metabolic steady-state conditions are maintained during contraction in the smooth muscle.

Compartmentation of intracellular nucleotides in mammalian cells

The important role of nucleotides in cellular metabolism requires that serious consideration be given to the question of the homogeneity or inhomogeneity of nucleotide pools in cells. The purpose of this review is to summarize the existing evidence for compartmentation of nucleotide pools, discuss the limitations of this evidence, and to discuss the implications of compartmentation for the interpretation of nucleotide concentration measurements. Evidence for nucleotide compartmentation comes from the following types of evidence: compartmentation of RNA precursors; compartmentation of deoxynucleoside triphosphates; mitochondrial compartmentation; the existence of tightly bound nucleotides; pools derived from alternative synthetic routes; compartmentation in cyclic nucleotide metabolism; channeling in the synthesis of pyrimidine nucleotides; and others. The types of evidence adduced for compartmentation will be considered critically and in detail, and alternative explanations considered, as well. Implications of the data and hypotheses on nucleotide compartmentation for the interpretation of nucleotide pool measurements in various types of experiments will be discussed.

The discovery of a rapidly metabolized polymeric tetraphosphate derivative of adenosine in perfused rat heart

The predicted presence in perfused rat hearts of a rapidly metabolized but hitherto unrecognized form of adenosine phosphate has been confirmed by specific radioactive labelling. The properties of the purified compound suggest that it is a heteropolymer of a small organic acid, phosphate and purine nucleoside in the proportions 1:4:1.

Creatine kinase as an intracellular regulator

Several recent studies have demonstrated the presence of creatine kinase and of phosphorylcreatine in a variety of cells besides striated muscle and brain cells. The total creatine kinase and phosphagen levels in these cells encompass a wide range of values. The available data are collected in this article to demonstrate that the variation of the enzyme and phosphagen concentrations is not random but that the two are interrelated. With both the major isoenzymes of creatine kinase, namely the muscle type and the brain type, the basal levels of phosphorylcreatine follow closely the cellular creatine kinase levels. A hypothesis is presented in which the enzyme itself is the major determinant of phosphorylcreatine content by virtue of its ability to act as an intracellular binding protein for creatine derived from extracellular fluid, and also for cellular ADP. The proposed mechanism further predicts that in cells that contain high levels of actin and thus sequester the cytoplasmic free ADP (e.g. most muscle cells), a high level of creatine kinase can effectively regulate the myokinase reaction by its ability to bind ADP. The net effect of such regulation is to conserve the adenine nucleotide pools in the cell. The evolutionary advantage of these two regulatory functions of creatine kinase in terms of energy conservation is discussed.

Metabolic recovery after exercise and the assessment of mitochondrial function in vivo in human skeletal muscle by means of 31P NMR

It has been suggested that the rate of phosphocreatine resynthesis after exercise is an index of mitochondrial oxidative metabolism in intact muscle. To investigate this hypothesis, the time courses of metabolite recovery following mild and more severe dynamic exercise of human forearm muscle were compared by means of 31P NMR. Severe exercise resulted in greater net hydrolysis of phosphocreatine and greater intracellular acidosis than light exercise. The rate of phosphocreatine resynthesis was significantly slower during recovery from the more severe exercise. To explain this it was noted that, as a consequence of the high activity of creatine kinase in the sarcoplasm, the [phosphocreatine] at any time is a function of the intracellular pH. Calculations demonstrate that the difference between rates of phosphocreatine recovery after the two exercise protocols was primarily determined by the rates of recovery of the intracellular pH to normal rest values. It is concluded that the calculated rate of recovery of the cytosolic free [ADP] to its pre-exercise concentration may provide a more specific measure of mitochondrial oxidative activity.

Adenine nucleotide metabolism and compartmentalization in isolated adult rat heart cells

The metabolism and intracellular compartmentalization of adenine nucleotides in a preparation of adult rat heart myocytes showing good morphology, viability, and tolerance to calcium ion has been examined by high performance liquid chromatography. These myocytes contain an average of 23 nmol adenine nucleotide per milligram protein which is about 60% of the adenine nucleotide content of intact rat heart tissue. The loss of adenine nucleotide occurs during the incubation and washing steps that increase the yield of viable cells, rather than during the collagenase perfusion. An analysis of cellular compartments shows that the adenine nucleotide of the cell consists of 17 nmol adenine nucleotide in the cytosol, 5 nmol in the mitochondria, and 1.3 nmol adenosine diphosphate bound to myofibrils per milligram cell protein. Myocytes lose both adenosine triphosphate and adenine nucleotide when incubated anaerobically in the absence of glucose, and the lost adenine nucleotide can be accounted for as increased inosine, adenosine, and inosine monophosphate. Myocytes that contain less than 0.1 nmol of cytosol adenosine triphosphate per milligram cell protein maintain an intact sarcolemma, but are unable to carry out anaerobic glycolysis. Reoxygenation of anaerobic cells results in restoration of energy charge and a net resynthesis of about 2 nmol adenine nucleotide per milligram protein. Adenosine and inosine monophosphate decrease on reaeration of anaerobic cells, whereas inosine levels increase. When iodoacetate is added to block glycolysis, the decline in adenine nucleotide and production of inosine monophosphate are accelerated and there is no resynthesis of adenine nucleotide when anaerobic cells are reoxygenated . Large accumulations of inosine monophosphate are also seen in myocytes treated with an uncoupler of oxidative phosphorylation.

Compartmentation of high-energy phosphates in resting and working rat skeletal muscle

The subcellular distribution of high-energy phosphates in various types of skeletal muscle of the rat was analysed by subfractionation of tissues in non-aqueous solvents. Different glycolytic and oxidative capacities were calculated from the ratio of phosphoglycerate kinase and citrate synthase activities, ranging from 25 in m. soleus to 130 in m. tensor fasciae latae. In the resting state, the subcellular contents of ATP, creatine phosphate and creatine were similar in m. soleus, m. vastus intermedius, m. gastrocnemius and m. tensor fasciae latae but, significantly, a higher extramitochondrial ADP-content was found in m. soleus. A similar observation was made in isometrically and isotonically working m. gastrocnemius. The extramitochondrial, bound ADP accounted fully for actin-binding sites in resting fast-twitch muscles, but an excess of bound ADP was found in m. soleus and working m. gastrocnemius. The amount of non-actin-bound ADP reached maximal values of approx. 1.2 nmol/mg total protein. It could not be enhanced further by prolonged isotonic stimulation or by increased isometric force development. It is suggested that non-actin-bound ADP is accounted for by actomyosin-ADP complexes generated during the contraction cycle. Binding of extramitochondrial ADP to actomyosin complexes in working muscles thus acts as a buffer for cytosolic ADP in addition to the creatine system, maintaining a high cytosolic phosphorylation potential also at increasing rates of ATP hydrolysis during muscle contraction.

The application of nuclear magnetic resonance to the study of cellular physiology

Nuclear magnetic resonance (NMR) is a form of absorption spectroscopy that can noninvasively monitor the intracellular concentration and several kinetic properties of numerous organic and inorganic compounds. Utilizing these characteristics, investigators have demonstrated that NMR is a useful tool in the study of cellular physiology. In this review, the techniques for using NMR to study isolated cells are outlined with suggestions for the determination of cellular viability within the NMR spectrometer. Whenever feasible, cell preparations that are continuously perfused are preferred, because they can be constantly fed and controlled. Results of various NMR experiments on isolated cells using several nuclides are reviewed to highlight the type of information NMR can provide about cellular physiology. Several important differences between NMR and chemical extraction data are noted. The reason for these differences is probably related to the chemical extraction techniques determining the total amount of a compound within the cell in comparison to NMR, which is somewhat more specific, theoretically, detecting only the free species within the cytosol.

Quantitative determination of myosin and actin in rabbit skeletal muscle

The myosin and actin content of muscle tissue and purified myofibrils from rabbit psoas muscle has been determined. Myofibrils were purified using Percoll gradients, which allowed rapid separation from nuclei and connective tissue proteins. Myosin and actin were quantitated by amino acid analysis of the appropriate bands from sodium dodecyl sulfate/polyacrylamide gels. Muscle tissue contained 94 and 619 nmol/g wet weight of myosin and actin, respectively, while myofibrils had 0.82 and 5.37 mumol/g protein. Thus myosin contributed 43% and actin 22% of the myofibril protein mass. The value of 2.5 myosins per 14.3 nm repeat as calculated from these results suggests that thick filament models with mixtures of two and three crossbridges per repeat should be considered.

The influence of the thyroid state on energy turnover during tetanic stimulation in the fast-twitch (mixed type) muscle of rats

The effects of thyroid hormones on energy metabolism and force development during tetanic stimulation of fast-twitch skeletal muscle (mixed type) were studied. Hypothyroid, euthyroid, and hyperthyroid (ten days of daily administration of 15 micrograms T3/100 gm body weight) rats showed the same tension-time integral (force x time) under tetanic stimulation. The ATP turnover rate was significantly lower in the hypothyroid group than in the euthyroid and hyperthyroid animals. The ratio difference in the energy/force ratio in the Tx group cannot be ascribed to differences in internal work. Lactate production was diminished in the hypothyroid group; this was probably not the result of a block in glycogenolysis but rather was a reflection of more economic energy expenditure during contraction. After tetanic stimulation, the phosphorylation potential ([ATP]/[ADP]f[Pi]) was decreased most strongly in the hyperthyroid group, mainly because of the higher accumulation of Pi compared with the other groups, which suggests that during the recovery phase the stimulation of oxidative phosphorylation is greater in this group.

The composition of animal cells: solutes contributing to osmotic pressure and charge balance

The cytoplasmic solutes of vertebrates and invertebrates, other than Na, K and Cl, are surveyed in relation to their influence on ionic regulation through osmolality and charge balance. The most abundant include MgATP, phosphagens, amino acids, various other nitrogen and phosphorus compounds and sometimes anaerobic end products and antifreeze agents. Differences in muscle osmolality, e.g. between marine and non-marine animals, affect mainly nitrogenous solutes of no net charge, such as certain amino acids, taurine, betaine, trimethylamine oxide and urea. The high osmolality of axoplasm in marine invertebrates is due more to anions such as aspartate, glutamate and isethionate.

The contents of high-energy phosphates in different fibre types in skeletal muscles from rat, guinea-pig and man

1. The contents of high-energy phosphates at rest have been measured in skeletal muscles with different fibre-type composition from rat, guinea-pig and man. All muscles studied biochemically have been characterized histochemically. 2. Fast-twitch muscles had a higher ATP/ADP ratio than slow-twitch muscles and, with the exception of the tongue in the rat, higher contents of ATP and phosphocreatine. 3. There was an inverse relationship between the content of phosphocreatine and the stainability for succinyl dehydrogenase, which is a marker enzyme for oxidative capacity. 4. The biochemical and histochemical data are discussed in relation to known morphological and functional properties of the different muscle-fibre types. It is concluded that fast-twitch fibres have a high ATP/ADP ratio favouring a fast acceleration of energy production. The content of phosphocreatine seems to be related to the glycolytic capacity but not to the contraction time. In addition to being an immediate energy source, phosphocreatine functions as a buffer against lactic acidosis.

Creatine kinase from the bovine myometrium: purification and characterization

Creatine kinase from the smooth muscle of the cow uterus was extracted and purified by procedures involving precipitation of the enzyme in the presence of ethanol, cation exchange chromatography on phosphocellulose, gel filtration in Sephadex G-150 and anion exchange chromatography on DEAE-cellulose. The purified enzyme eluted as a single active peak after rechromatography on Sephadex G-150 with a molecular weight of about 82 000. Electrophoresis in polyacrylamide gels in tris-glycine buffer (pH 8.6) under non-denaturing conditions revealed a single enzymatically active protein band. In the presence of sodium dodecyl sulphate, the enzyme migrated as a single band in polyacrylamide gels at a mobility corresponding to a molecular weight of about 40 000 per subunit. Reaction with iodoacetamide indicated the presence of sulphydryl groups of differing susceptibility to alkylation. The purified enzyme was optimally active over a wide pH range (6.5-8.0). The Michaelis constants (Km) of the enzyme for MgADP and phosphoryl creatine (PCr) are 0.12 mM and 0.7 mM respectively, which are significantly lower than those for skeletal muscle CK. MgADP lowered the dissociation constant of the enzyme for PCr (from about 3.6 mM to 0.7 mM). Evidence is presented that the high affinity of the smooth muscle CK to MgADP and the MgADP-mediated facilitation of PCr binding might be key factors in the role of this enzyme in harnessing the energy reserves of the cell.

Intracellular energy transport and control of cardiac contraction

The participation of the intracellular creatine kinase system in energy transport in cardiac cells was studied further. The functional behavior of different but kinetically indistinguishable isoenzymes of creatine kinase (CK) in muscle cells is determined by their intracellular localization as is shown in this report for mitochondrial and sarcolemmal creatine kinases. The kinetics of the forward mitochondrial creatine kinase reaction is influenced by oxidative phosphorylation which increases the apparent affinity for ATP but does not change the kinetics of the reverse creatine kinase reaction. The molar content of creatine kinase in heart mitochondria was determined and found to be close to the content of adenine nucleotide translocase, thus supporting the concept of the tight functional relationship between those two mitochondrial proteins as a basis for effective phosphocreatine (PCr) production in mitochondria. In the sarcolemmal preparation, the antiport of Na+ and K+ is much more effectively supported by the sarcolemmal creatine kinase reaction than by an externally added ATP-regenerating system consisting of phosphoenolpyruvate and pyruvate kinase. The results of these experiments are taken to show the ability of sarcolemmal creatine kinase to maintain a very high phosphorylation potential in the vicinity of the active centers of the Na+ -K+ ATPase necessary to support the active transport of Na+ and K+ across the plasma membrane and to avoid a reversal of the ion gradient. Finally, it is concluded in this chapter that a rapid decrease in PCr content in the cells under anoxic or ischemic conditions may be one of the important factors in the impairment of cardiac contractile function under those conditions.

Application of 31P-NMR spectroscopy to the study of striated muscle metabolism

This review presents the principles and limitations of phosphorus nuclear magnetic resonance (31P-NMR) spectroscopy as applied to the study of striated muscle metabolism. Application of the techniques discussed include noninvasive measurement of high-energy phosphate, intracellular pH, intracellular free Mg2+, and metabolite compartmentation. In perfused cat biceps (fast-twitch) muscle, but not in soleus (slow-twitch), NMR spectra indicate a substantially lower (1 mM) free inorganic phosphate level than when measured chemically (6 mM). In addition, saturation and inversion spin-transfer methods that enable direct measurement of the unidirectional fluxes through creatine kinase are described. In perfused cat biceps muscle, results suggest that this enzyme and its substrates are in simple chemical equilibrium.

In perfused rat hearts ischaemia promotes the reversible conversion of appreciable quantities of soluble adenine nucleotides to a stable trichloroacetic acid-precipitable form

Radioactivity from [14C] adenosine was linearly incorporated into tissue nucleotides in perfused rat hearts. All the TCA-extractable 14C was confined to the purine nucleoside phosphates for up to 1 h of perfusion. Radioactivity was also incorporated linearly into the TCA-insoluble fraction, which by 40 min accounted for 24% of the tissue 14 C. Estimates based on precursor specific radioactivity suggest that at least 0.6 micro mol/g of the mononucleotide is in this stable insoluble form. Following 2 min total ischaemia, the tissue nucleotide content and soluble radioactivity decreased while the insoluble radioactivity showed a corresponding increase to account now for 35% of the tissue radiolabel. This redistribution was rapidly reversed by post-ischaemic reperfusion. A possible function for the rapid reversible sequestration of adenine nucleotides in ischaemia is proposed.

Compartmentation of adenine nucleotides in the isolated working guinea pig heart stimulated by noradrenaline

Relationships between subcellular adenine nucleotides (ATP, ASP), heart function and oxidative myocardial metabolism were studied in the isolated working guinea pig heart. The heart preparations were stimulated by noradrenaline and utilized pyruvate alone or in combination with glucose as energy-providing substrates. Using density gradient centrifugation of lyophilized myocardial homogenates in non-aqueous media the following subcellular distribution of ATP and ADP, respectively, was obtained: The concentration of ATP in the cytosol was higher than in the mitochondria while the content of ADP was not different. The overall ATP/ADP ratio in the cytosol was more than 10-fold lower than the concentration ratio of free ATP and ADP in the cytosol as derived from the cytosolic creatine kinase equilibrium. Furthermore, the mitochondrial ATP/ADP ratio was much lower than the free cytosolic ATP/ADP ratio. The concentration term of the phosphorylation potential of ATP (RT in [ADP] x [Pi]/[ATP]) was thus higher in the cytosol than in the mitochondria. Myocardial function and substrate oxidation exhibited typical augmentations during infusion of 0.08 microM noradrenaline. However, increased heart performance and oxidative myocardial metabolism were not associated with major changes in the cytosolic ATP or ADP contents. On the other hand, the free ATP/ADP ratio and particularly the phosphorylation state of ATP, i.e. the ration [ATP]/[ADP] x [Pi], were decreased in the cytosol. In contrast, in the mitochondria adenine-nucleotide concentration ratios were not substantially changed under the same conditions. The results are compatible with an asymmetrical translocation of adenine nucleotides across the mitochondrial membrane in working hearts. The reciprocal relationship between rates of oxidative metabolism and free cytosolic ATP/ADP ratio indicates that mitochondrial respiration in the intact heart could be controlled by the phosphorylation state of the extramitochondrial ATP.

Protein synthesis in the isolated perfused rat heart. Effects of mechanical work load, diastolic ventricular pressure and coronary pressure on amino acid incorporation and its transmural distribution into left ventricular protein

The mechanisms involved in hypertrophy of the left ventricle were studied in Langendorff-perfused rat hearts by measuring the ventricular protein synthesis and its transmural distribution and by differentiating between the effects of changes in mechanical work load, intraventricular and coronary pressures. An increase in the aortic pressure from 7.85 kPa (80 cm of water) to 19.6 kPa (200 cm of water) in beating hearts increased phenylalanine incorporation into left ventricular protein from 1.4 to 2.0 mumol/g protein (p less than 0.02) during a two-hour perfusion. The protein synthesis was transmurally evenly distributed. A similar elevation in the perfusion pressure in potassium arrested hearts caused an increase in phenylalanine incorporation from 1.5 to 1.9 mumol/ (p less than 0.05) when the intraventricular pressure was adjusted to zero, indicating that the increase in aortic (coronary) pressure and not the work load per se was the reason for increased protein synthesis. Elevation of the end-diastolic pressure from zero to approximately 2 kPa in beating hearts at an aortic pressure of 7.85 kPa, or from 7.85 kPa to 17.3 kPa in K+-arrested hearts, at an aortic pressure of 19.6 kPa caused a significant reduction in subendocardial protein synthesis, whereas subepicardial phenylalanine incorporation was at most only slightly affected. The energetic parameters, oxygen consumption, output of vasoactive purine compounds and distribution of coronary flow indicate that the increase in protein synthesis via the elevation in aortic pressure was not due to the abolition of partial anoxia, whereas the same parameters indicate that the transmural gradient in protein synthesis observed under certain conditions was due to subendocardial ischemia when the intraventricular pressure approached the aortic pressure in arrested hearts, which are evidently of restricted use for extended periods without special measures to limit the build-up of intraventricular pressure.

Metabolic adaptation in phosphorylase kinase deficiency. Changes in metabolite concentrations during tetanic stimulation of mouse leg muscles

1. Glycogen, nucleotides and glycolytic intermediates and products were measured before and during tetanus in the hamstrings-muscle groups of normal (C3H) and phosphorylase kinase-deficient (ICR/IAn) mice. 2. Phosphorylase kinase-deficient muscles contained 3-4-fold more glycogen and sustained a larger (approx. 2-fold), more rapid (11 +/- 2 ng/s faster) and more prolonged glycogenolysis during 120s tetanus despite their lack of phosphorylase a. 3. No significant change in total adenine nucleotide contents occurred during tetanus in either strain, but there was a 60-100-fold rise in IMP concentration to approx. 2mM in both strains. The initial rate of IMP formation was 6-fold more rapid (112 nmol/s per g) in phosphorylase kinase-deficient muscle. 4. Adenylosuccinate content rose to 36 nmol/g in phosphorylase kinase-deficient muscle and to 9 nmol/g in normal muscle at 45s tetanus, but then fell. 5. In phosphorylase kinase-deficient muscle, glucose 6-phosphate, a powerful phosphorylase inhibitor, was 56% of that in normal muscle. 6. The mass-action ratio of the phosphoglucomutase-catalysed reaction [glucose 6-phosphate]/[glucose 1-phosphate] was markedly lower than Keq. (approx. 17) in relaxed muscle of both strains (approx. 5-7), but rose significantly during tetanus to the value for Keq. 7. The data for IMP satisfy the criteria put forward by Rahim, Perrett & Griffiths [(1976) FEBS Lett. 69, 203-206] for a nucleotide activator of phosphorylase b: it should be present at a higher concentration in phosphorylase kinase-deficient muscle, its concentration should rise during muscle work, and it should attain a concentration comparable with its activation constant for phosphorylase b.