The kinetics of anaerobic metabolism following the initiation of high-intensity exercise.

A mathematical study is made of the kinetics of anaerobic metabolism following the initiation of high-intensity exercise. Power and energy relationships are proposed for oxygen-independent glycolysis, phosphocreatine utilisation and the utilisation of endogenous ATP. The power relations consist of two components, one describing the build-up phase, the other the controlled-utilisation phase. The controlled-utilisation phase of oxygen-independent glycolysis and the build-up of aerobic metabolism are shown to be closely inter-related. The theoretical relations display trends consistent with published experimental results. Some property values are derived, but because of the scatter of the experimental results, the values are, in general, to be regarded as tentative.     

Microcalorimetric determination of the kinetics of substrate utilisation by non-growing suspensions of Neisseria sicca

The metabolism of various substrates by non-growing suspensions of Neisseria sicca was investigated by a flow-microcalorimetric technique. Substrate utilisation showed Michaelis kinetics allowing determination of saturation constants (Km) and maximum specific rates of substrate utilisation (Vmax). Pyruvate, lactate, a number of tricarboxylic acid cycle intermediates, and amino acids (aspartate, glutamate and proline) were rapidly metabolised [Vmax 5-35 mumol (g dry wt cells)-1 min-1]; Km values were between 4 and 20 microM. Glucose, glycerol, acetate and the other amino acids investigated gave only a slight or no increase in power. The pattern of substrate utilisation is discussed in relation to the role of carbonic anhydrase in N. sicca.     

Kinetic mechanism of fully activated S6K1 protein kinase

S6K1 is a member of the AGC subfamily of serine-threonine protein kinases, whereby catalytic activation requires dual phosphorylation of critical residues in the conserved T-loop (Thr-229) and hydrophobic motif (Thr-389). Previously, we described production of the fully activated catalytic kinase domain construct, His(6)-S6K1alphaII(DeltaAID)-T389E. Now, we report its kinetic mechanism for catalyzing phosphorylation of a model peptide substrate (Tide, RRRLSSLRA). First, two-substrate steady-state kinetics and product inhibition patterns indicated a Steady-State Ordered Bi Bi mechanism, whereby initial high affinity binding of ATP (K(d)(ATP)=5-6 microM) was followed by low affinity binding of Tide (K(d)(Tide)=180 microM), and values of K(m)(ATP)=5-6 microM and K(m)(Tide)=4-5 microM were expressed in the active ternary complex. Global curve-fitting analysis of ATP, Tide, and ADP titrations of pre-steady-state burst kinetics yielded microscopic rate constants for substrate binding, rapid chemical phosphorylation, and rate-limiting product release. Catalytic trapping experiments confirmed rate-limiting steps involving release of ADP. Pre-steady-state kinetic and catalytic trapping experiments showed osmotic pressure to increase the rate of ADP release; and direct binding experiments showed osmotic pressure to correspondingly weaken the affinity of the enzyme for both ADP and ATP, indicating a less hydrated conformational form of the free enzyme.     

Time-resolved charge translocation by the Ca-ATPase from sarcoplasmic reticulum after an ATP concentration jump

Time-resolved measurements of currents generated by Ca-ATPase from fragmented sarcoplasmic reticulum (SR) are described. SR vesicles spontaneously adsorb to a black lipid membrane acting as a capacitive electrode. Charge translocation by the enzyme is initiated by an ATP concentration jump performed by the light-induced conversion of an inactive precursor (caged ATP) to ATP with a time constant of 2.0 ms at pH 6.2 and 24 degrees C. The shape of the current signal is triphasic, an initial current flow into the vesicle lumen is followed by an outward current and a second slow inward current. The time course of the current signal can be described by five relaxation rate constants, lambda1 to lambda5 plus a fixed delay D approximately 1-3 ms. The electrical signal shows that 1) the reaction cycle of the Ca-ATPase contains two electrogenic steps; 2) positive charge is moved toward the luminal side in the first rapid step and toward the cytoplasmic side in the second slow step; 3) at least one electroneutral reaction precedes the electrogenic steps. Relaxation rate constant lambda3 reflects ATP binding, with lambda(3,max) approximately 100 s(-1). This step is electroneutral. Comparison with the kinetics of the reaction cycle shows that the first electrogenic step (inward current) occurs before the decay of E2P. Candidates are the formation of phosphoenzyme from E1ATP (lambda2 approximately 200 s[-1]) and the E1P --> E2P transition (D approximately 1 ms or lambda1 approximately 300 s[-1]). The second electrogenic transition (outward current) follows the formation of E2P (lambda4 approximately 3 s[-1]) and is tentatively assigned to H+ countertransport after the dissociation of Ca2+. Quenched flow experiments performed under the conditions of the electrical measurements 1) demonstrate competition by caged ATP for ATP-dependent phosphoenzyme formation and 2) yield a rate constant for phosphoenzyme formation of 200 s(-1). These results indicate that ATP and caged ATP compete for the substrate binding site, as suggested by the ATP dependence of lambda3 and favor correlation of lambda2 with phosphoenzyme formation.     

Effects of the mitogen concanavalin A on pathways of thymocyte energy metabolism.

The lectin concanavalin A (Con A) acts as a mitogen that preferentially activates T-cells. It stimulates the energy metabolism of thymocytes within seconds of exposure. We studied short-term effects (<30 min) of Con A on a conceptually simplified model system of rat thymocyte energy metabolism in the concentration range of 0-2 microg Con A per 107 cells, using metabolic control analysis. The model system consisted of three blocks of reactions, linked by the common intermediate mitochondrial membrane potential (Delta[psi]m): the substrate oxidation reactions, which produce the linking intermediate, and the proton conductance (or leak) and ATP turnover pathways which consume Delta[psi]m. Firstly, we used top-down elasticity analysis to establish which subsystems are targeted by Con A. Secondly, we quantitatively analysed the steady-state regulation of the system variables by Con A: how do the subsystem fluxes respond to Con A individually and as a whole? Our results indicate that: (1) steady-state respiration and Delta[psi]m increase as Con A concentration is raised, but at higher concentrations the increase in respiration is less and Delta[psi]m falls; (2) Con A independently changes the kinetics of the reactions that produce and consume Delta[psi]m: the Delta[psi]m-producing reactions are inhibited, and the reactions involved in ATP turnover are stimulated; and (3) the overall effects of Con A are mostly mediated by effects on ATP turnover.     

Effects of dinitrophenol and oligomycin on the coupling between anaerobic metabolism and anaerobic sodium transport by the isolated turtle bladder

Dinitrophenol (1 x 10^-5 M) has been found to inhibit anaerobic sodium transport by the isolated urinary bladder of the fresh water turtle. Concurrently, anaerobic glycolysis was stimulated markedly. However, tissue ATP levels diminished only modestly, remaining at approximately 75% of values observed under anaerobic conditions without DNP. The utilization of glucose (from endogenous glycogen) corresponded closely to that predicted from the molar quantities of lactate formed. Thus the glycolytic pathway was completed in the presence of DNP and if ATP were synthesized normally during glycolysis, synthesis should have been increased. On the other hand, the decrease in Na transport should have decreased ATP utilization. Oligomycin did not block sodium transport either aerobically or anaerobically, but ATP concentrations did decrease. When anaerobic glycolysis was blocked by iodoacetate, pyruvate did not sustain sodium transport thus suggesting that no electron acceptors were available in the system. Two explanations are entertained for the anaerobic effect of DNP: (a) Stimulation by DNP of plasma membrane as well as mitochondrial ATPase activity; (b) inhibition of a high energy intermediate derived from glycolytic ATP or from glycolysis per se. The arguments relevant to each possibility are presented in the text. Although definitive resolution is not possible, we believe that the data favor the hypothesis that there was a high energy intermediate in the anaerobic system and that this intermediate, rather than ATP, served as the immediate source of energy for the sodium pump.     

An overview of endosymbiotic models for the origins of eukaryotes, their ATP-producing organelles (mitochondria and hydrogenosomes), and their heterotrophic lifestyle.

The evolutionary processes underlying the differentness of prokaryotic and eukaryotic cells and the origin of the latter's organelles are still poorly understood. For about 100 years, the principle of endosymbiosis has figured into thoughts as to how these processes might have occurred. A number of models that have been discussed in the literature and that are designed to explain this difference are summarized. The evolutionary histories of the enzymes of anaerobic energy metabolism (oxygen-independent ATP synthesis) in the three basic types of heterotrophic eukaryotes those that lack organelles of ATP synthesis, those that possess mitochondria and those that possess hydrogenosomes--play an important role in this issue. Traditional endosymbiotic models generally do not address the origin of the heterotrophic lifestyle and anaerobic energy metabolism in eukaryotes. Rather they take it as a given, a direct inheritance from the host that acquired mitochondria. Traditional models are contrasted to an alternative endosymbiotic model (the hydrogen hypothesis), which addresses the origin of heterotrophy and the origin of compartmentalized energy metabolism in eukaryotes.     

Anaerobic energy expenditure and mechanical efficiency during exhaustive leg press exercise

Information about anaerobic energy production and mechanical efficiency that occurs over time during short-lasting maximal exercise is scarce and controversial. Bilateral leg press is an interesting muscle contraction model to estimate anaerobic energy production and mechanical efficiency during maximal exercise because it largely differs from the models used until now. This study examined the changes in muscle metabolite concentration and power output production during the first and the second half of a set of 10 repetitions to failure (10RM) of bilateral leg press exercise. On two separate days, muscle biopsies were obtained from vastus lateralis prior and immediately after a set of 5 or a set of 10 repetitions. During the second set of 5 repetitions, mean power production decreased by 19% and the average ATP utilisation accounted for by phosphagen decreased from 54% to 19%, whereas ATP utilisation from anaerobic glycolysis increased from 46 to 81%. Changes in contraction time and power output were correlated to the changes in muscle Phosphocreatine (PCr; r = −0.76; P<0.01) and lactate (r = −0.91; P<0.01), respectively, and were accompanied by parallel decreases (P<0.01-0.05) in muscle energy charge (0.6%), muscle ATP/ADP (8%) and ATP/AMP (19%) ratios, as well as by increases in ADP content (7%). The estimated average rate of ATP utilisation from anaerobic sources during the final 5 repetitions fell to 83% whereas total anaerobic ATP production increased by 9% due to a 30% longer average duration of exercise (18.4±4.0 vs 14.2±2.1 s). These data indicate that during a set of 10RM of bilateral leg press exercise there is a decrease in power output which is associated with a decrease in the contribution of PCr and/or an increase in muscle lactate. The higher energy cost per repetition during the second 5 repetitions is suggestive of decreased mechanical efficiency.     

Modeling ATP protonation and activity coefficients in NaClaq and KClaq by SIT and Pitzer equations

The acid-base properties of Adenosine 5'-triphosphate (ATP) in NaCl and KCl aqueous solutions at different ionic strengths (0相似文献   

Glycogen as a fuel: metabolic interaction between glycogen and ATP catabolism in oxygen-independent muscle contraction

The main role of muscular oxygen-independent glycolysis, starting from glycogen as the initial substrate, is the production of three ATP molecules from ADP and P_i per glucosyl moiety transformed into two lactate molecules. During this catabolic process not only there is no proton release, but one proton is consumed. Metabolic acidosis occurs because the three ATP molecules are immediately hydrolysed by myosin ATPase back to 3P_i and 3ADP, to sustain contraction. As a consequence of this ATP turnover, the ATP pool (~5?mmol?kg^?1 wet weight) should remain constant. However, a bulk of experimental evidence has clearly shown that depletion of the muscular ATP pool, and accumulation of ATP catabolites occur even during short sprint bouts. In the present article the interrelationship between glycogen and ATP catabolism in anaerobic contracting muscle is discussed. It is shown how myosin ATPase plays a role not only in the mechanisms of ATP recycling through glycogen anaerobic catabolism, but also in the process of ATP depletion.     

Anaerobic energy provision in aged skeletal muscle during tetanic stimulation

The effect of age on skeletal muscle anaerobic energy metabolism was investigated in adult (11 mo) and aged (25 mo) Fischer 344 rats. Hindlimb skeletal muscles innervated by the sciatic nerve were stimulated to contract with trains of supramaximal impulses (100 ms, 80 Hz) at a train rate of 1 Hz for 60 s, with an occluded circulation. Soleus, plantaris, and red and white gastrocnemius (WG) were sampled from control and stimulated limbs. All muscle masses were reduced with age (9-13%). Peak isometric tensions, normalized per gram of wet muscle, were lower throughout the stimulation in the aged animals (28%). The potential for anaerobic ATP provision was unaltered with age in all muscles, because resting high-energy phosphates and glycogen contents were similar to adult values. Anaerobic ATP provision during stimulation was unaltered by aging in soleus, plantaris, and red gastrocnemius muscles. In the WG, containing mainly fast glycolytic (FG) fibers, ATP and phosphocreatine contents were depleted less in aged muscle. In situ glycogenolysis and glycolysis were 90.0 +/- 4.8 and 69.3 +/- 2.6 mumol/g dry muscle (dm) in adult WG and reduced to 62.3 +/- 6.9 and 51.5 +/- 5.5 mumol/g dm, respectively, in aged WG. Consequently, total anaerobic ATP provision was lower in aged WG (224.5 +/- 20.9 mumol/g dm) vs. adult (292.6 +/- 7.6 mumol/g dm) WG muscle. In summary, the decreased tetanic tension production in aged animals was associated with a decreased anaerobic energy production in FG fibers. Reduced high-energy phosphate use and a greater energy charge potential after stimulation suggested that the energy demand was reduced in aged FG fibers.(ABSTRACT TRUNCATED AT 250 WORDS)     

Age-related changes in ATP-producing pathways in human skeletal muscle in vivo.

Energy for muscle contractions is supplied by ATP generated from 1) the net hydrolysis of phosphocreatine (PCr) through the creatine kinase reaction, 2) oxidative phosphorylation, and 3) anaerobic glycolysis. The effect of old age on these pathways is unclear. The purpose of this study was to examine whether age may affect ATP synthesis rates from these pathways during maximal voluntary isometric contractions (MVIC). Phosphorus magnetic resonance spectroscopy was used to assess high-energy phosphate metabolite concentrations in skeletal muscle of eight young (20-35 yr) and eight older (65-80 yr) men. Oxidative capacity was assessed from PCr recovery after a 16-s MVIC. We determined the contribution of each pathway to total ATP synthesis during a 60-s MVIC. Oxidative capacity was similar across age groups. Similar rates of ATP synthesis from PCr hydrolysis and oxidative phosphorylation were observed in young and older men during the 60-s MVIC. Glycolytic flux was higher in young than older men during the 60-s contraction (P < 0.001). When expressed relative to the overall ATP synthesis rate, older men relied on oxidative phosphorylation more than young men (P = 0.014) and derived a smaller proportion of ATP from anaerobic glycolysis (P < 0.001). These data demonstrate that although oxidative capacity was unaltered with age, peak glycolytic flux and overall ATP production from anaerobic glycolysis were lower in older men during a high-intensity contraction. Whether this represents an age-related limitation in glycolytic metabolism or a preferential reliance on oxidative ATP production remains to be determined.     

High-speed running performance is largely unaffected by hypoxic reductions in aerobic power.

We tested the importance of aerobic metabolism to human running speed directly by altering inspired oxygen concentrations and comparing the maximal speeds attained at different rates of oxygen uptake. Under both normoxic (20.93% O2) and hypoxic (13.00% O2) conditions, four fit adult men completed 15 all-out sprints lasting from 15 to 180 s as well as progressive, discontinuous treadmill tests to determine maximal oxygen uptake and the metabolic cost of steady-state running. Maximal aerobic power was lower by 30% (1.00 +/- 0.15 vs. 0.77 +/- 0.12 ml O2. kg-1. s-1) and sprinting rates of oxygen uptake by 12-25% under hypoxic vs. normoxic conditions while the metabolic cost of submaximal running was the same. Despite reductions in the aerobic energy available for sprinting under hypoxic conditions, our subjects were able to run just as fast for sprints of up to 60 s and nearly as fast for sprints of up to 120 s. This was possible because rates of anaerobic energy release, estimated from oxygen deficits, increased by as much as 18%, and thus compensated for the reductions in aerobic power. We conclude that maximal metabolic power outputs during sprinting are not limited by rates of anaerobic metabolism and that human speed is largely independent of aerobic power during all-out runs of 60 s or less.     

Fast transient currents in Na,K-ATPase induced by ATP concentration jumps from the P3-[1-(3',5'-dimethoxyphenyl)-2-phenyl-2-oxo]ethyl ester of ATP.

Electrogenic ion transport by Na,K-ATPase was investigated by analysis of transient currents in a model system of protein-containing membrane fragments adsorbed to planar lipid bilayers. Sodium transport was triggered by ATP concentration jumps in which ATP was released from an inactive precursor by an intense near-UV light flash. The method has been used previously with the P3-1-(2-nitrophenyl)ethyl ester of ATP (NPE-caged ATP), from which the relatively slow rate of ATP release limits analysis of processes in the pump mechanism controlled by rate constants greater than 100 s(-1) at physiological pH. Here Na,K-ATPase was reinvestigated using the P3-[1-(3,5-dimethoxyphenyl)-2-phenyl-2-oxo]ethyl ester of ATP (DMB-caged ATP), which has an ATP release rate of >10(5) s(-1). Under otherwise identical conditions, photorelease of ATP from DMB-caged ATP showed faster kinetics of the transient current compared to that from NPE-caged ATP. With DMB-caged ATP, transient currents had rate profiles that were relatively insensitive to pH and the concentration of caged compound. Rate constants of ATP binding and of the E1 to E2 conformational change were compatible with earlier studies. Rate constants of enzyme phosphorylation and ADP-dependent dephosphorylation were 600 s(-1) and 1.5 x 10(6) M(-1) s(-1), respectively, at pH 7.2 and 22 degrees C.     

Effect of hindlimb unweighting on anaerobic metabolism in rat skeletal muscle.

Female Sprague-Dawley rats (250 g) were hindlimb suspended for 14 days, and the effects of hindlimb unweighting (HU) on skeletal muscle anaerobic metabolism were investigated and compared with nonsuspended controls (C). Soleus (SOL), plantaris (PL), and red and white portions of the gastrocnemius (RG, WG) were sampled from resting and stimulated limbs. Muscle atrophy after HU was 46% in SOL, 22% in PL, and 24% in the gastrocnemius compared with nonsuspended C animals. The muscles innervated by the sciatic nerve were stimulated to contract with an occluded circulation for 60 s with trains of supramaximal impulses (100 ms, 80 Hz) at a train rate of 1.0 Hz. Peak tension development by the gastrocnemius-PL-SOL muscle group was similar in HU and C animals (13.0 +/- 1.2, 12.2 +/- 0.8 N/g wet muscle). Occlusion of the circulation before stimulation created a predominantly anaerobic environment, and in situ glycogenolysis and glycolysis were estimated from accumulations of glycolytic intermediates. Total glycogenolysis and glycolysis were higher in the RG muscle of HU animals (74.6 +/- 3.3, 58.1 +/- 1.1) relative to C (57.1 +/- 4.6, 46.1 +/- 2.9 mumol glucosyl units/g dry muscle). Consequently, total anaerobic ATP production was also increased (HU, 251.3 +/- 1.1; C, 204.6 +/- 8.9 mumol ATP/g dry muscle). Total ATP production, glycogenolysis, and glycolysis were unaffected by HU in SOL, PL, and WG muscles. The enhanced glycolytic activity in RG after HU may be attributed to a shift in the metabolic profile from oxidative to glycolytic in the fast oxidative-glycolytic fiber population.     

Reconstruction of steady state in cell-free systems. Interactions between glycolysis and mitochondrial metabolism: regulation of the redox and phosphorylation states

A reconstituted "open" system comprising respiring mitochondria and actively glycolyzing muscle extract was devised for studies of vectorially mediated interactions. Glycogen particles were the substrate for the glycolyzing enzymes. Purified soluble (F1) ATPase was added in varying quantities to establish a range of energetic steady states. The data generally confirm our recent conclusions (Wu and Davis, (1981) Arch. Biochem. Biophys. 208, 85-89) on the relative efficacy of the adenine nucleotides and their ratios, and of inorganic phosphate on flux through rate-controlling steps of glycolysis. When mitochondrial ATP synthesis was blocked, glycolytic flux was relatively rapid, and the lactate/pyruvate ratio increased with time to values up to greater than 300. If functional mitochondria were present, glycolytic flux was very strongly suppressed, provided the energy state (ATP/ADP) was high, and the phosphate concentration[Pi] was low. Adenine nucleotide control of glycolysis was to a large extent lost when the steady-state ATP/ADP was below about 10, or if [Pi] was elevated. In the two-phase system containing respiring mitochondria and components of the malate-aspartate shuttle, the ATP/ADP and both extra- and intramitochondrial NAD+/NADH ratios were maintained constant, and to various perturbable levels with varying energy load (ATPase). The gradient in reduction potentials attained values (delta Gredox) of up to about 2.5 kcal. The extramitochondrial redox state was not positively correlated with the external phosphorylation potential ([ATP]/[ADP] X [Pi]). The following conclusions are drawn on the basis of the present data, together with other reports (Davis, Bremer, and Akerman (1980) J. Biol. Chem. 255, 2277-2283) and (Klingenberg and Rottenberg (1977) Eur. J. Biochem. 73, 125-130): (a) the gradient in reduction potential is driven by the membrane potential (delta psi), mediated by the electrogenic glutamate-aspartate exchange, and the poise or set point of this gradient is a function of delta psi; and (b) the gradient of ATP/ADP ratios across the membrane is also driven principally by delta psi, mediated by the electrogenic ATP-ADP exchange. Hence, segregation of phosphorylation and reduction potentials is linked through a mutually shared electrical driving force.     

Phenol biodegradation by Pseudomonas putida DSM 548 in a batch reactor

Phenol biodegradation in a batch reactor using a pure culture of Pseudomonas putida DSM 548 was studied. The purpose of the experiments was to determine the kinetics of biodegradation by measuring biomass growth rates and phenol concentration as a function of time in a batch reactor. The Haldane equation μ=μ(m)S/((K(s)+S+S(2))/K(i)) adequately describes cell growth with kinetic constants μ(m)=0.436h(-1), K(s)=6.19mgl(-1), K(i)=54.1mgl(-1). These values are in the range of those published in literature for pure or mixed cultures degrading phenol.     

Influx and efflux kinetics of cationic dye binding to respiring mitochondria.

We have investigated the kinetics of interaction of cationic fluorescent lipophiles (dyes) rhodamine 123, rhodamine 6G, tetramethyl rhodamine ethyl ester, safranine O, 1,1'-diethyloxacarbocyanine, 1,1'-diethyloxadicarbocyanine, and 1,1'-diethylthiadicarbocyanine iodide with isolated respiring rat-liver mitochondria (RLM). Dye flux across the RLM inner membrane was measured by following the kinetics of fluorescence signal change after mixing of dye and RLM. The time course of fluorescence was analysed in terms of a kinetic model of the binding and transport processes involved. The rate constants of dye influx and efflux were extracted from the observed effect on the apparent time constant of fluorescence change to equilibrium intensity upon mixing dye with increasing concentrations of RLM. From the influx rate constants obtained, the apparent permeability constants for dye influx (at zero potential) across the membrane were calculated and ranged from 3 to 140 x 10(-4) cm/s. The influx rate constant was found to be linearly related to relative dye lipophilicity, as predicted by the model. As another test of the model, from the ratio of the influx and efflux rate constants, the apparent trans-membrane potential, psi, was calculated and found generally to agree with reported values, but to depend on the lipophilicity of the dye used. Not predicted by the simple model was a dissymmtry observed in the influx and efflux time constants for fluorescence change to equilibrium intensity. Inferences are made relating to the utility of these dyes as probes of psi.     

Ontogeny of Cerebral Oxidative Metabolism in the Chick Embryo

Abstract: The low cerebral energy requirements of most mammals at birth reflect an immaturity of the central nervous system, and it has been suggested that energy demands in fetuses are even less well developed than in newborns. Furthermore, fetal cerebral energy requirements are presumed to be met predominantly or exclusively by anaerobic glycolysis. To clarify these issues, we investigated cerebral oxidative metabolism in 9-, 14-, 16-, and 19-day-old chick embryos and in newly hatched peeps. Animals were decapitated and quick-frozen in liquid Freon 0-5 min post-mortem. Forebrain extracts were prepared and assayed for ATP, phosphocreatine, glucose, and lactate. Alterations in these metabolites post-decapitation were used to calculate cerebral metabolic rates (Δ∼P) and rates of maximal anaerobic glycolysis (Δ lactate). Rates of lactate accumulation during cerebral ischemia increased progressively from embryonic day 9 through hatching. Cerebral metabolic rates were not different in 9-, 14-, and 16-day-old embryos, but increased steadily thereafter. The extent to which total cerebral energy utilization could be derived from anaerobic glycolysis (Δ lactate/Δ∼ P) increased from a low at day 9 (0.29) to a maximum at day 16 (0.78). The data suggest that, despite the low cerebral metabolic activity of the chick embryo, at no time during development is anaerobic glycolysis capable of entirely supporting the energy needs of the developing brain.