The effect of iron limitation on glycerol production and expression of the isogenes for NAD(+)-dependent glycerol 3-phosphate dehydrogenase in Saccharomyces cerevisiae

The forearm flexor muscles of 56 untrained volunteers (26 women and 30 men) were examined by 31P magnetic resonance spectroscopy, during a rest-exercise-recovery protocol, in order to document the impact of gender on muscle energetics. Absolute concentrations of high-energy phosphate compounds, intracellular pH and rates of aerobic and anaerobic ATP production were calculated. An inverse correlation was found between body mass index (BMI) and power output in women but not in men. After correcting for power output and BMI, the measured energy cost of contraction was twice larger for women than for men. This increase was also reflected in larger ATP production from aerobic and anaerobic pathways. This higher energy cost might be explained in part by differences in local muscle mass, a higher impact of fatness, but also by a reduced metabolic efficiency of muscle fibers in untrained women.     

Skeletal muscle metabolic and ionic adaptations during intense exercise following sprint training in humans.

The effects of sprint training on muscle metabolism and ion regulation during intense exercise remain controversial. We employed a rigorous methodological approach, contrasting these responses during exercise to exhaustion and during identical work before and after training. Seven untrained men undertook 7 wk of sprint training. Subjects cycled to exhaustion at 130% pretraining peak oxygen uptake before (PreExh) and after training (PostExh), as well as performing another posttraining test identical to PreExh (PostMatch). Biopsies were taken at rest and immediately postexercise. After training in PostMatch, muscle and plasma lactate (Lac(-)) and H(+) concentrations, anaerobic ATP production rate, glycogen and ATP degradation, IMP accumulation, and peak plasma K(+) and norepinephrine concentrations were reduced (P<0.05). In PostExh, time to exhaustion was 21% greater than PreExh (P<0.001); however, muscle Lac(-) accumulation was unchanged; muscle H(+) concentration, ATP degradation, IMP accumulation, and anaerobic ATP production rate were reduced; and plasma Lac(-), norepinephrine, and H(+) concentrations were higher (P<0.05). Sprint training resulted in reduced anaerobic ATP generation during intense exercise, suggesting that aerobic metabolism was enhanced, which may allow increased time to fatigue.     

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.     

Association between body mass index and muscularity in healthy older Japanese women and men

Background

Body mass index (BMI), expressed as the ratio of body mass to height squared (kg/m²), involves not only fat but also lean mass. The present study aimed to clarify how BMI is associated with total muscle mass (TMM) in older Japanese women and men.

Findings

Using a B-mode ultrasound apparatus, muscle thickness was measured at nine sites (forearm, upper arm anterior and posterior, thigh anterior and posterior, lower leg anterior and posterior, abdomen, and subscapular) for 346 women (BMI 16.40 to 33.11 kg/m²) and 286 men (BMI 16.86 to 31.18 kg/m²) aged 60.0 to 79.5 yrs. TMM was estimated using the product of the sum of the muscle thicknesses at the nine sites with height as an independent variable. For both sexes, the estimated TMM relative to height squared was significantly correlated with BMI (r = 0.688, P<0.0001 for women; r = 0.696, P<0.0001 for men), but the percentage of the estimated TMM in body mass was not.

Conclusion

These results indicate that, for older Japanese women and men, BMI is a simple and convenient index for assessing total muscularity.     

Limitations on locomotor performance in squid

An empirical equation relating O2 consumption (power input) to pressure production during jet-propelled swimming in the squid (Illex illecebrosus) is compared with hydrodynamic estimates of the pressure-flow power output also calculated from pressure data. Resulting estimates of efficiency and stress indicate that the circularly arranged obliquely striated muscles in squid mantle produce maximum tensions about half those of vertebrate cross-striated muscle, that "anaerobic" fibers contribute to aerobic swimming, and that peak pressure production requires an instantaneous power output higher than is thought possible for muscle. Radial muscles probably contribute additional energy via elastic storage in circular collagen fibers. Although higher rates of aerobic power consumption are only found in terrestrial animals at much higher temperatures, the constraint on squid performance is circulation, not ventilation. Anaerobic power consumption is also among the highest ever measured, but the division of labor between "aerobic" and "anaerobic" fibers suggests a system designed to optimize the limited capacity of the circulation.     

Maximum aerobic power and body composition in healthy East African older male and female subjects

The relationship of body size and composition to maximum aerobic power output during work on a bicycle ergometer has been examined in older African subjects divided into three groups: Active men and inactive men and women. Comparison is made with similar data obtained on young African subjects (Davies, Mbelwa, Crockford and Weiner, '73). The results show that in the older men and women, in contrast to the young African subjects, there was complete lack of association between physiological function and body size and structure. In this latter group max was completely independent of body weight, lean body mass, and estimates of leg muscle volume. These findings confirm and extend the work of Davies ('72b) on Europeans and suggest that the main determinant of aerobic power output in older men irrespective of ethnic origin is more closely related to transport and utilization of O₂ within a given active muscle mass than to the total quantity of muscle available to perform the work.     

Reduced submaximal leg blood flow after high-intensity aerobic training.

This study evaluated the hypothesis that active muscle blood flow is lower during exercise at a given submaximal power output after aerobic conditioning as a result of unchanged cardiac output and blunted splanchnic vasoconstriction. Eight untrained subjects (4 men, 4 women, 23-31 yr) performed high-intensity aerobic training for 9-12 wk. Leg blood flow (femoral vein thermodilution), splanchnic blood flow (indocyanine green clearance), cardiac output (acetylene rebreathing), whole body O(2) uptake (VO(2)), and arterial-venous blood gases were measured before and after training at identical submaximal power outputs (70 and 140 W; upright 2-leg cycling). Training increased (P < 0.05) peak VO(2) (12-36%) but did not significantly change submaximal VO(2) or cardiac output. Leg blood flow during both submaximal power outputs averaged 18% lower after training (P = 0.001; n = 7), but these reductions were not correlated with changes in splanchnic vasoconstriction. Submaximal leg VO(2) was also lower after training. These findings support the hypothesis that aerobic training reduces active muscle blood flow at a given submaximal power output. However, changes in leg and splanchnic blood flow resulting from high-intensity training may not be causally linked.     

Influence of training status on maximal accumulated oxygen deficit during all-out cycle exercise

The influence of training status on the maximal accumulated oxygen deficit (MAOD) was used to assess the validity of the MAOD method during supramaximal all-out cycle exercise. Sprint trained (ST; n = 6), endurance trained (ET; n = 8), and active untrained controls (UT; n = 8) completed a 90 s all-out variable resistance test on a modified Monark cycle ergometer. Pretests included the determination of peak oxygen uptake ( O_2peak) and a series (5–8) of 5-min discontinuous rides at submaximal exercise intensities. The regression of steady-state oxygen uptake on power output to establish individual efficiency relationships was extrapolated to determine the theoretical oxygen cost of the supramaximal power output achieved in the 90 s all-out test. Total work output in 90 s was significantly greater in the trained groups (P<0.05), although no differences existed between ET and ST. Anaerobic capacity, as assessed by MAOD, was larger in ST compared to ET and UT. While the relative contributions of the aerobic and anaerobic energy systems were not significantly different among the groups, ET were able to achieve significantly more aerobic work than the other two groups, while ST were able to achieve significantly more anaerobic work. Peak power and peak pedalling rate were significantly higher in ST. The results suggested that MAOD determined during all-out exercise was sensitive to training status and provided a useful assessment of anaerobic capacity. In our study sprint training, compared with endurance training, appeared to enhance significantly power output and high intensity performance over brief periods (up to 60 s), yet few overall differences in performance (i.e. total work) existed during 90 s of all-out exercise.     

Metabolic Acclimation to Anoxia Induced by Low (2-4 kPa Partial Pressure) Oxygen Pretreatment (Hypoxia) in Root Tips of Zea mays

Young intact plants of maize (Zea mays L. cv INRA 508) were exposed to 2 to 4 kilopascals partial pressure oxygen (hypoxic pretreatment) for 18 hours before excision of the 5 millimeter root apex and treatment with strictly anaerobic conditions (anoxia). Hypoxic acclimation gave rise to larger amounts of ATP, to larger ATP/ADP and adenylate energy charge ratios, and to higher rates of ethanol production when excised root tips were subsequently made anaerobic, compared with root tips transferred directly from aerobic to anaerobic media. Improved energy metabolism following hypoxic pretreatment was associated with increased activity of alcohol dehydrogenase (ADH), and induction of ADH-2 isozymes. Roots of Adh1⁻ mutant plants lacked constitutive ADH and only slowly produced ethanol when made anaerobic. Those that were hypoxically pretreated acclimated to anoxia with induction of ADH2 and a higher energy metabolism, and a rate of ethanol production comparable to that of nonmutants. All these responses were insensitive to the presence or absence of NO₃⁻. Additionally, the rate of ethanol production was about 50 times greater than the rate of reduction of NO₃⁻ to NO₂⁻. These results indicate that nitrate reductase does not compete effectively with ADH for NADH, or contribute to energy metabolism during anaerobic respiration in this tissue through nitrate reduction. Unacclimated root tips of wild type and Adhl⁻ mutants appeared not to survive more than 8 to 9 hours in strict anoxia; when hypoxically pretreated they tolerated periods under anoxia in excess of 22 hours.     

Muscle metabolism, blood lactate and oxygen uptake in steady state exercise at aerobic and anaerobic thresholds

Muscle metabolites and blood lactate concentration were studied in five male subjects during five constant-load cycling exercises. The power outputs were below, equal to and above aerobic (AerT) and anaerobic (AnT) threshold as determined during an incremental leg cycling test. At AerT, muscle lactate had increased significantly (p less than 0.05) from the rest value of 2.31 to 5.56 mmol X kg-1 wet wt. This was accompanied by a significant reduction in CP by 28% (p less than 0.05), whereas only a minor change (9%) was observed for ATP. At AnT muscle lactate had further increased and CP decreased although not significantly as compared with values at AerT. At the highest power outputs (greater than AnT) muscle lactate had increased (p less than 0.01) and CP decreased (p less than 0.01) significantly from the values observed at AnT. Furthermore, a significant reduction (p less than 0.05) in ATP over resting values was recorded. Blood lactate decreased significantly (p less than 0.01) during the last half of the lowest 5 min exercise, remained unchanged at AerT and increased significantly (p less than 0.05-0.01) at power outputs greater than or equal to AnT. It is concluded that anaerobic muscle metabolism is increased above resting values at AerT: at low power outputs (less than or equal to AerT) this could be related to the transient oxygen deficit during the onset of exercise or the increase in power output. At high power outputs (greater than AnT) anaerobic energy production is accelerated and it is suggested that AnT represents the upper limit of power output where lactate production and removal may attain equilibrium during constant load exercise.     

Physiological Criteria in Defining the Standards for Training and Competition Loads in Elite Sports

Adaptation to training loads can be quantitatively described by a dose-effect dependence, with the gain in the training function over a certain period regarded as the effect and the dose expressed as a product of the energy spent during exercise and the stimulus duration. The duration combines the periods of exercises, pauses, and recovery needed to compensate for the fast fraction of the oxygen debt. In addition to direct measurements of the energy spent, quantitative assessment of the load intensity can be based on the total pulse cost of exercise, which accurately reflects the changes in the oxygen demand and the energy cost of the physical load. To quantitate and standardize training and competition loads, we suggest the use of correlations found between the pulse and energy costs of exercises and their relative power determined in critical modes of muscle activity: at the anaerobic threshold; the critical power, associated with the maximum oxygen consumption; the alactic anaerobic threshold; the power of exhaustion, when blood lactic acid reaches its maximum; or at maximum aerobic power, when the muscle reserves of ATP and creatine phosphate are the most depleted.     

Mitochondrial capacity is affected by glycemic status in young untrained women with type 1 diabetes but is not impaired relative to healthy untrained women

In this study, we examined whether glycemic status influences aerobic function in women with type 1 diabetes and whether aerobic function is reduced relative to healthy women. To this end, we compared several factors determining aerobic function of 29 young sedentary asymptomatic women (CON) with 9 women of similar age and activity level with type 1 diabetes [DIA, HbA1c range = 6.9-8.2%]. Calf muscle mitochondrial capacity was estimated by (31)P-magnetic resonance spectroscopy. Capillarization and muscle fiber oxidative enzyme activity were assessed from vastus lateralis and soleus muscle biopsies. Oxygen uptake and cardiac output were evaluated by ergospirometry and N(2)O/SF(6) rebreathing. Calf muscle mitochondrial capacity was not different between CON and DIA, as indicated by the identical calculated maximal rates of oxidative ATP synthesis [0.0307 (0.0070) vs. 0.0309 (0.0058) s(-1), P = 0.930]. Notably, HbA1c was negatively correlated with mitochondrial capacity in DIA (R(2) = 0.475, P = 0.040). Although HbA1c was negatively correlated with cardiac output (R(2) = 0.742, P = 0.013) in DIA, there was no difference between CON and DIA in maximal oxygen consumption [2.17 (0.34) vs. 2.21 (0.32) l/min, P = 0.764], cardiac output [12.1 (1.9) vs. 12.3 (1.8) l/min, P = 0.783], and endurance capacity [532 (212) vs. 471 (119) s, P = 0.475]. There was also no difference between the two groups either in the oxidative enzyme activity or capillary-to-fiber ratio. We conclude that mitochondrial capacity depends on HbA1c in untrained women with type 1 diabetes but is not reduced relative to untrained healthy women.     

Gender differences in anaerobic power tests

The purpose of this study was to determine if the differences in anaerobic power between males and females could be accounted for by differences in body composition, strength, and neuromuscular function. A total of 82 untrained men and 99 women took part in the study. Body composition, somatotype, isometric strength, neuromuscular function were measured, and four anaerobic power tests performed. The men were significantly different from the women on all strength, power, and neuromuscular measurements except reaction time and on all anthropometric and somatotype dimensions except ectomorphy. Strength and anthropometric dimensions were similarly related to anaerobic power values within each sex. Relative fat (%fat) exerted different degrees of influence on sprint and jump performances in each sex. Removing the influence of anthropometric, strength, and neuromuscular differences by analysis of covariance reduced, but did not remove, the significant differences between the sexes. Therefore, factors other than lean body mass, leg strength, and neuromuscular function may be operating in short-term, explosive power performances to account for the differences between the sexes. The task-specific nature of anaerobic power tests and the relatively large influence of anthropometric factors on power production were confirmed.     

Gender Differences in Anthropometric Predictors of Physical Performance in Older Adults

BackgroundBoth high body fat and low muscle mass have been associated with physical disability in older adults. However, men and women differ markedly in body composition; men generally have more absolute and relative lean muscle mass and less fat mass than women. It is not known how these anthropometric differences differentially affect physical ability in men and women.ObjectivesThis study examines differences in anthropometric predictors of physical performance in older women and men.MethodsParticipants were 470 older women and men 72.9 (7.9) years of age. Body composition was measured using dual-energy x-ray absorptiometry. Maximum leg strength and power were measured using a leg press. Muscle quality (MQ) was calculated as relative strength (leg press strength per kilogram of leg muscle mass). Gait speed and chair rise were used to assess mobility performance and functional strength.ResultsBody mass index (BMI), age, and MQ emerged as predictors (P < 0.05) of functional strength and mobility in men and women somewhat differently. After accounting for age and sample, leg MQ was related to chair rise time and gait speed in men but not women. BMI was related to gait speed in both men and women, but BMI was related to chair rise time only in women.ConclusionResults implicate the prioritized importance of healthy weight and muscle maintenance in older women and men for maintained physical functioning with aging.     

Metabolic and work efficiencies during exercise in Andean natives

Maximum O2 and CO2 fluxes during exercise were less perturbed by hypoxia in Quechua natives from the Andes than in lowlanders. In exploring how this was achieved, we found that, for a given work rate, Quechua highlanders at 4,200 m accumulated substantially less lactate than lowlanders at sea level normoxia (approximately 5-7 vs. 10-14 mM) despite hypobaric hypoxia. This phenomenon, known as the lactate paradox, was entirely refractory to normoxia-hypoxia transitions. In lowlanders, the lactate paradox is an acclimation; however, in Quechuas, the lactate paradox is an expression of metabolic organization that did not deacclimate, at least over the 6-wk period of our study. Thus it was concluded that this metabolic organization is a developmentally or genetically fixed characteristic selected because of the efficiency advantage of aerobic metabolism (high ATP yield per mol of substrate metabolized) compared with anaerobic glycolysis. Measurements of respiratory quotient indicated preferential use of carbohydrate as fuel for muscle work, which is also advantageous in hypoxia because it maximizes the yield of ATP per mol of O2 consumed. Finally, minimizing the cost of muscle work was also reflected in energetic efficiency as classically defined (power output per metabolic power input); this was evident at all work rates but was most pronounced at submaximal work rates (efficiency approximately 1.5 times higher than in lowlander athletes). Because plots of power output vs. metabolic power input did not extrapolate to the origin, it was concluded 1) that exercise in both groups sustained a significant ATP expenditure not convertible to mechanical work but 2) that this expenditure was downregulated in Andean natives by thus far unexplained mechanisms.     

Age and aerobic exercise training effects on whole body and muscle protein metabolism

Aging in humans is associated with loss of lean body mass, but the causes are incompletely defined. Lean tissue mass and function depend on continuous rebuilding of proteins. We tested the hypotheses that whole body and mixed muscle protein metabolism declines with age in men and women and that aerobic exercise training would partly reverse this decline. Seventy-eight healthy, previously untrained men and women aged 19-87 yr were studied before and after 4 mo of bicycle training (up to 45 min at 80% peak heart rate, 3-4 days/wk) or control (flexibility) activity. At the whole body level, protein breakdown (measured as [13C]leucine and [15N]phenylalanine flux), Leu oxidation, and protein synthesis (nonoxidative Leu disposal) declined with age at a rate of 4-5% per decade (P < 0.001). Fat-free mass was closely correlated with protein turnover and declined 3% per decade (P < 0.001), but even after covariate adjustment for fat-free mass, the decline in protein turnover with age remained significant. There were no differences between men and women after adjustment for fat-free mass. Mixed muscle protein synthesis also declined with age 3.5% per decade (P < 0.05). Exercise training improved aerobic capacity 9% overall (P < 0.01), and mixed muscle protein synthesis increased 22% (P < 0.05), with no effect of age on the training response for either variable. Fat-free mass, whole body protein turnover, and resting metabolic rate were unchanged by training. We conclude that rates of whole body and muscle protein metabolism decline with age in men and women, thus indicating that there is a progressive decline in the body's remodeling processes with aging. This study also demonstrates that aerobic exercise can enhance muscle protein synthesis irrespective of age.     

Energy absorption during impact on the proximal femur is affected by body mass index and flooring surface

Impact mechanics theory suggests that peak loads should decrease with increase in system energy absorption. In light of the reduced hip fracture risk for persons with high body mass index (BMI) and for falls on soft surfaces, the purpose of this study was to characterize the effects of participant BMI, gender, and flooring surface on system energy absorption during lateral falls on the hip with human volunteers. Twenty university-aged participants completed the study with five men and five women in both low BMI (<22.5 kg/m²) and high BMI (>27.5 kg/m²) groups. Participants underwent lateral pelvis release experiments from a height of 5 cm onto two common floors and four safety floors mounted on a force plate. A motion-capture system measured pelvic deflection. The energy absorbed during the initial compressive phase of impact was calculated as the area under the force–deflection curve. System energy absorption was (on average) 3-fold greater for high compared to low BMI participants, but no effects of gender were observed. Even after normalizing for body mass, high BMI participants absorbed 1.8-fold more energy per unit mass. Additionally, three of four safety floors demonstrated significantly increased energy absorption compared to a baseline resilient-rolled-sheeting system (% increases ranging from 20.7 to 28.3). Peak system deflection was larger for high BMI persons and for impacts on several safety floors. This study indicates that energy absorption may be a common mechanism underlying the reduced risk of hip fracture for persons with high BMI and for those who fall on soft surfaces.     

Deletion of Kinin B2 Receptor Alters Muscle Metabolism and Exercise Performance

Metabolic syndrome is a cluster of metabolic risk factors such as obesity, diabetes and cardiovascular diseases. Mitochondria is the main site of ATP production and its dysfunction leads to decreased oxidative phosphorylation, resulting in lipid accumulation and insulin resistance. Our group has demonstrated that kinins can modulate glucose and lipid metabolism as well as skeletal muscle mass. By using B2 receptor knockout mice (B2R^-/-) we investigated whether kinin action affects weight gain and physical performance of the animals. Our results show that B2R^-/- mice are resistant to high fat diet-induced obesity, have higher glucose tolerance as well as increased mitochondrial mass. These features are accompanied by higher energy expenditure and a lower feed efficiency associated with an increase in the proportion of type I fibers and intermediary fibers characterized by higher mitochondrial content and increased expression of genes related to oxidative metabolism. Additionally, the increased percentage of oxidative skeletal muscle fibers and mitochondrial apparatus in B2R^-/- mice is coupled with a higher aerobic exercise performance. Taken together, our data give support to the involvement of kinins in skeletal muscle fiber type distribution and muscle metabolism, which ultimately protects against fat-induced obesity and improves aerobic exercise performance.     

Effects of Obesity on Substrate Utilization during Exercise

Objective: The capacity for lipid and carbohydrate (CHO) oxidation during exercise is important for energy partitioning and storage. This study examined the effects of obesity on lipid and CHO oxidation during exercise. Research Methods and Procedures: Seven obese and seven lean [body mass index (BMI), 33 ± 0.8 and 23.7 ± 1.2 kg/m², respectively] sedentary, middle‐aged men matched for aerobic capacity performed 60 minutes of cycle exercise at similar relative (50% Vo _2max) and absolute exercise intensities. Results: Obese men derived a greater proportion of their energy from fatty‐acid oxidation than lean men (43 ± 5% 31 ± 2%; p = 0.02). Plasma fatty‐acid oxidation determined from recovery of infused [0.15 μmol/kg fat‐free mass (FFM) per minute] [1‐¹³C]‐palmitate in breath CO₂ was similar for obese and lean men (8.4 ± 1.1 and 29 ± 15 μmol/kg FFM per minute). Nonplasma fatty‐acid oxidation, presumably, from intramuscular sources, was 50% higher in obese men than in lean men (10.0 ± 0.6 versus 6.6 ± 0.8 μmol/kg FFM per minute; p < 0.05). Systemic glucose disposal was similar in lean and obese groups (33 ± 8 and 29 ± 15 μmol/kg FFM per minute). However, the estimated rate of glycogen‐oxidation was 50% lower in obese than in lean men (61 ± 12 versus 90 ± 6 μmol/kg FFM per minute; p < 0.05). Discussion: During moderate exercise, obese sedentary men have increased rates of fatty‐acid oxidation from nonplasma sources and reduced rates of CHO oxidation, particularly muscle glycogen, compared with lean sedentary men.     

Phosphofructokinase from foot muscle of the whelk, Busycotypus canaliculatum: Evidence for covalent modification of the enzyme during anaerobiosis

Phosphofructokinase (PFK) was purified from foot muscle of aerobic and anaerobic (24 h of anoxia) whelks, Busycotypus canaliculatum. Fructose-6-P kinetics were sigmoidal at pH 7.0 with affinity constants, S_0.5, of 2.18 ± 0.10 (n_H = 2.5 ± 0.1) and 2.48 ± 0.13 mm (n_H = 2.7 ± 0.1) for the enzyme from aerobic versus anaerobic muscle. Affinity for ATP, like that for fructose-6-P, did not differ for the two enzymes (0.031 ± 0.003 for the aerobic vs 0.041 ± 0.007 mm for the anaerobic enzyme), but S_0.5 for Mg²⁺ was significantly different for the two enzymes (0.060 ± 0.006 vs 0.130 ± 0.020 mm). Whelk muscle PFK was activated by NH₄⁺, P_i, AMP, ADP, and fructose-2,6-P₂. NH₄⁺ and fructose-2,6-P₂ were less effective activators of PFK from anoxic muscle, with apparent K_a's 1.6- and 3.5-fold higher for the anaerobic vs aerobic enzyme. Activators decreased S_0.5 for fructose-6-P and reduced n_H. With the exception of fructose-2,6-P₂, the effects of activators on S_0.5 were the same for the enzyme from aerobic and anaerobic muscle; fructose-2,6-P₂ at 2.5 μm reduced S_0.5 by only 3.3-fold for the anaerobic enzyme compared to 5.5-fold for the aerobic enzyme. ATP was a strong substrate inhibitor of PFK; the enzyme from anaerobic muscle showed greater ATP inhibition, with I₅₀'s 1.5- to 2.0-fold lower than those for the aerobic enzyme. The kinetic differences between PFK from anaerobic versus aerobic foot muscle (stronger ATP inhibition and decreased sensitivity to activators for the anaerobic enzyme) were consistent with kinetic differences reported for the phosphorylated versus dephosphorylated forms, respectively, of PFK in other systems. Treatment of PFK from anaerobic muscle with alkaline phosphatase resulted in a decrease in the K_a for fructose-2,6-P₂ to a level similar to that of the aerobic enzyme. The physiological stress of anoxia may, therefore, induce a covalent modification of PFK.