P. W. Hochachka

Allometric cascade as a unifying principle of body mass effects on metabolism

The power function of basal metabolic rate scaling is expressed as aMb, where a corresponds to a scaling constant (intercept), M is body mass, and b is the scaling exponent. The 3/4 power law (the best-fit b value for mammals) was developed from Kleibers original analysis and, since then, most workers have searched for a single cause to explain the observed allometry. Here we present a multiple-causes model of allometry, where the exponent b is the sum of the influences of multiple contributors to metabolism and control. The relative strength of each contributor, with its own characteristic exponent value, is determined by the control contribution. To illustrate its use, we apply this model to maximum versus basal metabolic rates to explain the differing scaling behaviour of these two biological states in mammals. The main difference in scaling is that, for the basal metabolic rate, the O2 delivery steps contribute almost nothing to the global b scaling exponent, whereas for the maximum metabolic rate, the O2 delivery steps significantly increase the global b value.

Invertebrate Facultative Anaerobiosis A reinterpretation of invertebrate enzyme pathways suggests new approaches to helminth chemotherapy

The unique pattern of anaerobic carbohydrate metabolism in invertebrate facultative anaerobes serves to couple other substrate-level phosphorylations to the glycolytic reactions, thus increasing the potential yield of high-energy phosphate compounds. Currently, two important coupling sites can be identified:

The anaerobic oyster heart: Coupling of glucose and aspartate fermentation

SummaryThe interrelationships of carbohydrate and amino acid metabolism during anaerobiosis were investigated in the ventricle of the intertidal oyster,Crassostrea gigas. While the ventricle accumulates alanine and succinate in a 2∶1 ratio during anoxia, these end products appear to arise from different precursors. Thus glucose-14C is metabolized mainly to alanine-14C (55% of glucose carbon appears in alanineversus 3% in succinate) by the anoxic ventriclein vitro while succinate-14C is the principle end product of aspartate-14C catabolism. Glutamate-14C is poorly metabolized by the anoxic ventricle, and correspondingly, while ventricular aspartate concentrations drop during anoxia, those of other amino acids do not. A metabolic scheme coupling glucose and aspartate catabolism in this facultative anaerobe is proposed. The detection of a third, as yet incompletely identified, anaerobic end product produced by the ventricle is reported.

The Brain at High Altitude: Hypometabolism as a Defense against Chronic Hypoxia?:

The brain of hypoxia-tolerant vertebrates is known to survive extreme limitations of oxygen in part because of very low rates of energy production and utilization. To assess if similar adaptations may be involved in humans during hypoxia adaptation over generational time, volunteer Quechua natives, indigenous to the high Andes between about 3,700 and 4,900 m altitude, served as subjects in positron emission tomographic measurements of brain regional glucose metabolic rates. Two metabolic states were analyzed: (a) the presumed normal (high altitude-adapted) state monitored as soon as possible after leaving the Andes and (b) the deacclimated state monitored after 3 weeks at low altitudes. Proton nuclear magnetic resonance spectroscopy studies of the Quechua brain found normal spectra, with no indication of any unusual lactate accumulation; in contrast, in hypoxia-tolerant species, a relatively large fraction of the glucose taken up by the brain is released as lactate. Positron emission tomographic measurements of [18F]2-deoxy-2-fluoro-d-glucose (FDG) uptake rates, quantified in 26 regions of the brain, indicated systematically lower region-by-region glucose metabolic rates in Quechuas than in lowlanders. The metabolic reductions were least pronounced in primitive brain structures (e.g., cerebellum) and most pronounced in regions classically associated with higher cortical functions (e.g., frontal cortex). These differences between Quechuas with lifetime exposure to hypobaric hypoxia and lowlanders, which seem to be expressed to some degree in most brain regions examined, may be the result of a defense adaptation against chronic hypoxia.

The purine nucleotide cycle as two temporally separated metabolic units: A study on trout muscle☆

Experimental results on fast-twitch muscle of rainbow trout following exercise and during subsequent recovery lead us to a reinterpretation for the function of the components of the purine nucleotide cycle (PNC). Exhaustive exercise depletes tissue ATP by more than 90% and results in a stoichiometric gain in IMP and ammonium ions. Simultaneously, white-muscle aspartate decreases by half, but its maximum contribution can account for less than 2% of the accumulated ammonium. Of the three enzymes of the purine nucleotide cycle, AMP deaminase, adenylosuccinate synthetase and adenylosuccinate lyase, only AMP deaminase is functional during exhaustive exercise. During the slow (greater than 15 hour) recovery, AMP deaminase is effectively shut off, while the other two enzymes replenish the adenylate pool. At all times, a tight inverse correlation exists between ATP and IMP concentrations. Tissue ammonium and malate supply the required aspartate. Theoretical treatment with special attention to proton dynamics in a potentially anaerobic tissue also leads to the conclusion that rather than constituting a true cycle, distinct parts of the PNC are temporally segregated. We hypothesize that during periods of high energy demand, exclusively AMP deaminase is activated as a means (1) to push the myokinase reaction toward ATP synthesis, (2) to supply allosteric effectors, and (3) to remove some of the accumulating protons through the formation of ammonium, all at the expense of the adenylate pool. The process leading to its replenishment, which involves the production of two protons and the consumption of a high-energy phosphate, can be active during aerobic recovery only.(ABSTRACT TRUNCATED AT 250 WORDS)

Capillary-to-fiber geometry and mitochondrial density in hummingbird flight muscle.

We investigated structural characteristics for high O2 flux in hummingbird flight muscle, i.e. the most O2 demanding skeletal muscle per unit tissue mass among vertebrates. Pectoralis and supracoracoideus muscles of 3-4 g hummingbirds (Selaphorus rufus) were perfusion fixed in situ, processed for electron microscopy and analyzed by morphometry. Small fiber size (group mean +/- SE, 201 +/- 14 microns 2 at 2.1 microns sarcomere length), large capillary length per fiber volume (8947 +/- 869 mm-2) and high mitochondrial volume density per volume of muscle fiber (34.5 +/- 0.9%) were characteristic features of the muscles. Considering capillary supply and mitochondrial volume on an individual fiber basis showed that the size of the capillary-to-fiber interface (i.e. capillary surface per fiber surface) was also high in the muscles. Comparison with mammalian hindlimb pointed to a major role of the size of the capillary-to-fiber interface in providing a great potential for O2 flux rate from capillary to muscle fiber mitochondria in hummingbird flight muscle.

INTERINDIVIDUAL VARIABILITY IN BODY COMPOSITION AND RESTING OXYGEN CONSUMPTION RATE IN BREEDING TREE SWALLOWS, TACHYCINETA BICOLOR

Basal metabolic rate is one of the most widely measured physiological traits. Previous studies on lab mice and field‐caught lizards suggest that individuals with relatively high basal metabolic rates or standard metabolic rates have relatively large masses of metabolically active tissues (e.g., heart, kidney, liver). As these are energetically expensive organs, there may be variability between breeding seasons dependent on, for example, availability of prey and capacity for energy intake. We present data from breeding tree swallows (Tachycineta bicolor) collected over two successive seasons. There was no difference between years in resting oxygen consumption rates, although there were significant interannual differences in the masses of all organs and tissues except the pectoralis. Interindividual differences in the masses of the kidney and small intestine explained 21% of the variation in oxygen consumption rates. Although individuals with relatively high resting oxygen consumption rates had relatively large, metabolically active kidneys, they had relatively small intestines and pectoral muscles. This is in contrast to all previous studies on mammals and to the single interspecific study of birds. Oxygen consumption rate also correlated positively with hematocrit. Our results suggest that assumptions of consistent positive relationships between resting oxygen consumption rate and organ masses cannot be extended intraspecifically for birds.

Energy metabolism in orchid bee flight muscles : carbohydrate fuels all

SUMMARY The widely accepted idea that bees fuel flight through the oxidation of carbohydrate is based on studies of only a few species. We tested this hypothesis as part of our research program to investigate the size-dependence of flight energetics in Panamanian orchid bees. We succeeded in measuring rates of O2 consumption and CO2 production in vivo during hovering flight, as well as maximal activities (Vmax values) in vitro of key enzymes in flight muscle energy metabolism in nine species belonging to four genera. Respiratory quotients (ratios of rates of CO2 production to O2 consumption) in all nine species are close to 1.0. This indicates that carbohydrate is the main fuel used for flight. Trehalase, glycogen phosphorylase and hexokinase activities are sufficient to account for the glycolytic flux rates estimated from rates of CO2 production. High activities of other glycolytic enzymes, as well as high activities of mitochondrial oxidative enzymes, are consistent with the estimated rates of carbohydrate-fueled oxidative metabolism. In contrast, hydroxyacylCoA dehydrogenase, an enzyme involved in fatty acid oxidation, was not detectable in any species. Thoracic homogenates displayed ADP-stimulated oxidition of pyruvate + proline, but did not oxidize palmitoyl l-carnitine + proline as substrates. A metabolic map, based on data reported herein and information from the literature, is presented. The evidence available supports the hypothesis that carbohydrate serves as the main fuel for flight in bees.

Metabolic sources of power for mantle muscle of a fast swimming squid

Abstract 1. 1. Squid mantle muscle utilizes carbohydrate as its primary carbon and energy source. 2. 2. Lactate does not accumulate under any circumstances because of a lack of lactate dehydrogenase. Instead, during imposed anoxic stress, alpha-glycerophosphate + pyruvate or metabolic derivatives of pyruvate accumulate in about a 1:1 ratio. 3. 3. The muscle is rich in mitochondria and displays high activities of citrate synthase, malate dehydrogenase, and aspartate aminotransferase. 4. 4. High α-glycerophosphate dehydrogenase activity in the cytoplasm and high rates of mitochondrial α-glycerophosphate oxidase indicate the the potential for an active α-glycerophosphate cycle comparable to that known in insect flight muscle. 5. 5. These characteristics, in addition to consistent control properties of several key regulatory enzymes, support the hypothesis of an obligatorily aerobic carbohydrate catabolism in squid mantle muscle.

Preparation and properties of rainbow trout liver mitochondria

SummaryMitochondria prepared from rainbow trout liver consistently display high respiratory control and ADP/O ratios. These appear to possess a complete Krebs cycle, since pyruvate, palmitoyll-carnitine, citrate, and various Krebs cycle intermediates can all be oxidized. Rapid oxidation of pyruvate and palmitoyll-carnitine requires the pressence of malate. Oxidation of palmitoyll-carnitine appears to inhibit pyruvate oxidation. Malate stimulates α-ketoglutarate oxidation while aspartate inhibits glutamate oxidation, indicating the presence of malate-α-ketoglutarate and glutamate-aspartate carriers. These properties are compared with those of liver mitochondria from other species of fish.