Peter W. Hochachka

Mechanism, origin, and evolution of anoxia tolerance in animals☆ ☆

Organisms vary widely in their tolerance to conditions of limiting oxygen supply to their cells and tissues. A unifying framework of hypoxia tolerance is now available that is based on information from cell-level models from highly anoxia-tolerant species, such as the aquatic turtle, and from other more hypoxia-sensitive systems. The response of hypoxia-tolerant systems to oxygen lack occurs in two (defense and rescue) phases. The first lines of defense against hypoxia include a drastic, if balanced, suppression of ATP demand and supply pathways; this regulation allows ATP levels to remain constant, even while ATP turnover rates greatly decline. The ATP requirements of ion pumping are down-regulated by generalized channel arrest in hepatocytes and by the arrest of specific ion channels in neurons. In hepatocytes, the ATP demands of protein synthesis are down-regulated on exposure to hypoxia by an immediate global blockade of the process (probably through translational arrest caused by complexing between polysomes and elongation factors). In hypoxia-sensitive cells, this translational arrest seems irreversible, but hypoxia-tolerant systems activate rescue mechanisms if the period of oxygen lack is extended by preferentially regulating the expression of several proteins. In these cells, a cascade of processes underpinning hypoxia rescue and defense begins with an oxygen sensor (a heme protein) and a signal transduction pathway that leads to the specific activation of some genes (increased expression of several proteins) and to specific down-regulation of other genes (decreased expression of several other proteins). The functional roles of the oxygen-sensing and signal-transduction system include significant gene-based metabolic reprogramming - the rescue process - with maintained down-regulation of energy demand and supply pathways in metabolism throughout the hypoxic period. We consider that, through this recent work, it is becoming evident how normoxic-maintenance ATP turnover rates can be down-regulated by an order of magnitude or more - to a new hypometabolic steady state, which is prerequisite for surviving prolonged hypoxia or anoxia. Because the phylogenies of the turtles and of fishes are well known, we are now in an excellent position to assess conservative vs. adaptable features in the evolution of the above hypoxia-response physiology in these two specific animal lineages.

Metabolic arrest and the control of biological time

Freshwater turtles and goldfish can survive for several days without oxygen, some diving turtles for several months; hibernating animals can exist without food for long periods; others can survive extreme conditions such as desiccation, freezing, and thawing. These creatures are, in effect, self-sustaining life-support systems, with a mysterious ability to regulate their own metabolisms. These capabilities raise important questions, which Hochachka and Guppy explore in this seminal new book. What mechanisms turn down (or off) cell metabolism and other cell functions? How does an animal such as an opossum know when to activate mechanisms for slowing or stopping tissue and organ functions? How does it know when to turn them on again? How extensive is metabolic arrest as a defense against harsh environmental conditions? Can we decipher universal principles of metabolic arrest from available data? The lessons to be learned are of potentially great interest to clinicians, because the authors provide a theoretical framework in which to organize an attack on the all-too-practical problem of protecting tissues against hypoxia. Areas that may be influenced include research on cardiac arrest, strokes, acute renal failure, liver ischemia, lung injury, respiratory defense syndrome, claudication, shock, and organ transplant. Investigation of other metabolic arrest mechanisms may be similarly useful in both clinical and agricultural fields. This is a pioneering book of great use to biomedical/clinical researchers and to biologists, biochemists, and physiologists generally.

Oxygen Sensing and Signal Transduction in Metabolic Defense Against Hypoxia: Lessons from Vertebrate Facultative Anaerobes

Earlier studies identified two main defense strategies against hypoxia in hypoxia tolerant animals: (1) reduction in energy turnover, and (2) improved energetic efficiency of those metabolic processes that remain. We used two model systems from the highly anoxia-tolerant aquatic turtle: (1) tissue slices of brain cortex (to probe cell level electrophysiological responses to oxygen limitation), and (2) isolated liver hepatocytes (to probe signalling and defense). In the latter, a cascade of processes underpinning hypoxia defense begins with an oxygen sensor that is probably a heme protein and a signal transduction pathway that leads to the specific activation of some genes (increased expression of several proteins) and to specific down-regulation of other genes (decreased expression of several other proteins). The pathway seems to have characteristics in common with oxygen-regulated control elements in other cells. The probable roles of the oxygen sensing and signal transduction system include coordinate down-regulation of energy demand and energy supply pathways in metabolism. Because of this coordination, hypoxia tolerant cells stay in energy balance even as they down-regulate to extremely low levels of ATP turnover. The main ATP-demanding processes in normoxia (protein synthesis, protein degradation, glucose synthesis, urea synthesis and maintenance of electrochemical gradients) are all turned down to variable degrees during anoxia or extreme hypoxia. Most striking is the observation that ion pumping is the main energy sink in anoxia-despite reductions in cell membrane permeability (channel arrest). Neurons also show a much lower permeability than do homologous mammalian cells but, in this case under acute anoxia, there is no further change in cell membrane conductivity. We consider that, through this recent work, it is becoming evident how normoxic maintenance ATP turnover rates can be down-regulated by an order of magnitude or more-to a new hypometabolic steady state that is prerequisite for surviving prolonged hypoxia or anoxia. The implications of these developments extend to many facets of biology and medicine.

Channels and pumps—determinants of metabolic cold adaptation strategies

Abstract 1. 1. In all cells the passive diffusion of ions through transmembrane channels must be precisely balanced by the active transport of ions against concentration gradients, i.e., the ratio of ion channel/ion pump fluxes must be controlled at unity. 2. 2. One reason why some cells are cold sensitive is because temperature differentially affects these two processes and the necessary balance between them breaks down. 3. 3. Polar fishes apparently maintain channel/pump flux ratios at unity by upward adjustments in functional pump densities. 4. 4. That is presumably why their maintenance metabolic requirements are higher, which is an important cause of their higher-than-expected metabolic rates. This is the causal basis for the paradigm of metabolic cold adaptation in polar fishes. 5. 5. Channel/pump flux ratios of unity in principle also could be achieved in cold tolerant organisms by downward adjustments in functional channel densities of cell (and organelle) membranes. 6. 6. However, organisms utilizing this strategy would be expected to display profoundly reduced metabolic rates, reduced scopes for activity, and reduced osmoregulatory capacities. 7. 7. Deep sea fishes, also normally functional at low temperatures, display drastically reduced metabolic rates compared to other fishes and may well illustrate the latter strategy.

Pyruvate kinase functions in hot and cold organs of tuna

SummaryThe skipjack tuna maintains its red skeletal musculature well above ambient temperatures while the temperature of the heart is within 1°C of that of the water. These two tissues exhibit tissue specific forms of pyruvate kinase. The red muscle has one form while the heart has two.TheKm(PEP) of the red muscle enzymes rises with temperature, within the normal temperature range of the tissue. The affinity of the major form of the heart enzyme for phosphoenolpyruvate is relatively independent of temperature over the physiological temperature range.Km(ADP) values are temperature independent for both enzymes.Inhibition by alanine of both enzymes is temperature dependent and rises with temperature. The same is true of ATP inhibition of the heart enzyme, but ATP inhibition of the red muscle enzyme is temperature independent. Fructose diphosphate reverses alanine and ATP inhibition at all temperatures.With both enzymes, temperature affects substrate affinities and the sensitivity of the enzyme to metabolite effectors. These effects can be rationalized in terms of physiological significance only in the case of the red muscle enzyme.