Effect of anoxia on ion distribution in the brain.

Summary
1. Freshwater fishes are the most northerly of freshwater ectotherms, followed by frogs. North American freshwater snakes, turtles, and salamanders do not range farther north than southernmost Canada.

2. Freezing and desiccation are the main challenges during terrestrial hibernation of ectotherms. Oxygen depletion, water balance, and ionic balance are the major problems for air breathing ectotherms that hibernate underwater.

3. The importance of accumulation of energy stores for overwintering among fishes depends upon the length and severity of the winters, whether or not there is springtime reproduction, body size, latitude, and the availability and use of food during overwintering.

4. Fishes can decrease energy demands during the winter by reductions in activity, metabolic depression, and entrance in semi-torpidity.

5. Adaptations for coping with hypoxia and anoxia among overwintering freshwater fishes may include metabolic depression, a decrease in blood O2 affinity, microhabitat selection, air breathing, short-distance migration, biochemical modifications aimed at adjusting glycolytic rates, and alcoholic fermentation.

6. Freshwater turtles have a worldwide northern limit of approximately 50° N, which means that some species spend about half of their lives hibernating.

7. Aquatic turtles normally hibernate underwater, although occasionally they hibernate on land. In water they usually hibernate in a hypoxic or anoxic (mud) environment and in relatively shallow water. Wintertime movements of unknown frequency occur in some species.

8. The hatchlings of many turtle species can overwinter in the nest. Among northern species this behaviour is most common among painted turtles, whose hatchlings can withstand freezing.

9. Mortality among adult turtles is probably highest during the hibernation cycle.

10. Temperature appears to the most important cue for entry and exit from hibernation among freshwater turtles.

11. Little is known of the energetics of overwintering turtles. Energy stores for overwintering may be more important at lower latitudes than at higher ones, due to the higher metabolic rates of overwintering, but non-feeding, southern turtles.

12. The ability of turtles to tolerate submergence is a function of temperature, degree of water oxygenation, latitude of origin, efficacy of extrapulmonary respiratory pathways, and metabolic rate.

13. For turtles that hibernate in an anoxic hibernaculum, and for those without sufficient extrapulmonary uptake of O2 to allow metabolism to be completely aerobic, the most important physiological perturbation is an acidosis developed from a continuing production of lactate. If sufficient O2 can be obtained, the most likely factors limiting hibernation time are water balance and ion balance.

14. Mechanisms of turtles for coping with acidosis include metabolic depression, integumental CO2 loss, bicarbonate buffering, and changes in ion concentrations that minimize the decrease in SID (strong ion difference). The most important among the latter are a decrease in plasma [Cl-] and large increases in plasma calcium and magnesium.

15. Turtles are unique among reptiles in their ability to maintain both cardiovascular and nervous system function during prolonged anoxia.

16. Turtles gain weight from water uptake during submerged hibernation, but apparently maintain some kidney function; however, osmoregulation is one of the least known areas of the physiology of hibernation.

17. Recovery of turtles upon emergence commences with a rapid hyperventilatory compensation of pH, followed by a slower adjustment of ion levels. Basking speeds recovery greatly.

18. While hibernation of turtles in the northern parts of their ranges is most likely very stressful physiologically, northern range limits are more likely to be determined by reproductive restraints than by the rigors of extended hibernation.

19. The superior ability of turtles to tolerate anoxia may be more the result of an annual hibernation than of their diving habits during active periods of the year.

20. Freshwater snakes usually hibernate on land. However, they appear to be capable of aquatic hibernation and may not do so because of the risk of death from anoxia.

21. Some species of terrestrial snakes are known to hibernate underwater, and are able to do so in the laboratory for months. In the field, this behaviour is considered opportunistic, as there is no evidence to suggest that any snakes can tolerate extended anoxia.

Ecology and physiology of hibernation and overwintering among freshwater fishes, turtles, and snakes

The action of neurohormones and drugs on polar lipids of the nervous system has mainly been explored by measuring the 32P metabolism (1). Hence studies regarding the hydrophobic moiety of membrane lipids were limited to survey the fatty acid distribution and the uptake of labeled precursors. Moreover very scarce information is available on lipid degradative enzymes acting on endogenous substrates and on the composition, concentration and changes taking place in endogenous lipid pools related with the non-polar side chains of membrane lipids.

Free Arachidonic Acid and Other Lipids in the Nervous System During Early Ischemia and After Electroshock

Newborn mammals exhibit a number of physiological reactions which differ from normal adult physiology and are often regarded as signs of immaturity. However, when looked upon from a comparative point of view, it becomes obvious that some of these ‘physiological peculiarities’ bear striking similarity to adaptation mechanisms known from hypoxia-tolerant animals and may thus contribute to the well-established, yet poorly understood, phenomenon of neonatal hypoxia tolerance. As the mammalian fetus lives at oxygen partial pressures corresponding to 8000 m altitude, the first line of perinatal hypoxia defense consists of long-term adaptations to limited intrauterine oxygen supply: (1) improved O2 transport by fetal acclimatization to high altitude, (2) reduced metabolic rate by hibernation-like deviation from metabolic size allometry, (3) diminished cerebral vulnerability by functional analogies to diving turtle brain, and (4) enhanced metabolic flexibility by optional repartitioning of energy supply from growth to maintenance metabolism. In the case of birth asphyxia, these background mechanisms are complemented by short-term responses to acute oxygen lack: (1) reduction of body temperature as in natural torpor, (2) reduction of heart rate and redistribution of circulation as in diving mammals, (3) reduction of respiration rate typical of ‘hypoxic hypometabolism’, and (4) reduction of blood pH according to the concept of ‘acidotic torpidity’. Although anaerobic metabolism is improved in neonatal mammals by increased glycogen stores, reduced metabolic demands, and sustained wash-out of acid metabolites, neonatal hypoxia tolerance seems to be primarily based on the ability to maintain tissue aerobiosis as long as possible. This is even reflected by isoenzyme patterns which do not consistently favour anaerobic glycolysis and, thus, are reminiscent of the ‘lactate paradox’ found in high altitude adaptation. Altogether, from a biological point of view, the perinatal period appears as a source of adaptive mechanisms that can be refound, in varying combinations, in many survival strategies. From a clinical point of view, the interplay of long- and short-term mechanisms offers a novel approach to estimation of the newborn’s ability to withstand temporary oxygen lack. However, most of these mechanisms are not unambiguous and, above all, not unlimited in their protective effect so that they do not release obstetricians or neonatologists from their obligation to counteract fetal or neonatal hypoxia without delay.

Neonatal tolerance to hypoxia: a comparative-physiological approach.

THURSTON,JEAN HOLOWACH, and DAVID B. MCDOUGAL, JR. Effect of &hernia on metabolism of the brain of the newborn mouse. Am. J. Physiol. 216(2):348-352. 1969.-Changes in six metabolites in the brains of newborn mice were followed during the ischemia induced by decapitation. Phosphocreatine fell rapidly, glucose and glucose 6-phosphate more slowly, and ATP and glycogen were the most stable of the substances measured. From the changes, the initial rate of use of highenergy phosphate (cerebral metabolic rate) was calculated to be 2.3 mmoles/kg per min, which is .09 of the adult rate following decapitation. Lactate accumulated much more slowly than in the adult. In the early stages (first 4 min) of ischemia the rate of disappearance of energy reserves was nearly linear, and much faster than the rate in the later stages of the experiment. The energy reserves of the brain arc about the same in young as in adult animals. In view of this, and the low rate of glycolysis, it is concluded that the chief factor in the resistance of very young animals to ischemia is the low cerebral metabolic rate, not high glycolytic capacity.

Effect of ischemia on metabolism of the brain of the newborn mouse

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The levels of the main cerebral energy reserves, ATP, P-creatine, glycogen and glucose, and of several glycolytic intermediates and lactate, were measured in the brains of fish (Carassius auratus), turtle (Pseudemys scripta elegans) and frog (Rana pipiens). The levels of glycogen in these brains were 2-9 times higher than those reported for mammals. In frog, cerebral glycogen levels were 35 per cent higher during the winter than in spring. The P-creatine: ATP ratios were 3 instead of the more usual (mammalian) value of 1. The levels of other intermediates were similar to those found in mammalian brain.



When anoxia was produced by decapitation, changes in the various substances measured were similar to those in mammalian brain, but were much slower. The initial rate at which high-energy phosphate was used could be calculated from these changes. Values of 1.1 m-equiv./kg/min for fish and frog and of 0.46 m-equiv./kg/min for turtle were found, which are 1/20 and 1/50, respectively, of the rate in mouse brain. The rate of disappearance of high-energy phosphate reserves followed first-order kinetics for 4 hr in turtle and for at least an hour in the other species.



Changes in metabolites as the experiment progressed were interpreted to indicate a progressively falling intracellular pH, prolonged inhibition of phosphofructokinase, and a long period of hexokinase inhibition.

C A Thomas

Papers

The effects of anoxia upon energy sources and selected metabolic intermediates in the brains of fish, frog and turtle.