Brain energetics and tolerance to anoxia in deep hypothermia.

The remarkable time-resolution enhancement by deep lethargic hypothermia (15 degrees C rectal temperature, "cold narcosis," "anesthesia by internal cold") of metabolic events in the rat brain after oxygen deprivation has been exploited to monitor metabolic changes by in vivo (31)P-NMR. A correlation was established between the bioenergetic status of the brain and physiological descriptors of tolerance (survival and revival times) determined in parallel experiments with large series of animals. Spectral peak integrals were transformed into absolute concentrations by comparison to biochemically determined time series of data obtained in freeze-trapping experiments conducted under identical conditions. Serial spectra were used to reconstruct the time-course kinetics of intracellular brain pH and of concentration changes of inorganic phosphate, phosphocreatine, ATP, and ADP. Both the biochemical and NMR time series of data were simultaneously fitted by a set of exponential kinetic equations accounting for relationships imposed by the Lohmann and adenylate kinase reactions. Depletion profiles were then computed for a number of descriptors of brain energy status (energy charge, phosphorylation potential, total adenylate, and primary energy stores expressed as the sum of high-energy phosphate-bond equivalents). The results contribute to the understanding of the role of brain energetics in tolerance to oxygen deprivation.

Equilibrium of nucleotides in the dogfish brain.

In the past, the results of experiments on the time course of concentration changes of adenylates, phosphocreatine, and free creatine in muscle appeared compatible with an equilibrium hypothesis involving only the Lohmann and the myokinase reactions. Other reports, however, denied the applicability of the equilibrium hypothesis to the same tissue. The controversy may have been due to the high probability of experimental errors since time sampling was performed at second intervals. We presently test the hypothesis in the living brain of the small-spotted dogfish shark (Scyliorhinus canicula), an animal-model allowing for timing of sampling at hourly intervals. According to our earlier work, the dogfish shark can easily be resuscitated 8.2 h on average after being brought into the state of "suspended animation" at 0 degree C body temperature and exposed, out of water, to an atmosphere of nitrogen gas. To obtain a complete mathematical description of the time course of concentration changes of brain adenylates and phosphocreatine, we devised a kinetic model based on principles of classical multicompartmental analysis and biochemical kinetics. Model testing of the equilibrium hypothesis resulted in very good agreement between the hypothesis and our experimental data. Time-course modeling, achieved by simultaneously fitting the time series of our data by the set of four equations constituting our model resulted in an excellent agreement between data points and the computed curves. Finally, modeling of the depletion profiles of brain energy status concerning three of its descriptors (energy charge, total adenylate, and primary energy stores expressed in high-energy phosphate equivalents) allowed for a correlation to be established between energy status and the "revival time," a valuable physiological descriptor of tolerance.

Mechanisms of immediate temperature compensation: experiments with brain synaptosomes from rat and ground squirrel.

Glucose conversion by brain synaptosomes can be regarded as a special case of intrinsic kinetic properties of the enzyme substrate system. Temperature modulation of apparent K(m) for this process can be described with our kinetic model. Using experimental data and the kinetic model, the minimal K(m) value for glucose conversion in ground squirrel synaptosomes was found at the lower temperature (6.5 degrees C), much lower than that for the rat (16.6 degrees C). The inversion temperatures (T(min)) closely coincided with the lowest body temperatures from which the unassisted recovery from hypothermia was demonstrated in both species. This study indicated that thermal modulation of enzyme affinities may have an adaptive role in endotherms that is linked to their tolerance to hypothermia.

Some components of hibernation rhythms.

At low ambient temperatures some small mammals drastically reduce their body temperature and enter a state of dormancy known as hibernation. They exhibit endogenous rhythms of heterothermy/homeothermy and body mass fluctuations with the period close to one year. At high ambient temperature only body mass cycling is expressed for almost one year as well. The effect of temperature on circannual periods of some biological rhythms in different hibernators in free-running conditions as well as the circaseptal character of the intrahibernation rhythm of periodic arousals are reviewed.

Photopic vision in eels: evidences of color discrimination.

Several classes of second-order retinal neurons have been studied electrophysiologically in European eel (Anguilla anguilla) from two different localities, Lake Seliger in Russia and the coastal waters of the Adriatic Sea in Montenegro. The majority of L-horizontal cells (68 explored) had both rod and cone inputs, an uncommon phenomenon among teleosts. Pronounced color-opponent properties, often taken as pointing to the capacity of color vision, were identified in one amacrine cell, apparently of the "blue/yellow" (or "blue/green") type. Microspectrophotometric measurements revealed two different spectral classes of cones with absorption maxima at about 525 and 434 nm. The existence of green-sensitive and blue-sensitive cone units was thus revealed by both electrophysiological and microspectrophotometric techniques.

Modeling of the migration of the European glass eel.

The characteristics of glass eel migration at the mouth of river Bojana have been investigated by seasonal field studies from February to April 1998. Samples were collected by two fyke nets, one on each riverside, for 12 hours each night. A total of about 3,300 individuals were caught. In this work we present the results obtained from three successive migration waves in 1998. The data obtained on the number of eels caught during one migration wave, as a function of time, had two components: a bell-shaped curve, lasting 7-14 days, over which an impulse (sudden burst in the number of caught eels, during 1-2 days) was observed. We propose a mathematical model for total number of captured eels within one migration wave. Impulse components were observed during the first and second migration waves. After their removal, the remaining experimental data of the three migration waves were fitted with normal distribution functions. A decrease in the values of fitting parameters as a function of time (migration wave number) was obtained. Our data indicate multiple causes for the observed migration waves.

Influence of photic environment on the form of the fish electroretinographic off-response.

Scotopic electroretinogram of dogfish shark (Scylliorhinus canicula) and eel (Anguilla anguilla) is characterized by a negative off-response, changing in sign under photopic condition. It increased under the effect of increased background illumination, but its amplitude never exceeded that of the b-wave. On the other hand, dark-adapted electroretinograms of two perch-like species, perch (Perca fluviatilis) and painted comber (Serranus scriba), exhibited a positive off-wave, exceeding the b-wave amplitude under bright photopic conditions.

Kinetic properties of the enzyme-substrate system: a basis for immediate temperature compensation.

One-minimum U-shaped temperature profiles of the dissociation constant (K(m)) have been observed experimentally with a variety of enzyme-substrate (E-S) systems. The increase of E-S affinity with falling temperature ("positive thermal modulation of affinity"), which opposes the cold-induced reduction in catalytic velocity, has been often interpreted as significant for both immediate and evolutionary temperature compensations and of major importance in setting thermal limits in ectothermic organisms. This role was denied to enzymes from endotherms, on the ground that their minimal K(m) values were situated well below their normal body temperature. Evidence is presented in this report that affinity changes described by U-shaped profiles can simply be the consequence of intrinsic kinetic properties of the E-S system. Theoretical modeling is achieved by combining the classical expression for the Michaelis constant with Transition State Theory expressions for the three rate constants involved. It provides for the U-shape of the K(m) vs. T profile and allows for the derivation of an equation for identifying its inversion point. Modeling of V(max) and V(min) (reaction velocity under conditions of substrate saturation and of dilution, K(m)>>[S], respectively) is also included. An expression was formulated for predicting the "critical temperature," T(C), corresponding to the low-temperature break in Arrhenius lines. Using existing K(m) data from literature, concerning a variety of E-S systems, our modeling proved to be highly satisfactory. Our own experiments show that glucose uptake by rat brain synaptosomes can be regarded as a special case of basically the same kinetic scheme, and that the U-shaped temperature modulation of apparent K(m) for glucose conversion is also in full agreement with our kinetic modeling. These experiments indicate that positive thermal modulation, although based on intrinsic kinetic properties of the underlying E-S system, may have an adaptive role in endotherms as well, linked, however, to their tolerance to hypothermia.