Temperature sensitivity of the sodium pump in red cells from various hibernator and non-hibernator species

Physiological adaptation of an Antarctic Na+/K+-ATPase to the cold

Because enzymatic activity is strongly suppressed by the cold, polar poikilotherms face significant adaptive challenges. For example, at 0°C the catalytic activity of a typical enzyme from a temperate organism is reduced by more than 90%. Enzymes embedded in the plasma membrane, such as the Na(+)/K(+)-ATPase, may be even more susceptible to the cold because of thermal effects on the lipid bilayer. Accordingly, adaptive changes in response to the cold may include adjustments to the enzyme or the surrounding lipid environment, or synergistic changes to both. To assess the contribution of the enzyme itself, we cloned orthologous Na(+)/K(+)-ATPase α-subunits from an Antarctic (Pareledone sp.; -1.8°C) and a temperate octopus (Octopus bimaculatus; ∼18°C), and compared their turnover rates and temperature sensitivities in a heterologous expression system. The primary sequences of the two pumps were found to be highly similar (97% identity), with most differences being conservative changes involving hydrophobic residues. The physiology of the pumps was studied using an electrophysiological approach in intact Xenopus oocytes. The voltage dependence of the pumps was equivalent. However, at room temperature the maximum turnover rate of the Antarctic pump was found to be 25% higher than that of the temperate pump. In addition, the Antarctic pump exhibited a lower temperature sensitivity, leading to significantly higher relative activity at lower temperatures. Orthologous Na(+)/K(+) pumps were then isolated from two tropical and two Arctic octopus. The temperature sensitivities of these pumps closely matched those of the temperate and Antarctic pumps, respectively. Thus, reduced thermal sensitivity appears to be a common mechanism driving cold adaptation in the Na(+)/K(+)-ATPase.

Physiological blocking of the mechanisms of cold death: theoretical and experimental considerations

The cold inhibited functions of skin thermoreceptors, of the thermoregulation centre, and the respiration centre during deep hypothermia can be restored without rewarming the body. The methods used were developed to test the hypothesis that during deep hypothermia calcium ion concentration [Ca(2+)](i) in the cytoplasm increases. This causes a perturbation of cell metabolism, the impairment of cell membrane function that cause the inhibition of cell functioning, resulting in cell death. Such an increase in [Ca(2+)](i) most likely would result from an energy deficit in a deeply cooled cell, which would compromise the processes that maintain the [Ca(2+)](i) at about 10(-7) M. These processes require large amounts of energy since they occur against a large concentration gradient. With the use of EDTA the extracellular concentration of Ca(2+) has been lowered by 15-27%, so reducing the concentration gradient for Ca(2+) between the cell and the medium and in consequence facilitated the process the extrusion of cell Ca(2+).During a period of cooling, sufficient to impair normal functioning, the experimental lowering of blood Ca(2+) allowed the restoration of normal function without the need to rewarm. In such cases the animals survived after cooling the body to temperatures at which they would normally have succumbed. The data presented support the stated hypothesis that the impairment of cellular function in mammals by low temperatures is the result of an uncorrected rise in [Ca(2+)](i).

Temperature effects on ion transport across the erythrocyte membrane of the frog Rana temporaria

Unidirectional K+ and Na+ influxes in the frog erythrocytes incubated in Cl- or NO(3)- media with 2.7 mM K+ were measured using 86Rb and 22Na as tracers. K+ influx was inhibited by 35-55% in the presence of 0.2-1.0 mM furosemide but it was unaffected by 0.1-0.2 mM bumetanide. Furosemide at a concentration of 0.5 mM had no effect on K+ loss from the frog red cells incubated in a nominally K(+)-free medium. Together with our previous studies the data support the existence of K-Cl cotransport and the absence of Na-K-2Cl cotransport in the frog erythrocyte membrane. Cell cooling from 20 to 5 degrees C caused a decrease in K+ influx and K+ efflux via the K-Cl cotransporter (3.2- and 3.7-fold, respectively) giving an apparent energy of activation (EA) of about 60 kJ/mol and Q10 value of 2.5. Only small decline (approximately 30%) in the ouabain-sensitive K+ influx was found as temperature was changed from 20 to 5-10 degrees C. Low values of Q10 (approximately 1.5) and EA (27.3 kJ/mol) were obtained for passive K+ influx in the frog erythrocytes (ouabain-insensitive in NO(3)- medium) at temperature within 5-20 degrees C. However, the temperature coefficients were greater for passive Na+ influx and passive K+ efflux (Q10 approximately 2.4-2.5 and EA approximately 56-58 kJ/mol). The temperature dependence of all ion transport components displayed discontinuities showing no changes at temperature between 5 and 10 degrees C. Thus, cooling of the frog red cells is associated with a greater decrease of Na+ influx and K+ efflux than passive and active K+ influx. These data indicate that the preservation of a relative high activity of the Na,K-pump during cell cooling and also the temperature-induced changes in the K-Cl cotransport activity and ion passive diffusion contribute to maintenance of ion concentration gradients in the frog erythrocytes at decreased temperature.

Decreased calcium accumulation in isolated nerve endings during hibernation in ground squirrels

Resting and depolarization-induced 45CaCl2 accumulation was compared for synaptosomes isolated from hibernating and nonhibernating ground squirrels. Channel subtype antagonists were used to identify the active voltage-sensitive calcium channel subtypes in these preparations. There was significantly less 45Ca2+ accumulation in synaptosomes isolated from hibernating as compared to cold-adapted nonhibernating ground squirrels in both basal (p < 0.005) and depolarizing (p < 0.03) media over a 30 sec to 5 min incubation period. The elevation in 45Ca2+ accumulation triggered by K+ depolarization was blocked by 50 microM CdCl2, 1 microM omega-conotoxin MVIIC or 1 microM omega-agatoxin IVA. Inhibition was not observed with 1 microM nifedipine or with 1 microM omega-conotoxin GVIA. These results suggest that hibernation is associated with reduced presynaptic 45Ca2+ conductance via voltage-sensitive channels with a pharmacological sensitivity that is different from the established L-, N-, and P-types in other systems but share features of the recently described Q-type calcium channel. This decrease may reflect a cellular adaptation that helps confer tolerance to the near total cerebral ischemia associated with hibernation.

Diversities of transport of sodium in rodent red cells

1. Na-K pumps of rodent red cells reveal variations among species in terms of kinetic properties such as ouabain sensitivity, Na/K coupling and temperature sensitivity and variations within an individual organism related to such physiological challenges as K deficiency, calorie deficiency and seasonal changes in temperature. 2. Passive Na entry among rodents collectively occurs through the same routes as in red cells of other mammals, but red cells of hamsters, rats and thirteen-lined ground squirrels lack or are deficient in an amiloride-sensitive, shrinkage-activated Na-H exchange. 3. In guinea-pig this pathway appears to be both activated and uncoupled by cooling from 37 to 20 degrees C. 4. Red cells of rodents in general and hamsters in particular are rich in a Na-Mg exchange pathway. In hamsters, this appears to be the only amiloride-sensitive pathway in simple media. 5. In hamster cells, Na entry through the amiloride-sensitive Mg-activated pathway exhibits the same kinetics as previously shown for Na activation of Mg extrusion.

Temperature effects on the Na and Ca currents in rat and hedgehog ventricular muscle

Cardiac transmembrane potentials and Na and Ca currents were recorded at different temperatures in rat and hedgehog ventricular muscle. At 35 degrees C in both species resting potential was about -80 mV and upstroke velocity (Vmax) of the action potential above 100 V/s. The shape of the action potential in hedgehog ventricular cells at 35 degrees C was similar to that in the rat showing a fast repolarization phase. When temperature was decreased, the membrane resting potential depolarized and action potential amplitude and Vmax declined. In rat ventricular cells at 10 degrees C, the resting potential was about -40 to -50 mV and Vmax was reduced to about 5 V/s. In hedgehog ventricular cells, however, the transmembrane potentials and Vmax were better maintained at low temperature. Phase 3 of the action potential was markedly prolonged below 20 degrees C in hedgehog but not in rat ventricular cells. When temperature was decreased to 10 degrees C the availability curve of the Na current shifted toward more negative potentials and ICa.peak declined in rat ventricular cells. In hedgehog cardiac preparations, the Na current was less influenced by the cooling and ICa.peak did not change very much at low temperatures. A transient inward current usually considered to induce cardiac arrhythmias could be recorded in rat ventricular cells below 20 degrees C but not in hedgehog preparations. These features of hedgehog cardiac membranes may contribute to the cold tolerance and the resistance to ventricular fibrillation during the hypothermia in mammalian hibernators.

Seasonal changes in cation transport in red blood cells of grey squirrel (Sciurus carolinensis) in relation to thermogenesis and cellular adaptation to cold

1. Unidirectional influx of 42K was measured in red cells of grey squirrels at seasonal intervals over two years. 2. Na/K pump-related (i.e. ouabain-sensitive) K influx at 37 degrees C was maximal in cells collected in January and was more than three times greater than cells collected in summer. Na/K pump activity, maximized by loading the cells with Na, exhibited a similar difference. 3. At 5 degrees C in fresh cells, ouabain-sensitive K influx, expressed as per cent of that at 37 degrees C, was highest in March. In Na-loaded cells it was lowest in summer. 4. Passive "leak" K influx (i.e., the residual influx remaining in presence of ouabain and bumetanide) was highest in October, and declined progressively to the summer months, when it was only 27% of that in October. 5. Cotransport (i.e., bumetanide-sensitive K influx) exhibited the same seasonal pattern as Na/K pump activity in fresh cells. 6. Net gain of Na in cells stored at 5 degrees C for three days in March was less than half of that in January or summer. 7. High transport activity in January may correlate with a requirement for increased non-shivering thermogenesis. However, red cells of grey squirrels exhibit maximum resistance to low temperature in March and at this time resemble the red cells of hibernating mammals.

Membrane function in mammalian hibernation

For homeotherms the maintenance of a high, uniform body temperature requires a constant energy supply and food intake. For many small mammals, the loss of heat in winter exceeds energy supply, particularly when food is scarce. To survive, some animals have developed a capacity for adaptive hypothermia in which they lower their body temperature to a new regulatory set-point, usually a few degrees above the ambient. This process, generally known as hibernation, reduces the temperature differential, metabolic activity, as well as the energy demand, and thus facilitates survival during winter. Successful hibernation in mammals requires that the enzymatic processes are regulated in such a manner that metabolic balance is maintained at both the high body temperature of the summer-active animal (37 degrees C) and the low body temperature of the winter-torpid animal (approx. 5 degrees C). This means that the cellular membranes have thermal properties capable of maintaining a balanced metabolism at these extreme physiological temperatures. The available evidence indicates that, for some tissues, preparation for hibernation involves an alteration in the lipid composition and thermal properties of cellular membranes. Marked differences in the thermal response of cellular membranes have been observed on a seasonal basis and, in some membranes, differences in lipid composition have been associated with the torpid state. However, to date, no consistent changes in lipid composition which would account for, or explain, the changes in membrane thermal response, have been detected. An important point to emphasize is that the process of 'homeoviscous adaptation', which occurs in procaryotes and some poikilotherms during acclimation to low temperatures, is not a characteristic feature of most membranes of mammalian hibernators.

Defense strategies against hypoxia and hypothermia

Because aerobic metabolic rates decrease in hypoxia-sensitive cells under oxygen-limiting conditions, the demand for glucose or glycogen for anaerobic glycolysis may rise drastically as a means of making up for the energetic shortfall. However, ion and electrical potentials typically cannot be sustained because of energy insufficiency and high membrane permeabilities; therefore metabolic and membrane functions in effect become decoupled. In hypoxia-tolerant animals, these problems are resolved through a number of biochemical and physiological mechanisms; of these metabolic arrest and stabilized membrane functions are the most effective strategies for extending tolerance to hypoxia. Metabolic arrest is achieved by means of a reversed or negative Pasteur effect (reduced or unchanging glycolytic flux at reduced O2 availability); and coupling of metabolic and membrane function is achievable, in spite of the lower energy turnover rates, by maintaining membranes of low permeability (probably via reduced densities of ion-specific channels). The possibility of combining metabolic arrest with channel arrest has been recognized as an intervention strategy. To date, the success of this strategy has been minimal, mainly because depression of metabolism through cold is the usual arrest mechanism used, and hypothermia in itself perturbs controlled cell function in most endotherms.