Red cell 2,3-diphosphoglycerate in hibernating ground squirrels

Hemoglobin function and allosteric regulation in semi-fossorial rodents (family Sciuridae) with different altitudinal ranges

Semi-fossorial ground squirrels face challenges to respiratory gas transport associated with the chronic hypoxia and hypercapnia of underground burrows, and such challenges are compounded in species that are native to high altitude. During hibernation, such species must also contend with vicissitudes of blood gas concentrations and plasma pH caused by episodic breathing. Here, we report an analysis of hemoglobin (Hb) function in six species of marmotine ground squirrels with different altitudinal distributions. Regardless of their native altitude, all species have high Hb-O2 affinities, mainly due to suppressed sensitivities to allosteric effectors [2,3-diphosphoglycerate (DPG) and chloride ions]. This suppressed anion sensitivity is surprising given that all canonical anion-binding sites are conserved. Two sciurid species, the golden-mantled and thirteen-lined ground squirrel, have Hb-O2 affinities that are characterized by high pH sensitivity and low thermal sensitivity relative to the Hbs of humans and other mammals. The pronounced Bohr effect is surprising in light of highly unusual amino acid substitutions at the C-termini that are known to abolish the Bohr effect in human HbA. Taken together, the high O2 affinity of sciurid Hbs suggests an enhanced capacity for pulmonary O2 loading under hypoxic and hypercapnic conditions, while the large Bohr effect should help to ensure efficient O2 unloading in tissue capillaries. In spite of the relatively low thermal sensitivities of the sciurid Hbs, our results indicate that the effect of hypothermia on Hb oxygenation is the main factor contributing to the increased blood-O2 affinity in hibernating ground squirrels.

Hibernation and gas exchange

Hibernation in endotherms and ectotherms is characterized by an energy-conserving metabolic depression due to low body temperatures and poorly understood temperature-independent mechanisms. Rates of gas exchange are correspondly reduced. In hibernating mammals, ventilation falls even more than metabolic rate leading to a relative respiratory acidosis that may contribute to metabolic depression. Breathing in some mammals becomes episodic and in some small mammals significant apneic gas exchange may occur by passive diffusion via airways or skin. In ectothermic vertebrates, extrapulmonary gas exchange predominates and in reptiles and amphibians hibernating underwater accounts for all gas exchange. In aerated water diffusive exchange permits amphibians and many species of turtles to remain fully aerobic, but hypoxic conditions can challenge many of these animals. Oxygen uptake into blood in both endotherms and ectotherms is enhanced by increased affinity of hemoglobin for O₂ at low temperature. Regulation of gas exchange in hibernating mammals is predominately linked to CO₂/pH, and in episodic breathers, control is principally directed at the duration of the apneic period. Control in submerged hibernating ectotherms is poorly understood, although skin-diffusing capacity may increase under hypoxic conditions. In aerated water blood pH of frogs and turtles either adheres to alphastat regulation (pH ∼8.0) or may even exhibit respiratory alkalosis. Arousal in hibernating mammals leads to restoration of euthermic temperature, metabolic rate, and gas exchange and occurs periodically even as ambient temperatures remain low, whereas body temperature, metabolic rate, and gas exchange of hibernating ectotherms are tightly linked to ambient temperature.

Decrease in the red cell cofactor 2,3-diphosphoglycerate increases hemoglobin oxygen affinity in the hibernating brown bear Ursus arctos

During winter hibernation, brown bears (Ursus arctos) reduce basal O(2) consumption rate to ∼25% compared with the active state, while body temperature decreases moderately (to ∼30°C), suggesting a temperature-independent component in their metabolic depression. To establish whether changes in O(2) consumption during hibernation correlate with changes in blood O(2) affinity, we took blood samples from the same six individuals of hibernating and nonhibernating free-ranging brown bears during winter and summer, respectively. A single hemoglobin (Hb) component was detected in all samples, indicating no switch in Hb synthesis. O(2) binding curves measured on red blood cell lysates at 30°C and 37°C showed a less temperature-sensitive O(2) affinity than in other vertebrates. Furthermore, hemolysates from hibernating bears consistently showed lower cooperativity and higher O(2) affinity than their summer counterparts, regardless of the temperature. We found that this increase in O(2) affinity was associated with a significant decrease in the red cell Hb-cofactor 2,3-diphosphoglycerate (DPG) during hibernation to approximately half of the summer value. Experiments performed on purified Hb, to which DPG had been added to match summer and winter levels, confirmed that the low DPG content was the cause of the left shift in the Hb-O(2) equilibrium curve during hibernation. Levels of plasma lactate indicated that glycolysis is not upregulated during hibernation and that metabolism is essentially aerobic. Calculations show that the increase in Hb-O(2) affinity and decrease in cooperativity resulting from decreased red cell DPG may be crucial in maintaining a fairly constant tissue oxygen tension during hibernation in vivo.

Effects of hibernation on blood oxygen transport in the golden-mantled ground squirrel

Isocapnic O2 equilibrium curves (O2EC) were generated for winter hibernating and summer active ground squirrels (Spermophilus lateralis) at 7 degrees and 37 degrees C using thin blood film techniques. Half-saturation PO2 at 7 degrees C and pHa 7.46 were 5.8 +/- 0.1 and 6.9 +/- 0.2 Torr for hibernating and summer squirrels, respectively; P50 values at 37 degrees C and pHa 7.49 were 15.3 +/- 0.1 and 18.1 +/- 0.5 Torr, respectively. This increased blood O2 affinity in the winter animal results, in part, from reductions of RBC organic phosphates. The molar ratio ([ATP] + [DPG])/[Hb4] decreased from 1.55 in summer squirrels to 0.91 in winter hibernators. O2EC shape and CO2 Bohr effect were similar for the two animal groups, but varied with blood temperature. At 7 degrees C, Hill plots were nonlinear; Hill's n increased from values of 2.2-2.4 below 40% S to 2.7-2.9 above 60% S. At 37 degrees C, Hill plots were reasonably linear (n = 2.5). CO2 Bohr slopes (delta log P50/delta pH) for hibernating and euthermic squirrels were -0.37 +/- 0.02 and -0.40 +/- 0.03 at 7 degrees C, respectively, and -0.62 +/- 0.04 and -0.60 +/- 0.02 at 37 degrees C, respectively. Blood O2 capacity was significantly greater (P < 0.001) in the hibernator; hematocrit (55%) and [Hb] (19.1 g/dl) exceeded the summer squirrel values by 20% and 25%, respectively. Estimated PvO2 values for summer and winter animals at 7 degrees C and pH 7.46 were 7.25 and 6.94 Torr, respectively. This suggests that the effect of increased Hb-O2 affinity on PvO2 is offset by increased circulating [Hb]. We conclude that seasonal changes in the O2 transport properties of squirrel blood do not contribute to the depression of aerobic metabolism during winter hibernation.

The effects of short term cold storage upon ATP and 2,3-BPG levels in the blood of euthermic and hibernating thirteen-lined ground squirrels Spermophilus tridecemlineatus

1. Whole blood was collected from hibernating and euthermic ground squirrels. 2. Blood from hibernating animals had approximately 50% less ATP and 2,3-biphosphoglycerate (2,3-BPG) than did that from euthermic animals. 3. In euthermic blood subjected to in vitro cold storage, levels of ATP decreased to those observed in hibernators, but 2,3-BPG did not decline significantly. 4. During cold storage of hibernator blood, no significant changes occurred in levels of these organophosphates.

Blood-gas properties and function in the fossorial mole rat under normal and hypoxic-hypercapnic atmospheric conditions

Blood and tissue gas exchange properties of mole rats in normoxic and hypoxic-hypercapnic conditions were compared to the common mammalian pattern. RBC count was 14.0 +/- 1.2-10(6)/microliter. Hb concentration was 15.0 +/- 0.4g/100 ml. P50 (at pH 7.4 and 37 degrees C) was 29.5 +/- 0.5 mm Hg. Oxygen capacity averaged 20.2 +/- 0.4 vol% and the Hill coefficient was 2.9 +/- 0.1. The Bohr effect was -0.53 +/- 0.02 (deltalog P/deltapH). The temperature coefficient was 0.0152 +/- 0.0014 (deltalog P/delta degrees C). The Haldane effect was 4.8 +/- 0.5 (deltaCCO2 vol%)at PCO2 =40 mm Hg. Steady-state partial pressures in gas pockets were PO2 = 15.1 +/- 1.4 mm Hg and PCO2 = 85.8 +/- 3.9 mm Hg in normoxia, and 11.5 +/- 3.0 and 101.8 +/- 3.5 repectively in hypoxia-hypercapnia (PIO2 congruent to 85 mm Hg). Under the same conditions 2,3-DPG dropped from 0.87 and 0.88 to 0.62 and 0.65 (mol/mol Hb) in the rat and in the white rat, respectively. Heart muscle myoglobin concentration of the mole rat (1.44 mg/g) did not differ significantly from that of the white rat (1.96 mg/g), whereas masseter myoglobin was 4.0 mg/g--significantly different from the rat (1.21 mg/g). Results indicate that the strategy used by the mole rat to maintain a normal metabolic rate under variable atmospheric conditions, besides having high oxygen affinity, is to expand the physiological range of the oxygen dissociation curve to very low oxygen tensions, at the expense of its acid-base regulation. The regulation of the shape of the oxygen dissociation curve is discussed.

Blood respiratory properties in the naked mole rat Heterocephalus glaber, a mammal of low body temperature

Respiratory properties of whole blood and Hb solutions have been studied in Heterocephalus glaber, a fossorial rodent, having a low body temperature (30.0-32.0 degrees C) and poor thermoregulatory ability. For comparison similar, measurements were made on laboratory mice, Mus musculus. Whole blood showed a distinctly higher O2 affinity for Heterocephalus at both 30 and 37 degrees C.P50 values were 23.3 mm Hg and 33.0 mm Hg at 37 degrees C for Heterocephalus and Mus, respectively, while at 30 degrees CP50's were 18.8 mm Hg and 24.9 mm Hg, all values at pH (b) 7.4. deltaH values (expressive of the effect of temperature on P50) were -5.8 kcal-mol-1 for Heterocephalus and -7.5 kcal-mol-1 for Mus. The CO2 Bohr effects (omega) were -0.43 and -0.50 for Heterocephalus at 37 and 30 degrees C. Corresponding values for Mus were -0.65 and -0.56. Both species had a Hill's n-value of 2.6. Red cell concentrations of 2,3-DGP were closely similar in the species being 7.3 mmol-L-1 rbc for Heterocephalus and 7.4 mmol-L-1 rbc for Mus. Stripped Heterocephalus Hb had a very high O2 affinity, at pH 7.25, 37 degrees C,P50 was 8.0 mm Hg whereas the corresponding value for Mus was 11.3 mm Hg. Addition of DPG to stripped Hb from the two species decreased O2 affinity to the same degree. The high O2 affinity of Heterocephalus blood is viewed as a possible adaptation to its burrowing habits. Its basis is inherent to the hemoglobin molecule itself and not dependent upon cofactor influence or the temperature sensitivity of the O2-Hb binding.

Respiratory function of blood in hibernating and non-hibernating hedgehogs

The oxygen affinity of the blood of hibernating hedgehogs has been investigated previously by Clausen and Ersland (1968) and Bartels et al. (1969). The results of these studies were conflicting in so far as Bartels et al. (1969) found an increase of the O2 affinity during hibernation resulting in a P50 of 23 mm Hg at pH 7.4 37 degrees C, while Clausen and Ersland reported a P50 of 33.6 mm Hg under the same conditions, a value which was only slightly different from the one found in nonhibernating animals (Bartels et al., 1969).