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

Abstract

consumption rate to ϳ25% compared with the active state, while body temperature decreases moderately (to ϳ30°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 O2 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 O2 affinity than in other vertebrates. Furthermore, hemolysates from hibernating bears consistently showed lower cooperativity and higher O2 affinity than their summer counterparts, regardless of the temperature. We found that this increase in O2 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-O2 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. metabolic suppression; body temperature; oxygen binding curves; heat of oxygenation; hibernation A DISTINCT TRAIT OF MAMMALIAN hibernation is the highly controlled decrease in body temperature and metabolic rate. A hibernating brown bear (Ursus arctos) routinely spends 5-7 mo per year in continuous dormancy with no food or water intake, no urination, and no defecation (19,36). During this time, bears appear to be resistant to loss of muscle mass, strength, or bone density (18,24,29,43,14,44). During hibernation bear body temperature is downregulated only slightly, fluctuating from ϳ37°C to a minimum of 30°C, as found in brown and in black bears (Ursus americanus) (17,26,27,36,43), whereas O 2 consumption rate is downregulated by 75% (43). In comparison, in most smaller hibernators, such as ground squirrels and marmots, body temperature drops dramatically to values close to ambient temperatures (32, 37, 48), with a consequent strong Q 10 -induced depression of metabolic rate (where Q 10 is the rate coefficient for a 10°C change in temperature). In spite of substantial downregulation of ventilation and heart rate, most hibernators likely experience only slight or no hypoxia and in some ground squirrels arterial O 2 tension (PO 2 ) is normal during torpor (16). As opposed to smaller hibernators that exhibit periods of spontaneous arousals back to normothermic temperature and metabolic rate (33...

Hemoglobin isoform differentiation and allosteric regulation of oxygen binding in the turtle,Trachemys scripta

“…Therefore, metabolic downregulation of red blood cell organic phosphate allosteric effectors found by us and others in turtles (28) and other hibernating (36,50) or estivating (26) vertebrates appears to be a consequence of an as yet poorly understood general reorganization of metabolism, and it is not necessarily a cause of increased Hb-O 2 affinity. Perhaps most interestingly, results of functional experiments yielded two findings with important implications for structure-function relationships of vertebrate Hbs, particularly of turtles and birds that express distinct HbA and HbD isoforms.…”

“…This effect would prevent a harmful mismatch between O 2 delivery and O 2 consumption. Interestingly, hibernation in bears that is a nonhypoxic metabolic depression is also associated with a left-shifted O 2 binding curve of the Hb, although the mechanism involves a combined decrease in temperature, as well as in the level of red blood cell organic phosphates (36). Furthermore, the observation that the heat of oxygenation remains constant in either hemolysate (this study) or whole blood (28), despite the large change in the red blood cell allosteric cofactor ATP, further supports the conclusion that in turtles Hb is fully saturated with ATP under normoxia as well as during anoxia.…”

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

“…In the brown bear (Ursus arctos), by contrast, changes in temperature and DPG jointly modulate Hb oxygenation during hibernation to prevent inappropriate O 2 delivery to tissues (Revsbech et al, 2013).…”

“…Likewise, in hibernating mammals, a left-shifted blood-O 2 equilibrium curve can help safeguard arterial O 2 saturation during apneic periods and prevent unfavorable O 2 release to tissues (Maginniss and Milsom, 1994;Milsom and Jackson, 2011;Revsbech et al, 2013). An increase in blood-O 2 affinity can be achieved by changes in the intrinsic O 2 affinity of hemoglobin (Hb), changes in the sensitivity of Hb to allosteric effectors and/or changes in the concentrations of allosteric effectors within the erythrocyte.…”

“…Other factors that modulate Hb-O 2 affinity are Cl -ions (Chiancone et al, 1972) and temperature (Antonini and Brunori, 1971), as well as protons and CO 2 , which also bind preferentially to deoxy-Hb and promote O 2 unloading in the tissue capillaries via the Bohr effect (Perutz, 1983;Weber and Fago, 2004). Increases in blood-O 2 affinity during winter hibernation have been documented in a number of mammals, and such changes are generally attributable to reductions in red cell DPG, increases in pH and/or decreased body temperature (Burlington and Whitten, 1971;Maginniss and Milsom, 1994;Musacchia and Volkert, 1971;Revsbech et al, 2013;Tempel and Musacchia, 1975 hibernation, experiments on purified Hbs are required to evaluate the independent and joint effects of DPG, pH and temperature on Hb-O 2 affinity under controlled conditions.…”