A Role of Erythrocytes in Adenosine Monophosphate Initiation of Hypometabolism in Mammals

Abstract

Biochemical and mechanistic aspects into how various hypometabolic states are initiated in mammals are poorly understood. Here, we show how a state of hypometabolism is initiated by 5-AMP uptake by erythrocytes. Wild type, ecto-5-nucleotidase-deficient, and adenosine receptor-deficient mice undergo 5-AMP-induced hypometabolism in a similar fashion. Injection of 5-AMP leads to two distinct declining phases of oxygen consumption (VO 2 ). The phase I response displays a rapid and steep decline in VO 2 that is independent of body temperature (T b ) and ambient temperature (T a ). It is followed by a phase II decline that is linked to T b and moderated by T a . Altering the dosages of 5-AMP from 0.25-to 2-fold does not change the phase I response. For mice, a T a of 15°C is effective for induction of DH with the appropriate dose of 5-AMP. Erythrocyte uptake of 5-AMP leads to utilization of ATP to synthesize ADP. This is accompanied by increased glucose but decreased lactate levels, suggesting that glycolysis has slowed. Reduction in glycolysis is known to stimulate erythrocytes to increase intracellular levels of 2,3-bisphosphoglycerate, a potent allosteric inhibitor of hemoglobin's affinity for oxygen. Our studies showed that both 2,3-bisphosphoglycerate and deoxyhemoglobin levels rose following 5-AMP administration and is in parallel with the phase I decline in VO 2 . In summary, our investigations reveal that 5-AMP mediated hypometabolism is probably triggered by reduced oxygen transport by erythrocytes initiated by uptake of 5-AMP. Mammals in deep hypometabolism (DH)5 with severe hypothermia and stupor are only observed naturally during hibernation (1). During hibernation, an animal's overall metabolic rate, based on oxygen consumption, is a small fraction of its euthermic needs. The biochemical event that triggers this hypometabolism is an enigma. It is thought that non-hibernating mammals cannot undertake a similar hypometabolic process because deep hypothermia is often associated with ventricular fibrillation and cardiac arrest.However, organs from non-hibernators can withstand a considerable ischemic period in a hypometabolic state. For example, during human organ transplantation, donated organs are transported in a cold ischemic state in the complete absence of blood circulation for many h (2), yet when implanted into a recipient, the restoration of blood flow and rewarming revives full organ function. This phenomenon also illustrates the important role of oxygen transportation by erythrocytes in metabolic activity. In addition, observations that exsanguination and prolonged hypoxia can result in hypothermia further demonstrate the importance of oxygen transportation and availability as factors that can modulate the metabolic rate in mammals (3, 4).We have demonstrated that the metabolite 5Ј-AMP can induce mice to undergo transient hypometabolism with T b as low as 26°C, similar to a torpor-like state (5). The T b of animals in torpor is defined to be at or moderately below 31°C (6). Here, we demonstrate tha...

The physiology of artificial hibernation

“…As indicated previously, the depth of the hypometabolic response is dependent on the Tb-Ta convective efficiency (section 2.2). This is further supported by the extent of the drop in Tb that is observed in 5'-AMP-treated mice, which is proportional to the difference between Tb and Ta (i.e., lower Tas induce a greater drop in Tb), demonstrating the Tb-Ta dependency [76].…”

“…The putative contention is that intraperitoneal administration of high 5'-AMP concentrations (e.g., 500 mg/kg) lead to extensive 5'-AMP uptake by erythrocytes [79], after which the high intracellular levels of 5'-AMP drive the adenylate equilibrium (ATP + AMP  2 ADP) towards production of ADP, thereby depleting erythrocyte ATP levels [76,77]. As a result, erythrocyte 2,3-disphosphoglycerate is upregulated, limiting the binding of oxygen to hemoglobin's oxygen binding sites (referred to as oxygen affinity hypoxia) [76]. In addition to the already impaired oxygen transport, the severe cardiovascular depression following 5'-AMP administration has the potential to further exacerbate this hypoxic state (referred to as circulatory hypoxia) [78].…”

“…Intraperitoneal injections of 5'-AMP have been shown to induce an artificial hypometabolic state in mice and rats as evidenced by a profound drop in Tb (Figure 8) [61,[76][77][78]. The putative contention is that intraperitoneal administration of high 5'-AMP concentrations (e.g., 500 mg/kg) lead to extensive 5'-AMP uptake by erythrocytes [79], after which the high intracellular levels of 5'-AMP drive the adenylate equilibrium (ATP + AMP  2 ADP) towards production of ADP, thereby depleting erythrocyte ATP levels [76,77].…”

Peripheral Adenosine A3 Receptor Activation Causes Regulated Hypothermia in Mice That Is Dependent on Central Histamine H1 Receptors

“…For example, hypothermia produced by N 6 -R-phenylisopropyladenosine was only partially attenuated in the Adora1 2/2 mouse (Yang et al, 2007). Also, hypothermia from high dose AMP was attributed to erythrocyte uptake of intact AMP, and this hypothermia was intact in mice lacking any one of the four adenosine receptors (Daniels et al, 2010). Most informative is the observation that N 6 -Rphenylisopropyladenosine-induced hypothermia was attenuated in adenosine A 3 receptor (A 3 AR) null (Adora3 2/2 ) mice, directly implicating the A 3 AR's involvement in hypothermia (Yang et al, 2010).…”

Adenosine 5′-monophosphate-induced hypothermia inhibits the activation of ERK1/2, JNK, p38 and NF-κB in endotoxemic rats

“…When administered at a sufficient dose, 5′-AMP allows nonhibernating mammals to rapidly undergo reversible and safe severely hypothermic reactions at ambient temperatures (ATs) of 15-16°C [9][10][11][12][13]. Recently, studies have shown that hypothermia induced by 5′-AMP has an anti-inflammatory effect [10,11,14].…”