Effect of anoxia and adenosine on cerebral blood flow in the leopard frog (Rana pipiens)

A nonlinear multi-scale model for blood circulation in a realistic vascular system

In the last decade, numerical models have become an increasingly important tool in biological and medical science. Numerical simulations contribute to a deeper understanding of physiology and are a powerful tool for better diagnostics and treatment. In this paper, a nonlinear multi-scale model framework is developed for blood flow distribution in the full vascular system of an organ. We couple a quasi one-dimensional vascular graph model to represent blood flow in larger vessels and a porous media model to describe flow in smaller vessels and capillary bed. The vascular model is based on Poiseuille's Law, with pressure correction by elasticity and pressure drop estimation at vessels' junctions. The porous capillary bed is modelled as a two-compartment domain (artery and venous) using Darcy's Law. The fluid exchange between the artery and venous capillary bed compartments is defined as blood perfusion. The numerical experiments show that the proposed model for blood circulation: (i) is closely dependent on the structure and parameters of both the larger vessels and of the capillary bed, and (ii) provides a realistic blood circulation in the organ. The advantage of the proposed model is that it is complex enough to reliably capture the main underlying physiological function, yet highly flexible as it offers the possibility of incorporating various local effects. Furthermore, the numerical implementation of the model is straightforward and allows for simulations on a regular desktop computer.

Mass Transport: Circulatory System with Emphasis on Nonendothermic Species

Mass transport can be generally defined as movement of material matter. The circulatory system then is a biological example given its role in the movement in transporting gases, nutrients, wastes, and chemical signals. Comparative physiology has a long history of providing new insights and advancing our understanding of circulatory mass transport across a wide array of circulatory systems. Here we focus on circulatory function of nonmodel species. Invertebrates possess diverse convection systems; that at the most complex generate pressures and perform at a level comparable to vertebrates. Many invertebrates actively modulate cardiovascular function using neuronal, neurohormonal, and skeletal muscle activity. In vertebrates, our understanding of cardiac morphology, cardiomyocyte function, and contractile protein regulation by Ca2+ highlights a high degree of conservation, but differences between species exist and are coupled to variable environments and body temperatures. Key regulators of vertebrate cardiac function and systemic blood pressure include the autonomic nervous system, hormones, and ventricular filling. Further chemical factors regulating cardiovascular function include adenosine, natriuretic peptides, arginine vasotocin, endothelin 1, bradykinin, histamine, nitric oxide, and hydrogen sulfide, to name but a few. Diverse vascular morphologies and the regulation of blood flow in the coronary and cerebral circulations are also apparent in nonmammalian species. Dynamic adjustments of cardiovascular function are associated with exercise on land, flying at high altitude, prolonged dives by marine mammals, and unique morphology, such as the giraffe. Future studies should address limits of gas exchange and convective transport, the evolution of high arterial pressure across diverse taxa, and the importance of the cardiovascular system adaptations to extreme environments. © 2017 American Physiological Society. Compr Physiol 7:17-66, 2017.

Adenosinergic modulation of neuronal activity in the pond snail Lymnaea stagnalis

Adenosine has been termed a retaliatory metabolite and its neuroprotective effects have been implicated in the hypoxia tolerance of several species; however, its role in the invertebrate CNS remains unclear. To determine if adenosine modulates neuronal activity in invertebrate neurons, we conducted whole-cell recordings from neurons in the central ring ganglia of the anoxia-tolerant pond snail Lymnaea stagnalis during exposure to adenosine and pharmacological compounds known to modulate the type I subclass of adenosine receptors (A1R). Action potential (AP) frequency and membrane potential (Vm) were unchanged under control conditions, and addition of adenosine decreased AP frequency by 47% (from 1.08±0.22 to 0.57±0.14 Hz) and caused significant hyperpolarization of Vm. The A1R agonist cyclopentyladenosine (CPA) mimicked the results obtained with adenosine whereas antagonism of the A1R with 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) had no effect on AP frequency or Vm but prevented the adenosine and CPA-mediated decreases in neuronal activity. Furthermore, Ca2+ measurements with fluo-4 revealed that A1R activation led to a 12% increase in intracellular Ca2+ concentration and this elevation was also antagonized by DPCPX. Our results suggest that adenosine acting via the adenosine receptor (type I subclass) depresses neuronal activity in the adult L. stagnalis CNS and this depression is correlated with an increase in cytosolic Ca2+ levels.

Hypoxia tolerance in reptiles, amphibians, and fishes: life with variable oxygen availability

The ability of fishes, amphibians, and reptiles to survive extremes of oxygen availability derives from a core triad of adaptations: profound metabolic suppression, tolerance of ionic and pH disturbances, and mechanisms for avoiding free-radical injury during reoxygenation. For long-term anoxic survival, enhanced storage of glycogen in critical tissues is also necessary. The diversity of body morphologies and habitats and the utilization of dormancy have resulted in a broad array of adaptations to hypoxia in lower vertebrates. For example, the most anoxia-tolerant vertebrates, painted turtles and crucian carp, meet the challenge of variable oxygen in fundamentally different ways: Turtles undergo near-suspended animation, whereas carp remain active and responsive in the absence of oxygen. Although the mechanisms of survival in both of these cases include large stores of glycogen and drastically decreased metabolism, other mechanisms, such as regulation of ion channels in excitable membranes, are apparently divergent. Common themes in the regulatory adjustments to hypoxia involve control of metabolism and ion channel conductance by protein phosphorylation. Tolerance of decreased energy charge and accumulating anaerobic end products as well as enhanced antioxidant defenses and regenerative capacities are also key to hypoxia survival in lower vertebrates.

Adenosine as a signal for ion channel arrest in anoxia-tolerant organisms

Certain freshwater turtles and fish are extremely anoxia-tolerant, capable of surviving hours of anoxia at high temperatures and weeks to months at low temperatures. There is great interest in understanding the cellular mechanisms underlying anoxia-tolerance in these groups because they are anoxia-tolerant vertebrates and because of the far-reaching medical benefits that would be gained. It has become clear that a pre-condition of prolonged anoxic survival must involve the matching of ATP production with ATP utilization to maintain stable ATP levels during anoxia. In most vertebrates, anoxia leads to a severe decrease in ATP production without a concomitant reduction in utilization, which inevitably leads to the catastrophic events associated with cell death or necrosis. Anoxia-tolerant organisms do not increase ATP production when faced with anoxia, but rather decrease utilization to a level that can be met by anaerobic glycolysis alone. Protein synthesis and ion movement across the plasma membrane are the two main targets of regulatory processes that reduce ATP utilization and promote anoxic survival. However, the oxygen sensing and biochemical signaling mechanisms that achieve a coordinated reduction in ATP production and utilization remain unclear. One candidate-signaling compound whose extracellular concentration increases in concert with decreasing oxygen availability is adenosine. Adenosine is known to have profound effects on various aspects of tissue metabolism, including protein synthesis, ion pumping and permeability of ion channels. In this review, I will investigate the role of adenosine in the naturally anoxia-tolerant freshwater turtle and goldfish and give an overview of pathways by which adenosine concentrations are regulated.

Vertebrate brains at the pilot light

While the brains of most vertebrates are unable to tolerate more than a few minutes of anoxia, some freshwater turtles (Trachemys and Chrysemys), crucian carp (Carassius carassius) and frogs (Rana pipens and Rana temporaria) can survive anoxia for hours to months. Obviously, anoxia tolerance has evolved separately several times and this is also reflected in the divergent strategies these animals utilize to survive without oxygen. The turtles and crucian carp defend their brain ATP levels and avoid a loss of ion homeostasis by reducing ATP use. In the turtles, the early release of adenosine and the activation of K(ATP) channels, a progressive release of GABA and a drastic reduction in electric activity and ion fluxes send the brain into a comatose like state. The crucian carp displays a more modest depression of ATP use, probably achieved through a moderated release of GABA and adenosine, allowing the animal to maintain physical activity in anoxia. The anoxic frog, on the other hand, seems to rely on mechanisms that greatly retard the anoxia induced fall in ATP levels and loss of ion homeostasis, so that the brain can be saved as long as the anoxia is limited to a few hours. The sequence of events characterizing the anoxic frog brain is similar to that of failing anoxic mammalian brain, although over a greatly extended time frame, allowing the frog to die slowly in anoxia, rather than survive. By contrast the only factor that limits anoxic survival in turtles and crucian carp may be the final depletion of their glycogen reserves.

Hypoxic survival strategies in two fishes: extreme anoxia tolerance in the North European crucian carp and natural hypoxic preconditioning in a coral-reef shark

Especially in aquatic habitats, hypoxia can be an important evolutionary driving force resulting in both convergent and divergent physiological strategies for hypoxic survival. Examining adaptations to anoxic/hypoxic survival in hypoxia-tolerant animals may offer fresh ideas for the treatment of hypoxia-related diseases. Here, we summarise our present knowledge of two fishes that have evolved to survive hypoxia under very different circumstances.

The crucian carp (Carassius carassius) is of particular interest because of its extreme anoxia tolerance. During the long North European winter, it survives for months in completely oxygen-deprived freshwater habitats. The crucian carp also tolerates a few days of anoxia at room temperature and, unlike anoxia-tolerant freshwater turtles, it is still physically active in anoxia. Moreover, the crucian carp does not appear to reduce neuronal ion permeability during anoxia and may primarily rely on more subtle neuromodulatory mechanisms for anoxic metabolic depression.

The epaulette shark (Hemiscyllium ocellatum) is a tropical marine vertebrate. It lives on shallow reef platforms that repeatedly become cut off from the ocean during periods of low tides. During nocturnal low tides, the water [O2] can fall by 80% due to respiration of the coral and associated organisms. Since the tides become lower and lower over a period of a few days, the hypoxic exposure during subsequent low tides will become progressively longer and more severe. Thus, this shark is under a natural hypoxic preconditioning regimen. Interestingly, hypoxic preconditioning lowers its metabolic rate and its critical PO2. Moreover, repeated anoxia appears to stimulate metabolic depression in an adenosine-dependent way.