Functional hypoxia: induced by high activity.

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Hypoxia
Functional hypoxia induced by high
activity. Environmental hypoxia surrounding PO2
decreases. ,
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Metabolic depression is a key adaptation in
intertidal marine invertebrates and it allows
long-term hypoxia/anoxia tolerance Oxygen
regulators undergo a Pasteur effect (top), but
facultative anaerobes demonstrate a reverse
Pasteur effect (bottom).
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Glucose utilization under AEROBIC and ANAEROBIC
conditions.
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Marine invertebrates have multiple anaerobic
pathways
There are also H consuming metabolic
processes Adenylate Deaminase reaction AMP
H2O IMP NH3 (forms NH4)
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Cellular Energetics Under Hypoxia/Anoxia 4
reaction systems are linked (3 supply, 1
demand) Aerobic metabolism Glucose ADP Pi
? ATP Creatine kinase rxn PCr ADP H ? ATP
Cr Glycolysis Glucose ADP Pi ? ATP
Lactate H ATPase ATP ? ADP Pi Net effect
of energy challenge ?PCr, ?Cr, ?Glucose,
?Lactate, ?pH, ?ADP, ?Pi
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Nuclear magnetic resonance can be used to assess
cellular energetics non-invasively
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Intracellular energetics in fish during hypoxia
31P-NMR analysis of muscle energetics in vivo in
goldfish (A) (ethanol producers) and tilapia (B)
(lactate producers). Note the decrease in PCr and
increase in Pi during hypoxia. Also note the Pi
peak has shifted to the left during hypoxia
(decreased pH). The ATP levels remain fairly
constant but ?G of ATP hydrolysis declines.
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Intracellular buffering of H Previously shown
that buffering capacity was correlated with
energy demand in fish (functional hypoxia)
(Castellini and Somero, 1981). Is habitat
related to the ability to regulate pH?
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Cellular pH regulation can be costly
Portner et al. 2000. J. Exp. Biol.
Extracellular pH decreases reduce the cost of
membrane transport processes associated with pH
regulation. Inhibitors Ouabain (Na/K ATPase),
DMA (amiloride, Na/H antiport), DIDS (anion
exchange, such as Na dependent Cl-/HCO3-
exchange), Bafilomycin (V-ATPase H pump
inhibitor).
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Gas exchange surfaces and hemoglobin binding
affinity are dependent on environment and behavior
Stewart, 1991
Innes and Taylor, 1986
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Other environmental factors can alter hemoglobin
P50
Wood, 1991
Brix et al. 1989
Euphausiid P50 is higher at lower T.
Goldfish move to lower T when PO2 drops, which
lowers metabolic rate and may help unload O2 at
tissues.
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Fish respond by invoking anaerobic metabolism and
increasing the ventilation rate Respiratory and
metabolic response to hypoxia in the epaulette
shark, which lives on reef platforms that become
hypoxic at low tide. The pattern is typical of
most vertebrates, lactate increases and VO2
decrease once a critical PO2 is reached.
Ventilation rate is increased to compensate for
the reduced oxygen (Routley et al. 2002).
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How is oxygen sensed by cells? Hypoxia-induced
gene expression via HIF. HIF1 is constitutively
expressed. A heme-based receptor detects O2
levels and leads to an oxygen-dependent
modification of HIF1. This modification allows
HIF1 to be ubiquinated, which targets it for
destruction. When HIF1 is not modified, it can
dimerize with ARNT to form a heterodimer
transcription factor that can bind enhancer sites
on DNA.
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Microarray analyses of Response to Hypoxia
Control Treatment
mRNA from tissue/cell cDNA (fluorescently
label) Hybridize to an array Determine
genes that are up-regulated (green),
down-regulated (red), or unchanged (yellow).
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Hypoxia-induced gene expression in the
hypoxia-tolerant goby Gillichthys mirabilis
examined using cDNA microarrays. PO2 was
approximately 10 of normoxia, which is below PO2
that induces a decrease in respiration. Gene
expression was then measured in a control, and at
times after hypoxia exposure (Gracey, Troll and
Somero, 2001).
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A Special Case The Diving Response in Marine
Mammals
From Hochachka and Somero (2002)
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Field studies of the Weddell seal showing
increase in hematocrit during diving. Also, in
the field, the lactate washout peak was often
small or absent during short dives, and large
during long dives. This led to the concept of
the aerobic dive limit (ADL).
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Myoglobin Function
1H-spectra 31P-spectra
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Top Mb (bars) and maximum dive duration
(circles) for species of cetaceans and pinnipeds.
Bottom Body mass relationship to maximum
dive duration. (Noren and Williams, 2000).
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How do marine mammals exceed the aerobic dive
limit without becoming anaerobic? Top Dive depth
vs. duration in marine mammals showing gliding
time (red) and swimming time (black). Bottom
Recovery oxygen costs (repaying the oxygen debt)
in the Weddell seal for a gliding dive is less
than for a swimming dive. A gliding dive extends
the aerobic dive time by 38. Williams et al.
(2000)
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Are marine mammals sensitive to pressure?
Glycolytic flux in RBCs from marine and
terrestrial mammals during a 2 h incubation under
high hydrostatic pressure (Castellini et al.
2001).