Lecture 8: Hypoxia & Metabolism Case Studies

Lecture 8: Hypoxia & Metabolism Case Studies

• Lecture date: January 22nd
• Readings: 620-621, 215-219, 202-203, 737-742

Differential Hypoxia Tolerance

• Anoxic survival time varies greatly between species and body temperature.
• Anoxia-tolerant vertebrates have significantly longer survival times than "normal" vertebrates at a variety of temperatures.
• The Trachemys turtle, Chrysemys turtle, crucian carp, blind goby, mouse, anole lizard, rainbow trout, and bullhead catfish are examples of species with varying hypoxia tolerances.
• Survival time increases with lower body temperature.

Maintaining Metabolic Balance in Hypoxia

• Glycolysis is an anaerobic process that produces small amounts of ATP from glucose.
• Fermentation occurs in the absence of oxygen, creating differing byproducts - lactic acid in animals and ethanol in yeast.
• In the presence of oxygen, the Krebs cycle and oxidative phosphorylation produce significantly more ATP.

Pigment Quantity: Adaptational

• Animals have varying amounts of oxygen stores (mL O2/kg) depending on their environment and needs.
• Measures of pigment quantities in various species (human, Weddell seal, ribbon seal, harbor seal, northern fur seal, and Steller sea lion) illustrate adaptations for low-oxygen conditions.

Pigment Quantity: Acute

• Seals' spleens act as "SCUBA tanks" to store oxygen reserves.
• Changes in spleen volume are observed in response to diving (increasing volume during dives).

Effects of Forced Diving

• Forced diving impacts the spleen and liver in northern elephant seals.

Metabolism & O2 Demand

• Aerobic dive limit (ADL) depends on the duration of oxygen stores being used to sustain the metabolic rate.
• Oxygen stores (mL O2/kg) differ between species.

Diving in Nature

• Dive duration varies depending on the animal, with high end-of-dive % Hb saturation.

ADL & Metabolic Rate

• Diving seals have strategies to minimize oxygen demand.
• Strategies include intermittent locomotion, postponing digestion, and selective vasoconstriction.

Hypoxia & Cells

• Cellular oxygen sensing involves proteins like prolyl hydroxylase.
• Hypoxia-inducible factor-1 (HIF-1) regulates gene transcription in response to low oxygen levels.

HIF-1: Hypoxia

• HIF-1 (hypoxia inducible factor) regulation
• HIF-1a levels are influenced by oxygen levels.

Hypoxia/Ischemia

• Hypoxia and ischemia trigger a complex response involving many genes and pathways.

Pigment Quantity: Acclimation

• Seasonal changes in muskrat hemoglobin levels influence acclimation to low oxygen conditions.

Model System

• Chrysemys picta belli, a turtle, is a suitable model system to study hypoxia tolerance.

Cellular Metabolism & Hypoxia

• Na+/K+ ATPase and protein synthesis are metabolic processes requiring oxygen (with the ATPase consuming a significant portion).

Suppressing Na+/K+ ATPase Activity

• Reduced Na+/K+ ATPase activity can be observed under hypoxic conditions

Hypoxia: Metabolic Strategies

• Anaerobic pathways produce ATP in the absence of oxygen, producing lactate.

Hypoxia: Anaerobic Strategies

• Some species utilize liver glycogen stores to support anaerobic metabolism.

Anaerobic Glycolysis

• Anaerobic glycolysis produces ATP in the absence of oxygen.

Increase Anaerobic Efficiency

• Oxidation pathways can increase ATP yield during anaerobic metabolism.

Avoiding Acidification

• Organisms avoid acidification by producing and excreting byproducts like ethanol and carbon dioxide.

Avoiding Acidification

• Increased tissue buffer capacity and mobilization of other buffers (e.g., carapace) help maintain pH.