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UBC Theses and Dissertations

Physiological responses associated with aquatic hypercarbia in the CO₂-tolerant white sturgeon, Acipenser transmontanus Baker, Daniel William


Through investigations conducted at the organismal, tissue and cellular levels, this thesis provides clear evidence that the white sturgeon, Acipenser transmontanus, is among the most CO₂ tolerant of all fishes investigated to date. During moderate increases in water CO₂ tension (PCO₂) (≤ 15 mm Hg PCO₂, hypercarbia), white sturgeon exhibited changes in gill morphology and restored blood pH (pHe) through net HCO₃⁻/Cl⁻, a process observed in most fishes (Chapter 3). At CO₂ tensions lethal to other fishes (≥ 22.5 mm Hg PCO₂), white sturgeon completely protected intracellular pH (pHi) of the heart, liver, brain and white muscle (termed preferential pHi regulation), despite a large reduction in pHe (up to 1 pH unit) (Chapter 3, 4). Tissue pHi regulation was activated in heart within minutes of the onset of hypercarbia (measured via NMR, Chapter 5), and completely protected pHi in this tissue even during exposure to potentially lethal CO₂ levels (i.e., 90 mm Hg PCO₂). In hearts examined in situ, maximum cardiac performance was well defended and associated with partial pHi compensation in ventricles (which exhibited only ~40% of predicted acidosis). Preferential pHi regulation was not associated with large increases in metabolic costs, as during exposure to severe hypercarbia (~45 mm Hg PCO₂), heart [ATP] and [CrP] had recovered to pre-exposure levels within 90 min, and whole animal was decreased (30%) when pHi was completely protected. Preferential pHi regulation of this magnitude and rapidity has not been documented before in any vertebrate in response to hypercarbia and represents a novel pattern of acid-base regulation among fishes. White sturgeon represent the first exclusively water-breathing fish to exhibit preferential pHi regulation during hypercarbia. Furthermore, white sturgeon are the most basal vertebrate to demonstrate complete pHi protection during severe pHe depression. As sturgeon may retain ancestral characteristics, I propose that preferential pHi regulation is the basis for enhanced CO₂ tolerance in other tolerant Osteichthyan fishes, and first arose in association with ionoregulatory and respiratory challenges experienced during freshwater invasion in the vertebrate lineage.

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