Submitted on September 12, 2008
Revised on March 12, 2009
Accepted on March 29, 2009
Complete intracellular pH protection during extracellular pH depression is associated with hypercarbia tolerance in white sturgeon, Acipenser transmontanus
Daniel William Baker1*, Victoria Matey2, Katie T. Huynh1, Jonathan M. Wilson3, John D. Morgan4, and Colin J. Brauner1
1 University of British Columbia
2 San Diego State University, 5500 Campanile Drive, San Diego, CA, 92182, USA
3 CIMAR, Portugal
4 Vancouver Island University
* To whom correspondence should be addressed. E-mail: baker{at}zoology.ubc.ca.
Sturgeons are among the most CO2 tolerant of fishes investigated to date. However, the basis of this exceptional CO2 tolerance is unknown. Here, white sturgeon, Acipenser transmontanus, were exposed to elevated CO2 to investigate the mechanisms associated with short term hypercarbia tolerance. During exposure to 1.5 kPa PCO2, transient blood pH (pHe) depression was compensated within 24 h and associated with net plasma HCO3- accumulation and equimolar Cl- loss, and changes in gill morphology, such as a decrease in apical surface area of mitochondrial rich cells (MRC). These findings indicate that pHe recovery at this level of hypercarbia is accomplished in a manner similar to most fresh water teleost species studied to date, although branchial mechanisms involved may differ. White sturgeon exposed to more severe hypercarbia (3 and 6 kPa PCO2) for 48 h exhibited incomplete pH compensation in blood and red blood cells. Despite pHe depression, intracellular pH (pHi) of white muscle, heart, brain, and liver did not decrease during a transient (6 h of 1.5 kPa) or prolonged (48 h at 3 and 6 kPa) blood acidosis. This pHi protection was not due to high intrinsic buffering in tissues. Such tight active cellular regulation of pHi in the absence of pHe compensation represents a unique pattern for non-air breathing fishes, and we hypothesize that it is the basis for the exceptional CO2 tolerance of white sturgeon and likely other CO2 tolerant fishes. Further research to elucidate the specific mechanisms responsible for this tremendous pH regulatory capacity in tissues of white sturgeon is warranted.
