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First published online July 6, 2005
Journal of Experimental Biology 208, 2693-2705 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01678
Alkaline tide and nitrogen conservation after feeding in an elasmobranch (Squalus acanthias)
1 Department of Biology, McMaster University, 1280 Main St West, Hamilton,
Ontario, Canada L8S 4K1
2 Rosenstiel School of Marine and Atmospheric Sciences, University of Miami,
Miami, Florida 33149, USA
3 Bamfield Marine Sciences Centre, 100 Pachena Drive, Bamfield, British
Columbia, Canada V0R 1B0
4 Department of Biology, University of Victoria, Victoria, British Columbia,
Canada V8W 2N5
* Author for correspondence (e-mail: woodcm{at}mcmaster.ca)
Accepted 10 May 2005
We investigated the consequences of feeding for acid–base balance,
nitrogen excretion, blood metabolites and osmoregulation in the Pacific spiny
dogfish. Sharks that had been starved for 7 days were surgically fitted with
indwelling stomach tubes for gastric feeding and blood catheters for
repetitive blood sampling and were confined in chambers, allowing measurement
of ammonia-N and urea-N fluxes. The experimental meal infused via the
stomach tube consisted of flatfish muscle (2% of body mass) suspended in
saline (4% of body mass total volume). Control animals received only saline
(4% of body mass). Feeding resulted in a marked rise in both arterial and
venous pH and HCO3– concentrations at 3–9 h
after the meal, with attenuation by 17 h. Venous
O2 also fell. As there were
negligible changes in CO2,
the response was interpreted as an alkaline tide without respiratory
compensation, associated with elevated gastric acid secretion. Urea-N
excretion, which comprised >90% of the total, was unaffected, while
ammonia-N excretion was very slightly elevated, amounting to <3% of the
total-N in the meal over 45 h. Plasma ammonia-N rose slightly. Plasma urea-N,
TMAO-N and glucose concentrations remained unchanged, while free amino acid
and ß-hydroxybutyrate levels exhibited modest declines. Plasma osmolality
was persistently elevated after the meal relative to controls, partially
explained by a significant rise in plasma Cl–. This marked
post-prandial conservation of nitrogen is interpreted as reflecting the needs
for urea synthesis for osmoregulation and protein growth in animals that are
severely N-limited due to their sporadic and opportunistic feeding lifestyle
in nature.
Key words: gastric acid secretion, metabolic alkalosis, ammonia, urea, osmolality, shark
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