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First published online March 31, 2007
Journal of Experimental Biology 210, 1335-1349 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.02736

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Osmoregulation, ionoregulation and acid–base regulation by the gastrointestinal tract after feeding in the elasmobranch (Squalus acanthias)

Chris M. Wood^1,2,3,*, Makiko Kajimura^1,3, Carol Bucking^1,3 and Patrick J. Walsh^2,3,4

¹ Department of Biology, McMaster University, 1280 Main St. West, Hamilton, Ontario, L8S 4K1, Canada
² Rosenstiel School of Marine and Atmospheric Sciences, University of Miami, Miami, FL 33149, USA
³ Bamfield Marine Sciences Centre, 100 Pachena Drive, Bamfield, British Columbia, Canada
⁴ Department of Biology, University of Ottawa, 30 Marie Curie, Ottawa, Ontario, K1N 6N5, Canada

^* Author for correspondence (e-mail: woodcm{at}mcmaster.ca)

Accepted 29 January 2007

In order to study the physiological consequences of voluntary feeding in the gastrointestinal tract of a ureotelic marine elasmobranch, dogfish (fasted for 96 h) were sampled at various times up to 360 h after consuming a 5–6% ration of teleost fish (hake) under natural feeding conditions. Digestion and absorption were completed between 120 and 360 h post-feeding. The tissue masses of different segments of the gastrointestinal tract increased and decreased markedly as the chyme moved through, mainly because of fluid engorgement rather than hyperplasia. In fasted dogfish, the cardiac and pyloric stomachs contained only small volumes of highly acidic fluid (pH 1.77±1.12, 2.05±0.08) similar in composition to seawater. Feeding resulted in gastric pHs of 3.20±0.31 and 3.95±0.40 at 6 h, followed by slow declines through 60 h. An alkaline tide in the blood also occurred at 6 h. In the face of large changing masses of highly acidic chyme in the stomachs, the pH (6.50±0.10), ionic composition and volume of chyme in the intestine (spiral valve) were precisely regulated from 6 to 60 h post-feeding at very different values from those in the stomachs, and intestinal HCO ^–₃ remained low (5.12±0.83 mmol l^–1). The colon was usually empty and its pH constant at 7.20±0.16 at all times. Despite the ingestion of strongly hypo-osmotic teleost tissue, the osmolality of the chyme remained in equilibrium with that of the blood plasma in all segments at all times after feeding. Much of the osmotic equilibration was because of the secretion of urea into the chyme, particularly in the intestine. After feeding, gastric fluid concentrations of Na⁺ and Mg²⁺ declined, K⁺ and Ca²⁺ increased, whereas Cl^– exhibited little change, indicating that additional drinking of seawater was minimal. Na⁺, K⁺, water and especially Cl^– were absorbed in the intestine, whereas Mg²⁺ and Ca²⁺ were largely excluded. Our results illustrate the complex integration of digestive and ionoregulatory function in the elasmobranch digestive tract, and marked differences from the teleost pattern.

Key words: gastric acid secretion, chyme composition, alkaline tide, urea, osmolality, dogfish, shark

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