The parr-smolt transformation is a life-history strategy that has evolved in salmon to prepare migratory juveniles for oceanic survival. A necessary adjustment for homeostasis is an increase in the uptake of fluid by the intestine, which will continuously replace the loss of body water in a salty environment. Vertebrate corticosteroids regulate hydromineral balance. Cortisol mediates adaptive changes in fish intestine, but much remains to be learned about the sites of action and participation of other endocrine signals.
To begin, I asked whether the numerous extensions of the salmonid intestine, pyloric caeca, are osmoregulatory sites? Fluid uptake rate and Na⁺, K⁺-ATPase activity (ion pump that drives solute and water transport) were measured on isolated caeca of chinook salmon (Oncorhynchus tshawytscha) after 9-10 days or 6 months residence in seawater, following transfer from fresh water. Na⁺, K⁺- ATPase and fluid uptake were concurrently elevated after seawater adaptation. Short-term cortisol implants in the peritoneal cavity of freshwater salmon increased circulating cortisol to high physiological concentrations and caused similar elevations in Na⁺, K⁺-ATPase activity and fluid uptake.
Tissue culture of intestine from freshwater-adapted sockeye salmon (Oncorhynchus nerkca) was used to assess potential direct effects of cortisol. Cortisol exposure maintained Na⁺, K⁺-ATPase activity above that of control explants and, in some cases, similar to levels before culture. A response was specific for the corticoid cortisol, elicited within 2 days, and dose-dependent over a physiological range.
Next, seasonal changes in endogenous Na⁺, K⁺-ATPase activity and tissue responsiveness to cortisol were determined for chinook salmon maintained in fresh water. There were pronounced increases in endogenous enzyme activity in summer for both intestinal regions, in underyearlings and yearlings. In pyloric caeca, a significant positive response of Na⁺, K⁺-ATPase activity to cortisol, in vitro, was restricted to the months preceding increases in endogenous Na⁺, K⁺-ATPase and the month afterward.
Several experiments were conducted in which chinook salmon were implanted with ovine growth hormone (5 (mu)g/g body weight), cortisol (50 (mu)g/g body weight), or both. Cortisol implants significantly elevated plasma cortisol concentrations and stimulated Na⁺, K⁺-ATPase activity in either caeca or posterior intestine, or both, after 7 and 14 days. Although growth hormone increased plasma cortisol concentrations after 14 days, no interaction occurred between cortisol and growth hormone on Na⁺, K⁺-ATPase activity.
The effect of insulin-like growth factor I on Na⁺, K⁺-ATPase activity was examined in tissue-cultured intestine of chinook salmon. Prior to culture, salmon were implanted with or without ovine growth hormone (5 (mu)g/g body weight) for 7 days. Intestinal explants were then exposed to recombinant human insulin-like growth factor I (rhIGF-I) for 3 or 6 days in culture. In caeca, rhIGF-I (0.01-1.0 (mu)g/ml) significantly maintained Na⁺, K⁺-ATPase activity above controls (0 (mu)g rhIGF-I /ml) in a dose-dependent manner, regardless of whether the fish were pretreated with growth hormone in vivo. The posterior intestine was not responsive to rhIGF-I.
These results demonstrate pyloric caeca are a major osmoregulatory site, evidenced by tissue responsiveness to cortisol (and IGF-I) and stimulation of functional changes indicative of parr-smolt transformation.
| Identifer | oai:union.ndltd.org:ADTP/217491 |
| Date | January 2005 |
| Creators | Veillette, Philip A., n/a |
| Publisher | University of Otago. Department of Zoology |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://policy01.otago.ac.nz/policies/FMPro?-db=policies.fm&-format=viewpolicy.html&-lay=viewpolicy&-sortfield=Title&Type=Academic&-recid=33025&-find), Copyright Philip A. Veillette |
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