Submitted on December 9, 2003
Accepted on March 17, 2004
A comparison of adrenergic stress responses in three tropical teleosts exposed to acute hypoxia
Steve F Perry1*, Stephen G Reid2, Kathleen M Gilmour1, Cheila L Boijink3, Jane M Lopes3, William K Milsom4, and Tadeu F Rantin3
1 Ottawa-Carleton Institute of Biology, University of Ottawa, Ottawa, Ontario, Canada
2 Department of Life Sciences, University of Toronto at Scarborough, Toronto, Ontario, Canada
3 Department of Physiological Sciences, Laboratory of Zoophysiology and Comparative Biochemistry, Federal University of Sao Carlos, Sao Carlos, Brazil
4 Department of Zoology, University of British Columbia, Vancouver, British Columbia, Canada
* To whom correspondence should be addressed. E-mail: sfperry{at}science.uottawa.ca.
Experiments were performed to assess the afferent and efferent limbs of the hypoxia-mediated humoral adrenergic stress response in selected hypoxia-tolerant tropical fishes that routinely experience environmental oxygen depletion. Plasma catecholamine levels and blood respiratory status were measured during acute aquatic hypoxia (water PO2 (PwO2) = 10 - 60 mm Hg) in three teleost species, the obligate water breathers, Hoplias malabaricus (traira) and Piaractus mesopotamicus (pacu), and the facultative air-breather, Hoplerythrinus unitaeniatus (jeju). Traira displayed a significant increase in plasma catecholamine levels (from 1.3 ± 0.4 to 23.3 ± 15.1 nmol l-1) at PwO2 levels below 20 mm Hg whereas circulating catecholamine levels were unaltered in pacu at all levels of hypoxia. In jeju denied access to air, plasma catecholamine levels were increased markedly to a maximum mean value of 53.6 ± 19.1 nmol l-1 as PwO2 was lowered below 40 mm Hg. In traira and jeju, catecholamine release into the circulation occurred at abrupt thresholds corresponding to arterial PO2 values of approximately 8.5 - 12.5 mm Hg. A comparison of in vivo blood O2 equilibration curves, revealed low and similar P50 values amongst the three species (7.7 - 11.3 mm Hg). Thus, catecholamine release in traira and jeju occurred as blood O2 concentration was reduced to approximately 50 - 60% of the normoxic value. Intravascular injections of nicotine (600 nmol kg-1) elicited pronounced increases in plasma catecholamine levels in traira and jeju but not in pacu. Thus, the lack of catecholamine release during hypoxia in pacu may reflect an inoperative or absent humoral adrenergic stress response in this species.
When allowed access to air, jeju did not release catecholamines into the circulation at any level of aquatic hypoxia. The likeliest explanation for the absence of catecholamine release in these fish was that air breathing, initiated by aquatic hypoxia, prevented arterial PO2 values from falling to the critical threshold required for catecholamine secretion.
The ventilatory responses to hypoxia in each species were similar, consisting generally of increases in both frequency and amplitude. These responses were not synchronized with, nor influenced by, plasma catecholamine levels. Thus, the acute humoral adrenergic stress response does not appear to stimulate ventilation during acute hypoxia in these tropical species.
