Keen, A.N.; Gamperl, A.K. (2012). Blood oxygenation and cardiorespiratory function in steelhead trout (Oncorhynchus mykiss) challenged with an acute temperature increase and zatebradine-induced bradycardia. Journal Of Thermal Biology. 37 (3) 201-210.
To explore whether temperature-dependent increases in cardiac output (Q) are mediated solely through heart rate (f(H)) in fish to ensure adequate/efficient blood oxygenation, we injected steelhead trout with saline (control) or zatebradine hydrochloride (1.0 mg kg(-1)), and measured blood oxygen status, cardiorespiratory variables and cardiorespiratory synchrony during a critical thermal maximum (CTMax) test. The increasing temperature regimen itself (from 12 degrees C to CTMax) resulted in large decreases in arterial oxygen partial pressure (PaO2) and content (CaO2) (by similar to 35% and 25%, respectively). Further, there was little evidence of cardiorespiratory synchrony at 12 degrees C, and the number of fish that showed synchrony at high temperatures only increased marginally (to 3 out of 7) despite the large decrease in PaO2. These results: (1) indicate that in some situations (e.g. when ventilation is exclusively/predominantly dependent on buccal-opercular pumping) the upper thermal tolerance of fish may be constrained by both cardiovascular and ventilatory performance; and (2) question the importance of cardiorespiratory synchrony (ventilation-perfusion matching) for gas exchange in salmonids, and fishes, in general.
Zatebradine injection decreased heart rate (f(H)) at 12 degrees C by 11% and limited maximum f(H) to 78.6 +/- 5.9 vs. 116.5 +/- 5.7 beats min(-1) in controls. However, it did not affect maximum cardiac output (due to a compensatory increase in S-V), ventilation, cardiorespiratory synchrony or PaO2. In contrast, metabolic scope and CTMax were lower in the zatebradine vs. control group [184.5 +/- 17.4 vs. 135.7 +/- 21.5 mL kg(-1) h(-1) (p < 0.05) and 23.7 +/- 0.2 vs. 22.6 +/- 0.4 degrees C (p < 0.08). respectively I. This result was unrelated to maximum f(H) or scope for f(H), and occurred despite higher values for blood oxygen content and haematocrit at > 18 degrees C in the zatebradine-treated fish. These latter findings suggest that zatebradine has non-pacemaker effects that limit tissue oxygen utilization and its usefulness for in vivo studies. (C) 2012 Elsevier Ltd. All rights reserved.