Proceedings of The Physiological Society

Physiology 2012 (Edinburgh) (2012) Proc Physiol Soc 27, C7

Oral Communications

Blood oxygenation and cardiorespiratory function in steelhead trout (Oncorhynchus mykiss) challenged with an acute temperature increase and zatebradine-induced bradycardia

A. N. Keen1, K. A. Gamperl2

1. Faculty of Life Sciences, University of Manchester, Manchester, United Kingdom. 2. Ocean Science Centre, Memorial University of Newfoundland, St. John's, Newfoundland, Canada.

In this study, steelhead trout were fitted with a Doppler blood flow probe around their ventral aorta, a dorsal aortic cannula (for drug injections and blood sampling), and an opercular cavity cannula for ventilatory measurements. Then we injected these fish with saline (control) or zatebradine hydrochloride (at a concentration of 1.0 mg kg-1), and measured blood oxygen status, cardiorespiratory variables and cardiorespiratory synchrony during a critical thermal maximum (CTMax) test to examine whether temperature-dependent increases in cardiac output (Q) are mediated solely through heart rate (fH) in fish to ensure adequate/efficient blood oxygenation. CTMax was determined as the temperature at which the fish lost equilibrium, at which point, water temperature was rapidly decreased to 10°C to allow the fish to recover before heart sampling. The increasing temperature regimen itself (from 12 °C to CTMax) resulted in large decreases in arterial oxygen partial pressure (PaO2) and content (CaO2) (by ~35% and 25%, respectively. Further, there was little evidence of cardiorespiratory synchrony at 12 °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 (fH) at 12 °C by 11% and limited maximum fH to 78.7 vs. 116.6 beats min-1 in controls. However, it did not affect maximum cardiac output (due to a compensatory increase in stroke volume), 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 °C (p<0.08), respectively]. These effects were unrelated to maximum fH or scope for fH, and occurred despite higher values for blood oxygen content and haematocrit at >18 °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.

Where applicable, experiments conform with Society ethical requirements