Long-term hypoxia exposure alters the cardiorespiratory physiology of steelhead trout (Oncorhynchus mykiss), but does not affect their upper thermal tolerance

It has been suggested that exposure to high temperature or hypoxia may confer tolerance to the other oxygen-limited stressor (i.e., 'cross-tolerance'). Thus, we investigated if chronic hypoxia-acclimation (>3 months at 40% air saturation) improved the steelhead trout's critical thermal maximum (CTMax), or affected key physiological variables that could impact upper thermal tolerance. Neither CTMax (24.7 vs. 25.3 °C) itself, nor oxygen consumption (ṀO2), haematocrit, blood haemoglobin concentration, or heart rate differed between hypoxia- and normoxia-acclimated trout when acutely warmed. However, the cardiac output (Q̇) of hypoxia-acclimated fish plateaued earlier compared to normoxia-acclimated fish due to an inability to maintain stroke volume (SV), and this resulted in a ~50% lower maximum Q̇. Despite this reduced maximum cardiac function, hypoxia-acclimated trout were able to consume more O2 per volume of blood pumped as evidenced by the equivalent ṀO2. These results provide additional evidence that long-term hypoxia improves tissue oxygen utilization, and that this compensates for diminished cardiac pumping capacity. The limited SV in hypoxia-acclimated trout in vivo was not associated with changes in cardiac morphology or in vitro maximum SV, but the affinity and density of myocardial ß-adrenoreceptors were lower and higher, respectively, than in normoxia-acclimated fish. These data suggest that alterations in ventricular filling dynamics or myocardial contractility constrain cardiac function in hypoxia-acclimated fish at high temperatures. Our results do not support (1) 'cross-tolerance' between high temperature and hypoxia when hypoxia is chronic, or (2) that cardiac function is always the determinant of temperature-induced changes in fish ṀO2, and thus thermal tolerance, as suggested by the oxygen- and capacity-limited thermal tolerance (OCLTT) theory.