The effects of progressive hypoxia and re-oxygenation on cardiac function, white muscle perfusion and haemoglobin saturation in anaesthetised snapper (Pagrus auratus)

The effects of progressive hypoxia and re-oxygenation on cardiac function, white muscle perfusion and haemoglobin saturation were investigated in anaesthetised snapper (Pagrus auratus). White muscle perfusion and haemoglobin saturation were recorded in real time using fibre optic methodology. A marked fall in heart rate (HR) was evoked when the water bath dissolved oxygen (DO) concentration decreased below 1.5 mg L⁻¹. This bradycardia deepened over the subsequent 20 min of progressive hypoxia and noticeable arrhythmias occurred, suggesting that hypoxia had direct and severe effects on the cardiac myocytes. Perfusion to the white muscle decreased below a DO concentration of 3 mg L⁻¹, and oxyhaemoglobin concentration decreased once the DO fell below ca. 2 mg L⁻¹. During re-oxygenation, heart rate and white muscle perfusion increased as the DO concentration exceeded 1.9 ± 0.1 mg L⁻¹, whereas haemoglobin saturation increased once the external DO concentration reached 2.9 mg L⁻¹. These changes occurred in anaesthetised fish, in which sensory function must be impaired, if not abolished. As white muscle perfusion both fell and increased prior to changes in white muscle oxyhaemoglobin saturation, a local hypoxia is more likely to be the consequence than the cause of the reduced blood delivery, and changes upstream from the tail vasculature must be responsible. HR and tissue haemoglobin concentrations did increase simultaneously on re-oxygenation suggesting an increased cardiac output as the cause.