SUMMARY
For most teleost fish, the majority of the myocardial oxygen supply is provided by the oxygen reserve remaining in venous blood after other tissues have extracted oxygen. We examined the effect of graded exercise and water temperature on this venous blood oxygen supply to the heart (termed the cardiac circulation) by performing novel on-line measurements of venous partial pressure of oxygen (PvO2) using a fibreoptic micro-optode implanted in the ductus Cuvier of rainbow trout(Oncorhynchus mykiss). As expected, PvO2 decreased progressively and significantly as swimming velocity approached the critical swimming speed(Ucrit). Unsteady swimming behaviours during the graded exercise, however, caused abrupt and generally short-lived decreases in PvO2. For the cold-acclimated (6-10°C)fish, PvO2 reached a minimum plateau value of 15.3±3.7 torr (1 torr=133.3 Pa) before Ucrit was reached, and so increased swimming effort near to Ucritdid not reduce PvO2 further. Warm-acclimated fish had a significantly higher PvO2(28.9±3.5 torr) at Ucrit. Despite this difference in the PvO2 at Ucrit, we estimated that there was little difference between warm- and cold-acclimated fish in terms of oxygen supply in the cardiac circulation because of a right-shift in the haemoglobin—oxygen dissociation curve at warm temperatures. Furthermore, although PvO2decreased significantly at Ucrit, our estimates suggest that the expected increase in cardiac output would easily maintain the oxygen supply in venous blood at a level similar to that found in resting fish. Although unsteady swimming behaviours decreased PvO2, unsteady swimming rarely decreased the minimum PvO2 value observed at Ucrit by more than 10%. The findings are discussed in terms of a threshold PvO2 required to maintain adequate rates of oxygen diffusion from the cardiac circulation to the myocardial tissues.
