ABSTRACT
Myocardial oxygen consumption 
and lactic acid release were measured in the isolated heart of a hagfish (Eptatretus cirrhatus Forster) perfused in vitro. Two different ranges of partial pressures of oxygen were employed (
3.87–5.87 and 1.60–2.67kPa). All hearts released lactate into the perfusate, but the rate of release was greater and 
was depressed at the lower 
.
When energy production through the glycolytic pathway to lactate is converted to oxygen equivalents and added to measured oxygen consumption rates, over a wide range of power outputs and different values of 
, the data can be fitted to a single linear regression line. The rate of oxygen consumption of the hagfish myocardium, so obtained, is similar to values reported for teleost fish. The unusual ability of the hagfish myocardium to support perhaps up to 50% of its maximal power output through anaerobic metabolism is related to its extremely low cardiac energy demand.
