Oxygen consumption in rabbit Langendorff hearts perfused with a saline medium

Isolated rabbit hearts were perfused according to Langendorff at a temperature of 38°C and a pressure of 5.9 kPa with gassed Tyrode solution. Gas mixtures containing 5 % CO₂ and 15, 20, 30, 60, or 95% O₂ in N₂ were used to saturate the perfusion medium. In some cases lactate (50 or 500 μM) was present in the medium perfusing the heart. Coronary flow (CF), oxygen pressure in the perfusion medium and in the cardiac effluent and lactate in the effluent were analysed in all experiments. The oxygen uptake in the hearts perfused with a medium equilibrated at atmospheric pressure with 95% O₂ and 5% CO₂ (oxygen pressure?87 kPa, oxygen content 19 ml × 1^-1), averaged 3 mix 100 g w.w. ^-1× min^-1. Reduction of the oxygen pressure in the perfusion medium resulted in an increase in CF and in the fractional extraction of oxygen from the medium, making it possible to maintain the heart's oxygen uptake (V?o₂) down to an oxygen pressure in the perfusion medium of about 24 kPa (oxygen content?5 ml × 1^-1, the perfusion medium equilibrated with 20% O₂ and 5%CO₂ in N₂). Myocardial lactate production was low during perfusion at pO₂? 87 kPa but increased rapidly when the oxygen pressure was lowered. The addition of lactate (500 /°M) to the perfusion medium at pO₂? 87 kP induced a fractional uptake of about 20%. It is concluded that the V?o₂ observed during perfusion at pO₂? 87 kPa mainly reflects aerobic myocardial metabolism in this preparation. This assumption is based on the facts that coronary flow and fractional oxygen extraction are submaximal and that a considerable uptake of lactate occurs concurrently with a very limited production. However, even moderate reduction of the oxygen pressure in the perfusion medium (to ? 61 kPa) is followed by a significant increase in lactate production, indicating that myocardial oxygenation is inefficient.