IS OXYGEN-SUPPLY SUFFICIENT TO INDUCE NORMOXIC CONDITIONS IN ISOLATEDRAT-HEART

The aim of this study is to assess whether oxygen supply is sufficient to induce normoxic conditions in isolated rat hearts. Hearts are perf used with a Krebs medium supplemented with 11 mM glucose, 0.6 mM lacta te, 0.06 mM pyruvate, non delipidated albumin (0.1 mM fatty acids), an d either 1.78 mM or 0.76 mM free calcium, at 10 ml.min(-1). Graded hyp oxia is induced by a stepwise decrease of partial pressure of oxygen ( PO2) from 660 to 52 mmHg. Contractile performance, oxygen uptake and l actate plus pyruvate balance are assessed. With high calcium, left ven tricular developed pressure, dP/dt max and oxygen uptake increase line arly with PO2 up to 660 mmHg; heart rate increases with PO2 up to 250 mmHg and then tends to stabilize. With low calcium, all parameters rea ch a plateau over 400 mmHg. Lactate plus pyruvate production suggests a stimulation of glycolysis with high calcium, even at 660 mmHg; conve rsely, there is no lactate plus pyruvate production with low calcium o ver 250 mmHg. In conclusion, our results demonstrate that, under a hig h level of calcium at a constant flow of 10 ml.min(-1), cardiac functi on is always limited by O-2 supply, except for heart rate. This raises the question as to the definition of a normoxic state. The better pre servation of heart rate during hypoxia, compared to other dynamic para meters, could be explained by a contribution of glycolytic ATP.