DYNAMICS OF MYOCARDIAL LACTATE EFFLUX AFTER A STEP IN HEART-RATE IN ISOLATED RABBIT HEARTS

We investigated whether a glycolytic burst contributes to the initial adaptation of ATP synthesis to increased cardiac metabolic demand. Six isolated rabbit hearts were perfused with glucose-containing Tyrode s olution at 28-degrees-C. In venous and arterial samples the lactate co ncentration was determined with a sensitive enzymatic cycling method. After the heart rate was doubled from 60 to 120 beats/min, lactate eff lux increased from 0.23 +/- 0.10 (SE) to 0.45 +/- 0.12 mumol.min-1.g-1 dry weight with a mean response time of 21.3 a but without an oversho ot. The transport time for lactate is longer than 15.7 s, suggesting t hat lactate production adapts with a mean response time of less than 6 s. Because no overshoot in lactate efflux was found, it is unlikely t hat a glycolytic burst after a step in heart rate contributes to the f ast adaptation of ATP synthesis to demand in the isolated rabbit heart , although it might be possible that a change in cytosolic lactate pro duction is not reflected in an increase in lactate efflux. Extrapolati on of the results of this study to the in vivo situation should be don e with caution.