The metabolic consequences of an increase in the frequency of stimulation in isolated ferret hearts

1. The metabolic consequences of an increase in the frequency of stimulation were examined in isolated ferret hearts. Intracellular pH (pH(i)) and the intracellular concentrations of phosphocreatine ([PCr](i)), inorganic phosphate ([P(i)](i)) and ATP were measured by 31P nuclear magnetic resonance (NMR) spectroscopy. 2. Increasing the stimulus rate from 0.1-0.7 to 2 Hz caused an increase in [P(i)](i) and a decrease in [PCr](i) in the first minute of 2 Hz stimulation. There was subsequently a partial recovery of both [PCr](i) and [P(i)](i) during continued stimulation. There was no change in [ATP](i) during stimulation at 2 Hz. Increasing the stimulus rate caused an intracellular acidosis of around 0.1 pH units. 3. Increasing the stimulus rate generally caused an initial increase in developed pressure, followed by a decrease over 1-2 min to a steady level slightly lower than developed pressure at the low (control) stimulus rate. The increase in stimulus rate caused a 4- to 6-fold increase in time-averaged muscle activity. 4. Both oxygen uptake and production of lactate increased on 2 Hz stimulation. Lactate production accounted for less than 5% of ATP production at low or high stimulus rates, suggesting that significant anoxia was not occurring during stimulation. The observed lactate production was, however, sufficient to explain most of the intracellular acidosis observed when the stimulus rate was raised. When glycolysis was prevented by removal of glucose and depletion of glycogen stores, 2 Hz stimulation was accompanied by an intracellular alkalosis rather than an acidosis, suggesting that lactate production by glycolysis was the cause of the intracellular acidosis. 5. Reducing the rate of glycolysis increased the size of the changes in [PCr](i) and [P(i)](i) evoked by stimulation at 2 Hz. Furthermore, there was now no partial reversal of the changes in [PCr](i) and [P(i)](i) during 2 Hz stimulation. 6. When oxidative phosphorylation was inhibited by replacing O2 with N2, increasing the rate of stimulation from 0.1-0.7 to 1-2 Hz caused an initial increase followed by a large fall in developed pressure, which declined to a level well below that at the control stimulus rate. The increase in stimulus rate was accompanied by a large fall in [PCr](i), an increase in [P(i)](i), and an intracellular acidosis of 0.1-0.3 pH units. The fall in developed pressure was consistent with the known effects of the rise in [P(i)](i) and the fall in pH(i) on the contractile apparatus.