METABOLIC CHANGES DURING ISCHEMIA AND THEIR ROLE IN CONTRACTILE FAILURE IN ISOLATED FERRET HEARTS

1. The effects of global ischaemia on phosphorus metabolites. intracellular pH (pH(i)) and developed pressure were measured in isolated whole ferret hearts using "P nuclear magnetic resonance (NMR) spectroscopy. 2. Brief (10 min) periods of global ischaemia reduced left ventricular developed pressure (LVDP) to undetectable levels. This fall in LVDP was accompanied by a fall in the intracellular concentration of phosphocreatine (PCr) and increases in the concentrations of inorganic phosphate (P(i)) and phosphomonoesters. There was no change in the intracellular ATP concentration ([ATP]i). pH(i) fell approximately linearly at a rate of 0.04 pH units min-1. 3. When ferret hearts were exposed to cyanide (CN-) in the presence of alpha-cyano-4-hydroxycinnamate (CHC), a blocker of lactate efflux. the changes in pH(i) and [P(i)]i which occurred were similar to those observed during global ischaemia. However, developed pressure only fell to around 15 % of the control value. 4. Removing the intracellular acidosis (by reducing the CO2 level of the gas with which the perfusate was equilibrated) during exposure to CN- and CHC caused an increase in developed pressure, consistent with the fall in pH(i) being responsible for a substantial fraction of the fall in developed pressure. 5. Taken together. these results suggest that most. but not all. of the fall in developed pressure during ischaemia can be explained by the effects of the changes in pH(i) and [P(i)]i on the contractile apparatus. 6. Action potential recordings made with a suction electrode during short periods of global ischaemia showed that there was no decrease in action potential duration over the period when developed pressure was falling, eliminating action potential shortening as a possible cause of the fall in developed pressure. 7. In hearts in which the rate of glycolysis had been reduced by glycogen depletion, global ischaemia led to a marked shortening of the action potential. NMR experiments showed that under these conditions [ATP]i decreased by around 50 % over the first 10 min of ischaemia, while the intracellular acidosis which occurred was smaller than that in a control ischaemic period. 8. The time course of the decline of [ATP]i was examined in several hearts during long (45 min and over) ischaemic periods without prior glycogen depletion. After 45 min of ischaemia [ATP]i fell to around two-thirds of the control value, while pH(i) declined to approximately 6.1. Resting pressure did not increase. On reperfusion pH(i) recovered rapidly to control levels. [ATP]i, however, did not recover. 9. If ischaemia was prolonged further, [ATP]i eventually became undetectable after 70-90 min. A rise in resting pressure occurred once [ATP]i fell below 0.5-1.0 mM. 10. Our results suggest that during ischaemia glycolysis can continue to function and maintain [ATP]i to some extent. However, if glycogen stores are depleted [ATP]i falls more rapidly, leading to action potential shortening and eventually, when [ATP]i reaches sub-millimolar levels, to a rise in resting pressure (rigor contracture).