Bioluminescent measurement in single cardiomyocytes of sudden cytosolic ATP depletion coincident with rigor

The sequence of events that leads to irreversible injury of the ischaemic myocardium is poorly under-stood but it is axiomatic that lack of oxygen will impair regeneration of ATP. In the globally-ischaemic heart a contracture develops which is independent of raised cytoplasmic free Ca 2+ and which has been attributed to activation of actomyosin by nucleotide-free actomyosin cross-bridges (‘rigor complexes’) which form at low ATP concentrations [1–3]. Single, metabolically-poisoned or anoxic cardiomyocytes show comparable behaviour, shortening before a significant rise in cytoplasmic free Ca 2+ occurs [4–7]. To explain the close temporal relation-ship that exists between cell shortening and the onset of the free Ca 2+ rise we have predicted [8, 9] that, during myocyte shortening, a precipitous fall in cytosolic ATP concentration occurs, the result of rigor-complexes activating myosin ATPase, which then perturbs ionic homeostasis. Here we show, by means of continuous measurements of cytosolic ATP using firefly luciferase microinjected into single, isolated cardiomyocytes, that cell shortening coincides with an abrupt fall in cytosolic ATP.