CONTINUOUS MEASUREMENTS OF CYTOPLASMIC ATP IN SINGLE CARDIOMYOCYTES DURING SIMULATION OF THE OXYGEN PARADOX

Objective: It is now possible to monitor cytoplasmic ATP in single car diomyocytes and it has recently been shown that cardiomyocytes exposed for several minutes to metabolic inhibitors undergo an abrupt rigor m ediated shortening which coincides with a sudden fall in cytoplasmic A TP, from approximately 150 mumol.litre-1 to a few micromolar or less. The objective of this work was to monitor cytoplasmic ATP during simul ated reoxygenation of a poisoned cardiomyocyte. Methods: Firefly lucif erase was injected into a single cell and the light signal generated w hen luciferin was superfused was monitored. Calibration of the signal is complicated by a transient enhancement of the signal (possibly the result of complex luciferase kinetics), and by uncertainties about cyt oplasmic pH. Results: The data indicate that millimolar levels of cyto plasmic ATP are restored within 1-2 min of cyanide removal. Conclusion s: Cytoplasmic free calcium is known to rise after poisoned cells unde rgo shortening, so it is conceivable that the restoration of cytoplasm ic ATP in a cell in which free calcium is at micromolar levels may pro vide a plausible cellular mechanism for the ''oxygen paradox''. Reoxyg enation induces large amplitude, but slow, oscillations in free calciu m which, together with the millimolar levels of ATP indicated here, co uld provide the stimuli for generating the uncoordinated mechanical fo rces that are prevalent in the oxygen paradox.