ROLE OF INCREASED CYTOSOLIC-FREE CALCIUM-CONCENTRATION IN MYOCARDIAL ISCHEMIC-INJURY

Increases in cytosolic free calcium concentration ([Ca2+]I) may play an important role in myocardial ischemic injury. An early effect of the rise in [Ca2+]I may be impaired postischemic contractile function if the ischemic myocardium is reperfused during the reversible phase of ischemic injury; furthermore, if the rise in [Ca2+]i is prolonged, a cascade of events may be initiated which ultimately results in lethal injury. With the development of methods for measuring [Ca2+]I, it has become possible to evaluate directly the role of increased [Ca2+]I in myocardial ischemic injury. Although it has been possible to show that inhibition of the transport processes which contribute to the early rise in [Ca2+]I attenuates stunning and the rise in [Ca2+]I concurrently, if increased [Ca2+]I plays an important role in ischemic injury, then it should be possible to show that interventions which alter the timecourse of ischemic injury also alter the timecourse of the rise in [Ca2+]I in a parallel manner. Recently, considerable effort has been expended to investigate the mechanisms underlying the preconditioning phenomenon, whereby repetitive brief periods of ischemia prior to a sustained period of ischemia protects the myocardium from injury during the sustained period of ischemia, and this has stimulated additional work to understand the possible involvement of adenosine as a mediator of preconditioning as well as to understand the protective effects of adenosine. Measurements of [Ca2+]I using 19F NMR of 5FBAPTA-loaded hearts have shown that preconditioning attenuates the rise in [Ca2+]I during 30 min of ischemia and reduces stunning during reflow. Adenosine pretreatment mimics the effects of preconditioning on the rise in [Ca2+]I and on stunning, but adenosine receptor antagonists do not eliminate the protective effects of preconditioning, although some adenosine antagonists also block hexose transport and under these conditions, the ability of preconditioning to attenuate the rise in [Ca2+]I is abolished and there is a corresponding loss of the protective effect of preconditioning on stunning. Although it has been suggested that the beneficial effect of preconditioning on infarct size can be eliminated by pretreatment with glibenclamide, in the isolated rat heart glibenclamide does not affect the attenuation of the rise in [Ca2+]I induced by preconditioning and does not affect stunning. All of these studies show a consistent relationship between the magnitude of the rise in [Ca2+]I during ischemia and the degree of stunning during reperfusion. The data suggest that increased [Ca2+]I plays a very important role in myocardial ischemic injury.