This proposal seeks to establish the basis for postischemic contractile dysfunction and reperfusion injury in rat and canine myocardium. The hypothesis is that postischemic contractile dysfunction or "stunning" differs significantly in rat and canine hearts. In the rat, Ca 2+ overload via Na/Ca 2+ exchange causes spontaneous and uncoordinated Ca 2+ release from the sarcoplasmic reticulum throughout the cardiac cycle, undermining diastolic filling and systolic ejection. Canine myocardium, however, is less dependent on reactions of the sarcoplasmic reticulum during excitation-contraction coupling. It is also more sensitive to beta-adrenergic stimulation. This suggests that stunning of the canine myocardium may instead involve a relative deficiency of cAMP. Such a deficiency could result from phosphorylation and internalization of sarcolemmal beta-receptors in response to transient elevations in catecholamines or intracellular free [Ca 2+] . There could also be oxygen radical activation of soluble guanylyl cyclase, with cGMP-dependent stimulation of cAMP phosphodiesterase. Increases in alpha-receptors or depletion of cytosolic GTP could also depress intracellular cAMP. It follows that stunning in canine myocardium could be caused by dislocations in the myocytes per se, as might be produced by intracellular acidosis, altered energy metabolism, or mitochondrial generation of reactive oxygen radicals. Alternatively, stunning could involve the release of injurious agents from non-muscle cells or excess catecholamines from tissue storage sites. To test this hypothesis and establish the mechanisms involved in postischemic dysfunction, ventricular myocytes will be isolated from rat and canine hearts and characterized in terms of energy metabolism, ion homeostasis, oxygen radical production, excitation-contraction coupling, and hormone responsiveness prior to and following simulated ischemia and reperfusion. Similar protocols will be used to characterize cells isolated from intact myocardium subjected to ischemia and reperfusion in vitro or in vivo. Isolated rat and canine myocytes incubated under identical controlled conditions overcome many of the technical difficulties inherent in comparing large and small animal hearts and in obtaining reliable data on membrane receptor density, intracellular free Ca2+ , and cAMP content.