Brief myocardial ischemia not only evokes a local cardioprotective or "preconditioning" effect but also can render remote myocardium resistant to sustained ischemia. We propose the following hypotheses: remote protection is initiated by a humoral trigger; brief ischemia-reperfusion will result in release of the humoral trigger (possibly adenosine and/or norepinephrine) into the coronary effluent; and transfer of this effluent to a virgin acceptor heart will elicit cardioprotection. To test these concepts, effluent was collected during normal perfusion from donor-control hearts and during preconditioning ischemia-reperfusion from donor-preconditioned (PC) hearts. After reoxygenation occurred and aliquots for measurement of adenosine and norepinephrine content were harvested, effluent was transfused to acceptor-control and acceptor-PC hearts. All hearts then underwent 40 min of global ischemia and 60 min of reperfusion, and infarct size was delineated by tetrazolium staining. Mean infarct size was smaller in both donor- and acceptor-PC groups (9% of left ventricle) than in donor- and acceptor-control groups (36% and 34%; P < 0.01). Protection in acceptor-PC hearts could not, however, be attributed to adenosine or norepinephrine. Thus preconditioning-induced cardioprotection can be transferred between rabbit hearts by transfusion of coronary effluent. Although adenosine and norepinephrine are apparently not responsible, these results suggest that remote protection is initiated by a humoral mechanism.

Neutron activation is an accurate analytic method in which trace quantities of isotopes of interest in a sample are activated and the emitted radiation is measured with high-resolution detection equipment. This study demonstrates the application of neutron activation for the measurement of myocardial perfusion using stable isotopically labeled microspheres. Stable labeled and standard radiolabeled microspheres (15 microm) were coinjected in an in vivo rabbit model of myocardial ischemia and reperfusion. Radiolabeled microspheres were detected with a standard gamma-well counter, and stable labeled microspheres were detected with a high-resolution Ge detection after neutron activation of the myocardial and reference blood samples. Regional myocardial blood flow was calculated from the deposition of radiolabeled and stable labeled microspheres. Both sets of microspheres gave similar measurements of regional myocardial blood flow over a wide range of flow with a high linear correlation (r = 0.95-0.99). Neutron activation is capable of detecting a single microsphere in an intact myocardial sample while providing simultaneous quantitative measurements of multiple isotope labels. This high sensitivity and capability for measuring perfusion in intact tissue are advantages over other techniques, such as optical detection of microspheres. Neutron activation also can provide an effective method for reducing the production of low-level radioactive waste generated from biomedical research. Further applications of neutron activation offer the potential for measuring other stable labeled compounds, such as fatty acids and growth factors, in conjunction with microsphere measured flow, providing the capability for simultaneous measurement of regional metabolism and perfusion.