The lumped constant--a term in the operational equation of the Sokoloff tracer kinetic model for deoxyglucose that accounts for the difference in transport and phosphorylation between glucose and its analog, deoxyglucose--could potentially vary from normal to ischemic conditions in the heart. To test the stability of the lumped constant during ischemia, we evaluated the ratio of the extraction fraction for (F-18)-fluorodeoxyglucose (FDG) to that for glucose (a measure of the lumped constant if there is no significant dephosphorylation of FDG-6-PO4) and the rate constant for dephosphorylation of FDG-6-PO4 (k4*) in the isolated, arterially perfused interventricular septum of the rabbit during moderate and severe demand-induced and reduced-flow ischemias. The lumped constant and k4* in each of the four ischemic experimental conditions were found not to be significantly different from the value obtained from the nonischemic controls.
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