We propose to study the transport and metabolism of glucose in the myocardium using our unique combination of an isolated, perfused, working rat heart preparation, glucose analogs labeled with positron emitting radionuclides, quantitive coincidence gamma-ray detection, rapid bolus injection, and digital compartmental modeling of tissue residue time courses measured at high temporal resolution. Estimates of membrane transport rates, steady state distribution volumes, and rates of phosphorylation and dephosphorylation of the analogs are derived from the fitted model parameters. Phosphorylation lumped constants are calculated using glucose utilization rates measured at regular intervals with tritiated glucose. Mechanical performance, oxygen utilization rates, and lactate production rates are also monitored using conventional techniques. Our approach provides a physical model of myocardial tissue isolated using positron emission tomography, and in addition demonstrates the merits of external counting of tissue radioactivity relative to those of the conventional use of beta-labeled analogs and liquid scintillation counting. The main focus of the proposed work is to examine the stability of the phosphorylation lumped constant and dephosphorylation rate constant of 2-deoxy-2-fluoro-D-glucose under a range of conditions, including diabetic, ischemic and hypoxic hearts, glucose or fatty acid as determined fuel, high or low work load, insulin present or absent, and other drug interventions. The relationship between lumped constant and analog cellular distribution volume will be studied using the transport analog 3-deoxy-3-fluoro-D-glucose. Subsidiary aims include performance of initial studies with 3-0-methylglucose and 2-deoxyglucose, setting an upper limit on the rate of dephosphorylation of glucose in our preparation, and further characterization of our preparation by determination of rates of utilization of endogenous substrate under various conditions. In addition to providing basic information about cardiac glucose utilization, its measurement, and its response to pathology and drug intervention, our results will bear directly on the future refinement of non-invasive measurements in the human using positron emission tomography.
