Dependence of 13NH3 myocardial extraction and clearance on flow and metabolism.

The isolated arterially perfused rabbit interventricular septum was used to study the relationship between flow (F) and myocardial uptake and subsequent clearance of bolus injections of 13N-labeled ammonia (13NH3). Multiple 13NH3 injections were delivered to a total of 19 septa at perfusion rates (F/g) ranging from 0.2-4.0 ml.min-1.g-1. Ten septa were studied at 37 degrees C and nine were studied at 28 degrees C to obtain uptake and clearance data at different metabolic rates. The septa were perfused without recirculation with a modified Tyrode solution. Analysis of the time-activity curves generated by the injections of 13NH3 revealed that the data was best described by three components. The first two components exhibited short clearance times (t1/2s less than 7 min) in contrast to prolonged clearance times (30-500 min) for component 3. Component 1 was consistent with a freely diffusible 13NH3 compartment. Component 3 persists and permits myocardial imaging 15 min or more after 13NH3 injection. At each perfusion rate the extraction fraction of component 3 was twice as large, and the clearance time of component 3 was twice as fast at 37 as at 28 degrees C, implying that the extraction and washout of this third component are dependent on enzymatic reactions. The total myocardial extraction into component 3 from a systemic injection is determined by multiplying the extraction fraction by the perfusion rate (EF(3) X F/g) and was directly related to the perfusion rate at both 28 and 37 degrees C.