Influence of specific activity on myocardial uptake of 123I-mIBG in rats

Heart and lung uptake of 123I-mIBG was studied in rats 1 h after injection as a function of specific activity (mCi mg-1) and the total loading dose (microgram kg-1) of labelled compound. Absolute lung uptake of the tracer was dependent upon specific activity, declining as the loading dose increased from 0.15 to 4300 micrograms kg-1. Uptake in the total heart was constant at 2.1% up to about 12 micrograms kg-1, at which point the heart uptake also began to decline as the loading dose increased. Myocardial uptake of 123I-mIBG, therefore, appears saturable in the rat and subject to competitive binding and/or displacement with increasing amounts of non-labelled compound.