Considerations in use of microspheres for myocardial blood-flow measurements in long-term experiments on rats

The loss of 15-μm radioactive microspheres from rat myocardium was determined in long-term experiments. Heterotopic heart transplantation was used for the evaluation of microsphere losses. Microspheres were injected into the left ventricle of a donor rat, and the number of microspheres entrapped in the heart was measured after its isolation. The heart was transplanted into the abdominal cavity of a recipient rat and left there for 1–3 wk. The number of microspheres entrapped in the donor heart before the transplantation was compared with the number of microspheres remaining there after 1–3 wk. The losses of radioactivity from the myocardium over 1–3 wk did not exceed 10%. In a control series of experiments, the loss of radioactivity from the microspheres placed in microporous capillaries implanted subcutaneously into rats was determined. The measured leach criterion in vivo was 0.58% instead of 0.30% indicated by the manufacturer for in vitro conditions. The loss of 15-μm microspheres from the rat myocardium taking into account the leach criterion in vivo was 1.54 ± 1.64%.