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

The loss of 15-micron 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-micron microspheres from the rat myocardium taking into account the leach criterion in vivo was 1.54 +/- 1.64%.