The variation with flow-rate of the extraction of bone-seeking tracers in recirculation experiments

Radioactive microspheres were injected into the hearts of anesthetized dogs and the reference organ technique used to determine cardiac output. Blood flow to the leg bones was then obtained from microsphere uptake. Simultaneous injections of the bone-seeking radiotracers 85Sr and, in fewer animals, 18F and 99mTc methylene diphosphonate similarly gave their clearances from the blood by bone over a period of 5 min. The ratio of clearance to flow (extraction ratio) was found to be flow-dependent, falling from near unity at low flow rates to about 0.4 at higher flow rates. The results are in general accordance with the Renkin theory of capillary permeability but do not fit exactly to the concept of a constant product of permeability and surface area, perhaps due to the heterogeneity of the model. The findings confirm conclusions based on tibial nutrient artery perfusion experiments and demonstrate that skeletal blood flow cannot be measured by the clearance of bone-seeking tracers. There were substantial differences of blood flow normalized for weight between the bones of a given animal.