Determination of blood volume in dogs using an enriched stable isotope 50Cr

For the measurement of canine blood volume, various experimental conditions and techniques have been investigated using a non radioactive stable isotope 50Cr. On the basis of the results in this preliminary work, erythrocytes were labeled using 50Cr. Five micrograms of 50Cr per 1 ml of blood was added and incubated for 60 min. The canine erythrocytes were tagged using 50Cr and injected into vein of the same dogs. The blood samples collected at 60 min after the injection were irradiated by thermal neutron for 20 min at the reactor of the JAERI. Activities of 51Cr (the 50Cr concentration method) and 51Cr/59Fe radioactivity ratios (the 51Cr/59Fe ratio method) in the samples were measured. There was a very high correlation (r = 0.97, P less than 0.001) between the blood volumes calculated by the 50Cr concentration method and the 51Cr/59Fe ratio method. The latter method is less complicated than the former, because measurement of the sample weight and correction of thermal neutron flux are unnecessary. The mean blood volumes calculated by the ratio method and the Evans blue method were 89.8 +/- 6.8 ml/kg B.W. (mean +/- SD) and 98.9 +/- 10.6 ml/kg, respectively, showing a significant difference between them (P less than 0.05). However, these values are almost in accord by correction of venous blood PCV values with factor 0.97. As a detection limit of 50Cr was approximately 0.1 ng per 1 ml of blood in this system, this method has been concluded to be applicable to the measurement of the blood volume of cattle.