Extravascular albumin in bone tissue.

1. The amount of albumin in extravascular tissue fluid in bone, kidney, intestine, skin and muscle and in plasma of young rabbits has been measured by radial immunodiffusion. 2. The majority of extravascular albumin in kidney, intestine, skin and muscle is exchangeable with plasma albumin, whereas in bone, only the proportion which is in tissue fluid is readily exchangeable; the remaining fraction in calcified matrix is more permanently fixed. 3. About 27% of the albumin in young bone is in tissue fluid, about 57% in calcified matrix and about 16% is intravascular. The total amount of extravascular albumin per unit mass of bone is similar to that found in soft tissues. 4. The volume of intravascular plasma in tissues was determined in two ways: from 51Cr-erythrocyte radioactivity and the venous haematocrit and from the '5 min 125I-fibrinogen space'. 5. The rate of egress of albumin from blood vessels has been estimated from the initial slope of the ratio of extravascular radioactivity in the tissue to plasma radioactivity plotted against time after injection of 125I-albumin. 6. The rate of clearance of the albumin in extravascular tissue fluid in bone is approximately once every hour. This is more rapid than in skin and muscle, comparable with intestine and less rapid than in kidney. 7. The amount of albumin incorporated into calcified matrix of bone per day is calculated to be less than 0-5% of the total albumin passing through the tissue fluid of bone per day.