Validation of [ 57Co]cyanocobalamin as an extracellular fluid marker and measurement of albumin exclusion from the interstitium in the rabbit

Abstract

The use of [ 57Co]cyanocobalamin as an extracellular marker has been validated. Its simultaneous use with 51Cr-labeled erythrocytes and 125I-human serum albumin allows quantitative measurement of tissue water and the fractional exclusion of albumin from the interstitial compartment. The extravascular extracellular spaces of rabbit tissue are lung, 32 ± 4%; heart, 25 ± 2%; gut, 24 ± 4%; and muscle, 13 ± 1%. The fractional excluded albumin spaces are lung, 0.78 ± 0.05; heart, 0.37 ± 0.03; gut, 0.69 ± 0.05; and muscle 0.58 ± 0.03. The oversimplification that the extravascular extracellular space is a homogeneous region defined by a small tracer molecule and has the composition of lymph must be reconsidered. Albumin has a vastly different distribution in the interstitial spaces of different organs. Use of the three tracers reported here allows efficient measurement of blood volume, interstitial space, and interstitial albumin exclusion in whole tissues and organs, and thus can lead to further understanding of the in vivo changes occurring in various physiologic and disease states.