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Initial plasma disappearance and distribution volume of [131I]albumin and [125I]fibrinogen in man
1Steno Memorial Hospital, Copenhagen, Denmark.
The simultaneous plasma disappearance curves of albumin and fibrinogen were recorded in eight normal subjects from 10 to 60 min following intravenous injection. Additional samples were taken at 3, 4, 5, 6 and 7 min. The initial distribution volume (IDV) of albumin calculated by semilogarithmic extrapolation to zero time was 5.56% (range 3.73-8.53) larger than that of fibrinogen, denoting an initial high-rate function of albumin efflux extending from zero to about 10 min after tracer injection. The following slower phase of the albumin curve from 10 to 60 min was found to be similar to the so-called transcapillary escape rate (TER) of single-tracer experiments. By introducing the value Cp(0) (i.e. the estimated curve height at t = 0, from the injected amount of albumin tracer divided by the IDVf), the entire initial part of the albumin curves was analysed. From this analysis the mean value of 0.0135 +/- 0.0038 min-1 was determined for initial slope, corresponding to a whole-body unidirectional albumin efflux [j(0)] of 0.0572 +/- 0.0160 ml 100 g-1 min-1. The result is about 16 times higher than normal estimates of total lymphatic albumin return, indicating a huge backflux of interstitial albumin at the whole-body capillary level. Both phases of efflux seem to reflect uptake in a variety of peripheral tissues, and the hypothesis that the second phase (TER) expresses the initial slope of albumin escape into non-liver tissues is not substantiated. Based on the difference in IDV of the tracers demonstrated, the uncritical use of albumin as a plasma volume marker is not justified.
The simultaneous plasma disappearance curves of albumin and fibrinogen were recorded in eight normal subjects from 10 to 60 min following intravenous injection. Additional samples were taken at 3, 4, 5, 6 and 7 min. The initial distribution volume (IDV) of albumin calculated by semilogarithmic extrapolation to zero time was 5.56% (range 3.73-8.53) larger than that of fibrinogen, denoting an initial high-rate function of albumin efflux extending from zero to about 10 min after tracer injection. The following slower phase of the albumin curve from 10 to 60 min was found to be similar to the so-called transcapillary escape rate (TER) of single-tracer experiments. By introducing the value Cp(0) (i.e. the estimated curve height at t = 0, from the injected amount of albumin tracer divided by the IDVf), the entire initial part of the albumin curves was analysed. From this analysis the mean value of 0.0135 +/- 0.0038 min-1 was determined for initial slope, corresponding to a whole-body unidirectional albumin efflux [j(0)] of 0.0572 +/- 0.0160 ml 100 g-1 min-1. The result is about 16 times higher than normal estimates of total lymphatic albumin return, indicating a huge backflux of interstitial albumin at the whole-body capillary level. Both phases of efflux seem to reflect uptake in a variety of peripheral tissues, and the hypothesis that the second phase (TER) expresses the initial slope of albumin escape into non-liver tissues is not substantiated. Based on the difference in IDV of the tracers demonstrated, the uncritical use of albumin as a plasma volume marker is not justified.