Tissue and Whole-Body Extracellular, Red Blood Cell and Albumin Spaces in the Rainbow Trout as a Function of Time: A Reappraisal of the Volume of the Secondary Circulation

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Abstract
[58Co]EDTA, [51Cr]RBC and [125I]albumin spaces in the whole body and 28 tissue samples were examined at timed intervals over 16 h in rainbow trout Oncorhynchus mykiss. [58Co]EDTA space (which approximates extracellular fluid volume; ECF) in fins, skin, gallbladder and eye are reported for the first time. After a 16 h equilibration, ECF volume was large (376–726 µl g−1 wet tissue mass) in kidney, swimbladder, skin and fins, moderate (219–313 µl g−1 wet tissue mass) in stomach, skull, spleen, liver, intestine, gills, eye and cecum, and small (53–181 µl g−1 wet tissue mass) in red muscle, fat, brain, gallbladder and white muscle. Whole-body ECF was 387±10.6 µl g−1 (mean ± S.E.M.; N=11). [51Cr]RBC space relative to [58Co]EDTA space was large in spleen, liver, intestine and gill, and low in skin, fins, stomach and skull. Whole-body [51Cr]RBC space was 9.9±0.6 µl g−1 body mass (N=17). Blood volume calculated from [51Cr]RBC space at 16 h and a dorsal aortic hematocrit of 24.5 % was 40.4 µl g−1 body mass. Whole-body [125I]albumin space at 16 h was 118.0±7.4 µl g−1 body mass (N=6), which resulted in an estimated blood volume of 156.6 µl g−1 body mass, nearly four times that estimated from the [51Cr]RBC space. Tissue hematocrits, calculated from [125I]albumin and [51Cr]RBC spaces, were significantly lower than dorsal aortic hematocrit in all tissues except spleen, kidney and liver. [58Co]EDTA and [51Cr]RBC spaces reached equilibrium in nearly all tissues within 1 h, whereas [125I]albumin continued to accumulate in many tissues up 24 h. The disparity between [125I]albumin distribution kinetics compared with the kinetics of [58Co]EDTA and [51Cr]RBC distribution, as well as the accumulation of [125I]albumin in tissues not known to have a secondary circulation, indicates that [125I]albumin is a poor marker of plasma volume in trout and that previous studies based on [125I]albumin clearance from the plasma have overestimated both the volume and the turnover rate of the secondary system. Revised estimates of secondary circulation volume, based on [58Co]EDTA distribution rate, indicate that it is no more than 10–20 % of the volume of the primary circulation.

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