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

[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 microl g-1 wet tissue mass) in kidney, swimbladder, skin and fins, moderate (219-313 microl g-1 wet tissue mass) in stomach, skull, spleen, liver, intestine, gills, eye and cecum, and small (53-181 microl g-1 wet tissue mass) in red muscle, fat, brain, gallbladder and white muscle. Whole-body ECF was 387+/-10.6 microl 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 microl 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 microl g-1 body mass. Whole-body [125I]albumin space at 16 h was 118.0+/-7.4 microl g-1 body mass (N=6), which resulted in an estimated blood volume of 156. 6 microl 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.