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

[Co-58]EDTA, [Cr-51]RBC and [I-125]albumin spaces in the whole body an d 28 tissue samples were examined at timed intervals over 16 h in rain bow trout Oncorhynchus mykiss. [Co-58]EDTA space (which approximates e xtracellular fluid volume; ECF) in fins, skin, gallbladder and eye are reported for the first time. After a 16 h equilibration, ECF volume w as large (376-726 mu l g(-1) wet tissue mass) in kidney, swimbladder, skin and fins, moderate (219-313 mu l g(-1) wet tissue mass) in stomac h, skull, spleen, liver, intestine, gills, eye and cecum, and small (5 3-181 mu l g(-1) wet tissue mass) in red muscle, fat, brain, gallbladd er and white muscle. Whole-body ECF was 387+/-10.6 mu l g(-1) (mean +/ - S.E.M.; N=11). [Cr-51]RBC space relative to [Co-58]EDTA space was la rge in spleen, liver, intestine and gill, and low in skin, fins, stoma ch and skull. Whole-body [Cr-51]RBC space was 9.9+/-0.6 mu l g(-1) bod y mass (N=17). Blood volume calculated from [Cr-51]RBC space at 16 h a nd a dorsal aortic hematocrit of 24.5% was 40.4 mu l g(-1) body mass. Whole-body [I-125]albumin space at 16 h was 118.0+/-7.4 mu l g(-1) bod y mass (N=6), which resulted in an estimated blood volume of 156.6 mu l g(-1) body mass, nearly four times that estimated from the [Cr-51]RB C space. Tissue hematocrits, calculated from [I-125]albumin and [Cr-51 ]RBC spaces, were significantly lower than dorsal aortic hematocrit in all tissues except spleen, kidney and liver. [Co-58]EDTA and [Cr-51]R BC spaces reached equilibrium in nearly all tissues within Ih, whereas [I-125]albumin continued to accumulate in many tissues up 24 h. The d isparity between [I-125]albumin distribution kinetics compared with th e kinetics of [Co-58]EDTA and [Cr-51]RBC distribution, as well as the accumulation of [I-125]albumin in tissues not known to have a secondar y circulation, indicates that [I-125]albumin is a poor marker of plasm a volume in trout and that previous studies based on [I-125]albumin cl earance from the plasma have overestimated both the volume and the tur nover rate of the secondary system. Revised estimates of secondary cir culation volume, based on [Co-58]EDTA distribution rate, indicate that it is no more than 10-20% of the volume of the primary circulation.