Determination of the extracellular fluid volume of a euryhaline flounder by kinetic and net retention methods using tritium-labeled inulin

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Abstract
The extracellular space of 10 euryhaline southern flounder, Paralichthys lethostigma, was determined with methoxy-H3 inulin. After a single intravenous injection, plasma inulin activity declined in a time-dependent, biphasic manner which could be resolved into two separate rate functions. An initial rapid phase of inulin distribution having an average half-value time of 1.87 h was followed by a slow exponential decline having a half-value time averaging 43.3 h in freshwater-adapted flounder and 119.3 h in marine flounder. The kinetic and net retention space estimates were in close agreement, averaging 15.2% of body weight for freshwater flounder and 16.85% for marine flounder. These figures are consistent with sucrose space estimates of 14.0% for freshwater teleosts and 15.4% for marine teleosts, reported by Thorson (1961). The methoxy-H3 inulin preparation used was shown to be highly stable and to remain unbound in the circulation. However, it penetrated gradually into a non-exchangeable space in kidney tissue. This undesirable characteristic does not appear to contribute significant error to the extracellular fluid volume (ECFV) estimate.

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