Extracellular fluid volume measurements in tissues of the rainbow trout (Oncorhynchus mykiss)in vivo and their effects on intracellular pH and ion calculations

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Abstract
Extracellular fluid volume (ECFV) estimates were determined in various tissues and whole body of resting, chronically cannulated rainbow trout (Oncorhynchus mykiss). Fish were infused with14C-inulin,3H-polyethylene glycol (PEG, M.W. 4000),3H-mannitol, or14C-mannitol, and values of ECFV determined from tissue: plasma water distribution ratios after 6 h or 13 h equilibration. Overall,3H-PEG provided the most conservative and reliable estimates after 13 h equilibration, with ECFV values in the order: brain < white muscle < red muscle < liver < heart < gill tissue.14C-inulin yielded generally similar values to3H-PEG at 13 h, but probably overestimated ECFV in liver.3H-mannitol and14C-mannitol spaces were similar to each other and far greater than3H-PEG or14C-inulin values in most tissues.3H-mannitol values increased significantly between 6 h and 13 h, in contrast to14C-inulin. Mannitol clearly overestimated ECFV in liver and gill, and probably also heart and whole body, but may have provided more realistic estimates in brain due to better penetration of the blood-brain barrier. The Cl−/K+ space technique overestimated ECFV in gills, but was satisfactory in white muscle. Measurements and model calculations evaluated sources of error in intracellular pH (by14C-DMO) and ion determinations. Trapped red cells in the gills have negligible influence. Errors in ECFV are much more influential in a tissue with a high ECFV (gills) than a low ECFV (white muscle).3H-PEG is the marker of choice for intracellular pH determinations. However, even when3H-PEG is used, the potential for absolute errors in intracellular ion concentrations remains high.

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