The Osmotically Functional Water Content of the Human Erythrocyte

Experiments were directed toward estimation of the magnitude of error incurred by the presumption of idealized osmometric behavior in the author's recent studies of monosaccharide transport through the human erythrocyte membrane. Thick suspensions of washed cells in isotonic buffered balanced salt medium were mixed in fixed proportions with varying dilutions of a concentrate of either (a) the mixed chlorides of the medium, or (b) glucose in the isotonic medium, and the resultant freezing point and hematocrit values determined. The form of the responses in the tonicity and the cell volume, as functions of the variable dilution of sugar or salts, conformed consistently with relations derived from the classical van't Hoff-Boyle-Mariotte pressure-volume relation. However, the effective cell water contents appeared substantially less than the weight lost in conventional drying, and varied somewhat according to the index used: expressed as grams of H₂O per milliliter of cells at isotonic volume, the cell water implied by the hematocrit behavior was 0.614 ± 0.015 (SD); by the salt tonicity response, 0.565 ± 0.027; by the immediate glucose tonicity response, 0.562 ± 0.044; and by the equilibrium glucose tonicities, 0.589 ± 0.043. Olmstead's reports of gross deviation from the van't Hoff relation, in the rabbit red cell's responses to tonicity displacement, are attributed primarily to a systematic aberration in his method of data analysis, the observations themselves agreeing substantially with the present findings.