Extracellular fluid in individual tissues and in whole animals: the distribution of radiosulfate and radiobromide

Abstract

Radiosulfate, ³⁵SO₄, and radiobromide, ⁸²Br, were administered simultaneously to rats and dogs. In rats, the apparent volume of distribution of ⁸²Br averaged 30% of body weight and was constant between 0.5 and 35 hr after injection. The apparent volume of distribution of ³⁵SO₄, corrected for urinary loss, increased by 6% body weight/hr: the extrapolated volume at zero time was 88% of bromide space. Analysis of individual tissues and carcasses for ⁸²Br and inorganic ³⁵SO₄ showed that equilibration of both isotopes in several organs and in the whole carcass was rapidly achieved within 1 to 2 hr: no further increase in measured spaces occurred in 24 hr. The carcass inorganic sulfate space was 92%±2% of the bromide space in intact rats, and showed no increase with time. However, a progressively greater fraction of the injected ³⁵SO₄ was not recovered, owing to metabolic alteration. In eviscerated rats, the inorganic sulfate space was a smaller and much more constant fraction (79.8% ±0.4%) of the bromide space, showing that at least 20% of body bromide (and hence chloride) is nonextracellular. The viscera chiefly responsible for the higher ratio of spaces in the intact animal were the liver, small bowel, and kidney. In the last two organs, excess inorganic ³⁵SO₄ (beyond the bromide space) was attributable to trapped transcellular fluid in which sulfate had been concentrated more than chloride (or bromide). Excess sulfate in liver and cartilage could not be explained in this manner: the results suggest passive binding of sulfate, but could reflect active cell uptake in these tissues. No excess sulfate was found in skin or tail. The implications of these observations with respect to the distribution of body chloride and the measurement of extracellular space are discussed. The extracellular volume of the rat is estimated to be 24% of body weight.