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Exchange of 3HHO in intact isolated muscle fiber of the giant barnacle.

(1) Efflux CUN~S of 3HH0 from intact isolated muscle fiber of the giant barnacle were studied by a continuous washout technique. (2) The accuracy of the washout technique was analyzed by studying the efflu of ‘HHO from two models: a) microsacs of collodion and, b) cylinders of agar gel. In both cases the experimental efflux kinetics follow the predictions of the respective models: surface-limited diffusion for the microsacs of collodion and bulk phase-limited diffusion for the filaments of agar gel. (3) The experimental efflux curves of 3HH0 from isolated muscle fiber can be described by a sum of at least two exponential terms. Several models have been analyzed to explain the distribution and kinetics of the ‘HHO in this preparation. Those models which consider the muscle water as distributed in two compartments, extracellular and intracellular water, require that at least 27% (two compartments in parallel) or 46% (two compartments in series) of the total water has to be located in the extracellular space. This fraction of water exceeds the maximum extracellular water content (9%) estimated on the basis of extracellular space measurements by a large margin. (4) It has been found that a model of bulk phase-limited diffusion and desorption mechanism can describe the 3HH0 effluxes adequately. This model basically suggests that the rate-limiting step of ‘HHO exchange is not permeation through a surface barrier but diffusion through the bulk of the intracellular water. According to this model, the apparent diffusion coefficient for 3HH0 has been estimated to be 55% of that in free solution. (5) The bulk phase-limited diffusion and desorption kinetics is compatible with recent evidence indicating that the cell water is in a different physical state than in free solution.