ONE FIGURE
For some time the double exponential curve has been accepted as describing the diffusion of radioactive molecules out of or into tissue slices in vitro. Levi and T'ssing ('48) appear to have been the earliest, to rcport such curves, using radioactive sodium and the isolated frog sartorius. A number of investigators (Harris and Burn, '49; Wesson e t al., '49 ; Abelson and Duryee, '49 ; Etlelman, '52 ; Mudge, '53 ; Shanes, '54; Shanes and Berman, '53 and '55; Demis and Rothstein, '55 ; Garoutte and Aird, '55) have confirmed the form of these diffusion curves, using different tissues and different tracer materials.
Many of the workers mentioned have concluded that double exponential diffusion curves arise partially OF entirely because of different rates of diffusion from extracellular and intracellular fluid spaces, although some have indicated the likelihood of ion binding as an additional or alternative mechanism. Harris and Burn ( '49) and Shanes and Berman ( '55) also mentioned the probable importance of diffusion processes in se. We have recently ( ' 5 5 ) published a brief report containing strong presumptive evidence that such curves are not related to tissue compartmentation or to ion binding, at least under the experimental conditions described. The present paper is an extension of this thesis to additional tracer materials, with additional experimental confirmation.