[Diffusion of fluorescein molecules in muscle fiber. I. The determination of the coefficient of diffusion].

Two methods for measuring the diffusion coefficient of fluorescein molecules (molecular weight 376 D) micropipetted to frog muscle fibre are discussed. The first (photoelectric) method implies that the fluorescein intensity is determined at a known distance from the site of injection using a photometric probe. The time dependence of this intensity has a maximum. If to apply the equation of heat conductance in a cylinder with impermeable walls on supply of point heat impulse, a simple formula is obtained for calculating the diffusion coefficient. The second method consists in photographing a stained portion of the fibre in fluorescence light. After densitometry of the film, the diffusion coefficient is calculated from the Gaussian equation. As demonstrated by the measurements, the diffusion coefficient values for fluorescein are three-fold lower than for water.