DIFFUSION OF ALBUMINS IN RAT CORTICAL SLICES AND RELEVANCE TO VOLUME TRANSMISSION

The apparent diffusion coefficient, D, was measured in rat cortical s lices and compared to the free diffusion coefficient, D, for three neg atively charged proteins, lactalbumin (mol. wt=14,500), ovalbumin (45, 000) and bovine serum albumin (66,000). The temporal evolution of the spatial distribution of albumin molecules labeled with the Texas Red f luorophore was determined using integrative optical imaging at interva ls after a brief pressure injection from a micropipette in slices of a dult rat cerebral cortex and dilute agarose gel. Diffusion coefficient s were obtained by fitting appropriate equations to the data. In slice s at 34 degrees C, the values of D (10(-7) cm(2)/s, mean+/-S.E.M.) fo r lactalbumin, ovalbumin and bovine serum albumin were 2.37+/-0.10, 1. 60+/-0.08 and 1.63+/-0.07, respectively. In agarose gel, values of D ( 10(-7) cm(2)/s) were 11.87+/-0.20, 10.02+/-0.25 and 8.29+/-0.17, respe ctively. From these data the tortuosity factors, (D/D)(0.5), were cal culated, with 2.24 obtained for lactalbumin, 2.50 for ovalbumin and 2. 26 for bovine serum albumin. Previous optical measurements using dextr ans with mol. wts of 40,000 and 70,000 gave tortuosities of 2.16 and 2 .25, but in contrast previous determinations with ion-selective microe lectrodes using the small cation tetramethylammonium (mol. wt=74.1) gi ve tortuosities of about 1.6. The results show that proteins as large as bovine serum albumin diffuse through brain extracellular space but are more hindered than smaller molecules. A simple model compared the differences in diffusion properties of bovine serum albumin, dopamine and nitric oxide in brain tissue and discussed the implications for vo lume transmission of chemical information between cells. The results a re also relevant to the behavior of diffusible factors in brain develo pment and the delivery of therapeutic agents. Copyright (C) 1996 IBRO.