Determination of microvessel permeability and tissue diffusion coefficient of solutes by laser scanning confocal microscopy

Interstitium contains a matrix of fibrous molecules that creates considerable resistance to water and solutes in series with the microvessel wall. On the basis of our preliminary studies (Adamson et al., 1994, Microcirculation 1(4), pp. 251-265; Fu et al., 1995 Am. J. Physiol 269(38), pp. H2124-H2140), by using laser-scanning confocal microscopy and a theoretical model for interstitial transport, we determined both microvessel solute permeability (P) and solute tissue diffusion coefficient (D_t) of α-lactalbumin (Stokes radius 2.01 nm) from the rate of tissue solute accumulation and the radial concentration gradient around individually perfused microvessel in frog mesentery, P^{α-lactalbumin} is 1.7 ± 0.7(SD) × 10^-6 cm/s (n = 6). D_t/D_free for α-lactalbumin is 27% ± 5% (SD) (n = 6). This value of D _t/D_free is comparable to that for small solute sodium fluorescein (Stokes radius 0.45 nm), while P^{α-lactalbumin} is only 3.4% of P^{sodium flourescein}. Our results suggest that frog mesenteric tissue is much less selective to solutes than the microvessel wall.