EFFECT OF CONCENTRATION ON ALBUMIN DIFFUSION IN LUNG INTERSTITIUM

The transport of macromolecules through the lung interstitium depends on both bulk transport of fluid and diffusion. In the present study, w e studied the diffusion of albumin. Isolated rabbit lungs were inflate d with silicon rubber via airways and blood vessels, and two chambers were bonded to the sides of a 0.5-cm-thick slab that enclosed a vessel with an intersititial cuff. One chamber was filled with either albumi n solution (2 or 5 g/dl) containing tracer I-125-albumin or with trace r I-125-albumin alone; the other was filled with Ringer solution. Unbo und I-125 was removed from the tracer by dialysis before use. The cham ber with Ringer solution was placed in the well of a NaI(Tl) scintilla tion detector. Diffusion of tracer through the interstitium was measur ed continuously for 60 h. Tracer mass (M) showed a time (t) delay foll owed by an increase to a steady-state flow (dM/dt constant). Albumin d iffusion co-efficient (D) was given by L-2/(6T), where T was the time intercept of the steady-state M-t line at zero M, and L was interstiti al length. Interstitial cuff thickness-to-vessel radius ratio (Th-o/R) was estimated by using Fick's law for steady-state diffusion. Both D and Th-o/R were independent of albumin concentration. D averaged 6.6 x 10(-7) cm(2)/s, similar to the free D for albumin. Values of Th-o/R a veraged 0.047 +/- 0.024 (SD), near the values measured histologically. Thus pulmonary interstitial constituents offered no restriction to th e diffusion of albumin.