Effect of concentration on restriction and diffusion of albumin in the excised rat diaphragm

In tissue samples of rat diaphragm mounted between two chambers, we measured the flow of albumin solution (0-5 g/dl) containing radioactive tracer (125)I-albumin in response to a driving pressure of 20 cmH(2)O. The ratio of the albumin concentration of the output solution to that of the input (sieving ratio, C(out)/C(in)) was measured from solution radioactivity. C(out)/C(in) increased monotonically from 0.5 with the flow of approximately 0 g/dl albumin solution (tracer) to 0.9 with the flow of 5 g/dl albumin solution. We modeled the tissue as a membrane subjected to flows of high Peclet No. with a reflection coefficient sigma = 1 - C(out)/C(in). Values of sigma decreased from 0.5 with Ringer solution to 0.1 with 5 g/dl albumin solution. Hydraulic conductivity measured with the flow of Ringer solution increased with the flow of 5 g/dl albumin solution. Wet-to-dry weight ratio and radioactivity of tissue samples immersed in 0.01-5 g/dl albumin solutions indicated a 40% increase in tissue water, associated with an albumin volume fraction of 0.3 measured at 0.5-2 h. The slower rate of albumin uptake occurring up to 20-30 h indicated intracellular diffusion that was equal with 1 and 5 g/dl albumin solution but reduced with a 0.01 g/dl albumin solution. The results suggest that interstitial pores increase in size in response to an increase in albumin concentration. We postulate a two-pore model made of intracellular pores that coalesce into a set of larger pores by osmotic flow.