Influence of perfusate oncotic pressure on the transcapillary clearance of albumin in maximally vasodilated rat skeletal muscle

An external detection technique was developed for repetitive and reliable measurements of clearance (Cl) of 99mTc-albumin during alterations of serum colloid osmotic pressure (pi p). Isolated, maximally vasodilated rat hindquarters were perfused with serum of two pi ps (20 mmHg and 45 mmHg) at four or five different filtration rates (Fv) for each pi p in each animal. The recorded accumulation rates of 99mTc-albumin (AR) were converted into dimensions of albumin clearance, setting the isogravimetric Cl at normal vascular pressures (pi p = 20 mmHg) at 0.0246 +/- 0.0012 ml min-1 per 100 g, which was obtained in defined muscle samples in 11 separate experiments. Serum perfusion with higher colloid osmotic pressure (45 mmHg) shifted the albumin clearance values upwards, without affecting the slope of the Cl vs. Fv relationship. Thus, the reflection coefficient (sigma) for albumin did not seem to be affected by the changes in pi p, while the isogravimetric albumin clearance was increased to roughly 0.058 ml min-1 per 100 g. Explicit two-pore equations were found to describe the experimental data fairly well, yielding an average sigma for albumin of 0.92 and a minor contribution of diffusion to overall transport even at low Fvs. Moreover, a coupling of macromolecular clearance to pi p may serve to minimize alterations in plasma protein concentration.