Use of flexible polymers as probes of glomerular pore size

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Abstract
The excretion of different polyethylene glycol and dextran fractions with a narrow MW range was compared to that of inulin in the anesthetized rat. The clearance of polyethylene glycol with MW up to 4000 was identical to that of inulin. The MW distribution of inulin in rat plasma and urine was identical, indicating no restriction to ultrafiltration of inulin in the rat. The ultrafiltrability of dextran declined from 100 towards 0% as Stokes radius was increased from 18-46 .ANG.. The decline of ultrafiltrability of polyethylene glycol occurred at a lower range of Stokes radii than for dextran. Furthermore, dextran enters a Sephadex G 200 gel to a higher extent than does polyethylene glycol of the same Stokes radius. The dependence of Stokes radius and intrinsic viscosity on MW indicates that polyethylene glycol is more expanded in aqueous solution than is dextran. This difference is attributed to chain branching of the latter polymer. Flexible polymers, probably because of steric hindrance, permeate through waterfilled pores to a lesser extent than compact molecules of similar Stokes radius. Assuming that the apparent molecular radii for steric hindrance of flexible polymers in glomerular membrane pores are determined by their penetration into Sephadex G-200 columns calibrated with globular proteins, the glomerular sieving data of the present paper suggest an effective channel half-width for the rat glomerulus of 55 .ANG.