Permeability studies with exogenous macromolecules: problems arising from the use of iodinated PVP and dextrans

The permeability of the glomerular filtration barrier with respect to molecular size could best be studied by substances which are neither secreted nor reabsorbed and have a mean molecular weight of about 40,000--80,000 daltons. Uncharged molecules are preferred because the charge definitely influences the glomerular filtration. Several such substances are available; however, in most permeability studies labelled PVP, mainly 125I- or 131I-PVP, and dextrans have been applied. Gel filtration and binding studies with 125I-PVP have demonstrated that this substance should not be used because of unspecific binding of radioactive material. The application of dextran gives more reliable results. Disadvantages, however, include the large amounts which must be administered and some cases of intolerance, particularly in rats. Therefore, experiments with dextran seem feasible in some species but not in others. Ficoll, a synthetic polysucrose, is better tolerated than dextran but still causes allergic reactions in susceptible animals.