Transport of charged macromolecules across a biological charged membrane.

The glomerular capillary wall of the kidney behaves as an electronegatively charged structure consisting of three layers, the lamina densa and the two laminae rarae, which are differently charged. Thus, a three layer model is proposed to analyse the transport of charged macromolecules across this wall. A modified Nernst-Planck equation describes the macromolecule flux across the wall and a Donnan equilibrium is assumed at each interface. For a given value of the fixed charge concentration in each layer, the local sieving coefficient of the macromolecule, i.e. the ratio between the concentrations in the filtrate and in the plasma, is calculated. A sieving curve which relates the sieving coefficient to the Einstein-Stokes radius of the macrosolute is obtained. The fixed charge concentrations in each layer are iteratively modified until simultaneous adjustment is achieved between calculated and experimental curves, for positively and negatively charged tracers and their neutral equivalent.