Tight junctions restrict the paracellular movement of ions, solutes, drugs, and larger material across epithelia and endothelia. For practical purposes, the barrier can be modeled as having two components. The first is a system of small 4 � radius pores lined or created by claudins. The pores show variable ionic charge selectivity and electrical resistance based on the pattern of claudin proteins expressed in a particular junction. Transport of compounds that are larger than 4 � are not subject to discrimination based on size or charge; they are likely passing through transient breaks in the tight junction barrier. The magnitude of the first and second pathways varies among epithelia and is altered in response to physiological and pathological stimuli. Unfortunately, most studies of permeability use few tracer sizes and thus provide limited information on size-dependent changes in permeability. Here we describe a method for simultaneously measuring the size-dependence of apparent permeability using a continuous series of polyethylene polymers which allows quantification of both the pore and leak pathways.
