Recycling of D-glucose in collagenous cuticle: A means of nutrient conservation?

Transport by an epithelium, possessing an accumulating saturable transport system in the apical membrane as well as a finite Fick permeability to the transported solute, was considered in the steady state in the case of zero cis concentration, and in the presence of a peripheral diffusion resistance in a layer apposing the cis face of the tissue (unstirred solution or structural coating). Under suitable conditions, the combination of peripheral diffusion resistance and accumulating epithelium transport may lead to recycling of solute at the cis face of the epithelium. This causes a decrease of the effective permeability to diffusional trans-cis flow across the tissue. The phenomenon is discussed in terms of epidermal D-glucose transport by the integument of aquatic animals with a collagenous cuticle, such as the seawater-acclimated polychaete worm Nereis diversicolor. The recycling phenomenon may be of significance to other epithelia with the function of maintaining large concentration gradients of permeating substances.