Transport of Exogenous D-glucose by the Integument of a Polychaete Worm (Nereis Diversicolor Müller)

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Abstract
Integumentary exchange of radio-labelled D-glucose in the brackish-water polychaete worm Nereis diversicolor (Annelida; Polychaeta) was investigated. In animals acclimated to 50% sea water, the influx of D-glucose was measured, and shown to occur largely across the outward-facing membranes of the epidermal cells. Transfer of exogenous D-glucose across the outer membranes occurs by facilitated transfer, involving two different transport systems. One of these has a Kt (transport constant) of ⋍ 5 μM, i.e. of the order of magnitude of environmental D-glucose concentrations. The Kt for the other system is at least ten times higher. The D-glucose is rapidly metabolized by the epidermal cells. Small amounts of unmetabolized D-glucose are released to the extracellular fluid, and probably to the medium. The maximal D-glucose outflux through the apical border of the epidermal cells is at least 4 times smaller than the outflux through the baso-lateral border. A maximum value is given for the diffusion permeability of D-glucose through the intercellular spaces of the integument. Applying this figure, the loss by intercellular diffusion was found not to exceed the D-glucose influx into the epidermal cells.