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.
Subsequent experiments have shown that the volume of incubation medium adhering to the surface after washing is only ≃; 1 μL (average 300 mg animal). Part of the [*H]inulin associated with the animal in the present experiment was possibly trapped in the cuticle (see Fig. 12, Plate 1), since a washing period of 10 sec, due to the low diffusivity of the inulin molecule, may be insufficient to remove all [3H]-inulin present after 30 min incubation.
Ferguson (1967, 1968, 1970) and Little & Gupta (1969) have provided autoradiographic evidence for a preferential incorporation of labelled material into the epidermis subsequent to incubation of some marine invertebrates in a medium containing radioactive DOM. These observations are compatible with the present results on Nereis.
