Paracellular water uptake and molecular sieving by the foot epithelium of terrestrial slugs

A paracellular pathway in the foot epithelium of Lehmannia valentiana can be opened by dehydrating the slug. Movement of water from a wet pad through the opened pathway into the haemolymph of this terrestrial slug is rapid. The sieving properties of this paracellular pathway have been determined using the reference isotope 3HOH and various 14C-labelled solutes. Paracellular uptake of 14C-insulin (Fig. 1) and 3HOH (Fig. 2) is initial rate for at least 3 min. If the wet pad contains 1,000 cpm of 14C per ml of 3HOH, slugs absorb only about 400 cpm of 14C with each ml of 3HOH absorbed representing a sieving ratio of 0.4 for insulin. The sieving ratio of 14C-inulin does not change when the concentration is increased from 0.1 to 2.5 mmol/l. Moreover, the sieving ratio of 14C-inulin was not affected significantly by the nature of the labelling, i.e., 14C-carboxyl vs 14C-methoxy. Sieving ratios for 14C-mannitol (182 Da), 14C-polyethylene glycol (4,000 Da), and 14C-inulin (5,250 Da) were 0.92, 0.63, and 0.39, respectively (Table 1), indicating that sieving is dependent on molecular size. 14C-Dextran (70,000 Da) and blue dextran (200,000 Da) were excluded from the paracellular pathway (Fig. 4). The effective pore size of the paracellular pathway was estimated using the relationships between sieving ratio and molecular weight of 3HOH and the various solutes that can pass through the pathway. The extrapolated pore size is equivalent to that of a sieve having a molecular weight cutoff of about 10,000 Da (Fig 3).