ABSTRACT
Mucus from the footsole of the freshwater snail Lymnaea stagnate behaves as a weak, negatively charged ion exchanger. Activities and concentrations of Na, K, Ca, and Cl were measured in mucus dialysed to equilibrium against artificial pond water or physiological saline. Observed activity coefficients (activity/concentration) in mucus were compared with those predicted by the Debye-Huckel theory to interpret the effects of electrostatic forces between the polyelectrolyte ions and small ions. The affinity of mucus for small ions decreased in the series, Ca2+, K+, Na+, Cl−.
The extent to which mucus can concentrate cations was measured using three different methods: by titrating the fixed acidic groups with K or Ca and by equilibrium dialysis after which the electrical potential difference was either measured directly or was calculated from the Nernst potential for Na. Ion exchange titration indicated a much smaller exchange capacity than did the other two methods.
Kinetics of cation uptake by the snail from dilute media were re-interpreted by considering the enhanced concentrations of cations in the mucus layer. It was shown that the presence of mucus in the unstirred layer adjacent to a transporting epithelium can result in an underestimate of the Michaelis constant (Km) determined from influx measurements.
