Active and electrogenic absorption of Na + and Cl- across posterior gills of Eriocheir sinensis: influence of short-term osmotic variations

Split lamellae of posterior gills of Chinese crabs (Eriocheir sinensis) acclimated to fresh water were mounted in a modified Ussing-type chamber, and the transepithelial short-circuit current and conductance were measured. The epithelium shows independent active and electrogenic absorption mechanisms for Na+ and Cl- that can be measured as positive and negative short-circuit currents, respectively, in the absence of the counter ion. Increasing the osmolarity of the haemolymph-side saline by addition of sucrose resulted in a marked decrease in active uptake of both Na+ and Cl-. In contrast, increasing the internal osmolarity by addition of urea or moderately decreasing the haemolymph-side osmolarity resulted in a marked increase in Na+ as well as Cl- transport. Circuit analysis revealed that Na+ current changes are mostly due to alterations in the apical amiloride-sensitive Na+ conductance, while Cl- current changes are caused not only by alterations in the transcellular conductance but also by changes in the electromotive force for Cl- absorption. Osmotic perturbations in the external bath induced current changes in the same directions, but the magnitudes of the effects were smaller than those after internal osmotic variations, indicating that the external barrier has a lower water permeability than the internal barrier. Short-term osmotic perturbations did not significantly affect the leak conductance, which is not associated with active transport and which may mostly reflect the paracellular conductance.