Effects of prolactin and environmental calcium on osmotic water permeability of the gills in the eel, Anguilla japonica.

The effects of prolactin and environmental calcium on branchial osmotic water permeability were examined using gill arches isolated from freshwater-adapted eels (Anguilla japonica). The gill arches were incubated for 1 hr in deionized water (DW) or in various concentrations of electrolyte solutions and the rate of osmotic water influx was calculated from the weight change. Addition of 1.0 mM calcium to the incubation medium (DW) resulted in an almost instantaneous reduction in the rate of water influx, whereas removal of calcium caused an immediate increase. The increased plasma calcium level after removal of the corpuscles of Stannius or after CaCl2 infusion did not affect water influx. The water permeability in DW increased after hypophysectomy; injection of ovine prolactin (0.15 U/g daily for 3 days) restored permeability toward normal. On the other hand, the prolactin effect was masked in the presence of calcium ions; no effect of hypophysectomy on prolactin treatment was seen when the gills were incubated in 1 mM CaCl2, and the rates of water influx were always lower than those observed in DW. Thus prolactin and/or environmental calcium ions seem to keep the osmotic water influx of the freshwater eel gills at a low level.