The Development of Ion Regulation in Embryonic Rainbow Trout, Oncorhynchus mykiss

Abstract

This study investigated the regulation of Na⁺ Ca²⁺ Cl⁻ during development in embryonic rainbow trout (Oncorhynchus mykiss). Because there is a close relationship between pH regulation and ion uptake mechanisms in adult teleosts, pH, pCO₂, and NH₄⁺ levels in unstirred layers (USLs) adjacent to whole eggs and dechorionated embryos were determined using double-barrel ion-selective microelectrodes (ISMEs). Whole eggs accumulated Na⁺, Ca²⁺ and Cl⁻ during the last 20 days of embryonic development, suggesting an ionoregulatory ability prior to hatching. Na⁺ uptake by whole eggs was linearly related to external [Na⁺], suggesting that Na⁺ crosses the chorion by diffusion. The uptake by dechorionated embryos was saturable, indicating the presence of active transport or facilitated diffusion mechanisms on the surface of embryos prior to hatching. Ca²⁺ uptake by whole eggs and dechorionated embryos was saturable, suggesting that specific pathways or binding sites are present in the chorion, and that active transport or facilitated diffusion mechanisms are present at the surface of embryos. The much higher Jₘₐₓ for whole eggs than dechorionated embryos suggests a role of the perivitelline fluid (pvf) or chorion in ion uptake. Jₘₐₓ was lower in dechorionated embryos than in hatchlings suggesting that Ca²⁺ uptake mechanisms may not be fully developed in dechorionated embryos. Low pH and high [NH₄⁺] were measured in the USLs adjacent to whole eggs and dechorionated embryos, relative to the bulk water. A high Pcₒ₂ measured adjacent to the gills suggests that CO₂ excretion was the primary source of USL acidification. pH was lower in the USL adjacent to the gills and trunk than in the USL adjacent to the yolk sac. [NH₄⁺] was highest adjacent to the gills and trunk than adjacent to the yolk sac. There was no change in the extent of USL acidification or in [NH₄⁺] over the last half of embryonic development. After hatching, however, there was a significant increase in [NH₄⁺] adjacent to the gills, which was not accompanied by a change in USL pH. It is possible that a change in permeability of the transporting surfaces (i.e. gills, trunk, yolk sac) occurs after hatching.