Ammonia toxicity in medaka embryos involves ion depletion and internal organ apoptosis and its mitigation is by sodium-driven ammonia excretion.

Elevated environmental ammonia poses a severe threat to fish, yet the physiological mechanisms of toxicity and tolerance in early developmental stages remain incompletely understood. In this study, we investigated the toxicological effects of ammonia (NH4Cl) on medaka (Oryzias latipes) embryos and the protective role of environmental Na+. Subchronic exposure (7 days) showed high survival before hatching due to chorion protection, but mortality increased rapidly after hatching. High ammonia loads disrupted whole-body ionic homeostasis, depleting Na+, K+, and Ca2+, and significantly reduced the density of skin ionocytes. Immunofluorescence staining for activated caspase-3 demonstrated that ammonia-induced apoptosis was predominantly localized to the liver and gut, with minimal or undetectable signals in the gills and skin ionocytes. Acute exposure (24 h) established a concentration-dependent lethality, which was significantly ameliorated by supplementation with Na+, but not K+. To elucidate the underlying physiological mechanism, we measured real-time surface NH4+ concentration gradients at the skin surface under increasing ambient NH4+ loads. This approach allowed us to identify the "reversal concentration"-the critical ambient threshold where active efflux equals passive influx-as a quantitative indicator of the embryo's capacity to resist external NH4+ entry. Our results demonstrated that Na+ supplementation significantly elevated this threshold, enhancing excretion capacity. Conversely, pharmacological inhibition of Na+/H+ exchangers (NHEs) with EIPA drastically reduced it, even with sodium. These findings provide direct physiological evidence that medaka embryos rely on an Na+-dependent active excretion mechanism to resist ammonia toxicity, and highlight the gut-liver as a critical target of injury during embryonic stages.