Physiological, metabolic, and histological responses of juvenile Schizothorax grahami to acute nitrite stress

Abstract

Nitrite is a prevalent environmental stressor in aquaculture, adversely impacting fish health and survival. This study investigated the effects of acute nitrite exposure (0, 20, 40, 60, 80 mg/L for 96 h) on juvenile Schizothorax grahami, specifically assessing survival, histological alterations, biochemical parameters, and the expression of related genes and proteins. The 96-h LC₅₀ was determined to be 42.3 mg/L, indicating a moderate sensitivity to nitrite. Histological analysis revealed dose-dependent gill damage, such as shortened filaments and epithelial oedema. Meanwhile, intestinal villi width and goblet cell density increased adaptively at moderate nitrite levels but exhibited mild disruption at higher concentrations. Blood analysis revealed elevated serum chloride levels at lower nitrite concentrations, indicating disrupted ionic homeostasis. Antioxidant activity (SOD) remained stable at lower nitrite concentrations but declined significantly at 80 mg/L, accompanied by a concurrent rise in malondialdehyde (MDA) levels, suggesting oxidative damage under high-concentration nitrite stress. Western blot analysis demonstrated upregulation of inflammatory cytokines (TNF-α, IL-1β) and the autophagy marker LC3b under high nitrite levels. Nitrite exposure altered hepatic glucose metabolism, characterized by increased g6pc and decreased gk expression. Lipid metabolism shifted toward oxidation (upregulation of pparaa and cebpa) and reduced biosynthesis (fas). These findings demonstrate that acute nitrite stress disrupts metabolic and physiological processes, underscoring the necessity of effective management strategies to mitigate nitrite toxicity in aquaculture.