Impact of Nitrite on the Molecular Physiology of the Guppy (Poecilia Reticulata Peters, 1859) Gill Epithelium Tight Junction Complex

Abstract

Nitrite (NO2-) levels in natural freshwater (FW) are typically low, but anthropogenic activity can elevate them, impairing osmoregulatory mechanisms in FW teleost gills. While transcellular disruption by NO2- is documented, its impact on paracellular osmoregulation via the gill tight junction (TJ) complex remains unexplored. This study examined how sublethal waterborne NO2- exposure (7 days) affects osmoregulation in FW guppies (Poecilia reticulata), focusing on the molecular physiology of the gill TJ complex. Gills were analyzed for TJ protein gene transcript abundance, Na+,K+-ATPase activity, and ultrastructural changes. Elevated NO2- reduced carcass Na+ and Cl- levels and gill Na+,K+-ATPase activity, indicating osmoregulatory imbalance. TJ gene transcript abundance was unaffected except claudin-10e, which decreased with NO2-, suggesting reduced mitochondria-rich cells (MRCs). Ultrastructural changes in MRCs revealed transcellular dysfunction (e.g., fewer mitochondria, tubular vacuolization) and compromised paracellular integrity (e.g., shallow junctions). Findings concur that NO2- primarily disrupts osmoregulation through transcellular pathways, with limited paracellular effects.