The effect of highly alkaline water (pH 9.5) on the morphology and morphometry of chloride cells and pavement cells in the gills of the freshwater rainbow trout: Relationship to ionic transport and ammonia excretion

Exposure of rainbow trout (Oncorhynchus mykiss) to alkaline water (pH 9.5) impairs ammonia excretion (J(Amm)) and gill-mediated ion-exchange processes, as characterized by decreased Cl^- (J(in)/(Cl)) and Na⁺ influx (J(in)/(Na)) across the gill. Scanning electron microscopy suggested that the depression of J(in)/(Cl) was concomitant with an early decrease in the population of the most active chloride cells (CCs), partly compensated for by an increasing number of immature CCs. However, within 72 h after the onset of exposure to alkaline water, there was a 2-fold increase in the fractional apical surface area of CCs that paralleled complete recovery of the maximal Cl^- influx rate (J(max)/(Cl)). These results suggest that recovery of J(max)/(Cl) was associated with greater CC surface area, resulting in more transport sites on the gill epithelium. Morphometric analysis of the outermost layer of pavement cells on the lamellar epithelium showed a greater density of microvilli during exposure to alkaline water, which may have contributed to partial restoration of the number of Na⁺ transport sites (J(max)/(Na)). Finally, the blood-to-water gill-diffusion distance decreased by 27% after 72 h at pH 9.5, and likely contributed to progressive restoration of ammonia excretion in alkaline water.