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 t he most active chloride cells (CCs), partly compensated for by an increasin g 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 sur face area of CCs that paralleled complete recovery of the maximal Cl- influ x 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 pav ement cells on the lamellar epithelium showed a greater density of microvil li during exposure to alkaline water, which may have contributed to partial restoration of the number of Na+ transport sites (J(max)(Na)). Finally, th e 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.