Summary The effect of cortisol on calcium (Ca2 ) transport across cultured rainbow trout gill epithelia composed of both pavement cells (PVCs) and mitochondria-rich cells (MRCs) was examined. Under symmetrical culture conditions (L15 media apical/L15 media basolateral), cortisol had subtle effects on gill epithelial preparations. Both control and cortisol treated epithelia exhibited Ca2 influx and efflux rates (measured radioisotopically using 45Ca) that were approximately balanced, with a slight inwardly directed net Ca2 flux. Ussing flux ratio analysis indicated active Ca2 transport in the inward direction across epithelia bathed symmetrically regardless of hormone treatment. In contrast, under asymmetrical conditions (freshwater apical/L15 media basolateral) control epithelia exhibited active Ca2 transport in the outward direction (basolateral to apical) throughout experiments conducted over a 24-h period, whereas cortisol-treated preparations exhibited active transport in the inward direction (apical to basolateral) during the early stages of an asymmetrical culture period (e.g., T0–6 h) and passive transport during the later stages (e.g., T18–24 h). When soft freshwater (with tenfold lower [Ca2 ]) was used for asymmetrical culture instead of freshwater, control epithelia developed outwardly directed active Ca2 transport properties, whereas cortisol-treated preparations did not. The results of this study support a hypercalcemic role for cortisol in rainbow trout and demonstrate that treating cultured gill epithelia composed of both PVCs and MRCs with cortisol can stimulate active Ca2 uptake under circumstances that more closely resemble natural conditions for fish gills (i.e., freshwater bathing the apical side of the epithelium).
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Vol. 44 • No. 3