Cortisol stimulates calcium transport across cultured gill epithelia from freshwater rainbow trout

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 Ca-45) 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).