Studies on lipid metabolism in trout (Oncorhynchus mykiss) branchial cultures

Abstract

Cultured branchial cell epithelia from freshwater rainbow trout were incubated with ((32)P)phosphate and ((14)C)acetate as lipid precursors under both symmetrical (L15 media apical/L15 media basolateral) and asymmetrical (freshwater apical/L15 media basolateral) culture conditions. Epithelia composed of pavement cells alone, or containing both pavement cells and chloride cells, were examined. Lipids (labeled with (32)P and (14)C) were isolated and assayed by thinlayer chromatography, and fatty acids (labeled with (14)C) were isolated and assayed by paper chromatography. The main goal was to see whether the loss of a major incorporation into ((32)P)phosphatidylethanolamine [((32)P)PE], previously seen in eel gills in vivo when the fish were transferred from an osmotic steady state to more dilute media, was the result of a hormonal regulation, i.e., did it only apply to gill tissue in vivo or could it also be seen in the absence of hormonal modulation after incorporation of ((32)P)phosphate in vitro? We likewise wished to see whether a major incorporation into ((32)P)PE was dependent upon the presence of chloride cells. Results show that it is possible to obtain a ((32)P)PE dominated incorporation pattern, even in the pavement cells alone, provided that ((32)P)phosphate is added specifically to freshwater on the apical side of epithelia bathed asymmetrically (freshwater/L15). This is identical to the pattern seen in vivo in trout adapted to freshwater. However, this pattern is not seen under symmetrical conditions (L15/L15) or when ((32)P)phosphate is added to the basolateral media. The shift from symmetrical (L15/L15) to asymmetrical (freshwater/L15) culture conditions thus leads to the establishment of a major incorporation into ((32)P)PE and not to the equivalent loss as seen in vivo in more dilute apical media. We conclude that hormonal control is not needed to change the pattern of short-term lipid formation but, nevertheless, the responses are not altogether the same in vitro and in vivo. Furthermore, cultured trout gill epithelia, in contrast to gills in vivo, do not exhibit a marked incorporation of ((14)C)acetate into palmitoleic acid.