Turnover of the fatty acyl and glycerol moieties of microsomal membrane lipids from liver, gill and muscle tissue of thermally acclimated rainbow trout,Salmo gairdneri

Abstract

Rainbow trout (Salmo gairdneri) acclimated to 5°C or 20°C were administered 2-3H-glycerol and 1-14C-acetate (63 μCi of each isotope/100 g body weight) via intraperitoneal injection, and subsequently maintained at their respective acclimation temperatures. Total lipid extracts (>80% phospholipid) were prepared from isolated microsomes of liver, gill and muscle tissue at various times over a three week period. Half-lives were determined independently for the fatty acyl and glycerol moieties from slopes of regression lines relating dpm/nmole phospholipidPi vs time. In liver tissue, rates of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) turnover were also determined. Membrane turnover was most rapid in liver followed by gill and muscle. In liver, membrane fatty acids turned over more rapidly in warm-(t1/2=3.4 days) than in cold-(t1/2=6.8 days) acclimated fish, whereas in gill, rates of fatty acid turnover, did not differ significantly between acclimation groups. In contrast, rates of glycerol turnover were independent of acclimation temperature in liver, but faster (t1/2=6.7 days) in warm- than cold- (t1/2=15.1 days) acclimated fish in gill. In total lipid extracts, rates of fatty acid and glycerol turnover were equivalent in warm-acclimated fish, however, in cold-acclimated trout, there was a tendency for fatty acids (t1/2=9.1 days) to turnover more rapidly than glycerol (t1/2=15.1 days) in gill tissue, but more slowly (t1/2=6.82 days) than glycerol (t1/2=4.1 days) in liver. Although rates of glycerol turnover were equivalent in PC and PE of liver microsomes, the fatty acyl component turned over significantly more rapidly in PC at both acclimation temperatures. In cold-acclimated trout, rates of fatty acid and glycerol turnover were equivalent in PE, but the fatty acyl moiety of PC (t1/2=4.7 days) turned over significantly more rapidly than glycerol (t1/2=7.5 days). These results were interpreted as indicating that: (1) acclimation temperature independently influenced rates of fatty acid and glycerol turnover in a tissue specific manner, (2) a deacylation-reacylation pathway was activated in both liver and gill as a consequence of cold acclimation, but that liver tissue was more effective than gill in reutilizing the fatty acids released by phospholipase activity, and (3), in liver microsomes, patterns of turnover were phospholipid specific, with PC and PE differing either in the susceptibility of their acyl groups to degradation, or in their ability to reutilize fatty acids cleaved during membrane turnover at cold temperatures.