Features of the lipid transport system of fish as demonstrated by studies on starvation in the rainbow trout

Summary

A comparison was made of the processes involved in the transport and uptake of lipids in starved and fed trout in order to gain a fuller understanding of the underlying mechanisms of these processes and their control in fish. Trout that had been starved for 8 weeks showed significantly lower lipoprotein lipase activities than control fed fish in their adipose tissue (64±45 and 546±205 units±SEM for starved and fed, respectively;P<0.05) and liver (22±6 and 147±56;P<0.05) but no significant difference in red muscle (22±6 and 88±35) or heart (0.53±0.20 and 0.89±0.27). A similar difference in salt-resistant lipase, present in extra-hepatic tissues in trout, was found, i.e. adipose tissue: 200±105 and 1,327±190 (P<0.05); liver: 133±16 and 404±78 (P<0.01); red muscle: 101±32 and 105±20 (n.s.); heart: 2.43±0.38 and 1.92±0.37 (n.s.). The plasma cholesterol esterifying activity of starved trout (1.79±0.36 units) was significantly lower (P<0.001) than the fed fish (3.74±0.65). The concentrations of plasma VLDL and LDL were 67% and 47% lower (P<0.001 andP<0.05, respectively) in the starved than in the fed trout, while the concentration of HDL was the same (163±15 and 165±20 mg cholesterol/100 ml for starved and fed fish, respectively), as was the concentration of nonesterified fatty acids (0.303±0.039 and 0.333±0.035 mEq/l, respectively). These observations demonstrate that, in spite of differences in the distribution of lipases between the various tissues, fish possess systems for the transport and uptake of lipids that broadly parallel those of mammals and are consistent with the greater use of lipid as a major energy source in fish.