Triglyceride Uptake in Muscles in Rats

Abstract

Exogenous lipid is assimilated with different priorities in adipose tissue regions and varies in the fasting and fed conditions. The quantitative role of uptake of lipid in muscle has not been evaluated. In order to examine the uptake in other than adipose tissues, U14C‐oleic acid in sesame oil was administered orally to conscious rats, and lipid label measured after different times in serum, heart, liver, mesenteric, retroperitoneal, inguinal and epididymal fat pads, as well as in red and white parts of gastrocnemius, extensor digitorum longus and soleus muscles. Lipid uptake in total adipose tissue was calculated from dissected adipose tissues plus lipids extracted from the eviscerated, skinned carcass. Lipid uptake in total muscle tissue was estimated from label in dissected muscles plus that in the carcass, assuming similar intracellular lipid contents and radioactivity as that averaged from dissected muscles. Lipid uptake in the liver was calculated from directly extracted lipid. Four hours after lipid administration to fed rats lipid radioactivity in heart and serum was minimal and had essentially disappeared at 8 hours. Liver label declined rapidly from peak values at or before 4 hours. Adipose tissue radioactivity increased gradually up to 16 hours and then decreased. Label in muscles was highest at 4 hours in the red gastrocnemius, and then decreased, while the other muscles showed a constant radioactivity over the observation period (24 hours). Radioactivity expressed per unit muscle mass seemed to be proportional to the oxidative capacity of muscles. In comparisons between fed and fasted rats at 16 hours, when adipose tissue label peaked, liver, individual muscles and carcass did not show any significant differences while adipose tissue label was fivefold higher in fed than fasted rats. The distribution of total measured lipid radioactivity between total adipose tissue, total muscle tissue and liver in fed rats at this time‐point was 76. 8, 14. 4 and 8. 8% respectively, and in the fasted state 26. 4, 51. 6 and 22. 0%. These estimations suggest that lipid uptake in the fed state is dominated by adipose tissue, while in the fasted state the lipid uptake is higher in muscles than adipose tissues. It was concluded that uptake of absorbed, exogenous triglyceride in muscle is of significance, particularly in the fasted state. This lipid has a half life of several days. It is suggested that this lipid is oxidized in situ, contributing with a hidden fraction to lipid energy needs, or partially transferred to adipose tissue. Lipid uptake in muscle probably constitutes a significant fraction of assimilated exogenous lipid, particularly in the fasting state. 1995 North American Association for the Study of Obesity (NAASO)