TRIGLYCERIDE UPTAKE IN MUSCLES IN RATS

Exogenous lipid is assimilated with different priorities in adipose ti ssue regions and varies in the fasting and fed conditions. The quantit ative role of uptake of lipid in muscle has not been evaluated. In ord er to examine the uptake in other than adipose tissues, (UC)-C-14-olei c acid in sesame oil was administered orally to conscious rats, and li pid label measured after different times in serum, heart, liver, mesen teric, retroperitoneal, inguinal and epididymal fat pads, as well as i n red and white parts of gastrocnemius, extensor digitorum longus and soleus muscles. Lipid uptake in total adipose tissue was calculated fr om dissected adipose tissues plus lipids extracted from the eviscerate d, skinned carcass. Lipid uptake in total muscle tissue was estimated from label in dissected muscles plus that in the carcass, assuming sim ilar intracellular lipid contents and radioactivity as that averaged f rom dissected muscles. Lipid uptake in the liver was calculated from d irectly extracted lipid. Four hours after lipid administration to fed rats lipid radioactivity in heart and serum was minimal and had essent ially disappeared at 8 hours. Liver label declined rapidly from peak v alues at or before 4 hours. Adipose tissue radioactivity increased gra dually up to 16 hours and then decreased. Label in muscles was highest at 4 hours in the red gastrocnemius, and then decreased, while the ot her muscles showed a constant radioactivity over the observation perio d (24 hours). Radioactivity expressed per unit muscle mass seemed to b e proportional to the oxidative capacity of muscles. In comparisons be tween fed and fasted rats at 16 hours, when adipose tissue label peake d, 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 b etween 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 f asted 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 tissue s. It was concluded that uptake of absorbed, exogenous triglyceride in muscle is of significance, particularly in the fasted state. This lip id 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 mu scle probably constitutes a significant fraction of assimilated exogen ous lipid, particularly in the fasting state.