Triglyceride Uptake in Muscles in Rats Miiz Li, Per Bjorntorp

Abstract MIN, LI AND PER BJORNTORP. Triglyceride uptake in muscles in rats. Obes Res. 1995;3:419-426. Exogenous lipid is assimilated with different priori- ties 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 oral- ly 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 dis- sected 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 grad- ually up to 16 hours and then decreased. Label in muscles was highest at 4 hours in the red gastrocne- mius, 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 oxida- tive capacity of muscles.

Submitted for publication December 20.1994. Accepted for publication in final form March 10,1595. From The Wallenberg Laboratory and Department of Heart and Lung Diseases, Sahlyren's Hospital, University of GiSteborg. Gateborg, Sweden. Reprint requests to Dr. Bjarntorp. Department of Heart and Lung Diseases, Sahlgrcn's Hospital, 431 45 Gatebag, Sweden. Copyright 01995 NAASO.

In comparisons between fed and fasted rats at 16 hours, when adipose tissue label peaked, liver, indi- vidual muscles and carcass did not show any signifi- cant differences while adipose tissue label was five- fold 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 a t 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, exoge- nous triglyceride in muscle is of significance, particu- larly 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 tis- sue. Lipid uptake in muscle probably constitutes a significant fraction of assimilated exogenous lipid, particularly in the fasting state.

Key words: triglyceride, adipose tissue, muscle, liver, rats, fed, fast

Introduction Upon absorption triglycerides are distributed

between various tissues. Adipose tissue is known to be a major site for direct uptake in the fed state (3,7). This is paralleled by activation of lipoprotein lipase (LPL). In the fasted state, however, LPL activity is low in adipose tissue and higher in muscle (9,10), suggesting a shift of uiglyceride uptake to muscle, The quantitative relation- ships in terms of uptake in total adipose and muscle t is- sues have, however, not ken evaluated.

Recent studies have shown that there is a priority of uptake of lipid among different adipose tissue regions, mesenteric fat assimilating more lipid per unit mass than other depots (8). In that study, we also observed that muscle tissue, obtained from the vastus lateralis muscle,

OBESITY RESEARCH Vol. 3 No. 5 Sept. 1995 419

Tissue Triglyceride Uptake, Li et al.

showed an uptake of orally administered triglycerides, which remained for several days, before oxidation or redistribution mainly to adipose tissue. This uptake was not possible to quantitize in terms of total muscular assimilation of triglyceride, but was estimated to be con- siderable due to the large mass of muscle (8).

Similar, although less detailed studies have been performed in man (10). After the immediate period of triglyceride uptake, lipid is transferred for several days to adipose tissue from other tissues. In analogy with the observations in rats, a considerable part of these triglyc- erides might well be originating from a primary storage site in muscle. Furthermore, calculations utilizing the half-life of labeled adipose tissue triglycerides result in a lower than expected contribution by adipose tissue triglycerides to lipid oxidation in humans (10). Therefore, it was considered likely that triglyceride energy from other sources than adipose tissue might contribute to the lipid energy supply of the body.

Muscles are not uniform, and differ in terms of oxidative and glycolytic metabolism, and their capacity for lipid oxidation is highly varying (1,2,7). Therefore it may be speculated that the uptake of triglyceride might vary in different muscles.

With this background, we decided to try to estimate the contribution of triglyceride uptake by other tissues than adipose tissue in quantitative terns. Calculations suggest that there is a comparably large fraction of triglycerides taken up in muscles, particularly in the fasted state, remaining for a rather prolonged period of time after absorption. In addition, triglyceride uptake and turn over are different in separate muscles.

Materials and Methods Animals Male Sprague-Dawley rats (Alab, Stockholm, Sweden), weighing 290g to 310 g (3 to 4 weeks of age), were used for the present study. The animals were kept under con- trolled conditions, with a fixed 12-hour artificial light cycle, a constant temperature (24-26°C) and humidity (50% to 60%). The animals were provided ad libitum with tap water and commercial rat food, containing 22.5% protein, 72.5% carbohydrates, 5% fat and vita- min and minerals (Ewos, Sodertiije, Sweden).

The rats were randomized into two groups. The first group was given chow and water ad libitum, the second group was fasted 40 hours with free access to water.

Determination of 14C oleic acid uptake Conscious rats were iven 0.75 mL sesame oil con-

Hertfordshire, UK) by gavage in the morning. The rats were sacrificed under anesthesia with Ketal- (Parke- Davis, Barcelona, Spain) 47 mg/kg, and Rompun@

taining 5pCi of (U- 18 C) oleic acid (NEC-672,

(Bayer, Stockholm, Sweden) 8.3 m a g , i.p. Blood sam- ples were taken by cardio-puncture and serum prepared by centrifugation. The following tissues were dissected out; adipose tissues; epididymal @PI), inguinal (ING) (the area between the umbilicus, inguinal fold and mid- line), retroperitoneal (RET) (around the kidney to diaphragm, midline and inguinal fold), and mesenteric (MES), and, muscle tissues; the deep red part of the gas- trocnemius (RG), the superficial white part of the gas- trocnemius (WG), the extensor digitorum longus (EDL) and soleus (SOL). The liver and heart were also dissect- ed out, the heart trimmed free of visible fat. All tissues were rinsed in saline, blotted, weighed and placed in ice- cold buffer for determination of uptake of radioactivity.

From the eviscerated carcass head, feet, tail and skin were removed. The remaining carcass was weighed, passed through a meat-grinder and then homogenized in 300 mL Dole's solvent (4) with an Ultraturrax (TP18/2, Stauffen, Germany) for about 5 minutes and left for at least 48 hours. 500 mLof heptane and 200 mL water were then added and the contents thoroughly stirred. After separation of phases, 5 mL of the heptane phase were taken in triplicate, evaporated, weighed, and the radioactivity measured in a liquid scin- tillator (Optiscint Hisafe, LKB, Stockholm, Sweden), by repeated counts, each of 20-minute duration to a count- ing error < 5% as previously described (7). Reextraction of tissue remnants showed no remaining lipid. Five hun- dred mg of other tissues or 0.5 mL serum were placed in 15 mL glass tubes and left overnight in 5 mL of Dole's solvent (4,8). After homogenization, and cen- trifugation, an aliquot of the heptane phase was evapo- rated at room temperature for 48 hours for determina- tion of triglyceride weight. Another similar aliquot was placed in liquid scintillation solution and counted as above. The results were expressed either as dpm per 0.5 g wet tissue weight, per mg triglyceride, per 0.5 mL serum, or per total tissue.

Statistical Methods Results were analyzed with ANOVA in multiple

comparisons between groups, otherwise with Student's r-test utilizing the Stat-view program of the Macintosh system. p c 0.05 was considered significant.

Results Figure la shows the radioactivity in the liver and

heart at 4, 8, 16 and 24 hours in fed rats after oral administration of label, expressed per total tissue. The amount of radioactivity was higher in liver and decreased with time. Heart radioactivity was very low from the outset and was not significantly different from zero at 8 hours. A low activity in serum also decreased with time and was not measurable with certainty at 8

420 OBESITY RESEARCH Vol. 3 No. 5 Sept. 1995

Tissue Triglyceride Uptake, Li et al.

hours (not shown). In adipose tissues (Figure lb) the radioactivity peaked at 16 hours and then declined. The uptake in ING was lower than in EPI and RET at 16 hours, and lower than in MES at 24 hours (pc0.05). Figure l c shows the radioactivity in muscles. The RG showed higher values than other muscles, significant in comparisons with EDL up to 16 hours (pcO.Ol), with borderline significance at 24 hours and in comparisons with WG and SOL at 4 and 16 hours (pc0.05). The radioactivity in the RG declined with time, but was apparently not changing in the other muscles up to 24 hours.

Figure 2 shows the radioactivity in liver, heart and serum 16 hours after oral administration of labeled oleic acid in sesame oil in fasted and fed rats. When expressed per relative mass of tissue (a) the liver showed higher radioactivity than the heart in the fasted (pc0.05) and in the fed state (pc0.001). There were no differences between fasted and fed rats. Only minor amounts of label were remaining in serum. Figure 2b shows the results expressed per total tissue, where the liver showed much higher radioactivity than the heart in the fasted (pc0.05) and in the fed state (pcO.OOl), with very little labeled oleic acid in heart. Liver radioactivity showed a trend to higher values in fed than fasted rats (O.l>p> 0.05).

Figure 3 shows the results in different adipose tis- sues. Fed rats showed higher radioactivity per unit weight (a) than in fasted rats in all tissues (pcO.OOl), and among the regions, MES showed higher values than the other tissues. RET contained more radioactivity than ING in fed rats. When expressed as uptake per total adi- pose tissue (b) the fed rats again showed much higher values than the fasted (pcO.001). In fed rats radioactivi- ty was higher in EPI and RET (borderline) than ING. In fasted rats MES and EPI contained more label (pc0.05) than RET.

The radioactivity in different muscles is shown in Figure 4. Per unit mass (a), the RG muscle showed higher uptake than WG in fasted (pc0.05), and the SOL than WG in fed rats (pcO.05). However, when expressed per total tissue (b) in fasted rats higher radioactivity was found in the RG than EDL, SOL (pcO.01) and WG (pc0.05). In fed rats RG values were higher than EDL (p<O.Ol), WG (pc0.05) and SOL (borderline). The radioactivity was higher in RG in fasted than fed rats (pcO.O5), but the other muscles were not different.

Table 1 shows the weight of carcasses, extracted TG from carcasses, total weight of dissected adipose tis- sues and the liver. Triglyceride weight was about 2% to 4% of carcass weights. The values at 4 hours fed was higher than that after 16 hours fasting (pcO.Ol), TG weight of adipose tissues and total liver weight were lower in fasted than fed rats (pcO.01).

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Figure 1: Radioactivity in lipids in different tissues 4 hours, 8 hours, 16 hours and 24 hours after oral admin- istration of labeled oleic acid in fed rats. The results are expressed as dpdtotal tissues, (Means k SEM, n=4-5). Figure l a ** p < 0.01 comparisons with heart. Figure l b * p < 0.05 epididymal and retroperitoneal, comparisons with inguinal, # p c 0.05 mesenteric comparison with inguinal. Figure lc ** p < 0.001 RG comparison with EDL, * p c 0.05 RG comparisons with WG and SOL, + 0.1 > p > 0.05 RG comparison with EDL. RG: red gastrocnemius. WG: white gastrocnemius. EDL: Extensor diaitorum lonnus. SOL: Soleus.

OBESITY RESEARCH Vol. 3 No. 5 Sept. 1995 421

Tissue Triglyceride Uptake, Li et al.

Fig. 2a ** *** W Liver

Fed I 6 his

Fast

Fig. 2b

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Figure 2: Radioactivity in triglyceride in liver, heart and serum 16 hours after oral administration of labeled oleic acid in fasted and fed rats. In Figure 2a results are expressed as dpm/0.5 g wwt or 0.5 mL serum, in Figure 2b as dpm/ total tissues, (Means k SEM, n=5). Figure 2a: * p c 0.05 comparisons with heart, ** p < 0.01 comparisons with serum in fasted rats, *** p c 0.001 comparisons with liver vs. heart and serum in fed rats. When results are expressed total tissues, Figure 2b ** p c 0.05 comparisons liver vs. heart in (fasted rats). *** p c 0.001 comparisons liver vs. heart (fed rats).

Table 2 shows the results of an attempt to evaluate and compare the uptake of label in adipose tissues, mus- cle tissues and the liver. When expressed per gram wet weight it is Seen that average label in dissected adipose tissues was higher in fed rats at 16 hours than 4 hours after administration of labeled oleic acid. At 16 hours fed rats showed about 8 time higher values than fasted rats. Dissected muscles were, however, not different in comparisons between groups. Liver label was higher at 4 hours than at 16 hours.

Uptake in total adipose tissue was then estimated as label in all dissected adipose tissues plus label in carcass triglycerides. Again fed rats at 16 hours showed signifi- cantly higher label than fed rats at 4 hours, and fed rats than fasted rats at 16 hours.

Uptake in total muscle tissue was estimated as fol- lows. Label in dissected muscle was first summed. From

the weight of the eviscerated carcass without skin, feet and head, triglyceride weight was subtracted, the rest then being taken to be mainly carcass muscle weight. Assuming that carcass muscle had taken up as much label as the average of dissected muscles, total carcass muscle label was calculated from the carcass weight times average label of dissected muscles. From Table 2 it is seen that this label was not different between groups. Total liver label was higher in the rats 4 hours than 16 hours after administration of label.

The percentage distribution of label between total adipose and muscle tissues and liver shows that the frac- tion assimilated in adipose tissue was clearly higher than in total muscle tissues in fed rats both at 4 hours (46.2% and 22.9%, respectively) and 16 hours (76.8% and 14.4%, respectively). This was, however, different

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Figure 3: Radioactivity in triglyceride in epididymal, inguinal, retroperitoneal and mesenteric adipose tissues, 16 hours after oral administration of labeled oleic acid in fasted and fed rats. In Figure 3a results are expressed as dpm/mg triglyceride (TG) and in Figure3b in total tissues, (Means * SEM, n=5). Figure 3a: * pc 0.05 comparisons with retroperitoneal and inguinal, *** p < 0.001 comparisons with epididy- mal, inguinal and retroperitoneal in fasted and fed rats. Fig 3b: ## p c 0.05 comparison with retroperitoneal, + 0 . b p >0.05 comparison with inguinal, * p c 0.05 com- parison with inguinal. * p < 0.05 comparison with inguinal.

422 OBESITY RESEARCH Vol. 3 No. 5 Sept. 1995

Tissue Triglyceride Uptake, Li et at.

Table 1. Wet weights of adipose tissues, muscle, carcass and liver, and triglyceride (TG) weight of carcass

Fed Fed Fast 4 hours (n=6) 16 hours (n=6)

carcass (g) 126 f 2.6 130 f 9.0 115 f 6.3 TG of carcass (g) 3.87 * 0.38 3.02 f 0.x 2.38 f 0.12* TG in adipose tissues (8) 14 f 0.32 13.8 k 0.89 8.9 f 0.74** Liver (g) 14.2 f 0.5 14.7 f 0.7 9.1 k 0.3**

*p < 0.01 vs. 4 hours of fed. **p < 0.01 vs. 4 hours of fed and 16 hours of fed. Means f SEM

in fasted rats were muscle uptake (51.6%) was almost twice that of adipose tissue (26.4%). The fraction of label in the liver was 30.9% in fed rats at 4 hours, decreasing to 8.8% at 16 hours. In fasted rats the 22.0% in the liver was of the same order as that in adipose tis- sue (26.4%).

It is furthermore seen in Table 2 that total label in all tissues was only about 1/4 in the fasted as compared with fed rats at 16 hours.

Discussion In order to estimate uptake of triglycerides in differ-

ent tissues, particularly total uptake in muscle, uniform- ly labeled oleic acid in carrier triglyceride was adminis- tered orally to conscious rats, and then followed with time in the fed and fasted condition. The rationale of this method has been discussed in detail previously (8,lO). In short, the labeled fatty acids are built into triglycerides during the absorption process, and then transported for uptake in different tissues, where storage or oxidation are occurring. Some label is subsequently recirculating for secondary uptake. Since oleic acid or fragments thereof cannot be transformed to carbohy- drates the measured radioactivity will be that of lipids, which is also ascertained by the lipid extraction proce- dure, which removes water-soluble radioactivity, includ- ing that in ketone bodies. The radioactivity measured will consequently be that of triglycerides, free fatty acids cholesterol and its esters and in phospholipid fatty acids, presumably with a majority in storage triglyc- erides (3,6).

In a previous work (8), utilizing this method, we found that during the period 4 to 24 hours after triglyc- eride administration in fed rats the label peaked in adi- pose tissue, liver and muscle. Liver label decreased sharply after 4 hours, found also in this work. Adipose tissue label showed a peak at 16 hours and then declined, as also shown previously (8). Label in muscle was found 4 hours after triglyceride label, decreasing

thereafter in RG, but not in WG, EDL or SOL. The uptake of label was different in these muscles when expressed per unit mass with highest values in SOL than other muscles and lowest in the WG. This uptake of triglycerides was therefore approximately proportional to the lipid oxidative capacity in the muscles (2,7). In fasted rats, radioactivity was not significantly different from that in fed rats in dissected muscles, liver or heart while there was a large difference in label in dissected adipose tissues with about 5 times higher values in the fed than fasted condition per gram or per total tissue. These observations suggest that triglyceride uptake in adipose tissue takes priority in the fed state, but does not allow comparisons between total uptake in the spread- out adipose and muscle tissues.

This was analyzed further in attempts to estimate uptake in total tissues. Radioactivity in dissected adi- pose tissues (ING, RET, EPI and MES) was summed. This lipid constituted 11 g to 17 g in the groups exam- ined. The eviscerated, skinned carcass contained another 2.6 g to 4.5 g lipid extractable material, which constitut- ed about 15% to 25% of dissected lipid. These masses were used for calculations of total adipose tissue radioactivity, utilizing the results of average radioactivi- ty per gram in dissected fat tissue.

Lipid label in the liver was estimated from the activity in a liver lobe and transformed to total liver weight.

Total uptake of lipid in muscle is more complicated to estimate. The eviscerated, skinned carcass without head and feet in rats of the size examined contains very little visible fat by inspection. Extractable lipid (2% to 4%) was subtracted from total carcass weight, the remaining weight was then assumed to be mainly mus- cles, with some contribution of skeletal weight, which was disregarded.

Triglycerides in muscle tissue are either localized in adipocytes among muscles, or as intracellular triglyc- eride. The dissected muscles were found to be essential-

OBESITY RESEARCH Vol. 3 No. 5 Sept. 1995 423

Tissue Triglyceride Uptake, Li et al.

ly free from adipose tissue when inspected in a dissec- tion microscope. The lipid radioactivity of these mus- cles was therefore assumed to be residing mainly in intracellular triglycerides, which was also checked by thin layer chromatography. The average lipid radioactiv- ity per unit muscle weight of dissected muscles was then multiplied with carcass weight, and after addition of label in the dissected muscles, the resulting radioactivity was assumed to be that of intracellular, total muscle lipid radioactivity, mainly residing in triglycerides.

From these estimations of total radioactivity the distribution of label from the orally administered triglyceride in major tissues for lipid uptake (adipose tissue, muscle and liver) was estimated. This was per- formed 4 hours after lipid administration, and compared with distribution at 16 hours, when uptake in adipose tissue is at a peak level (8), in both fed and fasted rats. It was then found that between 4 and 16 hours in fed rats uptake of radioactivity is rapid and dominating in adi- pose tissue, while liver radioactivity is declining sharply during this period. Muscle label was about the same in absolute counts, but apparently decreased from 22.9% of total measured radioactivity among the tissues at 4 hours, to about 14.4% at 16 hours.

In comparisons between fed and fasted animals at 16 hours total lipid uptake in the examined tissues was about 5 times higher in the fed animals, where it was dominated (76.8%) by adipose tissue. In the fasted rats the proportions of triglyceride label among adipose tis- sue (26.4%) and liver (22%) seemed to be of about the same magnitude. In these rats label in total muscle tis- sue constituted, however, the largest fraction (51.6%).

These results may be discussed in relation to the physiological uptake of triglycerides in the intact organ- ism, a study which to our knowledge has not been per- formed previously in conscious animals receiving lipid orally. It is then rather clear that adipose tissue assimi- lates most of absorbed triglycerides in the fed state. This is in agreement with previous results based on regional measurements of lipid uptake (3,8). Furthermore, the rate limiting enzyme for triglyceride uptake in adipose tissue, lipoprotein lipase (LPL), is activated in adipose tissue in the fed state (6,12). However, blood flow is diminished (12,13), which presumably would hamper rapid triglyceride uptake. As seen in the results present- ed here, the net result of the adaptations seems, however, to be a dominating uptake of triglyceride in adipose tissue.

In the fasted state less accumulated lipid label was found than in the fed state. This might have been due to immediate oxidation of lipid or/and uptake in tissues not measured. In this situation, however, proportionately more lipid is taken up by muscles, indeed this uptake was now apparently higher than in adipose tissue. Also in the fed condition muscle uptake was a significant part

of total uptake, about 22.9% at 4 hours and about 14.4% at 16 hours. The uptake of triglyceride label in muscle is thus occurring rapidly, remains at least in several mus- cles for at least 24 hours, probably longer (8), and con- stitutes a significant part of triglyceride uptake in tis- sues. These triglycerides are probably oxidized in situ, particularly in muscles with a high lipid oxidation capacity, but it also seems likely that parts are trans- ferred to other tissues, presumably mainly adipose tissue (8). This assumption is based on the observations that adipose tissue uptake of labeled lipid is occurring long after lipid absorption is finalized both in rats (8) and man (lo), while muscle label is diminishing (8). This label must be transferred to adipose tissue from other

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Figure 4: Radioactivity in lipids in WG, EDL, SOL and RG in muscle tissues 16 hours after administration of labeled oleic acid in fasted and fed rats. In Figure 4a results are expressed as dpdO.5 g wwt and in Fiureg 4b as dpm total tissues, (Mean&SEM, n=5). Figure 4a * p < 0.05 comparisons with WG in fasted and fed rats. Fig 4b. Fasted rats; ** p < 0.001 comparisons with EDL, SOL, * p < 0.05 comparisons with WG, ## p < 0.01 comparison with RG. Fed rats; ** p < 0.01 com- parison with EDL, * p < 0.05 comparison with WG, + 0.1 > p > 0.05 comparison with SOL.

424 OBESITY RESEARCH Vol. 3 No. 5 Sept. 1995

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tissues, probably in a first phase from the liver as seen here and previously (8), but also from other tissues, where muscles seem to be a likely candidate.

Taken together, this and previous work suggest that several tissues take part in the triglyceride assimilation process in the body with differences in rate and magni- tude of uptake. In the fed state adipose tissue is assimi- lating triglycerides both directly, and probably secon- darily from the liver and muscles. The half-life of adi- pose tissue triglycerides is much longer than that of the liver and muscles, as seen in this work and previously (8). The results obtained suggest that the order of half- lives of triglycerides is shortest in the liver, followed by muscles and longest in adipose tissue, varying also among different adipose tissues (11). This in turn sug- gests that several tissues participate significantly in triglyceride uptake, and that excess lipid, not needed for oxidative or other local purposes, is secondarily trans- ferred to adipose tissue.

The method used in this work, employing measure- ments of radioactivity from labeled oleic acid in triglyc- eride, administered orally, has also been utilized in men, where uptake and half-life of triglycerides were mea- sured in adipose tissue (10). In that study radioactivity in adipose tissue increased over a week after triglyceride administration, indicating a transfer of label to adipose tissue from other tissues. The results of the present study strongly suggests that muscle may have been at least one source of this late labeling of adipose tissue. The late, secondary uptake in adipose tissue was consid- erable, amounting to in the order of 20% of total (10). Based on the results obtained in the present study these findings then suggest that a fairly large fraction of absorbed triglycerides is stored primarily in other than adipose tissue stores, of which a significant part seems to be muscle.

In the human study (10) half-life of triglycerides was calculated in different adipose tissues and compared with estimations of the approximate need of lipid sub- strate for energy supply. It was then found that lipid energy must be delivered from other sources than adi- pose tissue triglycerides, even when fat in food was con- sidered. It is suggested that at least part of this lipid oxi- dation is occurring directly in muscle from locally stored triglycerides.

Muscular lipid oxidation has been pointed out as a regulator of insulin sensitivity in muscle, where mainly circulating free fatty acids have been considered as a substrate (9). This and previous studies (8,lO) strongly suggest that also local muscular triglyceride stores, assimilated at least partially from absorbed triglycerides, may in fact be an additional source of muscular lipid oxidation, which may be of importance for regulation of muscular insulin sensitivity.

References 1. Ariano MA, Armstrong RB, Ederton VR. Hindlimb

muscle fiber populations of five mammals. J Histochem Cytochem. 1973 ;2 1 :5 1 -55.

2. Baldwin KM, Winder WW, Terjung RL, Holloscy JO. Glycolytic enzymes in different types of skeletal muscle: adaptation to exercise. Am J Physiol. 1973;225:962-966.

3. Belfrage P, Borgstrom B, Olivecrona T. The tissue dis- tribution of radioactivity following the injection of vary- ing levels of free fatty acid labelled chylomicrons in the rat. Acfa Physwl Scud 1963;58:111-123.

4. Dole VP. Relation between non-esterified fatty acid in plasma and metabolism of glucose. J Clin Invest.

5 . Hollenberg CH. Effect of nutrition on activity and release of lipase from rat adipose tissue. Am J Physiol.

6. Karmen A, Whyte M, Goodman D. Fatty acid esterifi- cation and chylomicron formation during fat absorption: Triglycerides and cholesterol esters. J Lipid Res. 1963;4:312 -321.

7. Koerker DJ, Sweet IR, Baskln DG. Insulin binding to individual rat skeletal muscles. Am J Physiol. 1990;

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