- Email: [email protected]
Glycerokinase activity in brown and white adipose tissues of cold-adapted obese Zucker rats
BIOCHIMIE. 1984. 66, 569-5"72
Glycerokinase activity in brown and white adipose tissues of cold-adapted obese Zucker rats. Roseline BERTIN, Mamy ANDRIAMIHAJA, Ren6 PORTET. Laboratoire d'Adaptation Energ(tique fi i'Environnement, Ecole Pratique des Hautes Etudes (ERA 070413 CNRS), I!, Place Marcelin Berthelot, 75231 Paris Cedex 05. (Re¢'ue le 7-6-1984. accept(e aprd.~ rOvision le 19-9-1984).
R6sum6 - -
L'activit( glyc(rokinasique (GK) a (t( mesur(e dans les tissus adipeux bran interscapulaire et blanc (pididymaire du rat Zucker obdse g(n(tique, adapt( ou non au froid. Dans le tissu adipeux blanc, i'activitd totale est faible chez le Fa/- et plus (iev(e chez le f a / f a : e l l e n 'est pas modifi(e par I'adaptation au froid. Dans le tissu adipeux brun des rats maintenus f~ 220. elle est 2 fois plus (lev(e chez ie f a / f a que chez le Fa/-. L'adaptation au froid provoque une augmentation de l'activit( chez le Fa/- et une diminution chez le f a / f a . L'action oppos(e de l'adaptation au from sur I'activit( GK dans le tissu adipeux brun chez les rats Fa et f a / f a contribue, dans chaque cas, ,i assurer une meilleure efficacitd (nerg(tique du tissu. Mots-cl~s : activit6 glyc6rokinasique / tissus adipeux / acclimatation au froid / rats Zacker.
Summary - - Glycerokinase activi O, was measured in the brown and white adipose tissues compared with that in the liver o f obese Zucker rats adapted or not aa~,pted to coM. In white adipose tissue total activiO, was low but higher in the f a / f a rats than in the Fa/ones: coM adaptation did not modify this activiO,. In brown adipose tissue specific activity was higher than in white: specific activiO, was twice as high in the f a / f a rats than in the Fa/-. Cold-adaptation induced an increase in the activiO, in the Fa rats and a decrease in the f a / f a rats. The results are discussed with regard to the cold-induced increase in the energetic efficiency o f the tissue. Key-words : glyccrokinase activity / adipose tissues / cold acclimation / Zucker rats.
Introduction
The glycerophosphate required for triglyceride synthesis is normally formed through the glycolytic pathway, and, in some tissues such as the liver free glycerol resulting from lipolysis, may be phospharylated by the glycerokinase enzyme (GK). In white adipose tissue of normal animals the activity of this enzyme is very low. However numerous experimei'tts [I, 2] have indicated that in genetically obese rodents (ob/ob and db/db
mice and Zucker rats) enzyme activity, which may play an important role in the increased deposition of fat, is far from negligible. Brown adipose tissue (BAT) is a site of heat production in newborn mammals, hibernators or cold-adapted rodents via intensive lipid oxidation [3]. The oxidative efficiency of the tissue is enhanced by chronic cold exposure which leads to many adaptative modifications. Among these modifications glycerol reutilization is augmented by a considerable increase in GK activity [4].
570
R. Bertin and coll.
A few years ago, some authors [5] postulated that BAT might also play a role in the regulation of adiposity by oxidative elimination of energetic substrates. This would explain diet induced thermogenesis [6]. Some recent works [7,8] have shown that defective thermoregulation in genetically obese mice could be due to a deficiency of heat production in BAT: the result being an increase in fat deposition. The genetic obesity of the Zucker fa/fa rat results from its hyperphagia and is favoured by high hyperinsulinemia and hypertriglyceridemia [9]. However, in a previous study, we showed that cold acclimation of these animals inhibits the tendancy to obesity [10]. This fact may be due to the cold stimulation of BAT, as indicated by the in vitro increase in sensitivity through the lipolytic action of norepinephrine [11]. The aim of the present work is to study the effects of cold acclimation of Zucker Fa/- and fa/fa rats on the GK activity in brown and white adipose tissues.
Material and methods The animals studied were 4 month-old lean Fa/- and obese fa/fa Zucker rats adapted to cold (10"C) at 8 weeks and compared to controls kept at 22 °C. The rats were fed ad libitum and subjected to 12 hours of artificial light from 7 a.m. to 7 p.m. Rats were sacrificed by decapitation, blood was collected from the neck for insulin determination. interscapular brown adipose tissue, epididymal white adipose tissue, and liver were quickly excised. Glycerokinase activity was measured in cytosoi of brown and white adipose tissues, and compared with that of the liver following the Bublitz and Wieland method [121 Results are calculated in nanomoles of glycerol phosphorylated per minute; and expressed per g of wet tissue weight, per g of protein or for the total organ. Protein measurements were carried out according to the method of Lowry et al. [13].
Results and discussion The results are shown in Table I. As previously observed, cold acclimation of obese rats inhibits the tendency to obesity, this being partly due to the disappearance of hyperphagia [10]. The obesity of five-month old fa/fa rats kept at 22°C results in a large increase in weight of the 3 studied tissues: epididymal fat (6-fold), interscapula- brown fat, (3-fold) and
liver (2-fold). These increases are essentially du~ to a high lipid content. In epididymal white f a t (WAT) GK activity expressed per gram of tissue or per mg of protein was slightly higher in fa/fa than in Fa/- animals However total activity within the tissue was 1 times as high in fa/fa rats; it was not significantl! modified by cold acclimation. Giycerokinasq activity has been observed in white adiposq tissues of various animal species [14, 151. Enzymq activity is generally low in normal animals [6] however it is increased in a variety of obese state, [2, 16, 17]. It has been suggested that glyceroki nase could play a role in obesity by increasing thq supply of glycerophosphate required for fatt, acid esterification in white fat [181. Moreover, it has been observed that in norm~ and obese animals there is a correlation betwee~ enzyme activity and serum insulin levels [231 Glycerokinase levels may be a secondary conse quence of elevated levels of plasma insulin [91. ! has also been, reported that administration c insulin in vivo resulted in a rapid increase of Gl activity in normal rat adipose tissue [191. I~ genetically obese rodents it may be that hyperin sulinemia enhances GK activity in white adipos tissue. This effect seems to depend on the strain a large increase in specific activity of GK wa observed in obese ob/ob and d b / d b mice and small one in obese Zucker fa/fa rats [171. in ou exoeriment ,h.~o, .t.~ • -. . . . . . .~. . . . .c,h~.,-~ed ....... ,,,~ G K activity i: epididymal white fat of fa/fa, expresred per m of protein, was not higher than in Fa/- animal, However, total activity within :he ti~sue wa~ times as high, this being due to its increase i~ weight. Cold acclimation did not modify specifi and total GK activities in either fa/fa or Fa/- rat despite the fact that hyperinsulinemia in f a / f animals was considerably decreased. It might b concluded that in white fat of fa/fa rats hype~ insulinemia has a wcak effect on GK activity. In the liver the activity of GK, expressed pe mg of protein or per g of tissue, was about on hundred times that in white fat, no differenc being obsereed between fa/fa and Fa/- rat., However, since the organ was twice as heavy i~ fa/fa animals, total activity in the liver wa significantly higher in this group. A lack c stimulation of liver GK activity has also bee~ observed in o b / o b mice [15]. It has been observe, [19] that hepatic GK showed an unresponsivenes to lipogenic stimuli which may reflect the mai: function of GK in liver the metabolization c blood glycerol.
Glyce, okinase activi O, in cold-adapted obese Zucker rats
571
TABLE I
Glycerokinase activities in white and brown adipose tissues and in liver of lean (Fa/-) and obese (fa/fa) Zucker rats
ra/Ambient temperature (°C) Rats weight (g) Number of experiments
22 ° 396 +_. II 7
fa/fa 10o 366 ± 8b 7
22° 579 ± 33~ 8
10o 407 _ 24 ~~ 5
Epididymal white adipose tissue Tissue weight (g) GK activity nmol/min/g tissue nmol/min/mg protein Total activity nmol/min
2.0 ± 0.2
1.2 ± 0.1 b
12.2 + 0.4J
9.0 + 0.3 ~h
4.9 ± !.0 0.53 +_ 0.08 9.6 ± 1.7
7.5 +__ !.2 0.49 _ 0.10 9.0 + !.5
6.7 ± !.8 0.70 ± 0.15 82 + 18~
7.3 ± 2.0 0.76 ± 0.21 68 + !i J~
551 __. 32
741 __ 39 h
1464 ± 80~
17i0 ± 60 "~"
1130 ± 200 24 ± 3
1780 ± 15if' 27 _+ 3
1890 + 50~ 130 ± !0~
670 + 70J~ 18 ± 2"'~
630 ± 90
1230 +_ 190h
1820 +_ 230~
lnterscapular brown adipose tissue Tissue weight (mg) GK activity nmol/min/g tissue nmol/min/mg protein Total activity nmol/min
1310 ± 50t'
Liver Organ weight (g) GK activity nmol/min/g tissue nmol/min/mg protein Total activity nmol/min
Plasma insulin lxU/ml
13.32 ± 0.46
14.61 _+ 0.76
28.06 + 1.72~
21.27 ± 1.22~~
5020 ± 600 37 ± 8
3510 + 410 3t,± 7
5320 ± 560 41 ± 3
66160 _ 7100
51350 ± 7720
150300 + 19900"
88900 + 12000"~
142 ± 20
108 + 13
753 + 36~
593 + 39~h
4250 _+ 640 29 ± 6
- - E a c h v a l u e r e p r e s e n t s t h e m e a n _+ S . E . M . - - a : s i g n i f i c a n t d i f f e r e n c e s b e t w e e n F a / - a n d f a / f a rats, P < 0.05. - - b : s i g n i f i c a n t d i f f e r e n c e s b e t w e e n rats o f t h e s a m e g e n o t y p e a c c l i m a t e d at 2 2 " C o r I O ' C , P < 0.05.
C o l d acclimation h a d no effect on G K specific activity in the liver o f lean F a / - rats; a similar lack o f effect h a d previously been observed in L O N G E V A N S rats [4]. In obese f a / f a rats, decr,'ases in specific a n d total activities were observed and are p r o b a b l y due to the decrease in hyperinsulinemia. No experiments have previously been perform e d on G K activity in brown adipose tissue o f f a / f a rats. C o m p a r e d to F a / - , brown fat (BAT) was a b o u t three-fold heavier in f a / f a rats maint a i n e d at 22°C, this being due to a high lipid accumulation. C o l d acclimation of the rats led to an increase of 50 % in tissue weight in F a / - a n d o f only 20 % in f a / f a . Various laboratories including ours, have r e p o r t e d large G K activities in rat
BAT [4, 191 and in human BAT [201. In the present experiment we observed in lean Fa/- rats a GK
activity (expressed per mg protein) about fifty times higher than in epididymal fat. In obese f a / f a animals the specific a n d total activity were five and three times higher, respectively, than in lean animals. Following cold acclimation specific activity was not modified in F a / - but was largely decreased in f a / f a rats. in these animals total activity within the tissue was decreased compared to that o f w a r m a d a p t e d rats, despite the tissue showing hyperplasia. This fact might be due to the decreased hyperinrulinemia o f cold a d a p t e d f a / f a rats. In BAT, G K activity seems to be more sensitive to insulinemia than in white fat or in the liver. This conclusion was also reached by Persico et al. [19], who observed an increase in G K activity following glucose or insulin infusion greater in BAT than in white fat. As indicazed above, the role of brown fat is to produce heat
572
R. Bertin and coll.
under cold conditions and possibly to prevent obesity by burning the excess of energetic substrates. In cold-adapted animals, there is considerable lipolytic activity in the tissue [21], and the liberated glycerol is not released into the blood stream, this being demonstrated by arteriovenous measurements [22]. This fact is due to the coldinduced increase in GK activity [4]. This results in a greater production of glycerophosphate needed for reesterification of the lalge amount of free fatty acids drawn from the blood, these being rapidely oxidized. In fa/fa rats kept at 22°C the tissue is hypertrophied by lipid accumulation; moreover, the lipolytic activity is reduced compared to Fa/- animals [Ill. The observed high level of GK activity might favour lipid repletion of the tissue: in these animals, BAT seems to be a fat store. When fa/fa rats were adapted to cold, hyperplasia of the ti:,sue occurs, lipolytic activity is restored, and the relative amount of lipids is decreased (unpublished results). Large decreases in G K specific and total activities were observed which might limit fatty acid reesterification within the tissue and favour their oxidation. The opposite effect of cold acclimation on GK activity in BAT of lean and obese Zucker rats resuJ.ts, in both cases, in a permissive effect in the energetic efficiency of the tissue. This may depend on different regulatory factors which, except for insulinemia, are as yet unknown. REFERENCES i. Lochaya, S., Hamilton, J.C. & Mayer, J. (1963) Nature, (London), 197, 182-183. 2. Tilen n, S.W. & Mayer, J. (1975) Horm. Metab. Res., 8, 80-8 !.
3. Portet, R. (1981) Comp. Biochem. Physiol.. 70B, 679-688. 4. Berlin, R. (1976) Biochimie, 58, 431-434. 5. Rothwell, N. & Stock, J. (1979) Nature, (London), 281, 31-35. 6. Shetty, P. (1981) Clinical Sciences. 60, 519-525. 7. Goodbody, A. & Trayhurn, P. (1980) Symposium on Thermal Physiology, Pecs, Abst. 31. 8. Himms-Hagen, J. & Desautels, M. (1980) in: Thermoregulatory Mechanisms and Their Therapeutic Implications. Karger Basel, pp. 245-250. 9. Zucker, L.M. & Antoniades, H.N. (1972) Endocrino. log3". 90, 1320-1330.
10. Berlin, R., Razanamaniraka, G., De Marco, F. & Porter, R. (1983) Comp. Biochem. Physiol.. 74A, 855-860. l l. Berlin, R., Triconnet, M., Razanamaniraka, G. & Porlet, R. (1982) J. Physiol.. (Paris), 78, 30A. 12. Bublitz, C. & Wieland, O. (1962) in : Methods in Enzymology. V, S.P. Colowick and N.E. Kaplan Eds. Acad. Press N.Y., 355-361. 13. Lowry, O.H., Rosebrough, N.J., Farr, A.L. & Randall, R.J. (1951) J. Biol. Chem.. 193, 265-275. 14. Robinson, J. & Newsholme. A.E. (1967) Biochem. J., 104, 2c-4c. 15. Koshinsky, Th. & Gries, F.A. (1971) Physiol. Chem.. 352, 430-432. ,6. Koshinsky, Th., Gries, F.A. & Herberg, L. (1970) Horm. Metab. Res.. 2, 185-186. 17. Thenen, S.W. & Mayer, J. (1975) Proc. Soc. Exp. Biol. Med., 148, 953-957. 18. Mayer, J. (1963) Ann. Rev. Med.. 14, I I 1-132. 19. Persico, P.A., Cerchio, G.M. & Jeffay, H. (1975) Am. J. Physiol.. 228, 1868-1874. 20. Chakrabarty, K., Chaudhuri, B., Jeffay, H. (1983) J. Lipid Res.. 24, 381-390. 21. Berlin, R., Porlet, R. (1976) Eur. J. Biochem.. 69, 177-183. 22. Portet, R., Laury, M.C., Berlin, R., Senault, C., Hiuszko, M.T. & Chevillard, L. (1974) Proc. Soc Exp. Biol. Med.. 147, 807-814.
Glycerokinase activity in brown and white adipose tissues of cold-adapted obese Zucker rats. Roseline BERTIN, Mamy ANDRIAMIHAJA, Ren6 PORTET. Laboratoire d'Adaptation Energ(tique fi i'Environnement, Ecole Pratique des Hautes Etudes (ERA 070413 CNRS), I!, Place Marcelin Berthelot, 75231 Paris Cedex 05. (Re¢'ue le 7-6-1984. accept(e aprd.~ rOvision le 19-9-1984).
R6sum6 - -
L'activit( glyc(rokinasique (GK) a (t( mesur(e dans les tissus adipeux bran interscapulaire et blanc (pididymaire du rat Zucker obdse g(n(tique, adapt( ou non au froid. Dans le tissu adipeux blanc, i'activitd totale est faible chez le Fa/- et plus (iev(e chez le f a / f a : e l l e n 'est pas modifi(e par I'adaptation au froid. Dans le tissu adipeux brun des rats maintenus f~ 220. elle est 2 fois plus (lev(e chez ie f a / f a que chez le Fa/-. L'adaptation au froid provoque une augmentation de l'activit( chez le Fa/- et une diminution chez le f a / f a . L'action oppos(e de l'adaptation au from sur I'activit( GK dans le tissu adipeux brun chez les rats Fa et f a / f a contribue, dans chaque cas, ,i assurer une meilleure efficacitd (nerg(tique du tissu. Mots-cl~s : activit6 glyc6rokinasique / tissus adipeux / acclimatation au froid / rats Zacker.
Summary - - Glycerokinase activi O, was measured in the brown and white adipose tissues compared with that in the liver o f obese Zucker rats adapted or not aa~,pted to coM. In white adipose tissue total activiO, was low but higher in the f a / f a rats than in the Fa/ones: coM adaptation did not modify this activiO,. In brown adipose tissue specific activity was higher than in white: specific activiO, was twice as high in the f a / f a rats than in the Fa/-. Cold-adaptation induced an increase in the activiO, in the Fa rats and a decrease in the f a / f a rats. The results are discussed with regard to the cold-induced increase in the energetic efficiency o f the tissue. Key-words : glyccrokinase activity / adipose tissues / cold acclimation / Zucker rats.
Introduction
The glycerophosphate required for triglyceride synthesis is normally formed through the glycolytic pathway, and, in some tissues such as the liver free glycerol resulting from lipolysis, may be phospharylated by the glycerokinase enzyme (GK). In white adipose tissue of normal animals the activity of this enzyme is very low. However numerous experimei'tts [I, 2] have indicated that in genetically obese rodents (ob/ob and db/db
mice and Zucker rats) enzyme activity, which may play an important role in the increased deposition of fat, is far from negligible. Brown adipose tissue (BAT) is a site of heat production in newborn mammals, hibernators or cold-adapted rodents via intensive lipid oxidation [3]. The oxidative efficiency of the tissue is enhanced by chronic cold exposure which leads to many adaptative modifications. Among these modifications glycerol reutilization is augmented by a considerable increase in GK activity [4].
570
R. Bertin and coll.
A few years ago, some authors [5] postulated that BAT might also play a role in the regulation of adiposity by oxidative elimination of energetic substrates. This would explain diet induced thermogenesis [6]. Some recent works [7,8] have shown that defective thermoregulation in genetically obese mice could be due to a deficiency of heat production in BAT: the result being an increase in fat deposition. The genetic obesity of the Zucker fa/fa rat results from its hyperphagia and is favoured by high hyperinsulinemia and hypertriglyceridemia [9]. However, in a previous study, we showed that cold acclimation of these animals inhibits the tendancy to obesity [10]. This fact may be due to the cold stimulation of BAT, as indicated by the in vitro increase in sensitivity through the lipolytic action of norepinephrine [11]. The aim of the present work is to study the effects of cold acclimation of Zucker Fa/- and fa/fa rats on the GK activity in brown and white adipose tissues.
Material and methods The animals studied were 4 month-old lean Fa/- and obese fa/fa Zucker rats adapted to cold (10"C) at 8 weeks and compared to controls kept at 22 °C. The rats were fed ad libitum and subjected to 12 hours of artificial light from 7 a.m. to 7 p.m. Rats were sacrificed by decapitation, blood was collected from the neck for insulin determination. interscapular brown adipose tissue, epididymal white adipose tissue, and liver were quickly excised. Glycerokinase activity was measured in cytosoi of brown and white adipose tissues, and compared with that of the liver following the Bublitz and Wieland method [121 Results are calculated in nanomoles of glycerol phosphorylated per minute; and expressed per g of wet tissue weight, per g of protein or for the total organ. Protein measurements were carried out according to the method of Lowry et al. [13].
Results and discussion The results are shown in Table I. As previously observed, cold acclimation of obese rats inhibits the tendency to obesity, this being partly due to the disappearance of hyperphagia [10]. The obesity of five-month old fa/fa rats kept at 22°C results in a large increase in weight of the 3 studied tissues: epididymal fat (6-fold), interscapula- brown fat, (3-fold) and
liver (2-fold). These increases are essentially du~ to a high lipid content. In epididymal white f a t (WAT) GK activity expressed per gram of tissue or per mg of protein was slightly higher in fa/fa than in Fa/- animals However total activity within the tissue was 1 times as high in fa/fa rats; it was not significantl! modified by cold acclimation. Giycerokinasq activity has been observed in white adiposq tissues of various animal species [14, 151. Enzymq activity is generally low in normal animals [6] however it is increased in a variety of obese state, [2, 16, 17]. It has been suggested that glyceroki nase could play a role in obesity by increasing thq supply of glycerophosphate required for fatt, acid esterification in white fat [181. Moreover, it has been observed that in norm~ and obese animals there is a correlation betwee~ enzyme activity and serum insulin levels [231 Glycerokinase levels may be a secondary conse quence of elevated levels of plasma insulin [91. ! has also been, reported that administration c insulin in vivo resulted in a rapid increase of Gl activity in normal rat adipose tissue [191. I~ genetically obese rodents it may be that hyperin sulinemia enhances GK activity in white adipos tissue. This effect seems to depend on the strain a large increase in specific activity of GK wa observed in obese ob/ob and d b / d b mice and small one in obese Zucker fa/fa rats [171. in ou exoeriment ,h.~o, .t.~ • -. . . . . . .~. . . . .c,h~.,-~ed ....... ,,,~ G K activity i: epididymal white fat of fa/fa, expresred per m of protein, was not higher than in Fa/- animal, However, total activity within :he ti~sue wa~ times as high, this being due to its increase i~ weight. Cold acclimation did not modify specifi and total GK activities in either fa/fa or Fa/- rat despite the fact that hyperinsulinemia in f a / f animals was considerably decreased. It might b concluded that in white fat of fa/fa rats hype~ insulinemia has a wcak effect on GK activity. In the liver the activity of GK, expressed pe mg of protein or per g of tissue, was about on hundred times that in white fat, no differenc being obsereed between fa/fa and Fa/- rat., However, since the organ was twice as heavy i~ fa/fa animals, total activity in the liver wa significantly higher in this group. A lack c stimulation of liver GK activity has also bee~ observed in o b / o b mice [15]. It has been observe, [19] that hepatic GK showed an unresponsivenes to lipogenic stimuli which may reflect the mai: function of GK in liver the metabolization c blood glycerol.
Glyce, okinase activi O, in cold-adapted obese Zucker rats
571
TABLE I
Glycerokinase activities in white and brown adipose tissues and in liver of lean (Fa/-) and obese (fa/fa) Zucker rats
ra/Ambient temperature (°C) Rats weight (g) Number of experiments
22 ° 396 +_. II 7
fa/fa 10o 366 ± 8b 7
22° 579 ± 33~ 8
10o 407 _ 24 ~~ 5
Epididymal white adipose tissue Tissue weight (g) GK activity nmol/min/g tissue nmol/min/mg protein Total activity nmol/min
2.0 ± 0.2
1.2 ± 0.1 b
12.2 + 0.4J
9.0 + 0.3 ~h
4.9 ± !.0 0.53 +_ 0.08 9.6 ± 1.7
7.5 +__ !.2 0.49 _ 0.10 9.0 + !.5
6.7 ± !.8 0.70 ± 0.15 82 + 18~
7.3 ± 2.0 0.76 ± 0.21 68 + !i J~
551 __. 32
741 __ 39 h
1464 ± 80~
17i0 ± 60 "~"
1130 ± 200 24 ± 3
1780 ± 15if' 27 _+ 3
1890 + 50~ 130 ± !0~
670 + 70J~ 18 ± 2"'~
630 ± 90
1230 +_ 190h
1820 +_ 230~
lnterscapular brown adipose tissue Tissue weight (mg) GK activity nmol/min/g tissue nmol/min/mg protein Total activity nmol/min
1310 ± 50t'
Liver Organ weight (g) GK activity nmol/min/g tissue nmol/min/mg protein Total activity nmol/min
Plasma insulin lxU/ml
13.32 ± 0.46
14.61 _+ 0.76
28.06 + 1.72~
21.27 ± 1.22~~
5020 ± 600 37 ± 8
3510 + 410 3t,± 7
5320 ± 560 41 ± 3
66160 _ 7100
51350 ± 7720
150300 + 19900"
88900 + 12000"~
142 ± 20
108 + 13
753 + 36~
593 + 39~h
4250 _+ 640 29 ± 6
- - E a c h v a l u e r e p r e s e n t s t h e m e a n _+ S . E . M . - - a : s i g n i f i c a n t d i f f e r e n c e s b e t w e e n F a / - a n d f a / f a rats, P < 0.05. - - b : s i g n i f i c a n t d i f f e r e n c e s b e t w e e n rats o f t h e s a m e g e n o t y p e a c c l i m a t e d at 2 2 " C o r I O ' C , P < 0.05.
C o l d acclimation h a d no effect on G K specific activity in the liver o f lean F a / - rats; a similar lack o f effect h a d previously been observed in L O N G E V A N S rats [4]. In obese f a / f a rats, decr,'ases in specific a n d total activities were observed and are p r o b a b l y due to the decrease in hyperinsulinemia. No experiments have previously been perform e d on G K activity in brown adipose tissue o f f a / f a rats. C o m p a r e d to F a / - , brown fat (BAT) was a b o u t three-fold heavier in f a / f a rats maint a i n e d at 22°C, this being due to a high lipid accumulation. C o l d acclimation of the rats led to an increase of 50 % in tissue weight in F a / - a n d o f only 20 % in f a / f a . Various laboratories including ours, have r e p o r t e d large G K activities in rat
BAT [4, 191 and in human BAT [201. In the present experiment we observed in lean Fa/- rats a GK
activity (expressed per mg protein) about fifty times higher than in epididymal fat. In obese f a / f a animals the specific a n d total activity were five and three times higher, respectively, than in lean animals. Following cold acclimation specific activity was not modified in F a / - but was largely decreased in f a / f a rats. in these animals total activity within the tissue was decreased compared to that o f w a r m a d a p t e d rats, despite the tissue showing hyperplasia. This fact might be due to the decreased hyperinrulinemia o f cold a d a p t e d f a / f a rats. In BAT, G K activity seems to be more sensitive to insulinemia than in white fat or in the liver. This conclusion was also reached by Persico et al. [19], who observed an increase in G K activity following glucose or insulin infusion greater in BAT than in white fat. As indicazed above, the role of brown fat is to produce heat
572
R. Bertin and coll.
under cold conditions and possibly to prevent obesity by burning the excess of energetic substrates. In cold-adapted animals, there is considerable lipolytic activity in the tissue [21], and the liberated glycerol is not released into the blood stream, this being demonstrated by arteriovenous measurements [22]. This fact is due to the coldinduced increase in GK activity [4]. This results in a greater production of glycerophosphate needed for reesterification of the lalge amount of free fatty acids drawn from the blood, these being rapidely oxidized. In fa/fa rats kept at 22°C the tissue is hypertrophied by lipid accumulation; moreover, the lipolytic activity is reduced compared to Fa/- animals [Ill. The observed high level of GK activity might favour lipid repletion of the tissue: in these animals, BAT seems to be a fat store. When fa/fa rats were adapted to cold, hyperplasia of the ti:,sue occurs, lipolytic activity is restored, and the relative amount of lipids is decreased (unpublished results). Large decreases in G K specific and total activities were observed which might limit fatty acid reesterification within the tissue and favour their oxidation. The opposite effect of cold acclimation on GK activity in BAT of lean and obese Zucker rats resuJ.ts, in both cases, in a permissive effect in the energetic efficiency of the tissue. This may depend on different regulatory factors which, except for insulinemia, are as yet unknown. REFERENCES i. Lochaya, S., Hamilton, J.C. & Mayer, J. (1963) Nature, (London), 197, 182-183. 2. Tilen n, S.W. & Mayer, J. (1975) Horm. Metab. Res., 8, 80-8 !.
3. Portet, R. (1981) Comp. Biochem. Physiol.. 70B, 679-688. 4. Berlin, R. (1976) Biochimie, 58, 431-434. 5. Rothwell, N. & Stock, J. (1979) Nature, (London), 281, 31-35. 6. Shetty, P. (1981) Clinical Sciences. 60, 519-525. 7. Goodbody, A. & Trayhurn, P. (1980) Symposium on Thermal Physiology, Pecs, Abst. 31. 8. Himms-Hagen, J. & Desautels, M. (1980) in: Thermoregulatory Mechanisms and Their Therapeutic Implications. Karger Basel, pp. 245-250. 9. Zucker, L.M. & Antoniades, H.N. (1972) Endocrino. log3". 90, 1320-1330.
10. Berlin, R., Razanamaniraka, G., De Marco, F. & Porter, R. (1983) Comp. Biochem. Physiol.. 74A, 855-860. l l. Berlin, R., Triconnet, M., Razanamaniraka, G. & Porlet, R. (1982) J. Physiol.. (Paris), 78, 30A. 12. Bublitz, C. & Wieland, O. (1962) in : Methods in Enzymology. V, S.P. Colowick and N.E. Kaplan Eds. Acad. Press N.Y., 355-361. 13. Lowry, O.H., Rosebrough, N.J., Farr, A.L. & Randall, R.J. (1951) J. Biol. Chem.. 193, 265-275. 14. Robinson, J. & Newsholme. A.E. (1967) Biochem. J., 104, 2c-4c. 15. Koshinsky, Th. & Gries, F.A. (1971) Physiol. Chem.. 352, 430-432. ,6. Koshinsky, Th., Gries, F.A. & Herberg, L. (1970) Horm. Metab. Res.. 2, 185-186. 17. Thenen, S.W. & Mayer, J. (1975) Proc. Soc. Exp. Biol. Med., 148, 953-957. 18. Mayer, J. (1963) Ann. Rev. Med.. 14, I I 1-132. 19. Persico, P.A., Cerchio, G.M. & Jeffay, H. (1975) Am. J. Physiol.. 228, 1868-1874. 20. Chakrabarty, K., Chaudhuri, B., Jeffay, H. (1983) J. Lipid Res.. 24, 381-390. 21. Berlin, R., Porlet, R. (1976) Eur. J. Biochem.. 69, 177-183. 22. Portet, R., Laury, M.C., Berlin, R., Senault, C., Hiuszko, M.T. & Chevillard, L. (1974) Proc. Soc Exp. Biol. Med.. 147, 807-814.