Tissue Triglycerides, Insulin Resistance, And Insulin

8/12/2019 Tissue Triglycerides, Insulin Resistance, And Insulin

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Tissue triglycerides, insulin resistance, and insulin

production: implica t ions for hy perinsulinemia of obesity

KAZUNORI KOYAMA,1 G U O XU N C H E N ,1 Y O U N G L E E ,1 AN D R O G E R H . U N G E R 1,2

1Gifford L aborat ories, Center for Di abetes Resear ch, Depar tm ent of I nt er nal M edi cine,

U ni versit y of Texas Sout her n M edi cal Cent er, Dall as 75235;

an d 2Departm ent of Veter ans Affai rs M edi cal Center, Dal las, Texas 75216

Koyama, Kazunori, Guoxun Chen, Young Lee, andRoger H. Unger. Tissue t riglycerides, insulin resistan ce,an d insulin production: implications for hyperinsulinemia ofobesity. A m . J . P hy siol . 273 (End ocri nol. Metab.36): E708E713, 1997.Obesity is associated with both insulin resis-tance and hyperinsulinemia. Initially hyperinsulinemia com-p ens a t es for t he ins u lin res is t a nce a nd t hereb y ma int a insnormal glucose homeostasis. Obesity is also associated withincrea s ed t is s u e t r iglyceride (TG ) cont ent . To det erminewhether both insulin resistance and hyperinsulinemia mightbe secondary to increased tissue TG, we studied correlationsbetween TG content of skeletal muscle, liver, and pancreasa nd p la s ma ins u lin, p la s ma [ins u lin] [glucose], a nd -cellfu nct ion in fou r ra t models wit h widely va rying fa t cont ent :obese Zucker diabetic fat ty r at s, free-feeding lean Wista r ra ts,hyperleptinemic Wistar rats with profound tissue lipopenia,an d ra ts pa ir fed t o hyperleptinemics. Correlation coefficients0.9 (P 0.05) were obtained a mong TG of skeleta l muscle,l iver , a nd p a ncrea s a nd a mong p la s ma ins u lin, [ ins u lin] [glucose] product, and -cell fu nct ion a s ga u ged b y b a s a l ,glu cose-s t imu la t ed, a nd a rginine-s t imu la t ed ins u lin s ecre-tion by t he isolated perfused pan creas. Although t hese corre-lat ions can not prove cause and effect, they a re consistent w iththe hy pothesis th at the TG content of t issues sets t he level ofboth insulin resistan ce and insulin production.

tissue fat ; obese Zucker diabetic fatt y ra ts; -cell function

OBESITY is t h e m o st p r e va le n t h e a lt h p r o ble m in t h eU n i t ed S t a t e s a n d i s cu r r en t l y e st i m a t ed t o a f fl ict75,000,000 America ns (28). Although t he risk of diabe-t e s is h ig h , g lu cose h om e ost a sis r e m a in s r e la t ivelynormal for long periods of t ime despite insulin resis-t a n c e a n d e xce ss iv e i n t a k e of f ood . Th i s a b i li t y t oma inta in euglycemia despite diminished insulin effec-tiveness indicat es tha t insulin production is somehowmatched to insulin requirements. The mechanism bywhich -cells recognize the level of insulin resistanceth a t must be overcome is unknown . Because hyperinsu-linemia may be present in obesity even when glucosetoleran ce is normal, subtle glycemic elevat ions ar e not

the signal for the enhanced insulin production at thatst a g e o f t h e d iso r d e r . T h is st u d y w a s u n d e r t a k e n t oid en t i f y a n on g ly ce m ic sig n a l t h a t m ig h t l in k in su lina ction to insulin production.

Tissue t r ig lycer id e (TG) con t e n t se em e d t o be aplausible can didat e for this dual role. Long-chain freef a t t y a cid s (F F A) h a ve lo n g be e n k n o w n t o in t e r f e r ew ith insulin-media ted glucose meta bolism (2, 8, 13, 15,18, 24, 25, 29) a n d t o s t i m ul a t e i n su li n s ecr et i ona cutely (5, 11, 22, 26, 31). Moreover, high FFA levels invit r o h a ve be en sh ow n t o in d u ce in n or m a l is le t s t h esam e changes described in th e compensating -cells of

obesity, such as -cell hyperplasia and increased insu-lin secretion at substimulatory glucose levels (16, 26).Th ir d , i t h a s bee n sh o w n t h a t is le t s o f h y p e r in su lin -e m ic o bese r a t s h a ve a n e xt r e m ely h ig h TG con t e n tcompared with normal littermates (20).

There being no direct wa y t o test t he possibility t ha tthe coupling of insulin production to insulin need ismediated by t issue lipid content, w e relied on correla-tions between -cell function an d in sulin effectivenessand t issue TG content of groups of rats with a widelyv a r y in g t i s su e f a t con t en t . At on e e xt r em e of t h espectr um of fa t cont ent w ere obese ra ts, in w hich tissue

lipids a re m ar kedly increased (20). At t he other ex-treme w e exploited a novel syndrome of profound tissu elip id d e p le t io n t h a t is t h e a n t i t h e sis o f o be sit y ; t h issyndrome was produced by inducing chronic hyperlep-tinemia in normal rats by means of adenovirus-leptingene tra nsfer (3, 34). These an ima ls undergo ra pid an dselective loss of all grossly visible fat (3); additionallythe TG content in skeleta l muscle, liver, a nd pancreasdeclines t o 1/1,000 of tha t of obese ra ts a nd 1/10 of t ha tof pair-fed controls (34). Between these two extremes ofsevere lipopenia a nd obesity, we stu died free-feedingn o r m a l r a t s a n d n o r m a l r a t s w h o s e f o o d i n t a k e w a srestricted by pair feeding t o the h yperleptinemic rat s.The high correla tions betw een indexes of insulin resis-

tance and insulin production and the TG content in thet a r g e t t issu e s o f in su lin a n d in t h e t issu e o f in su linp r od u ct ion a r e con sist en t w it h t h e h y p ot h e sis t h a tt i ss u e f a t con t en t m i gh t p rov id e t h e l in k b et w e eninsulin resistance and hyperinsulinemia.

METHODS AND MATERIALS

A nim als . A l l a n i m a l s w e r e 7 9 w k o f a g e a t t h e s t a r t o fexperiments. Obese, prediabetic Zucker diabetic fatty (ZDF)r a t s (fa/ fa) were b red in ou r la b ora t ory from Z DF/Drt -fa(F10) stock purchased from R. Peterson (University of Indi-an a School of Medicine, India na polis, IN). Male ra ts exhibitedth e previously described ph enotype (30).

M a le Wis t a r ra t s p u rcha s ed from Cha rles R iver B reedingLa bora tories (Wilmington, MA) were used a s free-feedinglean controls an d diet-restricted controls. Diet-restricted ra tswere Wista r r at s pair fed to t he leptin-overexpressing lipope-nic ra t s des crib ed in t he next p a ra gra p h; t his res u lt ed in a42%reduction in t heir tota l caloric inta ke.

The hyperleptinemic, lipopenic anima ls were norm al Wista rra t s t ha t h a d received a n infu s ion of a recombina nt a denovi-rus containing the leptin cDNA (AdCMV-leptin). As reportedpreviously (3), leptin mR NA appeared in t he liver in associa-tion with a r ise in plasma leptin levels and a reduction in foodinta ke a nd body weight (see Fig. 1). Another gr oup of Wista rra t s wa s infu s ed w it h a denoviru s cont a ining t he cDN A of a n

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irrelevant protein, bacterial -galactosidase (AdCMV-G a l) ,as a contr ol for the viral infection.

Gene tr ansfer studi es. To clone t he ra t lept in cDN A a ndm e a s u r e i t s m R N A , t o t a l R N A w a s p r e p a r e d f r o m 1 g o fepididymal a dipose tissue of ra ts by extra ction with TRIzol asrecommended b y t he ma nu fa ct u rer . Oligo( dT) wa s u s ed t oprime first -stra nd cDNAsynt hesis by use of a cDNAsy nth esiskit ( Clont ech, P a lo A lt o, CA ) . A ft er t rea t ment of t he firs t -

strand cDNA with deoxyribonuclease-free ribonuclease, thel ep t in g en e p r od u ct w a s a m p l ifi e d b y p ol y m er a s e ch a i nreaction (P CR) wit h upstream sense primer 5-G G AGG AATC-CCTGCTCCAGC-3 a n d d o w n s t r e a m a n t i s e n s e p r i m e r 5-CTTCTCC TG AGG ATACCTGG -3 ba s ed on t h e r a t l ep ti ngene sequence (27). For both cDNA cloning a nd leptin m RNAmea s u rement , a mp lifica t ion w a s p erformed u s ing 1 cycle a t94C for 3 min, followed by 35 cycles at 92C for 45 s, at 53Cfor 45 s , a nd a t 72 C for 1 min, a nd t hen fina l ext ens ion a t70C for 10 min. We also measured -actin expression witht he s a me a mp lifica t ion condit ions a s for lep t in a nd a p revi-ously described oligonucleotide pair (23).

To p rep a re recombina nt viru s , a 640-b p P R fra gmentcontaining the entire leptin coding region was ligated to pCRTM 2.1 (Invitr ogen, San Diego, CA) according t o the ma nufa c-t u rer s p rot ocol. S equ ence a na lys is confirmed t ha t s evera lclones conta ined th e inta ct leptin cDNA. AB amH I - a nd X baI-restricted leptin cDNA fragment that included 60 bp of 5u nt ra ns la t ed region a nd 76 b p of 3 u nt ra ns la t ed region wa sligat ed to simila rly t reat ed pACC MVpLpA(10). The resultingplasmid wa s cotra nsfected wit h pJ M17 (23) into 293 cells bycalcium phosphat e/DNA coprecipitat ion to generat e t he newrecombina nt virus AdCMV-leptin by use of previously de-scribed methods (1). Virus DNA was isolated, and the pres-ence of the leptin gene insert w as confi rmed by P CR by use ofthe primers described above an d by Southern blotting w ith a noligonucleotid e (5-G G ATACC G ACTGC G TG TG TG AAATG T-C AT-3) comp lement a ry t o t h e ra t lep t in cDN A. S t ocks ofAdCMV-leptin were amplified and purified as described (1)a nd s t ored a t 70 C in phospha te-buffered sa line with 0.2%bovine serum a lbumin (BS A) and 10%glycerol at 13 1012

plaque-forming units (pfu)/ml. A virus conta ining the bacte-r i a l -ga lactosidase gene under contr ol of the C MV promoter,AdCMV-G a l, wa s p repa red a nd u t il iz ed a s des crib ed p revi-ously (14).

The viru s wa s infu s ed in ma le Wis t a r ra t s ob t a ined fromCha rles River La boratories (Wilmington, MA). B efore a deno-viru s infu s ion s t u dies , a l l ra t s received s t a nda rd ra t chow(Teklad F 6 8664, Madison, WI) a d libitum a nd ha d free accesst o wa t er. P olyet hylene t u b ing (P E -50, B ect on-Dickins on,Fra nklin Lakes, NJ ) wa s anchored in the left car otid artery of9-wk-old Wista r ra ts of 250300 g u nder p ent ob a rb it a lsodium a nesthesia (50 mg/kg, Abbott La boratories, NorthChica go, I L ) a nd ext erioriz ed via a s u b cu t a neou s t u nnel.Tubing w as fi lled w ith heparinized saline (1,000 U/dl) untilt h e v i ru s i n fu s ion w a s b eg u n 3 d a y s a f t e r s u r ge r y. B e for e

infu s ion, a denoviru s s a m p les w ere s u s p ended in s a line a ndfi ltered t hrough a 0.2-m fi lter. Two milliliters of AdCMV-leptin or AdCMV--G al containing a tota l of 1 1012 pfu wereinfused into a nesthetized a nima ls over a 1-h period. Anima lswere st udied in individua l meta bolic cages (Nalgene, Roches-t er , N Y) , a nd food int a ke a nd b ody weight were mea s u reddaily.

Plasma measurements. B eginning 1 da y a f t er a denoviru sinfusion, fasting blood sam ples (13 P M) were collected fromthe ta il vein in capillar y tubes coated wit h ED TA. Pla sma w ass t ored a t 20C until the time of leptin assay. Plasma leptinwa s assa yed using the Linco leptin assa y kit (Linco Research,S t . C h a r l es , M O ). P l a s m a i n s u l in w a s a s s a y e d b y s t a n d a r d

m e t h od s (3 7). P l a s m a g l ucos e w a s m e a s ur e d b y G l u cos eAna lyzer II (Beckman, B rea, CA).

Pancreas perfusion. P a ncrea t a were is ola t ed a nd p erfus edb y t h e m e t h od of G r o d sk y a n d F a n s k a (1 2) a s p r ev iou s lymodifi ed (17). The perfusa te consisted of Kr ebs-Ring er bicar-bonate buffer containing 4.5%Dextran T70, 5.6 mM glucose,1% B S A, a nd 5 mM ea ch of s odiu m p yru va t e, s odiu m glu t a -ma t e, a nd s odiu m fu ma r a t e. The fl ow ra t e w a s 3.0 ml/min.Aft er a 20-min equ ilib ra t ion p eriod, t he p a ncrea s wa s p er-fu s ed for 10 min wit h 5.6 m M glu cose. Then t he glu coseconcent ra t ion wa s increa s ed t o 20 mM for 10 min. A ft er a10-min rest, during wh ich the glucose level wa s retu rned t ob a s eline, a rginine wa s p erfu sed a t a concent ra t ion of 8 mMf or a t o t a l o f 10 m i n . S a m p l es w e r e c ol le ct e d a t 1 -m i nint erva ls for det ermina t ion of ins u lin concent ra t ion. Theywer e placed in chilled tubes conta ining 0.3 ml of 0.15 M Na Cl,0.05 M Na 2ED TA, an d 0.3 M benzam ide and were frozen untilthe time of assa y.

TG content of liver, skeletal muscle, pancreas, and islets.

Tissues were dissected and placed in liquid nitrogen. About100 mg of t issues were placed in 4 m l of homogenizing buffercontaining 18 mM of tris(hydroxymethyl)aminomethaneHCl(pH 7.5), 300 mM of D -ma nnit ol , a nd 5 mM of et hyleneglycol-bis(-aminoethyl ether)-N, N, N, N-tetra acetic acid and

were homogenized using a h an d-held polytron (Kontes Gla ss,Vineland, NJ ) for 10 s. Lipids were extra cted by the m ethod ofFolch et a l . (9). Tot a l TG were a s s a yed b y t he met hod ofDanno et al. (6).

Stati st i cal anal ysis.All data ar e expressed as mea ns S E .St a t is t ica l dif ference wa s a na lyz ed b y u np a ired t-test. P 0.05 was considered statistically significant. Regression linewa s ca lcu la t ed u s ing t he St a t V iew p rogra m ( A b a cu s Con-cepts, B erkeley, CA).

RESULTS

C l i n i c al a n d l a b or a t or y fin d i n g s. AdCMV-leptin-i n fu s ed r a t s d ev el op ed h y p er l ep t in e m ia a v e r a g i n g13.7 2.1 ng/ml dur ing t he 14 days of the st udy (Fig.

1A ). Compa red w ith pair-fed normoleptinemic controls,t h e se r a t s a p p ea r e d h y p er a ct ive a n d d isin t e r est e d inf ood bu t in ot h e r w ise g o od h e a lt h . Th e ir t o t a l f ee dinta ke over the period of 14 days avera ged 58%of tha tof Ga l-t r e a t e d con t r o ls (F ig . 1B). B o d y w e ig h t d e-clined during the 1st wk and failed to increase duringthe 14-day period of study (Fig. 1C), thus confirmingour previous report (3). Visible body fat was absent orprofoundly reduced in all sites after 1 wk of hyperlep-tinemia.

B lood glucose levels fell below 2.6 mM a nd r ema inedbelow n orma l throughout t he 14 da ys (Fig. 2A ). Fast ingplasma insulin levels a veraged only 33.6 5.0 pM 14days aft er the vira l infusion, compared w ith 79.3 5.7

and 95.7 3.6 pM in pair-fed and free-feeding G a l -trea ted controls, respectively (Fig. 2B).

T issue l i pid content i n th e vari ous groups.Tw o w eeksafter the AdCMV-leptin infusion, the tissue content ofTG w a s m ea s u r ed i n l iv er, s k el et a l m u s cl e, w h ol epancreas, and islets of age-matched hyperleptinemicrats, pair-fed and free-feeding control rats, and obeserat s. The TG content is shown in Table 1, expressedboth per gram of wet w eight a nd a s a percenta ge of thetissues of the three control groups. In the hyperleptin-emic rat s, t he TG content of liver avera ged 13.3% oftha t of free-feeding G a l - i n f u s e d r a t s a n d w a s o n l y

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6. 4% of t h a t of ob es e r a t s ; t h e f a t con t en t of t h ei rskeletal muscle was 8% of that of free-feeding G a l -infused controls. In whole pancreas, TG content of theh y p er lep t in em ics w a s 5.3% of t h a t of Ad CMV-G a lcontr ols an d 12.9%of tha t of pa ir-fed contr ols.

In islets isolat ed from hyperleptinemic rats, the TGcontent per islet wa s below the sensit ivity of the assa ya s e m pl oy e d . I s l et s of f r ee -f ee d in g Ad C M V-G a l -in f use d r a t s con t a in e d 28 n g /isle t of TG , t h o se ofpair-fed ra ts conta ined 14 n g/islet , whereas islets ofobese rat s a vera ged 990 ng/islet .

Corr elat ions betw een TG cont ent of pancreas and th e

p r o d u ct of p l a sm a i n s u l i n a n d p l a sm a g l u cose a n d

insulin production by isolated pancreata. To det ermin ethe relat ionship between -cell function and islet fatcontent, we examined the correlations between pancre-a t i c f a t con t en t a n d i n su li n p rod u ct i on b y i sol a t e dp er f u sed p a n cr ea t a f r om a l l g r ou p s. Th e p er f u sa t econsist ed of either 5.6 mM g lucose, 20 mM g lucose, or 8mM ar ginine plus 5.6 mM glucose. We a lso measuredt h e cor r el a t i on b et w e en p a n cr ea t i c f a t a n d p la s m a[insulin] plasma [glucose], which distinguished be-tw een h ypoinsulinemia tha t results in increased bloodglucose concent ra tions an d hypoinsulinemia a ssociat edw ith increas ed insulin effectiveness.

A highly signifi cant relat ionship betw een the plasma[insulin] [glucose] and TG content of t issues w asobserved (r 0.93, P 0.0001). A similar correlationwa s observed between plasm a insulin alone and t issueTG content (r 0.93, P 0.0001). The relationshipsbetw een TG cont ent of the three tissues a nd [insulin] [g lu cose ] a r e sh ow n in F ig . 3 . C or r e la t ion w it h TGcontent of isolated islets is not shown because islets

could not be weighed accurately and varied in size by afactor of 4.

In the st udies of perfused pa ncreat a, insulin produc-t i on w a s l ow e s t i n t h e h y per l ep t in em i c g r ou p a n dincreased progressively as the TG content of the pan-creas increased. Thus, in the hyperleptinemic group,insulin secretion at 5.6 mM glucose was low but notsignificant ly different from tha t of pair-fed controls,w h e r e a s t h e r e sp o n se s t o bo t h g lu co se a n d a r g in in ewere completely absent (Fig. 4). At the other extreme oftissue TG, insulin production was greatest in the obeseZD F g r ou p a n d i n t er m ed ia t e i n t h e Ad C M V-G a l -in fu sed a n d p a ir -f ed con t r ol g r ou p s. P a n cr ea t a f r omobese ZDF rats were highest in fat content and exhib-it e d t h e h ig h e st ba sa l in su lin se cr e t io n r a t e a n d t h emost brisk responses to glucose and arginine in abso-lut e ter ms (Fig . 4). The coefficients of correla tion in TGcontent among the three parameters of -cell fu nction(basal, glucose-stimulated, and arginine-stimulated in-sulin secretion) ranged from 0.89 to 0.99.

DISCUSSION

The goal of this st udy w as to determine whether thef a t co n t e n t in t h e t a r g e t t issu e s o f in su lin a n d in t h epancreas is correlated with both the effectiveness ofinsulin and the rate of its secretion. If so, it would beconsistent w ith t he notion t ha t t he responses of-cells

are t ailored t o insulin need by a derivative of TG suchas FFA. This w ould facilita te ma intenance of euglyce-mia despite t he chan ging body composit ion a ssociat edwit h obesity. It should, however, be str essed tha t corre-

Fig. 1. Comparison of plasma leptin levels (A ), body weight (B), an df oo d i n t a k e (C) i n r a t s i n fu s ed w i t h a r eco mb in a n t a d e nov i ru scontaining leptin cDNA (AdCMV-leptin), normal rats pair fed to theformer, free-feeding norma l ra ts infused w ith a n a denovirus contain-ing cDNA of -gala ctosidase (AdCMV-gal) , and obese Zucker dia-betic fa t t y ra ts [ZDF (f a/ f a)].

Fig. 2. Comparison of plasma insulin (A ) and glucose (B) in AdC MV-leptin-infused rat s , norma l ra ts pair fed to t he former, free-feedingAdCMV-gal-infused normal ra ts , and ZDF(f a/ f a) rat s .

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lat ions cannot prove cause and effect and that alterna-t ive in t er p r et a t ion s of t h e se cor r e la t ion s a r e e q u a llypossible. For exa mple, primary hyperinsulinemia couldin cr e a se l ip o g e n e sis a n d ca u se in cr e a se d t issu e T G,wh ich could in turn give rise to insulin r esistance (24,25).

We employed both obese ZDF rats and lean Wistarrats. The availability of a unique fat-free animal, thehyperleptinemic rat, a novel model of extreme lipope-nia tha t is t he ant ithesis of obesity, ma de it possible toexamine t hese correlations over t he broadest possiblespectrum of TG content, ranging from extreme lipope-nia to severe obesity. In acute perfusion experiments,leptin ha d n o effect on insulin production (dat a notshown), but in chronic studies of cultured islets, leptinabolished the insulin responses to both glucose- andarginine-stimulated insulin secretion while depletingisle t TG; F F A r est o re d t h e in su lin r e spon se a lm o stimmediately (19).

We observed tha t t he production of the fast ing plasm a

in su lin a n d blood g lu cose levels , a cr u d e in d ex ofinsulin sensitivity, was significantly correlated with TGcon t en t , a s w a s t h e f a s t i n g p la s m a i n su li n l e ve l b yitself . Insulin sensit ivity wa s ma rkedly enha nced (low

insulin-glucose product) in the lipopenic hyperleptin-emic group. Fa sting glucose levels in t hese groups werein a hypoglycemic range despite reduction in fast ingplasma insulin levels; th e insulin-glucose product ofhyperleptinemic ra ts avera ged 13%of normal controlsa n d 4% of obese r a t s , e vid en ce o f e xq u isit e in su lin

sensit ivity. Clearly lipid content in target organs wascor r e la t e d w it h t h is in d ex of in su lin se n sit ivi t y ; rvalues ra nged from 0.96 to 0.99 and were sta t ist icallysign ifi ca n t (P 0.05). However, because serum cortisollevels were not measured, a role of hypocortisolism int h e i n su li n s en s it i v it y of t h e h y per l ep t in em i c r a t scann ot be excluded.

P a n cr ea t ic TG con t e n t w a s cor r e la t e d w it h -cellfunction as reflected by the fasting insulin level (r 0.97; P 0.037). All pha ses of insulin secretion by t heisolated perfused pancreas, ba sal a nd both glucose-a ndarginine-stimulated insulin secretion, also varied di-rectly wit h th e TG content of the pancrea s.

Ta ble 1. TG cont ent i n m uscle, l i ver, and pancreas of AdCM V-lepti n-in fused r ats com par ed w it h 3 cont r ol groups

Tissue AdC MV- L ept in P a ir F ed Ad C MV- G a l ZD F ( fa/fa)

mg/ g wet wt

Muscle 0.330.12 (6)* 1.310.26 (3) 4.121.16 (5) 13.803.45L iver 0.640.19 (4)* 4.591.17 (5) 4.821.03 (4) 10.021.15P a n cr ea s 0.470.39 (4)* 3.632.00 (5) 8.861.86 (3) 14.911.89

% of control groups

Muscle 25.2 8.0 2.4L iver 13.9 13.3 6.4P a n cr ea s 12.9 5.3 3.2

Values ar e means SE . Nos. of experiment s a re in pa renth eses. AdCMV-leptin a nd AdCMV-Ga l, adenovirus containing leptin cDNA and-galactosidase, respectively; pair fed, normal rats pair fed to AdCMV -leptin-infused rats ; ZDF ( fa/fa) , obese Zucker diabetic fa t ty rats .S i g n ifi c a n t (P0.05) differences: *vs. pair fed; vs. AdCMV-Ga l; vs . ZDF ( fa/fa).

Fig. 3. Correlat ions between t riglyceride (TG) content of a ll t issuessurveyed (skeletal muscle, l iver, a nd pancreas) in AdCMV-leptin-infused rats , ra ts pair fed to the former, free-feeding AdCMV -g a l -i n fu s ed n or m a l r a t s , a n d Z D F (f a/ f a) r a t s a n d t h e [i n su li n ] [glucose] product . Correlat ion wit h plasma insulin a lone gave a nidenticalrva lue (not shown ).

F i g . 4 . C o m pa r i s on o f TG c on t e n t o f p a n c r ea s (A ) a n d i n su li nproduction by perfused pancreata (B) isolated from hyperleptinemicrats, normal Wistar rats pair fed to the former, free-feeding AdCMV-gal-infused rats , a nd ZDF(f a/ f a) rat s .



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The results a re consist ent w ith, but do not prove, thehypothesis that tissue TG content determines the levelof in su lin p r o du ct ion a n d m a t ch es i t t o t h e leve l ofinsulin effectiveness, perhaps by providing an intracel-lular source of FFA retrieved from intracellular (TG)stora ge sites. Becaus e the TG content of skeletal mus cleand pancreas is also posit ively correlated, it is likelyth a t it refl ects generalized chang es in tissue fat . Teleo-

logically, this postulated linkage between insulin re-quirement and insulin secretion would serve to reducet h e r i s k o f d i a b e t e s i n o b e s i t y a n d t o m i n i m i z e t h epossibility of hypoglycemia in un dernut rition.

These observa tions follow more tha n t hree decades ofresearch linking fat t y a cid meta bolism to car bohydra temet a bolism a nd d ia betes (2, 4, 5, 7, 8, 11, 13, 15, 16, 18,2022, 2426, 29, 3135, 38). Most at tent ion ha s beendirected a t the effects of plasm a lipids in reducing theinsulin sensit ivity of target t issues (3, 8, 13, 18, 23).However, the init ial reports of st imulatory effects ofF F A on isle t s (5, 11, 22) a n d , m or e r e ce n t ly, t h e irinhibitory actions on -cell function (2, 7, 16, 38) havelaid t he groundwork for t he present studies. The ma indifference is the emphasis on tissue TG content ratherthan perfusing levels of FFAand TG.

We tha nk K ay McCorkle, who provided t echnical support , a ndSha rryn H arr is , who provided secretaria l assistance.

This s tudy was supported by National I nst itute of Diabetes andDigest ive and Kidney Diseases Grant DK-0270037, the NationalInst itut es of Hea lth/J uvenile Diabetes Founda tion Dia betes Interdis-ciplinary Research P rogram, a nd th e Departm ent of Veterans AffairsI nst itut ional Research Support Gr ant SMI 821109.

Address for reprint requests : R. H. Unger, Center for DiabetesResearch, Un iv. of Texas S outhwestern Medical Center, 5323 H arr yHin es B lvd., Da llas, TX 752358854.

Received 14 Mar ch 1997; accepted in fi na l form 19 J une 1997.

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Tissue Triglycerides, Insulin Resistance, And Insulin

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