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Departments of Physiology (J.P.W., H.F.H., N.C.P., A.B.G., S.F.A., M.F.D.) and Surgery (E.C.W., A.B.), University of California, San Francisco, San Francisco, California 94143
Address all correspondence and requests for reprints to: Dr. James Warne, Department of Physiology, Box 0444, University of California, San Francisco, 513 Parnassus Avenue, San Francisco, California 94143. E-mail: james.warne{at}ucsf.edu.
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Introduction |
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We reported that in ADX rats presented with a choice of lard or chow, B replacement results in a dose-dependent increase in lard, but not chow, intake concomitant with an increase in insulin secretion (5). In contrast, ADX rats made diabetic with streptozotocin (STZ) exhibit a B dose-related increase in chow but not lard intake (5). When diabetic rats were treated systemically with insulin, the small increases in insulin correlated positively with lard intake without reducing high circulating glucose concentrations (5). These data suggest that insulin resulted in preference for lard. However, it is unclear where insulin acts to facilitate the choice of macronutrient intake.
The liver is an important target for insulin action and contributes to the regulation of body weight and food intake. After secretion into the portal vein, the liver removes some insulin before the remainder reaches the general circulation (8). In liver, insulin inhibits glycogenolysis and gluconeogenesis and promotes fat synthesis. This is exemplified in mice with hepatocyte-specific ablation of the insulin receptor (IR), which show large elevations in blood glucose, glucose intolerance, hyperinsulinemia, decreased insulin clearance, and lack of normal insulin-induced suppression of hepatic gluconeogenesis (9). In rats, hepatic vagotomy abolishes the stimulatory effects of mercaptoacetate, which inhibits fatty acid oxidation, on food intake (10). Furthermore, hepatic vagotomy prevents lard-induced inhibition of food intake and changes in hypothalamic neuropeptide expression in diabetic rats (11, 12). These data strongly suggest that there are major inputs to brain-feeding centers mediated by the hepatic vagus that can be activated by some aspect of altered insulin-stimulated hepatic metabolism.
Insulin also acts directly to inhibit orexigenic and excite anorexigenic neurons in the arcuate nucleus of the hypothalamus (13). However, the hormone clearly acts at other sites throughout the brain (14). In addition, rats, only when presented with a choice of lard and chow and not a single mixture of both, exhibit reduced ACTH and elevated insulin concentrations after restraint stress; other variables did not differ between groups (15). Hence, small increases in systemic insulin can act on the brain when there is a choice among food calories.
It is clear that insulin-stimulated direct and indirect (via the liver) signaling to the brain impacts food intake. However, the contribution of both signaling routes to insulin-induced lard intake under conditions in which there is a choice of macronutrient intake was unknown. We sought to examine the role of a liver insulin signal by replacing insulin in STZ-diabetic rats into the hepatic portal system via the superior mesenteric vein, such that the liver would be the first major organ to receive the insulin. This was compared with a venous insulin replacement into the right external jugular vein. Due to the important effects of B, concentrations were clamped at steady-state stress/peak circadian levels. After a 5-d period of recovery after surgery, the rats were given a macronutrient choice. In addition to chow, lard and 32% sucrose solution were provided. The later was also included because it was not known whether an insulin signal also regulates sucrose intake. Body weight and caloric intake were monitored throughout the study and at the end of the 5-d choice period, plasma and liver samples were collected for analysis of GC- and insulin-sensitive variables and the fat pads were excised and weighed. Our results show distinct site-dependent and -independent actions of insulin to prevent the deleterious consequences of STZ-diabetes and point to the liver as being important in the insulin-induced stimulation of lard intake.
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Materials and Methods |
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Surgical procedures and treatments
All procedures were performed in one surgery (d 0). All rats were anesthetized using ketamine (75 mg/kg, im) and xylazine (10 mg/kg, im). Ketoprofen (10 mg/kg, sc) was provided as an analgesic after surgery but before the rat regained consciousness. The rats were then allowed 5 d to recover, during which incisions, body weight, and food and water intake were monitored daily. All rats were also presented with lard and 32% sucrose on d 5. Body weight and solid and liquid intakes were monitored daily for a further 5 d. On d 10, all rats were killed by decapitation and samples were collected. Five groups of rats (n = 6/group) were studied: nondiabetic control (ctrl), STZ-diabetic with vehicle (saline) infused into the jugular (veh-jug) or superior mesenteric vein (veh-mes), STZ-diabetic with insulin (3 U/d) infused into the jugular (ins-jug), or mesenteric vein (ins-mes). All rats received a sc pellet of B.
Preliminary studies (data not shown) were conduced to develop the final model used in this study. The original paradigm was identical to that outlined above with the exception that it involved a 5-d recovery period, followed by a 7-d period during which lard and 32% sucrose were available. However, the effects on lard and 32% sucrose intake appeared to wane after 5 d because the maximum pumping duration was being reached. Consequently, to reduce this technical limitation and minimize any detrimental effects of a high-fat, high-carbohydrate diet, the period during which lard and 32% sucrose were available was reduced to 5 d. Similar effects on organ weights (fat pads, spleen, thymus, adrenals), blood (insulin, leptin, glucose), and liver (glycogen) parameters at the end of either the 10- or 12-d study were observed. Hence, the shorter paradigm was adopted. These model developing, preliminary experiments were performed using an insulin replacement dose of 2 U/d. This dose was based on previous studies using the same dose but applied by a sustained-release insulin implant placed sc in the back (5). Similar to those studies, 2 U/d insulin did not modify plasma glucose levels. Because insulin action was important to observe in this paradigm, insulin replacement was increased to 3 U/d. Small, but significant, effects on glucose were observed, with this dose illustrating the efficacy of insulin action.
B-treatment.
Circulating B levels were clamped by placing a 100-mg pellet of B (100%; Steraloids Inc., Newport, RI) sc through a small incision in the back that was subsequently sealed using silk suture. Previous studies have shown this treatment produces sustained, steady-state, stress/circadian maximum levels of B (16).
STZ-induced diabetes.
Diabetes was induced by a sc injection of STZ (Sigma Chemicals, St. Louis, MO; 65 mg/kg in citrate buffer pH 4.2). Control rats were injected with citrate buffer (2 ml/kg). Diabetes was confirmed by analysis of urinary glucose levels (Multistix 9 SG; Bayer Corp., Elkhart, IN).
Insulin replacement.
Insulin (3 U/d; Humulin R U500; Eli Lilly & Co., Indianapolis, IN) or saline was infused in the STZ-treated animals at two locations (jugular or superior mesenteric veins) via the insertion of catheters (PE5 tubing, 1.5 cm, fused to PE60 tubing, 1.5 cm) attached to osmotic minipumps (Alzet, model 2002; Alza, Palo Alto, CA) based on a method previously described (17). For the jugular vein, a small incision was made into the neck and the right external jugular vein was exposed. The vein was gently elevated, a small incision was made, and the catheter was inserted. The vein was sealed using sterile glue (Vetabond; 3M Animal Care Products, St. Paul, MN). A sc pocket from the neck to the back was then created to hold the catheter and attached osmotic minipump. The neck incision was then sealed using silk suture. For the superior mesenteric vein, a ventral midline incision was made and the cecum was externalized onto gauze soaked in sterile saline and superior mesenteric vein was gently exposed. The catheter was inserted into the vein and immediately sealed into place using sterile glue. The cecum and osmotic minipump were then quickly internalized, such that the minipump nestles close to the cecum and small intestine. The muscle and skin layers were then closed separately with silk suture. The presence of an osmotic minipump at either site did not cause any obvious signs of discomfort for the rats.
Methylene blue infusion
To ascertain that superior mesenteric vein infusions reached the liver and observe any local spill from the superior mesenteric and jugular vein infusions, rats (n = 2/group) were implanted with osmotic minipumps containing methylene blue [1.2 mg/d; (18)] in sterile saline into either the superior mesenteric or right jugular veins, as outlined above for insulin replacement. Methylene blue was selected due to its staining (blue) and biological (oxygenation through induction of cytochrome oxidase) activities that can be easily visualized. The rats were allowed to recover for 4 d before they were killed by decapitation and the tissues (liver, fat pads, and brain) were removed and photographed.
Sample collection
After rats were killed, trunk blood was collected into chilled tubes containing 100 µl EDTA (65 mg/ml). Tubes were centrifuged and plasma collected and stored at –80 C. One hundred milligram liver biopsies were quickly collected from the same lobe (lobus sinister lateralis), snap frozen, and stored at –80 C. The rest of the body was put onto ice for subsequent dissection and weighing of the white adipose tissue (WAT) fat pads [sc (scWAT), epididymal (eWAT), perirenal (pWAT), and mesenteric (mWAT)], thymus, spleen, and adrenals. At the same time, position of the catheters and osmotic minipumps was verified. In all cases, one end of the catheter was secured in the desired vein and the other attached to the minipump.
Plasma assays
Plasma B, insulin, and leptin concentrations were assessed by RIAs at half volumes (Linco Research Inc., St. Charles, MO; MP Biomedicals, Orangeburg, NY), whereas plasma glucose, triglycerides, glycerol, and free fatty acids (FFAs) were measured colorimetrically on a plate reader using kits (Mega Diagnostics, Los Angeles, CA; Sigma-Aldrich, St. Louis, MO; Wako Chemicals, Neuss, Germany), all as previously described (5).
Liver measurements
Hepatic phosphoenolpyruvate carboxykinase (PEPCK) and fatty acid synthase (FAS) enzyme activity assays were performed as previously described (19, 20) using snap-frozen tissue. Enzyme activity was standardized to milligrams soluble protein, as determined by the Bradford method. Liver glycogen content was assessed by a colorimetric plate assay outlined elsewhere (21) and was standardized to milligrams wet weight. Liver triglyceride content was determined by chloroform: methanol [2:1 (vol/vol)] extraction of total liver lipids (22), followed by determination of triglycerides as described for plasma measures.
Immunoprecipitation and Western blotting
Liver IR protein expression was assessed by immunoprecipitation followed by Western blot analysis as previously described (23). Briefly, frozen liver biopsies were homogenized in ice-cold lysis buffer [50 mM Tris HCl (pH 7.5), 120 mM NaCl, 1% Nonidet P-40, 1 mM EDTA supplemented with the Complete protease inhibitor cocktail (EDTA free; Roche Diagnostics, Indianapolis, IN)]. The samples were then incubated for 1 h at 4 C under constant rotation and insoluble material was removed by centrifugation (16,000 x g, 20 min, 4 C). Five hundred micrograms of protein were incubated in 0.5 ml of immunoprecipitation buffer [20 mM Tris HCl (pH 7.5), 150 mM NaCl, 1% Nonidet P-40, 1% Triton X-100, 1 mM EGTA, 1 mM EDTA supplemented with the Complete protease inhibitor cocktail] with 0.25 µg of a polyclonal antibody directed against the IR ß-subunit (BD Transduction Laboratories, San Diego, CA) overnight at 4 C under constant agitation. Immunocomplexes were bound to protein-A-Sepharose beads [25 µl of a 50% (wt/vol) slurry] during a further 2-h incubation at 4 C under constant agitation and washed four times in ice-cold immunoprecipitation buffer. Proteins were eluted by boiling for 10 min in 20 µl of 2x Laemmli buffer, separated on an 8% polyacrylamide gel, and transferred onto a nitrocellulose membrane. After blocking for nonspecific binding, the membrane was incubated overnight with the anti-IR antibody (0.25 µg/ml). The following day, the membrane was washed and incubated with a secondary antibody (IRDye 800 conjugated antirabbit IgG, 0.1 µg/ml; Rockland Inc., Gilbertsville, PA) for 1 h at room temperature and visualized using the Odyssey infrared imaging system (LI-COR Biosciences, Lincoln, NE).
Statistical analyses
All data, except those in Fig. 1
, are presented as the mean ± SE of the mean. Data were analyzed by one-way ANOVA to test for differences from the control, nondiabetic group. Significant (P < 0.05) effects were followed by post hoc tests of individual group differences (Tukey’s test). In addition, two-way ANOVA was also performed on the four STZ-treated groups to test for significant interactions (P < 0.05) between insulin replacement (vehicle vs. insulin) and site of replacement (superior mesenteric vs. right external jugular).
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Results |
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, show that the liver of a rat treated with methylene blue into the superior mesenteric vein was redder (more oxygenated) than the liver of a rat infused into the right jugular vein. This shows that superior mesenteric infusions have comparatively greater impact on the liver than jugular infusions due to the oxygenating capacity of methylene blue. Interestingly, the brain also showed a similar color change. This could also be associated with increased stain or oxygenation, either within the brain itself or the capillaries of the brain parenchyma. Both proximal (mWAT for superior mesenteric, scWAT for jugular) and distal fat (pWAT and eWAT) pads were similar in color after the two sites of infusion. Additionally, for both superior mesenteric and jugular methylene blue infusions, no stain was observed in the proximal ip and sc regions, suggesting venous delivery.
The effects of STZ-diabetes and site-specific insulin replacement on caloric intake are shown in Fig. 2. The intake of chow (Fig. 2A) steadily increased in the STZ-treated groups after surgery such that, by d 3, chow intake exceeded the control group. The daily intake of lard and sucrose during the period these were given (Fig. 2, C and E) is summed across the last 4 d (Fig 2, D and F) because the first day represented the acclimation day to novel foods. The controls increased caloric intake with introduction of the lard and sucrose and derived approximately one third of the calories from each source. STZ treatment, without exogenous insulin supplementation, led to significantly lower intakes of both lard and sucrose (P < 0.05). Insulin replacement, regardless of site, resulted in an increase in lard intake (two-way ANOVA). This was significant for the insulin-mesenteric (P < 0.05) but not the insulin-jugular group. Sucrose intake was very low in all diabetic groups. The increase in lard intake in the insulin groups was accompanied by a reduction in chow intake (P < 0.05; Fig. 2B) such that total caloric intake (Fig. 2H) for the last 4 d was unchanged among the diabetic groups. In all of the diabetic groups, total caloric intake remained greater (P < 0.05) than the control group, largely due to the greater (P < 0.05) chow intake (Fig. 2B).
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shows the effects of the experimental manipulations on body weight. Due to the presence of high B, all rats lost weight during the study. STZ treatment further exacerbated this weight loss. Only insulin, replaced into the jugular vein, prevented this STZ effect. Introduction of lard and sucrose on d 5 was accompanied by no further weight loss in all groups. Both jugular infused groups began to gain weight from d 8 onward, whereas the other groups remained relatively stable.
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, A–D). Diabetes, without exogenous insulin replacement, produced a reduction in the weight of all fat pads examined (P < 0.05). The vehicle pumps, at either location, did not produce any marked differential effects except in the case of mWAT (Fig. 4B), in which the presence of a vehicle pump in the abdominal cavity resulted in higher mWAT weight, compared with that of the vehicle-jugular group. Insulin replacement increased depot weight of the pWAT and eWAT similarly and independently of the infusion site (Fig. 4, C and D). In the case of mWAT, insulin infusion into either site produced a significant increase in weight (two-way ANOVA; Fig. 4B). However, only superior mesenteric insulin infusion maintained mWAT weight to nondiabetic levels, whereas jugular insulin infusion only partially achieved this, resulting in a weight similar to that observed with the vehicle pump into the superior mesenteric vein. Only scWAT showed a distinct site-dose effect (two-way ANOVA; Fig. 4A) with insulin replacement into the jugular vein maintaining the weight of this fat pad similar to that in the nondiabetic controls. Additionally, insulin replacement into the superior mesenteric vein maintained mWAT and eWAT weights similar to those of the nondiabetic controls, an effect that was only partially achieved by insulin infused into the jugular vein. These site-specific effects of exogenous insulin infusion on fat pad weights were, however, not evident in other GC and insulin-sensitive tissues examined. STZ treatment increased spleen weight (P < 0.05); the effects on spleen were not counterbalanced by exogenous insulin replacement at either site (Table 1). Adrenal and thymus weights were very low, as anticipated from the constant exposure to high B (Table 1).
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and Fig. 5. Plasma B was not significantly different among the treatment groups, showing the efficacy of the pellet-derived infusion. STZ treatment produced a marked reduction in both circulating insulin and leptin concentrations (P < 0.05; Fig. 5, A and B) when compared with nondiabetic controls. There was a site-dose interaction for plasma insulin levels (P < 0.05; two-way ANOVA). Specifically, only insulin into the jugular, but not superior mesenteric, vein produced an increase in circulating insulin levels above those of vehicle infusion (P < 0.05). In contrast, there was a significant dose effect of insulin into both sites on plasma leptin levels (two-way ANOVA), but only the insulin-jugular group had elevated plasma leptin (P < 0.05) when compared with the corresponding control group. STZ-diabetes significantly elevated plasma glucose and triglyceride levels. Insulin infusion into both the superior mesenteric and jugular veins was sufficient to reduce plasma glucose levels (P < 0.05; Fig. 5C). However, plasma triglycerides were reduced only by insulin infused into the jugular, but not superior mesenteric, vein (Fig. 5D). None of these variables reached nondiabetic levels. Glycerol and FFA concentrations were unaffected by any of the treatment regimens examined (Table 1).
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, A and C). Insulin replacement at both sites restored liver glycogen content (P < 0.05), but not FAS enzyme activity, to that of control, nondiabetic levels. PEPCK enzyme activity (Fig. 6B) was not altered by STZ treatment or exogenous insulin supplementation. Liver triglyceride content (Fig. 6D) was similarly unchanged by STZ treatment and insulin replacement, with the exception that the control group was higher than the vehicle-jugular group (P < 0.05). Liver IR expression (Fig. 6, E and F) was similarly unchanged across treatment groups.
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Discussion |
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The finding that exogenous replacement of 3 U/d insulin into the superior mesenteric vein does not elevate circulating insulin levels above vehicle infused suggests that there was a primary action on liver in this treatment group. Several studies have shown that portal venous infusion of insulin can result in reduced circulating insulin levels, compared with peripheral venous infusion (24), probably associated with greater liver insulin extraction (25). It therefore stands that an insulin-induced, liver-mediated signal may have resulted in the restoration of lard intake to nondiabetic levels. Superior mesenteric infused insulin was active on the liver, as confirmed by the reduction of the STZ-induced elevated glucose concentrations and restoration of liver glycogen content.
The brain integrates a diverse set of sensory inputs to regulate food intake. The hepatic vagus is an important peripheral controller in this regulation (26) and has been shown to mediate the lard-induced inhibition of diabetic hyperphagia (11). In this study, however, provision of lard in addition to chow did not significantly reduce the STZ-induced hyperphagia in terms of total caloric intake, although chow intake was considerably reduced. There could be several reasons for this distinction. First, hepatic vagotomy severs both afferent and efferent fibers, although the unchanged liver ATP-ADP data and changes in hypothalamic neuropeptide expression from the studies of la Fleur et al. (11) argue for a predominantly afferent effect. Second, hepatic vagotomy is likely to affect the stomach, pancreas, and proximal duodenum as well because the common hepatic branch also innervates these organs (26). Provision of insulin into the portal circulation probably produced a local, liver-specific afferent signal to the brain. Alternatively, the high B, which drives food intake (2), could be the factor that distinguishes the studies.
The jugular insulin infusion might also be acting at the liver or on hepatic vagal afferents. It is clear that insulin from this source has equal actions on plasma glucose and liver glycogen. This parallels studies showing that portal and peripheral insulin administration can produce similar concentrations of plasma glucose (27), although the kinetics of the two methods of delivery differ in rats (28). However, insulin could also be acting on other organs to elicit a similar final outcome, with direct actions on the brain being a prime candidate. Insulin (2 µU) into the arcuate nucleus, but not paraventricular nucleus or third ventricle, can cause a selective reduction in fat-saturated but not carbohydrate- or protein-saturated diets (29). Significantly higher concentrations of insulin (>2 mU intracerebroventricularly) have been shown to reduce chow intake without other available calorie sources (30, 31). This was not evident in our study because insulin increased the proportion of fat ingested. Choice appears to be the key in the lard-induced inhibition of the stress response (15). However, the reduction in chow intake observed in our study could reflect known insulin actions at the arcuate nucleus.
The hypothalamus is certainly not the sole target for insulin action in the brain. Low-level circulating insulin, under conditions of choice, might sway macronutrient intake toward lard via actions on other parts of the brain. The ventral tegmental area, which contains dopaminergic neurons and is involved in reward (32), could be important in this regard. The ventral tegmental area contains IRs and insulin action can increase expression of dopamine transporters (33, 34). However, results using insulin delivered directly into the brain shows that insulin appears to inhibit caloric intake and conditioned place preference for a high-fat diet (35). Thus, the effects of insulin might be dose dependent, a possibility alluded to in our previous studies (15). Alternatively, other brain systems might come into play, such as opiatergic systems that are activated by palatable food choice (36).
It is possible that other insulin-regulated metabolic signals might regulate lard intake. Glucose, a prime candidate, is not likely to in this instance. Whereas exogenous insulin replacement slightly reduced the STZ-induced elevated glucose levels, our previous study showed that exogenous insulin replacement promote lard intake without affecting plasma glucose (5). Hence, insulin-induced alterations in glucose metabolism and lard intake are likely to occur via independent mechanisms. Lipids represent other possible candidates. Plasma lipid levels are certainly insulin sensitive (37), and, furthermore, lipids can traverse the blood brain barrier and affect food intake (38). Neither triglycerides nor FFAs were consistently changed by insulin infusion into both sites. However, it does not preclude the possibility that the same outcome of lard intake could be generated by two different pathways activated by an insulin signal in the two sites of replacement.
Our studies provide the novel observation that prevention of STZ-induced reduction in fat pad weight critically depends, in a depot specific manner, on the site of insulin replacement. This furthers studies showing that insulin is an important regulator of adiposity, as exemplified in mice lacking IR specifically in adipose tissue (39) and with STZ-diabetes (5, 40). Although insulin treatment similarly prevented loss of weight to the epididymal and perirenal fat pads, the weight of sc and mesenteric fat pads was highly dependent on the site of exogenous insulin replacement. Complete prevention of the STZ effect was evident in the fat pads near the site of insulin infusion. This could be associated with local and/or centrally mediated insulin effects. Insulin acts directly on adipose tissue to promote adipogenesis [the maturation of preadipocytes into mature, fat storing adipocytes (41)] and lipid metabolism [by enabling the uptake of glucose, an important substrate in the process of lipogenesis (42)], collectively increasing tissue weight. However, our studies using methylene blue infusions open another possibility: a centrally mediated effect. Because no methylene blue staining or biological activity was visually evident in the fat pads proximal to the infusion site, a requirement for local action, an insulin-induced signal to the brain could be important. Distinct neural projections emanate from the brain that provide sympathetic innervation of sc and retroperitoneal fat depots (43), and white adipose tissue is extensively innervated by sympathetic outflow (44). Thus, this distinctive signaling to different brain regions from insulin infused into different sites could consequently regulate the pattern of fat deposition.
Further site-dependent effects of exogenous insulin replacement were evident when examining body weight changes and plasma leptin and triglyceride levels. Patients with type 1 diabetes mellitus and STZ-diabetic rats exhibit abnormalities in the GH-IGF-I axis (45, 46), suggesting an important role for insulin in the regulation of growth. Insulin also prevents the decrease in circulating leptin (47) and the hypertriglyceridemia (37) induced by STZ treatment. The finding that exogenous insulin replacement only into the jugular vein prevented the STZ-induced exacerbation of body weight loss and partially attenuated the STZ-induced reduction in leptin and elevation in triglyceride levels is likely attributed to the elevated circulating insulin levels evident only in this insulin replaced group.
Examination of liver showed that, similar to the effects on plasma glucose, the reduction in glycogen content due to STZ-diabetes was prevented by insulin infusions into both sites. Insulin promotes glycogen synthesis and activates glycogen synthase in the liver (48). Hence, the concomitant reduction in circulating glucose in insulin replaced STZ-treated rats can be accounted for by the increase in liver glycogen. This increased storage was insufficient to reduce circulating glucose levels to nondiabetic levels, suggesting that the hyperphagia exhibited led to an excessive glucose load such that the maximum capacity for storage was exceeded. The liver has limited capacity to phosphorylate glucose, hence ultimately produce glycogen, at even physiological concentrations of glucose (49). However, the dose of insulin used was insufficient to restore hepatic FAS activity. In comparison, hepatic PEPCK activity was unchanged by STZ. Both FAS and PEPCK are GC and insulin sensitive (50, 51). Under our experimental conditions, FAS appears inherently more insulin sensitive, whereas PEPCK is more GC sensitive. In contrast, liver triglyceride content was largely unaffected by STZ and insulin replacement. In addition, the lack of change of hepatic IR protein expression suggests that the insulin-induced changes within the liver can be attributed to insulin action, as opposed to alterations in liver insulin signaling via the IR.
In conclusion, our data open the possibility that an insulin-stimulated, liver-derived signal is important in regulating fat, but not carbohydrate, intake. The siphoning off of superior mesenteric infused insulin by the liver precludes other insulin-induced actions on other organs. Hence, it is possible that in addition to influencing liver glucose metabolism, a superior mesenteric insulin infusion produces a signal via the liver that stimulates lard intake. Because the liver is not the first, but is an eventual and effective target of the jugular infused insulin, this enables insulin-stimulated effects on growth, adiposity, and plasma variables in addition to the effects on the liver, thus rescuing many of the deranged metabolic parameters of STZ-diabetic rats. However, it is also possible the same outcome on food intake may be derived from distinct, but not mutually exclusive, pathways. Further delineating the mechanisms by which superior mesenteric vs. jugular insulin infusions may differentially act to regulate the choice of macronutrient intake, as they do for growth and adiposity, is important to the understanding of food intake behavior in health and disease and provide novel considerations for the treatment of diabetes.
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Acknowledgments |
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Footnotes |
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Disclosure statement: the authors have nothing to disclose.
First Published Online July 27, 2006
Abbreviations: ADX, Adrenalectomized; B, corticosterone; eWAT, epididymal WAT; FAS, fatty acid synthase; FFA, free fatty acid; GC, glucocorticoid; IR, insulin receptor; mWAT, mesenteric WAT; PEPCK, phosphoenolpyruvate carboxykinase; pWAT, perirenal WAT; scWAT, subcutaneous WAT; STZ, streptozotocin; WAT, white adipose tissue.
Received May 25, 2006.
Accepted for publication July 18, 2006.
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