Primary insulin antagonism of glucose transport in muscle from the older-obese rat

Regulation of IRS-1/SHP2 interaction and AKT phosphorylation in animal models of insulin resistance

Insulin stimulates tyrosine kinase activity of its receptor, resulting in phosphorylation of its cytosolic substrate, insulin receptor substrate-1, which, in turn, associates with proteins containing SH2 domains, including phosphatidylinositol 3-kinase (PI 3-kinase) and the phosphotyrosine phosphatase SHP2. The regulation of these associations in situations of altered insulin receptor substrate-1 (IRS-1) phosphorylation was not yet investigated. In the present study, we investigated insulin-induced IRS-1/SHP2 and IRS-1/PI 3-kinase associations and the regulation of a downstream serine-kinase AKT/PKB in liver and muscle of three animal models of insulin resistance: STZ diabetes, epinephrine-treated rats, and aging, which have alterations in IRS-1 tyrosine phosphorylation in common. The results demonstrated that insulin-induced IRS-1/PI 3-kinase association has a close correlation with IRS-1 tyrosine phosphorylation levels, but insulin-induced IRS-1/SHP2 association showed a modulation that did not parallel IRS-1 phosphorylation, with a tissue-specific regulation in aging. The integration of the behavior of IRS-1/PI 3-kinase and with IRS-1/SHP2 associations may be important for insulin signaling downstream as AKT phosphorylation. In conclusion, the results of the present study demonstrated that insulin-induced IRS-1/SHP2 association can be regulated in insulin-sensitive tissues of animal models of insulin resistance and may have a role in the control of AKT phosphorylation, which may be implicated in the control of glucose metabolism.

Attenuation of age-related declines in glucagon-mediated signal transduction in rat liver by exercise training

This study investigated alterations in glucagon receptor-mediated signal transduction in rat livers from 7- to 25-mo-old animals and examined the effects of exercise training on ameliorating these changes. Sixty-six young (4 mo), middle-aged (12 mo), and old (22 mo) male Fischer 344 rats were divided into sedentary and trained (treadmill running) groups. Isolated hepatic membranes were combined with [(125)I-Tyr(10)]monoiodoglucagon and nine concentrations of glucagon to determine maximal binding capacity (B(max)) and dissociation constant (K(d)). No alterations were found in B(max) among groups; however, middle-aged trained animals had significantly higher glucagon affinity (lower K(d); 21.1 +/- 1.8 nM) than did their untrained counterparts (50.2 +/- 7.1 nM). Second messenger studies were performed by measuring adenylyl cyclase (AC) specific activity under basal conditions and with four pharmacological stimulations to assess changes in receptor-dependent, G protein-dependent, and AC catalyst-dependent cAMP production. Age-related declines were observed in the old animals under all five conditions. Training resulted in increased cAMP production in the old animals when AC was directly stimulated by forskolin. Stimulatory G protein (G(s)) content was reduced with age in the sedentary group; however, training offset this decline. We conclude that age-related declines in glucagon signaling capacity and responsiveness may be attributed, in part, to declines in intrinsic AC activity and changes in G protein [inhibitory G protein (G(i))/G(s)] ratios. These age-related changes occur in the absence of alterations in glucagon receptor content and appear to involve both G protein- and AC-related changes. Endurance training was able to significantly offset these declines through restoration of the G(i)/G(s) ratio and AC activity.

Calorie restriction increases insulin-stimulated tyrosine phosphorylation of insulin receptor and insulin receptor substrate-1 in rat skeletal muscle

A moderate reduction in calorie intake (calorie restriction, CR) improves insulin-stimulated glucose transport in skeletal muscle. Therefore, we studied muscle insulin signalling in ad libitum (AL) and CR ( approximately 60% AL intake for 20 days) fed rats, which received a control injection (sterile water) or an insulin injection (30 U kg-1 body weight). In control (not insulin-treated) rats, there was no detectable tyrosine phosphorylation of insulin receptor (IR), regardless of diet; no diet effect on tyrosine phosphorylation of insulin receptor substrate-1 (IRS1) or IRS1-associated phosphatidylinositol 3-kinase (PI3K) protein and 21% higher IRS1-associated PI3K activity in AL vs. CR. In insulin-treated rats, tyrosine-phosphorylated IR was 79% higher for CR vs. AL; tyrosine-phosphorylated IRS1 was 109% higher for CR vs. AL; IRS1-associated PI3K protein and IRS1-associated PI3K activity were unaffected by diet. Calorie restriction amplifies early insulin signalling steps without changing IRS1-associated PI3K, suggesting enhanced glucose transport is mediated by altering: IRS1-PI3K localization, PI3K associated with proteins other than IRS1 or post-PI3K events.

Lower calorie intake enhances muscle insulin action and reduces hexosamine levels

Previous studies have demonstrated enhanced insulin sensitivity in calorie-restricted [CR, fed 60% ad libitum (AL) one time daily] compared with AL-fed rats. To evaluate the effects of reduced food intake, independent of temporal differences in consumption, we studied AL (unlimited food access)-fed and CR (fed one time daily) rats along with groups temporally matched for feeding [fed 3 meals (M) daily]: MAL and MCR, eating 100 and 60% of AL intake, respectively. Insulin-stimulated glucose transport by isolated muscle was increased in MCR and CR vs. AL and MAL; there was no significant difference for MCR vs. CR or MAL vs. AL. Intramuscular triglyceride concentration, which is inversely related to insulin sensitivity in some conditions, did not differ among groups. Muscle concentration of UDP-N-acetylhexosamines [end products of the hexosamine biosynthetic pathway (HBP)] was lower in MCR vs. MAL despite unaltered glutamine-fructose-6-phosphate aminotransferase activity (rate-limiting enzyme for HBP). These results indicate that the CR-induced increase in insulin-stimulated glucose transport in muscle is attributable to an altered amount, not timing, of food intake and is independent of lower triglyceride concentration. They further suggest that enhanced insulin action might involve changes in HBP.

Effect of calorie restriction on in vivo glucose metabolism by individual tissues in rats

We evaluated the effects of 8 mo of calorie restriction [CR: 60% of ad libitum (AL) food intake] on glucose uptake by 14 tissues in unanesthetized, adult (12 mo) F344xBN rats. Glucose metabolism was assessed by the 2-[3H]deoxyglucose tracer technique at 1500 or 2100. Despite an approximately 60% decline in insulinemia with CR, plasma 2-[3H]deoxyglucose clearance for CR was greater than for AL at both times. A small, CR-related decrease in glucose metabolic index (R'g) occurred only at 1500 in the spleen and heart, and this decrease was reversed at 2100. In some tissues (cerebellum, lung, kidney, soleus, and diaphragm), R'g was unaffected by diet, regardless of time. In the other tissues (brown fat, 3 white fat pads, epitrochlearis, plantaris, and gastrocnemius), R'g was higher or tended to be higher for CR vs. AL at one or both times. These findings indicate that 8 mo of CR did not cause a continuous reduction in in vivo glucose uptake by any tissue studied, and, in several insulin-sensitive tissues, glucose uptake was at times greater for CR vs. AL rats.

Effect of extracellular palmitate on 2-deoxy-d-glucose uptake in muscle from Ad libitum fed and calorie restricted rats

We studied the effect of a high physiologic concentration of palmitate (1mM) on in vitro 2-deoxy-D-glucose (2DG) uptake by flexor digitorum brevis (FDB) muscle from ad libitum fed rats (AL) and rats fed 60% of ad libitum intake (CR) for 20 days. CR did not alter muscle 2DG uptake in the absence of insulin, but relative to AL, CR significantly (p<0.01) increased 2DG uptake in the presence of 20,000 microU/ml insulin. This effect of CR persisted in the presence of 1mM palmitate. The presence of 1mM palmitate significantly (p<0.01) impaired 2DG glucose uptake, both in the presence and absence of insulin, to the same extent in AL and CR muscle, despite an 18% decrease in FABPpm expression with CR. Thus, although CR profoundly affects insulin-mediated muscle glucose uptake, it does not alter the ability of extracellular fatty acid to modulate glucose utilization by skeletal muscle.

Chronic high-fat feeding and middle-aging reduce in an additive fashion Glut4 expression in skeletal muscle and adipose tissue

The interaction of middle-aging and chronic high-fat feeding on glucose transport in skeletal muscle and adipose tissue was examined. To this end, we studied the effects of 6 month treatment with a high-fat diet in 12-month old rats. Chronic high-fat feeding led to a substantial reduction in GLUT4 glucose transporter expression both in adipose tissue and in skeletal muscle, which was additive to the reduction in GLUT4 protein content detected in aged rats. In parallel, the high-fat diet led to a reduced insulin-stimulated glucose transport in the incubated soleus muscle and isolated adipocytes; insulin resistance induced by high-fat feeding was superimposed to the decreased insulin response detected in aged rats. Different mechanisms were responsible for GLUT4 repression in response to high-fat feeding or aging in skeletal muscles and adipose tissue.

Gender differences in diaphragmatic metabolic properties of the adult Sprague-Dawley rat

Adult male Sprague-Dawley rats (> or = 180 days old) develop an obesity-exacerbated insulin resistance in contrast with female animals of the same strain. Given the fact the maintenance of muscle mass requires an adequate supply of insulin and active insulin receptors, we postulated that gender differences might exist in both protein content and metabolic properties of skeletal and cardiac muscle in adult Sprague-Dawley rats. Therefore, to test this hypothesis, we examined activities of bioenergetic enzymes and total protein content in the diaphragm, the heart and the plantaris muscle in 12-month-old male and female animals. Mean (+/- SD) body weights of male animals were significantly (P < 0.05) greater than female animals (598 +/- 8 vs. 362 +/- 19 g) and the diaphragm weight/body weight ratio was significantly lower in males compared to females (2.36 +/- 0.05 vs. 3.02 +/- 0.13 mg/g). The activities of isocitrate dehydrogenase (NADP-specific) and succinate dehydrogenase were significantly lower (P < 0.05) in male animals compared to females in both the crural and costal regions of the diaphragm, the heart, and the plantaris muscle. In contrast, no gender differences (P > 0.05) existed in lactate dehydrogenase activity in any of the muscles studied. Finally, muscle protein concentration was significantly higher in female animals when compared to males (P < 0.05) in all muscles studied except the heart. These data support the hypothesis that gender differences exist for adult Sprague-Dawley rats in general and specific protein content of the diaphragm, locomotor muscles, and the heart.

Glucose transport with brief dietary restriction: heterogenous responses in muscles

The time course (1, 5, or 20 days) for the effect of dietary restriction (DR; approximately 25% reduction below ad libitum intake) on epitrochlearis and flexor digitorum brevis (FDB) muscle glucose transport activity was studied in female Fischer 344 rats (8 mo old). Epitrochlearis glucose transport activity with 100 microU/ml insulin was increased by 38% after 5 days of DR (P < 0.05) despite no change in glucose transport activity with 0 or 20,000 microU/ml insulin. The increase with 100 microU/ml insulin was not further enhanced by 20 days of DR. DR did not result in a significant increase in the glucose transport activity of the FDB with 0, 100, or 20,000 microU/ml insulin. Abdominal fat content was significantly (P < 0.01) reduced below ad libitum levels only after 20 days of DR. These results demonstrate that DR-induced improvement in epitrochlearis glucose transport activity with a physiological insulin concentration can occur very rapidly, preceding detectable changes in basal or maximal insulin-stimulated glucose transport activity or abdominal fat pad mass, and the enhancement of insulin action does not occur simultaneously in all muscles.

Circadian neuroendocrine role in age-related changes in body fat stores and insulin sensitivity of the male Sprague-Dawley rat

A role for circadian neuroendocrine rhythms in the age-related development of obesity and insulin resistance was investigated in the male Sprague-Dawley rat. The phases and amplitudes of the plasma rhythms of several metabolic hormones (i.e., corticosterone, prolactin, insulin, and triiodothyronine) differed in lean, insulin-sensitive (3-week-old rats), insulin-resistant (8-week-old rats) and obese, insulin-resistant (44-week-old rats) animals. Simulation of the daily rhythms of endogenous corticosterone and prolactin by daily injections of the hormones at times corresponding to the peak levels found in 3-week-old rats reversed age-related increases in insulin resistance and body fat in older (5-6-month-old) rats. Ten such daily injections of corticosterone and prolactin in 12-14-week-old rats produced long-term reductions in body fat stores (30%), plasma insulin concentration (40%), and insulin resistance (60%) (determined by a glucose tolerance test) measured 11-14 weeks after the treatment. Alterations in circadian neuroendocrine rhythms may account for age-related changes in carbohydrate and lipid metabolism in the male Sprague-Dawley rat, and resetting of these rhythms by appropriately timed daily injections of corticosterone and prolactin may help maintain metabolism characteristic of younger animals.

Glucose transporters and glucose transport in skeletal muscles of 1- to 25-mo-old rats

It is widely thought that aging results in development of insulin resistance in skeletal muscle. In this study, we examined the effects of growth and aging on the concentration of the GLUT-4 glucose transporter and on glucose transport activity in skeletal muscles of female Long-Evans rats. Relative amounts of immunoreactive GLUT-4 protein were measured in muscle homogenates of 1-, 10-, and 25-mo-old rats by immunoblotting with a polyclonal antibody directed against GLUT-4. In the epitrochlearis, plantaris, and the red and white regions of the quadriceps muscles, GLUT-4 immunoreactivity decreased by 14-33% between 1 and 10 mo of age and thereafter remained constant. In flexor digitorum brevis (FDB) and soleus muscles, GLUT-4 concentration was similar at all three ages studied. Glucose transport activity was assessed in epitrochlearis and FDB muscles by incubation with 2-deoxyglucose under the following conditions: basal, submaximal insulin, and either maximal insulin or maximal insulin combined with contractile activity. Glucose transport in the epitrochlearis muscle decreased by approximately 60% between 1 and 4 mo of age and then did not decline further between 4 and 25 mo of age. Transport activity in the FDB assessed with a maximally effective insulin concentration decreased only slightly (< 20%) between 1 and 7 mo of age. Aging, i.e., the transition from young adulthood to old age, was not associated with a decrease in glucose transport activity in either the epitrochlearis or the FDB.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of wheel running on glucose transporter (GLUT4) concentration in skeletal muscle of young adult and old rats

We examined the effects of voluntary exercise on glucose transporter concentration in skeletal muscle from young adult and old female Long-Evans rats. Rats had free access to voluntary running wheels beginning at 4 months of age or remained sedentary. Exercising rats ran approximately 7.5, 6.2, 5.6 and 5.3 km/day during their 6th, 8th, 9th and 10th month of age, respectively. During the 23rd, 24th and 25th month of age running distance averaged 3.0, 2.8 and 2.4 km/day, respectively. At 10 and 25 months of age, glucose transporter protein concentration was assessed in epitrochlearis and flexor digitorum brevis muscles with a polyclonal antibody directed against the GLUT4 transporter isoform. GLUT4 protein concentration was not altered by the aging process (i.e., comparing 10- and 25-month-old rats) in either muscle type. Wheel running increased GLUT4 protein concentration by 45% in epitrochlearis muscles of 10-month-old rats relative to age-matched sedentary controls. The training-induced adaptation in GLUT4 protein was no longer present at age 25 months, probably because the running distance had declined by 50%. In the flexor digitorum brevis, exercise did not alter GLUT4 concentration at either 10 or 25 months, presumably due to insufficient recruitment of this muscle during wheel running as assessed by measurement of citrate synthase and hexokinase enzyme activities. Wheel running induced cardiac and soleus muscle hypertrophy in 10- and 25-month-old rats. In summary, voluntary wheel running can induce an increase in skeletal muscle GLUT4 protein concentration in adult rats. Older rats that run less exhibit cardiac and soleus muscle hypertrophy, but do not maintain an elevated GLUT4 protein concentration in the epitrochlearis muscle. Aging does not alter GLUT4 protein concentration in the epitrochlearis or FDB muscles.

Effects of aging on the responsiveness and sensitivity of glucose metabolism to insulin in the incubated soleus muscle isolated from Sprague-Dawley and Wistar rats

1. The effects of aging on the sensitivity and responsiveness of glucose transport, lactate formation and glycogen synthesis to insulin were studied in the incubated stripped soleus muscle isolated from aging Sprague-Dawley and Wistar rats. 2. As Sprague-Dawley rats aged from 5 to 13 weeks, there were marked increases in the concentrations of insulin that were required for half-maximal stimulation (i.e. EC50 value, which is a measure of sensitivity) of glucose transport, lactate formation and glycogen synthesis. 3. In marked contrast, there were no alterations in sensitivities of any of these processes to insulin in soleus muscle prepared from Wistar rats aged between 6 and 12 weeks. 4. However, in soleus muscles from 85-week-old Wistar rats the rates of glycogen synthesis in response to basal, sub-maximal and maximal concentrations of insulin were markedly decreased. The insulin EC50 value of glycogen synthesis was increased 4-fold, but was unchanged for lactate formation. 5. The insulin-stimulated rates of glucose transport in soleus muscles from 5- or 85-week-old Wistar rats were not significantly different.

Impaired renal and extrarenal potassium adaptation in old rats

Young (3 to 4 months) and old (21 to 22 months) rats were fed either a regular or high potassium (K) diet. After acute potassium chloride infusion, the fraction of infused K excreted (K efficiency) was similar in rats on a normal diet (57 +/- 3%, young, vs. 61 +/- 2%, old). With high K feeding there was a significant increase in the young, 69 +/- 4%, but not in the old rats, 62 +/- 2%. Na-K ATPase activity was markedly reduced in the renal medulla of old rats on a regular or high K diet. In addition, the response to acute K loading was compared in acutely nephrectomized rats. In the young rats on a regular diet plasma K increased from 3.72 +/- 0.09 to 5.28 +/- 0.16 mEq/liter while with K ingestion the increase was significantly less, 3.62 +/- 0.07 to 4.75 +/- 0.12 mEq/liter. In the old rats plasma K increased similarly on a regular or high K diet, 3.68 +/- 0.10 to 5.68 +/- 0.33 mEq/liter and 3.76 +/- 0.06 to 5.97 +/- 0.30 mEq/liter, respectively. Thus, old rats have impaired renal and extrarenal adaptation, but they have a normal response to an acute K challenge. A reduction in Na-K ATPase may account for the defect in renal adaptation in the aged rats.

The effect of aging on carbohydrate metabolism: a review of the English literature and a practical approach to the diagnosis of diabetes mellitus in the elderly

There seems little doubt that the disposal of a glucose load is progressively impaired during aging. The mechanism(s) for this alteration remains unclear. Five possibilities have been raised: (1) poor diet, (2) physical inactivity, (3) decreased lean body mass in which to store the carbohydrate load, (4) decreased insulin secretion, and (5) insulin antagonism. Although poor diet and physical inactivity may contribute to some of the abnormal glucose tolerance tests of the older population, these two factors do not provide a full explanation. Diminished lean body mass may play some role but there is almost certainly an additional effect due to aging. A few papers have suggested that glucose-induced insulin secretion may be impaired as the population ages, but the bulk of studies in this area conclude that normal or increased amounts of insulin are released by the pancreatic beta-cell during aging. If abnormalities of insulin secretion exist, either in degree or timing, they are subtle and would not seem sufficient to account for the great number of older subjects who manifest impaired glucose tolerance. The evidence for insulin antagonism seems the strongest but the data are certainly not conclusive. In actuality, the aging effect on carbohydrate metabolism may be heterogeneous in nature. Either some or all of these five factors may contribute to the aging effect to varying degrees in individual subjects. Alternatively, the glucose intolerance of aging may represent a heterogeneous group of disorders. In any event, until better methods to identify possible subgroups of these subjects and/or a marker for diabetes mellitus independent of glucose concentration become available, this problem will remain difficult to resolve. Based on the currently available data, it seems prudent to diagnose diabetes mellitus only if fasting hyperglycemia is present.

Lipid metabolism by muscle of diabetic rats

This study was undertaken to ascertain whether enhanced oxidation of intracellular lipids could explain the impaired carbohydrate metabolism of diabetes. Pieces of diaphragms removed from diabetic (60--75 mg/kg streptozotocin i.v.) and control rats were incubated for 1 h with palmitate-1-14C. Tissue lipids from one piece were separated on silicic acid columns and the amount and specific activity of free fatty acids (FFA), triglycerides (TG) and phospholipids (PL) were measured. 14CO2 production was also assessed in some experiments. The other pieces of tissue were incubated for a subsequent hour (without radioactivity) at which time measurements of tissue lipid content and specific activity and 14CO2 production were again performed. FFA incorporation into CO2, tissue TG and PL was normal. TG content was moderately and PL content was slightly reduced in diabetic tissue. Changes in diaphragm TG and PL content and specific activity during the 2nd h of incubation strongly suggested that most of the 14CO2 produced during this period was derived from TG. Approximately 25% of tissue TG in both control and diabetic muscle was oxidized to CO2 during the 2nd h of incubation. In diaphragms from diabetic rats, (+)-octanoylcarnitine (an inhibitor of FFA oxidation) decreased TG oxidation considerably but had no effect on the impaired glucose uptake. Thus, these data do not support the hypothesis that the glucose-fatty acid cycle (utilizing either extra- or intracellular lipids) may account for the altered carbohydrate metabolism of diabetic muscle.