Sensitization of diabetic and obese mice to insulin by retinoid X receptor agonists.

Retinoic acid receptors (RAR), thyroid hormone receptors (TR), peroxisome proliferator activated receptors (PPARs) and the orphan receptor, LXR, bind preferentially to DNA as heterodimers with a common partner, retinoid X receptor (RXR), to regulate transcription. We investigated whether RXR-selective agonists replicate the activity of ligands for several of these receptors? We demonstrate here that RXR-selective ligands (referred to as rexinoids) function as RXR heterodimer-selective agonists, activating RXR: PPARgamma and RXR:LXR dimers but not RXR:RAR or RXR:TR heterodimers. Because PPARgamma is a target for antidiabetic agents, we investigated whether RXR ligands could alter insulin and glucose signalling. In mouse models of noninsulin-dependent diabetes mellitus (NIDDM) and obesity, RXR agonists function as insulin sensitizers and can decrease hyperglycaemia, hypertriglyceridaemia and hyperinsulinaemia. This antidiabetic activity can be further enhanced by combination treatment with PPARgamma agonists, such as thiazolidinediones. These data suggest that the RXR:PPARgamma heterodimer is a single-function complex serving as a molecular target for treatment of insulin resistance. Activation of the RXR:PPARgamma dimer with rexinoids may provide a new and effective treatment for NIDDM.

Nutritional status modifies insulin-mediated glucose uptake in spontaneously hypertensive rats.

In this study we quantified insulin-mediated glucose uptake in weight-matched (260-330 g) fed (6-8 h fast) and fasted (24 h fast) male rats with spontaneous hypertension (SHR) and control Wistar-Kyoto (WKY) rats. To accomplish this goal, rats were infused continuously for 165 min with glucose and insulin. Blood was taken at frequent intervals from 120-165 min, and the values averaged to determine the steady-state plasma glucose (SSPG) and insulin (SSPI) concentrations. In some studies epinephrine and propranolol were added to the infusate in order to suppress endogenous insulin secretion. Steady-state plasma insulin (SSPI) concentrations were similar in SHR and WKY during the three infusion studies (382-483 pmol/L). However, SSPG was significantly higher in fed SHR as compared to fed WKY during infusions performed with (9.4 +/- 0.8 vs 7.0 +/- 0.4 mmol/L, p < 0.05) or without (8.6 +/- 0.2 vs 7.0 +/- 0.6 mmol/L, p < 0.05) epinephrine and propranolol in the infusate. In contrast, SSPG concentrations (mmol/L) were similar in SHR (6.8 +/- 0.3) and WKY rats (6.5 +/- 0.6) when they were studied after a 24 h fast. These results demonstrates that differences in insulin-mediated glucose removal from plasma between SHR and WKY rats will vary as a function of nutritional status.

Hypertriglyceridemic mice transgenic for the human apolipoprotein C-III gene are neither insulin resistant nor hyperinsulinemic.

Plasma glucose and insulin concentrations and in vivo and in vitro estimates of insulin action were compared in hypertriglyceridemic apolipoprotein C-III transgenic mice (mean +/- SE triglyceride concentration = 11.8 +/- 0.9 mmol/l) and their normotriglyceridemic (1.1 +/- 0.1 mmol/l) littermates. There were no differences in the glucose (8.9 +/- 0.2 vs. 9.3 +/- 0.5 mmol/l) or insulin (172 +/- 21 vs. 203 +/- 17 pmol/l) concentrations of the transgenic and control mice, respectively. Steady-state plasma glucose concentrations at the end of a 150-min period of physiological hyperinsulinemia were also similar in transgenic (6.2 +/- 0.5 mmol/l) and control mice (6.7 +/- 0.5 mmol/l). As the steady-state plasma insulin levels were essentially identical in the two groups (approximately 1000 pmol/l), these results show that whole body insulin-mediated glucose disposal was unchanged in the transgenic mice. Finally, values for isoproterenol-stimulated lipolysis, insulin-inhibition of lipolysis, and insulin-stimulated glucose disposal were similar in adipocytes isolated from transgenic and control mice. It can be concluded from these data that insulin resistance does not develop in hypertriglyceridemic mice transgenic for the human apolipoprotein C-III gene.

Glucose transporter (GLUT 4) content and insulin receptor kinase activity in muscles of the LA/N-cp rat.

In this study we compared insulin binding activity, insulin receptor tyrosine kinase activity, and GLUT 4 protein content in six muscles from LA/N-cp rats and their lean controls. LA/N-cp rats had an approximate 20-fold increase in insulin concentration (837 +/- 113 vs 40 +/- 1), associated with significant (p < 0.01) decreases in both insulin binding activity per mg muscle and in muscle GLUT 4 content. Maximum insulin tyrosine kinase activity was also lower in muscle from LA/N-cp rats, but no difference was noted when tyrosine kinase activity was expressed per receptor. These data indicate that there are at least two defects in the insulin action cascade in muscle from LA/N-cp rats that contribute to the insulin resistance in these animals.

Why does insulin resistance develop during maturation?

We compared skeletal muscle glucose uptake between young and mature rats. Hindlimb perfusions at insulin concentrations of 0, 100, 250, or 10,000 microU/mL were performed on male Sprague-Dawley rats at 5 weeks or 4 months of age. Basal glucose uptake, and glucose uptake at all insulin concentrations were significantly lower in the 4-month-old mature rats (p < .05). This difference was most pronounced at maximally stimulating insulin concentrations. Skeletal muscle insulin receptor binding, autophosphorylation, and tyrosine kinase activity did not differ between young and mature rats. Surprisingly, GLUT-4 glucose transporter content was significantly higher in several muscles of the mature rats (p < .05). Therefore, the decline in insulin-stimulated glucose uptake in hindlimbs of mature rats cannot be explained by decreased activity of these steps in the glucose transport system.

Mechanism of hypertriglyceridemia in Dahl rats.

Plasma triglyceride concentrations were shown to be higher in hypertensive (153 +/- 2 mm Hg) male Dahl salt-sensitive rats than in control Sprague-Dawley rats (122 +/- 2 mm Hg). These differences in triglyceride concentrations were seen when blood was drawn at 9 AM from unfasted animals (229 +/- 27 versus 111 +/- 8 mg/dL), at 1 PM after a 4-hour fast (186 +/- 13 versus 88 +/- 4 mg/dL), or at 9 AM after a 13-hour fast (151 +/- 6 versus 90 +/- 6 mg/dL), all p < 0.001. Total triglyceride secretion was also compared in groups of rats by determining the increment in plasma triglyceride concentration for 2 hours after blocking triglyceride removal from plasma by injecting Triton. Studies performed at 1 PM and 9 AM, after the 4- and 13-hour fast, demonstrated that total triglyceride secretion was greater (p < 0.05) in Dahl rats only when studied at 1 PM. Direct estimates of hepatic triglyceride secretion at 1 PM also demonstrated a significant (p < 0.02) increase in secretion rate by perfused livers from Dahl rats, due in part to their increased liver size. In addition, removal of prelabeled very low density lipoprotein-triglyceride in the intact rat was significantly (p < 0.05) decreased in Dahl rats. Lipoprotein lipase activity measured in skeletal muscle, heart, and adipose tissue was also significantly decreased at 9 AM and 1 PM (after 0 and 4 hours of fasting) in tissue from Dahl rats. These data confirm that Dahl rats have higher plasma triglyceride concentrations than Sprague-Dawley rats. Since both total and hepatic triglyceride secretion were somewhat greater in Dahl rats, in association with a decrease in both removal of very low density lipoprotein from plasma and decreased muscle and adipose tissue lipoprotein lipase activity, it seems likely that hypertriglyceridemia in Dahl rats results from a combination of increased triglyceride secretion and decreased triglyceride removal.

Lactate production and pyruvate dehydrogenase activity in fat and skeletal muscle from diabetic rats.

This study was initiated to explore the possibility that an increase in the supply of gluconeogenic precursors contributes to the overproduction of glucose by the liver in NIDDM patients. To address this issue, a form of experimental NIDDM was produced in rats by injecting a low dose (38 mg/kg) of STZ and comparing lactate and alanine production and PDH activity in skeletal muscle and isolated adipocytes from normal and diabetic rats. Skeletal muscle lactate production was measured by using a hindlimb perfusion technique and was significantly greater (P < 0.01) in the diabetic rats compared with two groups of control rats: one perfused at normal glucose levels and the other perfused at glucose concentrations comparable with those observed in diabetic rats. Alanine production by hindlimb from diabetic rats was 46% greater than hindlimbs from control rats perfused at normal glucose levels (P < 0.01) but was not significantly greater than control rats perfused at diabetic glucose levels. The percentage of glucose converted to lactate by muscle from both control groups was 4-5%, significantly lower than the 18% conversion rate observed in diabetic animals (P < 0.001). An increase in the ratio of lactate produced/glucose transport by isolated adipocytes from diabetic rats also was observed when measured in both the basal state (0.65 +/- 0.12 vs. 0.15 +/- 0.03, P < 0.01) and in the presence of maximal amounts of insulin (0.15 +/- 0.02 vs. 0.04 +/- 0.01, P < 0.02).(ABSTRACT TRUNCATED AT 250 WORDS)

Differential metabolic responses to tumor necrosis factor with increase in age.

Previous studies have shown varied responses to the effects of tumor necrosis factor (TNF) on glucose and lipid metabolism. To elucidate possible causes for this variation, the present study compared sequential changes in plasma glucose, lactic acid, triglyceride (TG), free fatty acids (FFA), and plasma insulin levels in 1.5- and 16-month-old, normal, fed, male rats, 1 to 6 hours after different doses of intravenous (IV) TNF. In addition, assessment was made of TNF injected intraperitoneally (IP) in precannulated and intact young (1.5 months) rats and of the dose-response (0.25 to 50 micrograms/100 g rat) and the sensitivity to insulin in intact rats. Finally, the metabolic responses and changes in serum insulin and corticosterone concentration after IP TNF were compared in 1.5-, 5-, and 16-month-old rats. Data show that metabolic responses vary with increase in age and experimental conditions. Dose-dependent decreases in plasma glucose (1.4 to 3.9 mumol/mL) and elevations in lactic acid (0.8 to 3.0 mumol/mL) were greater in 1.5- versus 16-month-old rats, were delayed in cannulated rats, and were preceded by hyperglycemia following larger IV doses. Plasma TG levels were elevated after TNF in all groups except precannulated rats, and also showed differences with age. In young rats, the elevation in TG peaked 2 hours after IP injection with return to baseline and was preceded by an elevation in FFA levels. In older rats, which were hypertriglyceridemic at base line, the elevation in TG by TNF occurred by suppressing the decrease in TG of controls, was not accompanied by an increase in FFA levels, was sustained for 5 hours, and was of greater magnitude than in young rats. Significant changes in plasma insulin did not occur in young and older rats after IV TNF. However, young rats had a significant decrease (P less than .02) in plasma insulin and an elevation in corticosterone levels after IP TNF, whereas older rats exhibited an increase in plasma insulin (P less than .02) and a comparable elevation in plasma corticosterone. Young rats also showed an increase in plasma insulin following IP TNF when hypoglycemia was prevented by the infusion of glucose. However, when insulin levels were held comparable (2.4 ng/mL), glucose uptake was enhanced (P less than .05) compared with controls. These findings indicate that mobilization of energy substrates occurs during the initial exposure to TNF, which is altered by the nutritional state of the rats and the dose and route of administration.(ABSTRACT TRUNCATED AT 400 WORDS)

Alterations in glucose and protein metabolism in animals subjected to simulated microgravity.

Reduction of physical activity due to disease or environmental restraints, such as total bed rest or exposure to spaceflight, leads to atrophy of skeletal muscle and is frequently accompanied by alterations in food intake and the concentration of metabolic regulatory hormones such as insulin. Hindlimb suspension of laboratory rats, as a model for microgravity, also shows marked atrophy of gravity dependent muscles along with a reduced gain in body weight. Suspended rats exhibit enhanced sensitivity to insulin-induced glucose uptake when compared with normal control rats and resistance to insulin action when compared with control rats matched similarly for reduced body weight gain. These changes are accompanied by decreased insulin binding and tyrosine kinase activity in soleus but not plantaris muscle, unchanged glucose uptake by perfused hindlimb and decreased sensitivity but not responsiveness to insulin-induced suppression of net proteolysis in hindlimb skeletal muscle. These findings suggest that loss of insulin sensitivity during muscle atrophy is associated with decreased insulin binding and tyrosine kinase activity in atrophied soleus muscle along with decreased sensitivity to the effects of insulin on suppressing net protein breakdown but not on enhancing glucose uptake by perfused hindlimb.

Effectiveness of a purified human hemoglobin as a blood substitute in the perfused rat liver.

The efficacy of purified cross-linked human hemoglobin solution in maintaining the metabolic integrity of perfused livers from fed rats was determined and compared with that of livers recirculated with red blood cells, perfluorocarbon, and Krebs' Ringer bicarbonate buffer media at normal and accelerated flow rates. The data indicate that oxygen utilization was comparable in livers perfused with red blood cell medium (53.5 +/- 4.0 microL.g liver-1.min-1), hemoglobin (45.7 +/- 1.9), and perfluorocarbon (57.2 +/- 6.1) and less in livers perfused with Krebs' Ringer bicarbonate buffer solution at normal (17.4 +/- 1.4) and high (27.7 +/- 1.4) flow rates. Bile flow, the outflow of glucose and lactic acid, and residual glycogen levels were similar when livers were perfused with red blood cells (hematocrit, 19) and hemoglobin solutions containing 7 g hemoglobin/dL at flow rates of 1.1-1.2 mL.g liver-1.min-1. However, livers perfused with perfluorocarbon at 1.1 mL.g-1.min-1 showed a significantly greater (P less than 0.01) decline in bile flow and outflow of glucose and lactic acid. Livers perfused with Krebs' Ringer bicarbonate buffer at normal (1.3 mL.g-1.min-1) and accelerated flow rates (3.0 mL.g-1.min-1) also showed a progressive decrease in bile flow and marked glycogenolysis as well as depletion of adenosine triphosphate content. In addition, morphological studies (light and electron microscopy) showed more vacuoles, membrane alterations, and increased mitochondrial swelling in livers perfused with Krebs' Ringer bicarbonate buffer and perfluorocarbon. These findings suggest that hepatocyte function in livers from fed rats is maintained equally well with hemoglobin solutions and with red blood cell medium, suggesting that cross-linked hemoglobin solution may serve as an effective blood substitute for maintaining adequate oxygenation and metabolic integrity of the isolated perfused rat liver.

Effect of denervation or unweighting on GLUT-4 protein in rat soleus muscle.

The purpose of this study was to test the hypothesis that the decreased capacity for glucose transport in the denervated rat soleus and the increased capacity for glucose transport in the unweighted rat soleus are related to changes in the expression of the regulatable glucose transporter protein in skeletal muscle (GLUT-4). One day after sciatic nerve sectioning, when decreases in the stimulation of soleus 2-deoxyglucose (2-DG) uptake by insulin (-51%, P less than 0.001), contractions (-29%, P less than 0.05), or insulin and contractions in combination (-40%, P less than 0.001) were observed, there was a slight (-18%, NS) decrease in GLUT-4 protein. By day 3 of denervation, stimulation of 2-DG uptake by insulin (-74%, P less than 0.001), contractions (-31%, P less than 0.001), or the two stimuli in combination (-59%, P less than 0.001), as well as GLUT-4 protein (-52%, P less than 0.001), was further reduced. Soleus muscle from hindlimb-suspended rats, which develops an enhanced capacity for insulin-stimulated glucose transport, showed muscle atrophy similar to denervated soleus but, in contrast, displayed substantial increases in GLUT-4 protein after 3 (+35%, P less than 0.05) and 7 days (+107%, P less than 0.001). These results indicate that altered GLUT-4 expression may be a major contributor to the changes in insulin-stimulated glucose transport that are observed with denervation and unweighting. We conclude that muscle activity is an important factor in the regulation of GLUT-4 expression in skeletal muscle.

Characterization of insulin receptor kinase activity and autophosphorylation in different skeletal muscle types.

We have examined insulin binding, autophosphorylation, and tyrosine kinase activity in detergent-solubilized and wheat germ agglutinin-purified insulin receptor preparations from four rat muscles of different fiber composition (i.e., tensor fascia latae, soleus, vastus intermedius, and plantaris). Insulin binding activity was similar in three of the four muscles but lower in tensor fascia latae. No significant differences were noted in the affinity of insulin for its receptor from various muscle types. Insulin receptor tyrosine kinase activity measured in the absence (basal) and presence of insulin (0.3-300 nM) was comparable in all muscle types (normalized to the amount of insulin bound). Insulin sensitivity, measured as the dose of insulin required for half-maximal activation of kinase activity, was also similar in all muscle types. Likewise, incubation of receptor preparations with [gamma-32P]ATP, Mn2+, and insulin (0.25-100 nM) resulted in a dose-dependent autophosphorylation of the beta-subunit (relative molecular weight approximately 95 kDa) with similar kinetics in all muscle types. In conclusion, these results show that the functional behavior of the insulin receptor autophosphorylation-kinase system (in vitro) is not changed by alterations in muscle fiber composition, indicating that differences in insulin sensitivity between different skeletal muscle types is probably not due to modulation of the insulin receptor phosphorylation system.

Effects of exercise training on insulin-regulatable glucose-transporter protein levels in rat skeletal muscle.

Exercise training has been shown to enhance the ability of insulin to stimulate glucose uptake in responsive tissues. The purpose of this study was to determine the effects of exercise training on the levels of the insulin-regulatable glucose transporter (IRGT) in rat skeletal muscle. After 6 wk of voluntary running in exercise-wheel cages, male Sprague-Dawley rats were rested for approximately 27 h and fasted overnight before removal of plantaris and soleus muscles. The concentration of glucose transporters per unit of muscle protein or DNA was quantitated by immunoblotting with an anti-IRGT polyclonal antibody raised against a synthetic peptide. The IRGT protein was increased by 60% (141 +/- 14 vs. 229 +/- 24 counts/min [cpm]/25 micrograms protein, P less than 0.01) in plantaris muscle from exercise-trained rats compared with controls. Total protein yield, DNA content, and 5'-nucleotidase activity were not different in plantaris muscle from control and exercise-trained rats. In contrast, there was no significant increase in the IRGT protein in soleus muscle after training when data were expressed per unit of muscle protein (292 +/- 22 vs. 346 +/- 16 cpm/25 micrograms protein). These data indicate that the increase in the IRGT in plantaris muscle is a selective response to exercise training that does not reflect an overall increase in muscle protein. The changes in IRGT for these muscles with exercise training parallel changes observed in insulin-mediated glucose uptake. We propose that this increase in the total number of glucose transporters may be a major component of the increase in insulin-mediated glucose uptake that is observed with exercise training.

Effects of insulin on carbohydrate and protein metabolism in voluntary running rats.

The goal of this study was to assess the effects of voluntary running activity in rats on various aspects of carbohydrate and protein metabolism. After 6 wk of exercise training, rats (ET) were rested for 24 h and their hindquarters, along with those from sedentary control (SC) and dietary control (DC) rats, were perfused with 0, 60, 250, or 6,000 microU/ml insulin. At 0 insulin, glucose clearance was similar for all groups, and it was increased with added insulin. However, the insulin effect was 20-40% greater for ET rats at all insulin concentrations (P less than 0.05). Muscle glycogen deposition also increased with added insulin but showed muscle-specific differences. Specifically, glycogen content of the plantaris muscle was significantly higher in ET compared with SC or DC rats, whereas this pattern was reversed in soleus muscle. In plantaris muscle, insulin stimulated glucose 6-phosphate (G-6-P)-independent (-G-6-P) glycogen synthase activity only in SC and DC rats and increased its affinity for G-6-P at 250 microU/ml in all groups. In contrast, the -G-6-P synthase activity was not increased in soleus muscle and was actually decreased in all groups at 6,000 microU/ml. Tyrosine release was suppressed by insulin in all groups, but this effect was significantly greater at insulin levels of 60 microU/ml (P less than 0.02) in hindquarters from ET rats compared with SC and DC rats. Neither insulin nor exercise training decreased 3-methylhistidine release from perfused hindquarters.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of perfluorochemical artificial blood on lipoprotein metabolism in rats.

Experiments were performed to evaluate the utility of a perfluorochemical emulsion as an artificial blood substitute for studies of lipoprotein metabolism in rats. Perfusing the liver of fed rats with perfluorochemical emulsion FC-34 at the same rate as a 20% red blood cell (RBC) perfusate, there was comparable oxygen uptake; however, there was a greater release of glucose and production of lactate than in RBC perfused livers. Under the stimulation of a low level of free fatty acid, there was less free fatty acid uptake and less triglyceride secretion in emulsion perfused livers. The lipoprotein secreted contained similar apoprotein, but there was a lower triglyceride to cholesterol ratio in the emulsion perfused liver. In addition to these moderate metabolic alterations, the uptake of radiolabeled chylomicron remnants by the perfused liver was almost completely suppressed when the perfluorochemical emulsion was used as an oxygen carrier. In vivo the presence of the perfluorochemical emulsion (5% of blood volume) decreased the rate of clearance of chylomicron remnants, beta-very-low-density lipoprotein (beta-VLDL) and cholesterol-rich high-density lipoprotein (HDLc), but not of low-density lipoprotein (LDL). In the presence of the emulsion, the degradation of 125I remnants, but not of [125I]LDL, by rat hepatoma cells was inhibited. The perfluorochemical emulsion did not inactivate lipoprotein lipase. The perfluorochemical emulsion did not change the triglyceride concentration or apoprotein composition of chylomicron remnants when they were incubated with the perfluorochemical emulsion at 37 degrees C for 1 hour and reisolated. The detergent used to solubilize the fluorocarbon FC-43, Pluronic F-68, did not affect the removal of chylomicron remnants in vivo.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of body weight gain on insulin sensitivity after retirement from exercise training.

The objectives of this study were to determine how long increased insulin sensitivity, elicited by exercise training, persists after the end of training and what the effect of weight gain is on this retention. Exercise-trained (ET) rats ran voluntarily in freely rotating wheel cages, and insulin sensitivity was assessed by oral glucose tolerance tests (OGTT) and insulin suppression tests (IST). After training, ET rats were retired for 1, 3, or 7 days (R1, R3, or R7). Initial OGTT and IST studies indicated that sensitivity to insulin-induced glucose uptake was increased in ET rats compared with sedentary control (C) rats and was progressively lost with retirement: ET greater than R1 and R3 greater than R7 and C rats, and this reaction was generally associated with a rapid gain in body weight. Subsequent IST tests were performed on C and R7 rats fed laboratory chow or a hypocaloric diet consisting of equal parts of cellulose and chow for 7 days before the test. The results of these tests showed that steady-state serum glucose (SSSG) levels averaged 165 +/- 12 mg/dl for chow-fed C rats and 172 +/- 11 mg/dl for chow-fed R7 rats that gained body weight at rates twice those of C rats. Chow-fed R7 rats, gaining weight at rates comparable to C rats, had SSSG levels of 104 +/- 6 mg/dl. C and R7 rats fed the hypocaloric diet had SSSG values of 102 +/- 6 and 59 +/- 4 mg/dl, respectively. Muscle glycogen levels were comparable in all groups, and liver glycogen was lower in C and R7 rats fed the hypocaloric diet.(ABSTRACT TRUNCATED AT 250 WORDS)

Differences in insulin-induced glucose uptake and enzyme activity in running rats.

To evaluate the relationship between enhanced insulin action and level of exercise training, in vivo glucose uptake was assessed in the absence of added insulin and during insulin-stimulated conditions for three activity levels of voluntarily trained rats (low 2-5 km/day, medium 6-9 km/day, high 11-16 km/day). After rats rested for 24 h and fasted overnight, glucose uptake was estimated by comparing steady-state serum glucose (SSSG) levels at low insulin (SSSI) concentrations achieved during an insulin suppression test. In the absence of added insulin, SSSI averaged approximately 20 microU/ml and glucose uptake was similar for high runners and younger weight-matched controls. However, with insulin added to sustain SSSI at approximately 35 microU/ml, SSSG was significantly reduced in all runners (P less than 0.02), with the lowest value attained in high runners. Fasting serum triglycerides were also reduced in all runners (P less than 0.05), with the lowest values seen in medium and high runners. The concentration of glycogen in liver and select skeletal muscles at the start of the study was not different between trained and control rats, suggesting that enhanced insulin-stimulated glucose uptake was not the result of lower glycogen levels. In addition, glycogen synthase and succinate dehydrogenase activities in biceps femoris muscle were only elevated for high runners, but glycogen synthase activity was not enhanced in plantaris muscle and was decreased in soleus muscle. These findings indicate that enhanced insulin-stimulated glucose uptake and reduced serum triglyceride concentrations induced in exercise-trained rats at varying activity levels are dissociated from changes in glycogen synthase and oxidative enzyme activity for skeletal muscle.

Abnormal insulin metabolism by specific organs from rats with spontaneous hypertension.

Spontaneously hypertensive rats (SHR) have been shown to be both insulin resistant and hyperinsulinemic after oral glucose administration or infusion of exogenous insulin during an insulin suppression test. To determine if this hyperinsulinemia may be due to decreased removal of insulin, the metabolic clearance (k) of insulin was measured in isolated perfused liver, kidney, and hindlimb skeletal muscle from SHR and Wistar-Kyoto (WKY) control rats. The data indicate that the k for insulin removal by liver was similar in SHR and WKY rats, averaging 287 +/- 18 and 271 +/- 10 microliters.min-1.g-1 liver, respectively. In contrast, the k for insulin removal by hindlimbs from SHR was decreased 37% (P less than 0.001) compared with WKY rats (8.6 +/- 0.5 vs. 13.7 +/- 0.7 microliters.min-1.g-1 muscle), and this decrease was not accompanied by decreased binding of insulin to its receptor in plantaris muscle. Although the removal of insulin by glomerular filtration was similar in SHR and WKY rats (653 +/- 64 microliters/min vs. 665 +/- 90 microliters.min-1.kidney-1), total insulin removal by kidney was significantly lower (P less than 0.05) in SHR (710 +/- 78 microliters/min) compared with WKY rats (962 +/- 67 microliters/min), due to decreased peritubular clearance of insulin in SHR (56 +/- 73 vs. 297 +/- 59 microliters/min, P less than 0.05). These findings suggest that the decreased clearance of insulin in SHR rats was possibly not due to impaired hepatic removal of insulin but rather to decreased removal by skeletal muscle and kidneys.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of exercise training on the relationship between insulin binding and insulin-stimulated tyrosine kinase activity in rat skeletal muscle.

The effect of exercise training on insulin binding and insulin receptor tyrosine kinase activity was studied using detergent solubilized wheat germ agglutinin (WGA)-agarose purified receptor preparations from rat biceps femoris (BF) and tensor fascia lata (TFL) muscles. Insulin receptor activity, as assessed by A14 [125I] insulin binding, was significantly elevated in BF of exercise-trained rats when compared with similar preparations from a sedentary control group. This increase in binding activity was due to change in Bmax not KD. In contrast, neither the Bmax nor the KD of insulin binding to TFL changed with exercise training. The structure of insulin receptors isolated from BF or TFL was unaltered by exercise training as determined by affinity labeling (alpha-subunit, molecular weight (mol wt) approximately 131 kilodaltons [kDa]) and electrophoretic mobility of the alpha- and beta-subunit. Furthermore, basal tyrosine kinase activity was not affected by exercise training in extracts from either BF or TFL. However, the insulin dependent increase in maximal tyrosine kinase activity (Vmax) of the BF, but not TFL, was enhanced by exercise training. Specifically, insulin stimulated phosphorylation of both the beta-subunit of the insulin receptor (auto phosphorylation) and of a synthetic peptide (exogenous phosphorylation) were increased over control values in BF from exercise-trained rats, whereas both measurements of tyrosine kinase activity of TFL from the two experimental groups were similar. In contrast, both insulin-stimulated autophosphorylation and tyrosine kinase activity were significantly decreased in BF of exercise-trained rats when normalized to insulin binding activity. This disassociation was only seen in BF from exercise-trained rats, and was not true of TFL. These data indicate that exercise training can lead to increases in insulin receptor number and tyrosine kinase activity, as well as modifying the relationship between these two variables. The changes noted are not observed in all exercising muscles, and their development seems to depend upon the fiber composition. These results emphasize the complex relationship that exists in the regulation of insulin action at the level of its receptor.

Variations in running activity and enzymatic adaptations in voluntary running rats.

The running behavior and biochemical markers of oxidative and glycolytic activities associated with voluntary running activity were studied in male Sprague-Dawley rats after 6 wk of training in exercise wheel cages. Twenty-four-hour recordings of running activity were used to quantify the number of individual running bouts, their duration and running speed, and the distance run per day. We then established three categories of voluntary running activity based on the mean distance run per day during the last 3 wk of training: low-activity runners averaged 2-5 km/day, medium runners 6-9 km/day, and high runners greater than 11 km/day. Each group demonstrated an intermittent, nocturnal running pattern, at relatively high intensities, with a similar mean running speed for all groups (avg approximately 45 m/min). Differences in total distance run per day were the result of variations in both the number and duration of individual running bouts. Specifically, high runners (n = 7) had 206 +/- 30 individual running bouts per 24 h, each lasting 87 +/- 7 s; medium runners (n = 7) 221 +/- 22 running bouts, lasting 47 +/- 5 s; and low runners (n = 7) 113 +/- 7 bouts, each lasting 40 +/- 7 s. Voluntary running depressed the rate of body weight gain compared with sedentary control rats, despite an increased food and water intake for all runners. Furthermore, drinking activity was temporally associated with running periods.(ABSTRACT TRUNCATED AT 250 WORDS)