Hyperinsulinemia produces both sympathetic neural activation and vasodilation in normal humans.

Hyperinsulinemia may contribute to hypertension by increasing sympathetic activity and vascular resistance. We sought to determine if insulin increases central sympathetic neural outflow and vascular resistance in humans. We recorded muscle sympathetic nerve activity (MSNA; microneurography, peroneal nerve), forearm blood flow (plethysmography), heart rate, and blood pressure in 14 normotensive males during 1-h infusions of low (38 mU/m2/min) and high (76 mU/m2/min) doses of insulin while holding blood glucose constant. Plasma insulin rose from 8 +/- 1 microU/ml during control, to 72 +/- 8 and 144 +/- 13 microU/ml during the low and high insulin doses, respectively, and fell to 15 +/- 6 microU/ml 1 h after insulin infusion was stopped. MSNA, which averaged 21.5 +/- 1.5 bursts/min in control, increased significantly (P less than 0.001) during both the low and high doses of insulin (+/- 5.4 and +/- 9.3 bursts/min, respectively) and further increased during 1-h recovery (+15.2 bursts/min). Plasma norepinephrine levels (119 +/- 19 pg/ml during control) rose during both low (258 +/- 25; P less than 0.02) and high (285 +/- 95; P less than 0.01) doses of insulin and recovery (316 +/- 23; P less than 0.01). Plasma epinephrine levels did not change during insulin infusion. Despite the increased MSNA and plasma norepinephrine, there were significant (P less than 0.001) increases in forearm blood flow and decreases in forearm vascular resistance during both doses of insulin. Systolic pressure did not change significantly during infusion of insulin and diastolic pressure fell approximately 4-5 mmHg (P less than 0.01). This study suggests that acute increases in plasma insulin within the physiological range elevate sympathetic neural outflow but produce forearm vasodilation and do not elevate arterial pressure in normal humans.

Cyclic nucleotide phosphodiesterase 3 expression in vivo: evidence for tissue-specific expression of phosphodiesterase 3A or 3B mRNA and activity in the aorta and adipose tissue of atherosclerosis-prone insulin-resistant rats.

With a view of understanding the potential roles of phosphodiesterase (PDE)3 in the acceleration of atherosclerosis in diabetes, we have analyzed the in vivo levels of low Km cAMP PDE3 and PDE4 activities as well as PDE3A and PDE3B mRNA in a relevant animal model. The JCR:LA-cp rat is a unique strain that develops obesity, insulin resistance, and vasculopathy when homozygous for the autosomal recessive cp gene (cp/cp). Lean rats, bred (designated +/?) as a 2:1 mixture of animals that are heterozygous (cp/+) or homozygous normal (+/+), are metabolically normal. We find that PDE3 activity is the major low Km cAMP activity in the aorta of cp/cp rats and is approximately twofold higher than that in lean +/? rats. PDE3A mRNA levels in middle-aged cp/cp rats are also elevated, approximately threefold, compared with those of +/? rats or young 12-week-old cp/cp rats. Thus, in the aorta of atherosclerosis-prone insulin-resistant cp/cp rats, PDE3A gene expression is upregulated, resulting in significantly higher PDE3 activity. This upregulation of PDE3A mRNA levels was a rather unique phenomenon to the aorta of JCR:LA-cp rats compared with that in the aorta of other rat strains. This result is consistent with our hypothesis that an increased PDE3 activity in aortic smooth muscle cells may contribute to accelerated atherosclerosis in diabetes. Furthermore, determination of PDE3 activity and PDE3A and PDE3B mRNA levels in heart and white and brown fat tissues of JCR:LA-cp rats revealed that PDE3B mRNA and activity in white adipose tissue is downregulated in this diabetic animal model, and that PDE3A and PDE3B genes are tissue-specifically expressed and differentially regulated in aorta and adipose tissue, respectively, under hyperinsulinemic conditions.

Insulin increases sympathetic activity but not blood pressure in borderline hypertensive humans.

We have previously demonstrated that physiological hyperinsulinemia in normotensive humans increases sympathetic nerve activity but not arterial pressure since it also causes skeletal muscle vasodilation. However, in the presence of insulin resistance and/or hypertension, insulin may cause exaggerated sympathetic activation or impaired vasodilation and thus elevate arterial pressure. This study sought to determine if insulin causes a pressor response in borderline hypertensive humans by producing exaggerated increases in sympathetic neural outflow or impaired vasodilation. We recorded muscle sympathetic nerve activity (microneurography, peroneal nerve), forearm blood flow, heart rate, and blood pressure in 13 borderline hypertensive subjects during a 1-hour insulin infusion (38 microunits/m2/min) while holding blood glucose constant. Plasma insulin rose from 12 +/- 3 microunits/ml (mean +/- SEM) during control to 73 +/- 7 microunits/ml during insulin infusion and fell to 9 +/- 2 microunits/ml 2 hours after insulin infusion was stopped. Muscle sympathetic nerve activity, which averaged 25 +/- 2 bursts per minute in control, increased significantly during insulin infusion (+9 bursts per minute) and remained elevated 1.5 hours into recovery (+7 bursts per minute, p less than 0.001). Despite increased muscle sympathetic nerve activity, there were significant (p less than 0.001) increases in forearm blood flow and decreases in forearm vascular resistance during insulin infusion. Further, systolic and diastolic pressures fell approximately 3 and 6 mm Hg, respectively, during insulin infusion (p less than 0.01). This study suggests that acute physiological increases in plasma insulin elevate sympathetic neural outflow in borderline hypertensive humans but produce vasodilation and do not elevate arterial pressure.

Dietary magnesium prevents fructose-induced insulin insensitivity in rats.

Increased dietary fructose may produce insulin insensitivity and elevate blood pressure in rats. It is possible that the reduced magnesium content of the high-fructose commercial diet used in some studies may play a role in these abnormalities because it is known that magnesium deficiency can produce insulin insensitivity and increased angiotensin II action in humans. To study this, we maintained rats for 9 weeks on either a normal control diet, a standard high-fructose diet, or the same high-fructose diet supplemented with magnesium. Glucose uptake was assessed using a perfused rat hindquarter preparation sequentially with 0, 900, and 120,000 pmol/L of added insulin. Basal serum glucose, plasma insulin, and basal glucose uptake in the absence of insulin were similar among all three groups. However, insulin sensitivity, defined as glucose uptake in the presence of 900 pmol/L insulin minus basal, was depressed in the high-fructose compared with the control group (1.02 +/- 0.38 to 1.77 +/- 0.57 mumol/g per hour, P < .05). In contrast, the high-fructose group supplemented with normal magnesium had similar insulin sensitivity as the control group (2.09 +/- 0.69 mumol/g per hour). Total serum magnesium was reduced in the high-fructose group compared with control or high-fructose plus magnesium-supplemented groups. Blood pressure and fasting insulin levels were also lower in the magnesium-supplemented group. These results suggest that magnesium deficiency and not fructose ingestion per se leads to insulin insensitivity in skeletal muscle and changes in blood pressure.

Cyclic nucleotide PDE-3. Quantitation of PDE-3A and -3B mRNAs in rat tissues by RNase protection assay.

Type 3 cyclic nucleotide phosphodiesterase (PDE-3) isoforms exhibit a high affinity ("low K(m)") for cAMP and are specifically inhibited by cGMP and a number of pharmacological agents, which increase myocardial contractility, inhibit platelet aggregation, and increase smooth muscle relaxation. The PDE-3 family consists of at least two isozymes, PDE-3A (cardiac type) and PDE-3B (adipocyte type), with distinct tissue-specific distributions. PDE-3A mRNA is highly expressed in the cardiovascular system, whereas PDE-3B mRNA is primarily expressed in adipocytes and hepatocytes. Toward understanding potential roles of PDE-3 in diabetes mellitus, we have established a specific and sensitive RNase protection assay (RPA) for quantitating PDE-3A and PDE-3B mRNA in rat diabetic models. In fatty Zucker diabetic (ZDF) rats, PDE-3A mRNA, but not PDE-3B mRNA, was expressed in heart, whereas liver and white and brown fat tissues predominantly expressed PDE-3B mRNA. Unexpectedly, PDE-3B mRNA expression was approximately 2.5 times higher than PDE-3A mRNA in aorta from both ZDF and Sprague-Dawley (SD) rats. In contrast, expression levels of PDE-3A mRNA in heart were similar in both species. With this RPA, we were thus able to compare PDE-3A and -3B mRNA levels in different tissues as well as in different rat species.

Prior exercise potentiates the thermic effect of a carbohydrate load.

It is unclear whether dietary-induced thermogenesis (DIT) is increased after exercise. To test this possibility, six healthy volunteers, male and female, exercised for 45 minutes at 70% of maximal aerobic capacity (VO2 max) in the morning after an overnight fast. Two hours after the end of the exercise, by which time VO2 had returned to near baseline levels, subjects ingested a 100-g glucose load. Blood samples and respiratory gas exchange data were collected over the next three hours. On a separate day on which the subjects did not exercise, the test procedure was repeated. Glucose tolerance and the insulin response to the glucose load were not significantly different between the two trials; however, VO2 increased by 15.5% over baseline on the exercise day, compared with only 8.9% when exercise was not performed. The net increase in energy expenditure for the three-hour period following glucose ingestion was 15 kcal/180 min greater on the exercise than on the control day, with increases upwards of 20 kcal/180 min in several individuals. No correlation was found between the magnitude of exercise-enhanced DIT and VO2 max, suggesting that this effect is independent of the state of training. The results indicate that the thermic effect of exogenous carbohydrate can be potentiated by prior exercise.

Effects of prior exercise on the thermic effect of glucose and fructose.

It has been previously observed that the thermic effect of a glucose load is potentiated by prior exercise. To determine whether this phenomenon is observed when different carbohydrates are used and to ascertain the role of insulin, the thermic effects of fructose and glucose were compared during control (rest) and postexercise trials. Six male subjects ingested 100 g fructose or glucose at rest or after recovery from 45 min of treadmill exercise at 70% of maximal O2 consumption. Measurements of O2 consumption, respiratory exchange ratio, and plasma concentrations of glucose, insulin, glycerol, and lactate were measured for 3 h postingestion. Although glucose and fructose increased net energy expenditure by 44 and 51 kcal, respectively, over baseline during control trials, exercise increased the thermic effect of both carbohydrate challenges an additional 20-25 kcal (P less than 0.05). Glucose ingestion was associated with large (P less than 0.05) increases in plasma insulin concentration during control and exercise trials, in contrast to fructose ingestion. Because fructose, which is primarily metabolized by liver, and glucose elicited a similar postexercise potentiation of thermogenesis, the results indicate that the thermogenic phenomenon is not limited to skeletal muscle. These results also demonstrate that carbohydrate-induced postexercise thermogenesis is not related to an incremental increase in plasma insulin concentration.

Evidence that nitric oxide increases glucose transport in skeletal muscle.

Nitric oxide synthase (NOS) is expressed in skeletal muscle. However, the role of nitric oxide (NO) in glucose transport in this tissue remains unclear. To determine the role of NO in modulating glucose transport, 2-deoxyglucose (2-DG) transport was measured in rat extensor digitorum longus (EDL) muscles that were exposed to either a maximally stimulating concentration of insulin or to an electrical stimulation protocol, in the presence of NG-monomethyl-L-arginine, a NOS inhibitor. In addition, EDL preparations were exposed to sodium nitroprusside (SNP), an NO donor, in the presence of submaximal and maximally stimulating concentrations of insulin. NOS inhibition reduced both basal and exercise-enhanced 2-DG transport but had no effect on insulin-stimulated 2-DG transport. Furthermore, SNP increased 2-DG transport in a dose-responsive manner. The effects of SNP and insulin on 2-DG transport were additive when insulin was present in physiological but not in pharmacological concentrations. Chronic treadmill training increased protein expression of both type I and type III NOS in soleus muscle homogenates. Our results suggest that NO may be a potential mediator of exercise-induced glucose transport.

Nitric oxide release is present from incubated skeletal muscle preparations.

To determine whether nitric oxide (NO) synthase activity exists in rat skeletal muscle, media from incubated rat extensor digitorum longus muscle preparations were assayed for NO with a chemiluminescent detection system. Although small amounts of NO were detected in media alone, the addition of muscle increased NO concentration in the media by 30-fold. The release of NO into the media diminished over time. Either arginine (10(-6) M), sodium nitroprusside (10(-6) M), or prior electrical stimulation in vivo caused 50-200% increases (P < 0.05) in NO concentration. NG-monomethyl-L-arginine monoacetate (10(-6) M), an NO synthase inhibitor, decreased both basal 2-deoxyglucose transport and NO efflux, indicating that NO may play a role in modulating skeletal muscle carbohydrate metabolism. These data indicate that NO is released from an incubated skeletal muscle preparation and presents the possibility that muscle-derived NO may play an important metabolic role.

Effects of insulin and exercise on rat hindlimb muscles after simulated microgravity.

This study was designed to examine insulin- and exercise-stimulated glucose uptake and metabolism in the hindlimb muscles of rats after conditions of simulated microgravity. To simulate microgravity, male Sprague-Dawley rats were suspended in a head-down (45 degrees) position with their hindlimbs non-weight bearing (SUS) for 14 days. In addition, rats were assigned to suspension followed by exercise (SUS-E), to cage control (CC), or to exercising control (CC-E) groups. Exercise consisted of five 10-min bouts of treadmill running at the same relative intensity for the CC-E and SUS-E rats (80-90% of maximum O2 consumption). Hindlimb perfusion results indicated that glucose uptake for the entire hindquarter at 24,000 microU/ml insulin (maximum stimulation) was significantly higher in the SUS (8.9 +/- 0.5 mumol.g-1.h-1) than in the CC (7.6 +/- 0.4 mumol.g-1.h-1) rats, signifying an increased insulin responsiveness. Glucose uptake at 90 microU/ml insulin was also significantly higher in the SUS (48 +/- 4; % of maximum stimulation over basal) than in the CC (21 +/- 4%) rats. In addition, exercise-induced increases in glucose uptake for the hindlimbs (133%) and glucose incorporation into glycogen for the plantaris (8.4-fold), extensor digitorum longus (5.4-fold), and white gastrocnemius (4.8-fold) muscles were greater for the SUS-E rats than for the CC-E rats (39% and 1.9-, 1.9-, and 3.0-fold, respectively). Therefore, suspension of the rat with hindlimbs non-weight bearing leads to enhanced muscle responses to insulin and exercise when they were applied separately. However, insulin action appeared to be impaired after exercise for the SUS-E rats, especially for the soleus muscle.

Effect of prior exercise and insulin on potential thermogenic systems in rat skeletal muscle.

We previously reported that insulin stimulates oxygen consumption by the perfused rat hindquarter after high-intensity exercise. The purpose of the present study was to examine whether fructose 6-phosphate-fructose 1,6-bisphosphate cycling or an uncoupling of mitochondrial respiration contributes to this phenomenon. Hindquarter skeletal muscle was analyzed after perfusion in the absence or presence of insulin (150-200 microU/ml) for high-energy phosphate content, fructose 6-phosphate-fructose 1,6-bisphosphate cycling of glucose before incorporation into glycogen, and mitochondrial respiratory control. Muscle from exercised rats perfused with insulin did not display greater rates of glucose cycling or mitochondrial uncoupling; in fact, insulin decreased the rate of fructose 6-phosphate cycling and tended to increase respiratory control in skeletal muscle mitochondria. In addition, the concentrations of ATP and creatine phosphate and the calculated free ADP level in muscle of previously exercised rats perfused with insulin were similar to those of control rats. The results do not exclude the possibility that localized subcellular changes in ADP occurred, however. In conclusion, the results suggest that insulin-induced increases in other substrate cycles, ion transport systems, and/or as yet unidentified energy-requiring processes account for the 25-30% increase in hindquarter oxygen consumption after intense exercise.

Role of dihydropyridine sensitive calcium channels in glucose transport in skeletal muscle.

Glucose transport in skeletal muscle is a carrier-mediated process activated by insulin and by contractile activity. Since previous evidence suggests a role for calcium influx in the activation of this process, the purpose of this study was to determine if glucose transport is mediated by muscle's voltage dependent (dihydropyridine sensitive) calcium channels. Soleus and extensor digitorum longus (EDL) muscles, isolated from rats, were incubated with the calcium channel blocker nifedipine. Basal glucose transport was decreased in both soleus and EDL by nifedipine. Treatment with nifedipine effectively blocked both insulin and contraction stimulated glucose transport in soleus. Conversely, glucose transport in EDL, although reduced, was still significantly increased over basal by both insulin and contraction, due, perhaps, to a relatively greater number of dihydropyridine receptors in EDL. These results provide evidence that contraction stimulated, as well as insulin stimulated, glucose transport is mediated in-part by dihydropyridine receptors in skeletal muscle.

Antioxidant status of young children: response to an antioxidant supplement.

OBJECTIVE: To study oxidative stress indicators in healthy young children and their response to a commercially available fruit- and vegetable-based antioxidant supplement. DESIGN: Healthy children were randomly assigned to a placebo and a supplement (commercial antioxidant supplement produced from dried fruit and vegetable extracts and fortified with antioxidants, resembling a gummy-type candy). The placebo and the supplement were taken in 2 doses per day for 21 days. SUBJECTS: Participants were 39 children (26 boys and 13 girls) aged 5 to 10 years. Research was conducted at Primary Children's Medical Center and the University of Utah, Salt Lake City. MAIN OUTCOME MEASURES: Breath and urine samples were collected on days 1 and 21 and assayed for breath pentane and urine 8-hydroxydeoxyguanosine, malondialdehyde, nitrites, and 8-isoprostane as noninvasive indicators of oxidative stress. Urine oxygen radical absorbance capacity was measured at days 1 and 21 as an indirect indicator of the antioxidant capacity of the body. Three-day food records were collected at the beginning and end of the study to measure intake of dietary fruit; vegetable; and antioxidant vitamins A, C, and E. STATISTICAL ANALYSIS: Descriptive statistics, repeated measures analysis of variance, paired t tests, and Pearson r correlations. RESULTS: Markers of oxidative stress were not significantly different between the placebo and supplement groups at day 1 or day 21. The oxidative stress indicators of the healthy children in this study appear to be similar to those of healthy adults and were not changed by antioxidant supplementation. The diet record analyses indicated that mean fruit and vegetable intakes (2.75 servings/day) were similar to the national average intake for children in the United States. APPLICATIONS/CONCLUSIONS: This research presents original information on the subject of oxidative stress in healthy children. The results of this study may be useful as reference baseline markers to use in conjunction with clinical dietary evaluations and for future research with healthy children and with children in disease states who are subject to elevated levels of oxidative stress.

Redox regulation of skeletal muscle glucose transport.

Although the control of carbohydrate metabolism may be regulated by numerous factors, the redox state of the cell is of primary importance. The redox state may be influenced by a number of different factors, including different reactive oxygen species (ROS) and reactive nitrogen species (RNS) collectively, called reactive oxygen/nitrogen species (RONS). This review attempts to summarize the importance of redox regulation in relation to glucose transport and regulation of carbohydrate metabolism in skeletal muscle. In addition, prior studies implicating the role of different RONS in the control of glucose transport in skeletal muscle will be presented. Finally, the possible involvement of the cGMP, p21ras, and mean arterial pressure (MAP) kinase signal transduction cascades, which have been implicated with redox-sensitive alterations in glucose transport, will also be discussed.

Role of nitric oxide in contraction induced glucose transport.

Nitric oxide (NO) is a vasoactive substance, which was first described as endothelium derived relaxing factor (EDRF). Subsequently, NO has been found to be a messenger molecule abundantly present in the nervous system. Functioning as a neurotransmitter in the peripheral nervous system, NO mediates an array of physiological functions such as gastrointestinal motility, regional blood flow, smooth muscle contraction, neuroendocrine activity and immune function. Recently NO biosynthesis has been found in skeletal muscle, where NO exerts an effect on both the metabolic and contractile processes. This review will focus on the actions of NO in skeletal muscle metabolism. NO donors have been shown to increase glucose transport in skeletal muscle. Inhibition of NOS activity blunts contraction-stimulated glucose transport but has no effect on insulin-stimulated glucose transport. NOS protein expression is enhanced by chronic exercise suggesting that NO may play a role in the improved glucose tolerance and increased insulin sensitivity characteristic of the trained state.

Acute exposure to AICAR increases glucose transport in mouse EDL and soleus muscle.

AMP-activated protein kinase (AMPK) may regulate a number of metabolic processes including glucose transport. 5-Aminoimidazole-4-carboxamideribonucleoside (AICAR), an AMPK activator, has been used to study the potential role of AMPK in rat skeletal muscle; however, its effects on glucose transport in mouse skeletal muscle are unknown. Incubation with 2 mM AICAR increased 2-deoxyglucose transport in EDL muscle from both rats and mice by 86 and 37%, respectively. In contrast, AICAR did not increase 2-deoxyglucose transport in rat soleus muscle. However, AICAR induced a large (81%) increase in 2-deoxyglucose transport in soleus muscles obtained from mice. It is proposed that nonspecificity of the stimulation of glucose transport in mouse muscle may be due to a greater percentage of fast-twitch muscle fibers within the muscles.

Effects of carbohydrate loading and weight-lifting on muscle girth.

Bodybuilders have used different carbohydrate loading regimens in conjunction with resistance exercise prior to competition in the belief that this would result in increased muscle size. To investigate this possibility, muscle girth measurements were obtained from nine weight-trained males before and after a control (standard isocaloric diet) and an experimental trial (carbohydrate loading). The latter regimen consisted of 3 days of intense weight-lifting while the subjects ingested a diet of 10% carbohydrate (CHO), 57% fat (F), and 33% protein (P), followed by 3 days of light weight-lifting and a day of rest while ingesting a diet of 80% CHO, 5% F, and 15% P. The control trial consisted of an identical weight-lifting regimen while subjects ingested an isocaloric (45 kcal/kg BW/day) diet. Body weight and girths (forearm, upper arm, chest, thigh, waist, and calf) were obtained before and after each trial in a relaxed and flexed state. The results indicate that an exercise/carbohydrate loading regimen had no significant effect on muscle girth as compared to the control trial. It is concluded that CHO loading has no additional advantage to enhancing muscle girth in bodybuilders over weight-lifting alone.

Effects of chronic N(omega)-nitro-L-arginine methyl ester administration on glucose tolerance and skeletal muscle glucose transport in the rat.

It has been suggested that nitric oxide (NO) is a key regulator of carbohydrate metabolism in skeletal muscle. The present study was undertaken to examine the effects of chronic in vivo competitive antagonism of NO synthase (NOS) by the administration of N(omega)-nitro-L-arginine methyl ester (L-NAME) in the drinking water (1 mg/ml) for 14 days on glucose tolerance and skeletal muscle glucose transport in rats. Oral glucose tolerance tests (OGTT) revealed an impaired glucose tolerance in the L-NAME-treated rats as reflected by the area under the glucose curve (4675 +/- 514 mg% x 120 min (control) vs 6653 +/- 571 mg% x 120 min (L-NAME treated); P < 0.03). While a large rise in plasma insulin concentration was present in the control rats (0.87 +/- 0.34 ng/ml, P < 0.001) during the first 15 min of the OGTT, rises in plasma insulin concentration were absent in the L-NAME-treated rats (0.18 +/- 0.13 ng/ml, P = NS). Intravenous glucose tolerance tests confirmed an impaired insulin secretion in the L-NAME-treated rats. In contrast, insulin-stimulated 2-deoxyglucose transport was enhanced (P < 0.03) by chronic NOS inhibition (5.29 +/- 0.83 nmol/g/min) compared to control rats (2.21 +/- 0.90 nmol/g/min). Plasma sodium concentrations were lower and plasma potassium concentrations were higher in the L-NAME-treated group, indicating an impaired electrolyte status. We conclude that chronic in vivo administration of a NOS inhibitor, while not impairing basal parameters of carbohydrate metabolism, may manifest different responses than acute exposure to the same agent in vitro.

Nonuniform regional sympathetic nerve responses to hyperinsulinemia in rats.

The insulin hypothesis of hypertension proposes that hyperinsulinemia increases sympathetic nerve activity (SNA) and raises arterial pressure. The goals of this study were 1) to determine if hyperinsulinemia produces regionally uniform or nonuniform increases in SNA and 2) to test the hypothesis that spontaneously hypertensive rats (SHR) have exaggerated sympathoadrenal responses to hyperinsulinemia. We measured plasma insulin, blood glucose, mean arterial pressure, and adrenal, renal, and lumbar SNA in alpha-chloralose-anesthetized SHR and normotensive Wistar-Kyoto (WKY) rats before and during infusion of two doses of insulin for 60 min each while maintaining euglycemia. In WKY rats, graded increases in plasma insulin from 27 +/- 5 (SE) to 200 +/- 29 microU/ml increased lumbar SNA from 100% to 285 +/- 26% but failed to significantly increase adrenal or renal SNA. In SHR rats, similar increases in plasma insulin from 27 +/- 4 to 213 +/- 33 microU/ml caused significant increases in adrenal (100% to 174 +/- 16%) and lumbar (100% to 307 +/- 26%) SNA but not in renal SNA. Despite increases in SNA, mean arterial pressure did not increase significantly in either group of rats. We conclude that 1) hyperinsulinemic euglycemic clamp produces regionally nonuniform increases in sympathetic nerve activity, and 2) there is a potentiated increase in adrenal SNA in SHR compared with WKY rats during hyperinsulinemia, whereas lumbar SNA responses were similar in the two strains, and renal SNA did not increase in either strain.

Effects of contractile activity on tyrosine phosphoproteins and PI 3-kinase activity in rat skeletal muscle.

Insulin stimulates signaling reactions that include insulin receptor autophosphorylation and tyrosine kinase activation, insulin receptor substrate-1 (IRS-1) tyrosine phosphorylation, and phosphatidylinositol 3-kinase (PI 3-kinase) activation. Muscle contraction has metabolic effects similar to insulin, and contraction can increase insulin sensitivity, but little is known about the molecular signals that mediate the effects of contraction. To investigate the effects of muscle contraction on insulin signaling, rats were studied after contraction of hindlimb muscles by electrical stimulation, maximal insulin injection in the absence of contraction, or contraction followed by insulin injection. Insulin increased tyrosine phosphorylation of the insulin receptor and IRS-1, whereas contraction alone had no effect. Contraction before insulin injection decreased the insulin effect on receptor and IRS-1 phosphorylation by 20-25%. Increased tyrosine phosphorylation of other proteins by insulin and/or contraction was not observed. Contraction alone had little effect on PI 3-kinase activity, but contraction markedly blunted the insulin-stimulated activation of IRS-1 and insulin receptor-immunoprecipitable PI 3-kinase. In conclusion, skeletal muscle contractile activity does not result in tyrosine phosphorylation of molecules involved in the initial steps of insulin signaling. Although contractile activity increases insulin sensitivity and responsiveness in skeletal muscle, contraction causes a paradoxical decrease in insulin-stimulated tyrosine phosphorylation and PI 3-kinase activity.