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Soluble epoxide hydrolase (sEH) is an important enzyme for metabolic and cardiovascular health. sEH converts FFA epoxides (EpFAs), many of which are regulators of various cellular processes, to biologically less active diols. In human studies, diol (sEH product) to EpFA (sEH substrate) ratios in plasma or serum have been used as indices of sEH activity. We previously showed these ratios profoundly decreased in rats during acute feeding, possibly reflecting decreases in tissue sEH activities. The present study was designed to test which tissue(s) these measurements in the blood represent and if factors other than sEH activity, such as renal excretion or dietary intake of EpFAs and diols, significantly alter plasma EpFAs, diols, and/or their ratios. The results show that postprandial changes in EpFAs and diols and their ratios in plasma were very similar to those observed in the liver but not in other tissues, suggesting that the liver is largely responsible for these changes in plasma levels. EpFAs and diols were excreted into the urine, but their levels were not significantly altered by feeding, suggesting that renal excretion of EpFAs and diols may not play a major role in postprandial changes in circulating EpFAs, diols, or their ratios. Diet intake had significant impacts on circulating EpFA and diol levels but not on diol-to-EpFA (D-to-E) ratios, suggesting that these ratios, reflecting sEH activities, may not be significantly affected by the availability of sEH substrates (i.e., EpFAs). In conclusion, changes in FFA D-to-E ratios in plasma may reflect those in the liver, which may in turn represent sEH activities in the liver, and they may not be significantly affected by renal excretion or the dietary intake of EpFAs and diols.

Oxylipins, oxidation products of unsaturated free fatty acids (FFAs), are involved in various cellular signaling systems. Among these oxylipins, FFA epoxides are associated with beneficial effects in metabolic and cardiovascular health. FFA epoxides are metabolized to diols, which are usually biologically less active, by soluble epoxide hydrolase (sEH). Plasma epoxide-diol ratios have been used as indirect measures of sEH activity. This study was designed to examine the effects of acute elevation of individual plasma FFAs on a variety of oxylipins, particularly epoxides, diols, and their ratios. We tested if FFA epoxide-diol ratios are altered by circulating FFA levels (i.e., substrate availability) independent of sEH activity. Wistar rats received a constant intravenous infusion of olive (70% oleic acid (OA)), safflower seed (72% linoleic acid (LA)), and fish oils (rich in ω-3 FFAs) as emulsions to selectively raise OA, LA, and ω-3 FFAs (eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA)), respectively. As expected, olive, safflower seed, and fish oil infusions selectively raised plasma OA (57%), LA (87%), EPA (70%), and DHA (54%), respectively (p < 0.05 for all). Raising plasma FFAs exerted substrate effects to increase hepatic and plasma epoxide and diol levels. These increases in epoxides and diols occurred to similar extents, resulting in no significant changes in epoxide-diol ratios. These data suggest that epoxide-diol ratios, often used as indices of sEH activity, are not affected by substrate availability or altered plasma FFA levels and that epoxide-diol ratios may be used to compare sEH activity between conditions of different circulating FFA levels.

Epoxides of free fatty acids (FFAs), especially epoxyeicosatrienoic acids (EETs), are lipid mediators with beneficial effects in metabolic and cardiovascular (CV) health. FFA epoxides are quickly metabolized to biologically less active diols by soluble epoxide hydrolase (sEH). Inhibition of sEH, which increases EET levels, improves glucose homeostasis and CV health and is proposed as an effective strategy for the treatment of diabetes and CV diseases. Here, we show evidence that sEH activity is profoundly reduced in postprandial states in rats; plasma levels of 17 sEH products (i.e., FFA diols), detected by targeted oxylipin analysis, all decreased after a meal. In addition, the ratios of sEH product to substrate (sEH P/S ratios), which may reflect sEH activity, decreased ~70% on average 2.5 h after a meal in rats (P

Maintaining extracellular potassium (K+) within narrow limits, critical for membrane potential and excitability, is accomplished through the internal redistribution of K+ between extracellular fluid (ECF) and intracellular fluid (ICF) in concert with the regulation of renal K+ output to balance K+ intake. Here we present evidence from high-precision analyses of stable K+ isotopes in rats maintained on a control diet that the tissues and organs involved in the internal redistribution of K+ differ in their speed of K+ exchange with ECF and can be grouped into those that exchange K+ with ECF either rapidly or more slowly ("fast" and "slow" pools). After 10 days of K+ restriction, a compartmental analysis indicates that the sizes of the ICF K+ pools decreased but that this decrease in ICF K+ pools was not homogeneous, rather occurring only in the slow pool (15% decrease, p < 0.01), representing skeletal muscles, not in the fast pool. Furthermore, we find that the dietary K+ restriction is associated with a decline in the rate constants for K+ effluxes from both the "fast" and "slow" ICF pools (p < 0.05 for both). These results suggest that changes in unidentified transport pathways responsible for K+ efflux from ICF to ECF play an important role in buffering the internal redistribution of K+ between ICF and ECF during K+ restriction. Thus, the present study introduces novel stable isotope approaches to separately characterize heterogenous ICF K+ pools in vivo and assess K+ uptake by individual tissues, methods that provide key new tools to elucidate K+ homeostatic mechanisms in vivo. [ABSTRACT FROM AUTHOR]

Hypertriglyceridemia is a risk factor for type 2 diabetes and cardiovascular disease (CVD). Plasma triglycerides (TGs) are a key factor for assessing the risk of diabetes or CVD. However, previous lipidomics studies have demonstrated that not all TG molecules behave the same way. Individual TGs with different fatty acid compositions are regulated differentially under various conditions. In addition, distinct groups of TGs were identified to be associated with increased diabetes risk (TGs with lower carbon number [C#] and double-bond number [DB#]), or with decreased risk (TGs with higher C# and DB#). In this study, we examined the effects of high-fat feeding in rats on plasma lipid profiles with special attention to TG profiles. Wistar rats were maintained on either a low-fat (control) or high-fat diet (HFD) for 2 weeks. Plasma samples were obtained before and 2.5 h after a meal (n = 10 each) and subjected to lipidomics analyses. High-fat feeding significantly impacted circulating lipid profiles, with the most significant effects observed on TG profile. The effects of an HFD on individual TG species depended on DB# in their fatty acid chains; an HFD increased TGs with low DB#, associated with increased diabetes risk, but decreased TGs with high DB#, associated with decreased risk. These changes in TGs with an HFD were associated with decreased indices of hepatic stearoyl-CoA desaturase (SCD) activity, assessed from hepatic fatty acid profiles. Decreased SCD activity would reduce the conversion of saturated to monounsaturated fatty acids, contributing to the increases in saturated TGs or TGs with low DB#. In addition, an HFD selectively depleted ω-3 polyunsaturated fatty acids (PUFAs), contributing to the decreases in TGs with high DB#. Thus, an HFD had profound impacts on circulating TG profiles. Some of these changes were at least partly explained by decreased hepatic SCD activity and depleted ω-3 PUFA. [ABSTRACT FROM AUTHOR]

As the most abundant cation in archaeal, bacterial, and eukaryotic cells, potassium (K+) is an essential element for life. While much is known about the machinery of transcellular and paracellular K transport–channels, pumps, co-transporters, and tight-junction proteins—many quantitative aspects of K homeostasis in biological systems remain poorly constrained. Here we present measurements of the stable isotope ratios of potassium (41K/39K) in three biological systems (algae, fish, and mammals). When considered in the context of our current understanding of plausible mechanisms of K isotope fractionation and K+ transport in these biological systems, our results provide evidence that the fractionation of K isotopes depends on transport pathway and transmembrane transport machinery. Specifically, we find that passive transport of K+ down its electrochemical potential through channels and pores in tight-junctions at favors 39K, a result which we attribute to a kinetic isotope effect associated with dehydration and/or size selectivity at the channel/pore entrance. In contrast, we find that transport of K+ against its electrochemical gradient via pumps and co-transporters is associated with less/no isotopic fractionation, a result that we attribute to small equilibrium isotope effects that are expressed in pumps/co-transporters due to their slower turnover rate and the relatively long residence time of K+ in the ion pocket. These results indicate that stable K isotopes may be able to provide quantitative constraints on transporter-specific K+ fluxes (e.g., the fraction of K efflux from a tissue by channels vs. co-transporters) and how these fluxes change in different physiological states. In addition, precise determination of K isotope effects associated with K+ transport via channels, pumps, and co-transporters may provide unique constraints on the mechanisms of K transport that could be tested with steered molecular dynamic simulations. [ABSTRACT FROM AUTHOR]

AMP-activated protein kinase (AMPK), activated by an increase in intracellular AMP-to-ATP ratio, stimulates pathways that can restore ATP levels. We tested the hypothesis that AMPK activation influences extracellular fluid (ECF) [K.sup.+] homeostasis. In conscious rats, AMPK was activated with 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR) infusion: 38.4 mg/kg bolus then 4 mg x [kg.sup.-1] x [min.sup.-1] infusion. Plasma [[K.sup.+]] and [glucose] both dropped at 1 h of AICAR infusion and [[K.sup.+]] dropped to 3.3 [+ or -] 0.04 mM by 3 h, linearly related to the increase in muscle AMPK phosphorylation. AICAR treatment did not increase urinary [K.sup.+] excretion. AICAR lowered [[K.sup.+]] whether plasma [[K.sup.+]] was chronically elevated or lowered. The [K.sup.+] infusion rate needed to maintain baseline plasma [[K.sup.+]] reached 15.7 [+ or -] 1.3 [micro]mol [K.sup.+] x [kg.sup.-1] x [min.sup.-1] between 120 and 180 min AICAR infusion. In mice expressing a dominant inhibitory form of AMPK in the muscle (Tg-KD1), baseline [[K.sup.+]] was not different from controls (4.2 [+ or -] 0.1 mM), but the fall in plasma [[K.sup.+]] in response to AICAR (0.25 g/kg) was blunted: [[K.sup.+]] fell to 3.6 [+ or -] 0.1 in controls and to 3.9 [+ or -] 0.1 mM in Tg-KD1, suggesting that ECF [K.sup.+] redistributes, at least in part, to muscle ICF. In summary, these findings illustrate that activation of AMPK activity with AICAR provokes a significant fall in plasma [[K.sup.+]] and suggest a novel mechanism for redistributing [K.sup.+] from ECF to ICF. potassium homeostasis; sodium-potassium-ATPase; muscle; urine

Lee FN, Oh G, McDonough AA, Youn JH. Evidence for gut factor in K+ homeostasis. Am J Physiol Renal Physiol 293: F541-F547, 2007. First published May 23, 2007; doi: 10.1152/ajprenal.00427.2006.--We tested the hypothesis that [K.sup.+]-intake is sensed by putative [K.sup.+] sensors in the splanchnic areas, and renal [K.sup.+] handling is regulated by this signal. [K.sup.+] was infused for 2 h into overnight-fasted rats via the jugular vein (systemic infusion), hepatic portal vein (intraportal infusion), or stomach (intragastric infusion) (n = 5 each), and plasma [K.sup.+] concentration ([[K.sup.+]]) and renal [K.sup.+] excretion were measured during the 2-h preinfusion, 2-h [K.sup.+] infusion, and 3-h washout periods. During systemic [K.sup.+] infusion, plasma [[K.sup.+]] increased by ~ 1.3 mM (P < 0.05), and, on cessation of the [K.sup.+] infusion, plasma [[K.sup.+]] fell to the preinfusion level within 1-2 h. Renal [K.sup.+] excretion changed in proportion to the changes in plasma [[K.sup.+]]. During intraportal or intragastric [K.sup.+] infusion, plasma [[K.sup.+]] and renal [K.sup.+] excretion profiles were similar to those with systemic infusion. The effects of [K.sup.+] infusions via the different routes (n = 5 or 6 each) were also studied during simultaneous feeding of overnight-fasted rats with a [K.sup.+]-deficient diet. During the meal, intraportal infusion resulted in increases in plasma [[K.sup.+]] similar to those with the systemic [K.sup.+] infusion, while intragastric [K.sup.+] infusion did not significantly increase plasma [[K.sup.+]]. Thus, when the intragastric [K.sup.+] infusion was combined with a meal, there was marked enhancement of clearance of the [K.sup.+] infused, which was associated with an apparent increase in renal efficiency of [K.sup.+] excretion. These data suggest that there may be a gut factor that enhances renal efficiency of [K.sup.+] excretion during meal (or dietary [K.sup.+]) intake. feedback control; feedforward control; potassium sensor; potassium excretion

Extracellular [K.sup.+] concentration ([[K.sup.+]]) is closely regulated by the concerted regulatory responses of kidney and muscle. In this study, we aimed to define the responses activated when dietary [K.sup.+] was moderately reduced from a control diet (1.0% [K.sup.+]) to a 0.33% [K.sup.+] diet for 15 days. Although body weight and baseline plasma [[K.sup.+]] (4.0 mM) were not reduced in the 0.33% [K.sup.+] group, regulatory responses to conserve plasma [[K.sup.+]] were evident in both muscle and kidney. Insulin- stimulated clearance of [K.sup.+] from the plasma was estimated in vivo in conscious rats with the use of tail venous and arterial cannulas. During infusion of insulin*(50 mU*[kg.sup.-1] * [min.sup.-1]), plasma [[K.sup.+]] level fell to 3.2 [+ or -] 0.1 mM in the 1.0% [K.sup.+] diet group and to only 3.47 [+ or -] 0.07 mM in the 0.33% [K.sup.+] diet group (P < 0.01) with no reduction in urinary [K.sup.+] excretion, which is evidence of insulin resistance to cellular [K.sup.+] uptake. Insulinstimulated cellular [K.sup.4] uptake was quantitated by measuring the [K.sup.+] infusion rate necessary to clamp plasma [K.sup.+] at baseline (in [micro] mol * [kg.sup.-1] * [min.sup.-1]) during 5 mU of insulin * [kg.sup.-1] * [min.sup.-1 infusion: 9.7 [+ or -] 1.5 in 1% [K.sup.+] diet was blunted to 5.2 [+ or -] 1.7 in the 0.33% [K.sup.+] diet group (P < 0.001). Muscle [[K.sup.+]] and [Na.sup.+]-[K.sup.+]-ATPase activity and abundance were unchanged during the 0.33% [K.sup.+] diet. Renal excretion, which was measured overnight in metabolic cages, was reduced by 80%, from 117.6 [+ or -] 10.5 [micro]mol/h/animal (1% [K.sup.+] diet)to 24.2 [+ or -] 1.7 [micro]mol/h/animal (0.33% [K.sup.+] diet) (P < 0.001). There was no significant change in total abundance of key renal [K.sup.+] transporters, but 50% increases in both renal PTK cSrc abundance and ROMK phosphorylation in the 0.33% [K.sup.+] vs. 1% [K.sup.+] diet group, previously established to be associated with internalization of ROMK. These results indicate that plasma [[K.sup.+]] can be maintained during modest K restriction due to a decrease in insulin-stimulated cellular [K.sup.+] uptake as well as renal [K.sup.+] conservation mediated by inactivation of ROMK, both without a detectable change in plasma [[K.sup.+]]. The error signals inciting and maintaining these responses remain to be identified. potassium homeostasis; [Na.sup.+]-[K.sup.+]-ATPase; [H.sup.+]- [K.sup.+]- ATPase; protein tyrosine kinase; cSrc

Dexamethasone treatment causes resistance to insulin-stimulated cellular potassium uptake in the rat. Am J Physiol Cell Physiol 287: C1229-C1237, 2004. First published June 22, 2004; doi:10.1152/ajpcell.00111.2004.--Patients treated with glucocorticoids have elevated skeletal muscle ouabain binding sites. The major [Na.sup.+]-[K.sup.+]-ATPase (NKA) isoform proteins found in muscle, [[alpha].sub.2] and [[beta].sub.1], are increased by 50% in rats treated for 14 days with the synthetic glucocorticoid dexamethasone (DEX). This study addressed whether the DEX-induced increase in the muscle NKA pool leads to increased insulin-stimulated cellular K+ uptake that could precipitate hypokalemia. Rats were treated with DEX or vehicle via osmotic minipumps at one of two doses: 0.02 mg*[kg.sup.-1]*[day.sup.-1] for 14 days (low DEX; n = 5 pairs) or 0.1 mg*[kg.sup.-1]*[day.sup.-1] for 7 days (high DEX; n = 6 pairs). Insulin was infused at a rate of 5 mU*[kg.sup.-1]*[min.sup.-1] over 2.5 h in conscious rats. Insulin-stimulated cellular [K.sup.+] and glucose uptake rates were assessed in vivo by measuring the exogenous [K.sup.+] infusion ([K.sup.+.sub.inf]) and glucose infusion ([G.sub.inf]) rates needed to maintain constant plasma [K.sup.+] and glucose concentrations during insulin infusion. DEX at both doses decreased insulin-stimulated glucose uptake as previously reported. [G.sub.inf] (in mmol*[kg.sup.-1]*[h.sup.-1]) was 10.2 [+ or -] 0.6 in vehicle-treated rats, 5.8 [+ or -] 0.8 in low-DEX-treated rats, and 5.2 [+ or -] 0.6 in high-DEX-treated rats. High DEX treatment also reduced insulin-stimulated [K.sup.+] uptake. [K.sup.+.sub.inf] (in mmol*[kg.sup.-1]*[h.sup.-1]) was 0.53 [+ or -] 0.08 in vehicle-treated rats, 0.49 [+ or -] 0.14 in low-DEX-treated rats, and 0.27 [+ or -] 0.08 in high-DEX-treated rats. DEX treatment did not alter urinary [K.sup.+] excretion. NKA [[alpha].sub.2]-isoform levels in the low-DEX-treated group, measured by immunoblotting, were unchanged, but they increased by 38 [+ or -] 15% (soleus) and by 67 [+ or -] 3% (gastrocnemius) in the high-DEX treatment group. The NKA [[alpha].sub.1]-isoform level was unchanged. These results provide novel evidence for the insulin resistance of [K.sup.+] clearance during chronic DEX treatment. Insulin-stimulated cellular [K.sup.+] uptake was significantly depressed despite increased muscle sodium pump pool size. skeletal muscle; sodium pump; [Na.sup.+]-[K.sup.+]-ATPase

Starvation and experimental diabetes induce a stable increase in pyruvate dehydrogenase kinase (PDK) activity in skeletal muscle, which is largely due to a selective upregulation of PDK-4 expression. Increased free fatty acid (FFA) level has been suggested to be responsible for the upregulation. Because these metabolic states are also characterized by insulin deficiency, the present study was designed to examine whether insulin has a significant effect to regulate PDK mRNA expression in rat skeletal muscle. In study 1, overnight-fasted rats received an infusion of saline or insulin for 5 h (n = 6 each). During the insulin infusion, plasma glucose was clamped at basal levels (euglycemic hyperinsulinemic clamp). A third group (n = 6) received Intralipid infusion during the clamp to prevent a fall in plasma FFA. PDK-2 mRNA level in gastrocnemius muscle was not altered by insulin or FFA (i.e., Intralipid infusion). In contrast, PDK-4 mRNA level was decreased 72% by insulin (P < 0.05), and Intralipid infusion prevented only 20% of the decrease. PDK-4 protein level was decreased ~20% by insulin (P < 0.05), but this effect was not altered by Intralipid infusion. In study 2, overnight-fasted rats were refed or received an infusion of saline or nicotinic acid (NA, 30 [micro]mol/h) for 5 h (n = 5 each). During the refeeding and NA infusion, plasma FFA levels were similarly (i.e., 60-70% vs. saline control) lowered. Muscle PDK-4 mRNA level decreased 77% after the refeeding (P < 0.05) but not after the NA infusion. In conclusion, the present data indicate that insulin had a profound effect to suppress PDK-4 expression in skeletal muscle and that, contrary to previous suggestions, circulating FFA had little impact on PDK-4 mRNA expression, at least within 5 h. pyruvate dehydrogenase kinase; free fatty acid; insulin resistance; forkhead transcription factor

Elevation of plasma lactate levels induces peripheral insulin resistance, but the underlying mechanisms are unclear. We examined whether lactate infusion in rats suppresses glycolysis preceding insulin resistance and whether lactate-induced insulin resistance is accompanied by altered insulin signaling and/or insulin-stimulated glucose transport in skeletal muscle. Hyperinsulinemic euglycemic clamps were conducted for 6 h in conscious, overnight-fasted rats with or without lactate infusion (120 [micro]mol * [kg.sup.-1] * [min.sup.-1]) during the final 3.5 h. Lactate infusion increased plasma lactate levels about fourfold. The elevation of plasma lactate had rapid effects to suppress insulin-stimulated glycolysis, which clearly preceded its effect to decrease insulin-stimulated glucose uptake. Both submaximal and maximal insulin-stimulated glucose transport decreased 25-30% (P < 0.05) in soleus but not in epitrochlearis muscles of lactate-infused rats. Lactate infusion did not alter insulin's ability to phosphorylate the insulin receptor, the insulin receptor substrate (IRS)-1, or IRS-2 but decreased insulin's ability to stimulate IRS-1- and IRS-2-associated phosphatidylinositol 3-kinase activities and Akt/protein kinase B activity by 47, 75, and 55%, respectively (P < 0.05 for all). In conclusion, elevation of plasma lactate suppressed glycolysis before its effect on insulin-stimulated glucose uptake, consistent with the hypothesis that suppression of glucose metabolism could precede and cause insulin resistance. In addition, lactate-induced insulin resistance was associated with impaired insulin signaling and decreased insulin-stimulated glucose transport in skeletal muscle. glucose transport; euglycemic clamp; rat GLUT4

Maintaining extracellular fluid (ECF) [K.sup.+] concentration ([[K.sup.+]]) within a narrow range is accomplished by the concerted responses of the kidney, which matches [K.sup.+] excretion to [K.sup.+] intake, and skeletal muscle, the main intracellular fluid (ICF) store of [K.sup.+], which can rapidly buffer ECF [[K.sup.+]]. In both systems, homologous P-type ATPase isoforms are key effectors of this homeostasis. During dietary [K.sup.+] deprivation, these P-type ATPases are regulated in opposite directions: increased abundance of the H,K-ATPase 'colonic' isoform in the renal collecting duct drives active [K.sup.+] conservation while decreased abundance of the plasma membrane Na,K-ATPase [[alpha].sub.2]-isoform leads to the specific shift of [K.sup.+] from muscle ICF to ECF. The skeletal muscle response is isoform and muscle specific: [[alpha].sub.2] and [[beta].sub.2], not [[alpha].sub.1] and [[beta].sub.1], levels are depressed, and fast glycolytic muscles lose >90% [[alpha].sub.2], whereas slow oxidative muscles lose ~50%; however, both muscle types have the same fall in cellular [[K.sup.+]]. To understand the physiological impact, we developed the '[K.sup.+] clamp' to assess insulin-stimulated cellular [K.sup.+] uptake in vivo in the conscious rat by measuring the exogenous [K.sup.+] infusion rate needed to maintain constant plasma [[K.sup.+]] during insulin infusion. Using the [K.sup.+] clamp, we established that [K.sup.+] deprivation leads to near-complete insulin resistance of cellular [K.sup.+] uptake and that this insulin resistance can occur before any decrease in plasma [[K.sup.+]] or muscle [Na.sup.+] pump expression. These studies establish the advantage of combining molecular analyses of P-type ATPase expression with in vivo analyses of cellular [K.sup.+] uptake and excretion to determine mechanisms in models of disrupted [K.sup.+] homeostasis. sodium, potassium-adenosine triphosphatase; hydrogen, potassium-adenosine triphosphatase; potassium clamp; hypokalemia; ion homeostasis

Skeletal muscle releases intracellular fluid K+ to buffer the drop in extracellular fluid (ECF) K+ when K+ output exceeds input. To elucidate on the mechanism and muscle specificity of the K+ shift, rats were fed K+-deficient food for 2-10 days and two muscles at phenotypic extremes were studied: slow-twitch oxidative soleus and fast-twitch glycolytic white gatrocnemius. Findings revealed that the patterns of regulation of Na+ pump isoforms in oxidative and glycolytic muscles during K+ deprivation mediated by posttranscriptional regulation of alpha2beta1 and alpha2beta are distinct and that reductions in alpha2-isoform pools can occur early enough in both muscles to explain the shift of K+ to the ECF.

Rats treated with varying doses of streptozotocin (STZ) were used to investigate the relationships among postabsorptive glucose concentration, hepatic glucose output and glucose clearance. Results reveal that the pathophysiology of STZ-induced diabetes in rats involves primarily reduced glucose clearance although there is a contribution from the hepatic glucose output in experimental animals receiving the highest dose of STZ.

Male Wistar rats subjected to prolonged hyperinsulinemic euglycemic clamps were analyzed to determine the effects of suppression of glycolysis on insulin-stimulated whole body and skeletal muscle glucose fluxes. Metabolic suppression of glycogen synthesis in rats triggered a cellular adaptive mechanism that produced insulin resistance in the skeletal muscles. The mechanism of skeletal muscle insulin resistance was mediated by the prolonged elevation of glucose 6-phosphate.

The effect of insulin on whole body glucose distribution kinetics was investigated in anesthetized dogs. Whole body kinetic analysis of plasma L-glucose was performed using a three- or four-compartment model. Lymph L-glucose data were compared with the results of plasma L-glucose analysis. Data indicated that insulin does not affect whole body glucose distribution. Glucose dynamics were best predicted by a four-compartment model.

Insulin does not make any impact on either extracellular glucose distribution kinetics or volumes in conscious rats. Insulin's influence nce on cellular glucose transport, in vivo, can be assessed by the analysing plasma L-glucose and 3-O-methylglucose(3-MG) kinetics. A 5.5-fold increase in 3-MG transport activity is stimulated maximally effective insulin concentrations.

Glucose transport is stimulated by the contractions/hypoxia pathway by W-7, which produces Ca2+ from sarcoplasmic reticulum. This indicates that cytosolic Ca2+ increment stimulates contractile activity in manner similar to hypoxia initiating glucose transport in striated muscles. W-7 also inhibits glucose transport stimulation by insulin.

The GLUT-4 glucose transporter concentration and the glucose transport activity in skeletal muscles of female Long-Evans rats are influenced by growth and aging. The presumption that skeletal muscle shows the formation of insulin resistance due to aging is analyzed. Immunoblotting with a polyclonal antibody directed against GLUT-4 is used to calculate the varying quantities of immunoreactive GLUT-4 protein in muscle homogenates of 1-, 10-, and 25-month-old rats. A fall in GLUT-4 immunoreactivity is seen in the epitrochlearis, plantaris and the red and white areas of the quadriceps muscles. Identical GLUT-4 concentration is observed in flexor digitorum brevis and soleus muscles.