Biomedical subjects

A Bonen

Publications and source records attributed to A Bonen.

At least 73 records · Page 4Linked to original sources

PubMed1997

Cellular and subcellular expression of the monocarboxylate transporter MCT1 in rat heart. A high-resolution immunogold analysis.

An antibody to the C-terminus of the monocarboxylate transporter MCT1 was used to study the precise cellular and subcellular distribution of this transporter in rat heart. Postembedding immunogold procedures revealed that the labeling in the heart was restricted to cardiomyocytes and concentrated along the plasma membrane, including the transverse tubules. Gold particles occurred with highest densities in intercalated disks, where they avoided desmosomes and gap junctions. Labeling was also associated with plasmalemmal invaginations having ultrastructural features typical of caveolae. Internal membrane compartments were unlabeled. Quantitative analyses following postembedding labeling showed that the distribution of gold particles across the plasma membrane was nearly symmetrical, indicating that the C-terminus of the transporter is situated very close to the cell membrane. In preembedding immunogold experiments, the gold particles were localized at the external aspect of the plasma membrane, suggesting that the C-terminus is extracellular. From the present data, it can be concluded that even under basal conditions the majority of the MCT1 molecules in heart is present in the myocyte plasma membrane, implying that there is a constitutive functional expression of this transporter. It follows that the increased transmembrane flux of lactate during exercise or in pathological conditions such as ischemia must be a result of altered substrate gradients rather than of translocation of MCT1 molecules to the plasma membrane.

Animals↗

PubMed1996

Chronic muscle stimulation increases lactate transport in rat skeletal muscle.

The aim of this study was to examine the effects of chronic low frequency stimulation on the lactate transport across the plasma membrane of the tibialis anterior (TA) muscle of the rat. Stimulating electrodes were implanted on either side of the peroneal nerve in one hindlimb. Chronic stimulation (10 Hz, 50 microsecond bursts, 24h/day) commenced 7 days after surgery, and were continued for 7 days. Animals were then left for 24 h, and thereafter muscles were obtained. Cytochrome C-oxidase activity was increased 1.9-fold in the stimulated TA compared to the control TA (p < 0.05). Lactate transport (zero-trans) was measured in giant sarcolemmal vesicles obtained from the chronically stimulated TA and the control TA. At each of the concentrations used in these studies a significant increase in lactate transport was observed; 2.8-fold increase at 1 mM lactate p < 0.05); 2-fold increases at both 30 mM and 50 mM lactate p < 0.05). These studies have shown that lactate transport capacity is markedly increased in response to chronic muscle contraction.

Action Potentials↗

PubMed1996

Increments in skeletal muscle GLUT-1 and GLUT-4 after endurance training in humans.

We investigated the time course of training-induced changes in the expression of GLUT-1 and GLUT-4 in human skeletal muscle. Seven healthy males trained for 2 h/day (approximately 60% pretraining VO2peak) for 31 days (31D). Muscle biopsies were obtained before training (PRE) and after 5 (5D) and 31 days (31D) of training. Training resulted in progressive increases in muscle GLUT-4 with increasing training duration (PRE<5D<31D; P<0.01). Muscle GLUT-1 content was also increased (P<0.05) after training; however, the increase was not observed until 31D (131%). Increases in muscle hexokinase (HK) activity were complete by 5D (P<0.01). Muscle malate dehydrogenase activity was not elevated after 5D of training but was increased (+35%; P<0.01) at 31D. Results from this study show that increases in both GLUT-4 and HK represent early training-induced adaptations to prolonged exercise training. As training progresses, further increases in GLUT-4, but not HK, occur in conjunction with an increase in muscle mitochondrial potential and GLUT-1.

Adult↗

PubMed1996

Role of the lactate transporter (MCT1) in skeletal muscles.

We used an antibody, constructed against the monocarboxylate transporter 1 (MCT1) protein (L. Carpenter, R. C. Poole, and A. P. Halestrap. Biochim. Biophys. Acta 1279: 157-165, 1996), to study the expression and role of MCT1 in rat skeletal muscles. MCT1 was higher in red than in white muscles (P < 0.05) and was highly correlated with the oxidative fiber content (%slow-twitch oxidative + %fast-twitch oxidative glycolytic) of skeletal muscles (r = 0.91). MCT1 was highly related to lactate uptake in skeletal muscles (r = 0.90). Total lactate dehydrogenase (LDH) activity, an index of glycolysis, was negatively correlated with MCT1 in rat muscles (r = -0.80). MCT1 was also strongly correlated with the heart-type forms of LDH (LDH-1 vs. MCT1, r = 0.83; LDH-2 vs. MCT1, r = 0.89). There was no relationship between MCT1 and the muscle form of LDH (LDH-5; P > 0.05). MCT1 was highly correlated with citrate synthase activity, a marker of the oxidative capacity of muscle (r = 0.82). Therefore, MCT1 may have kinetics that favor the uptake of L-lactate into the muscle cell for oxidative metabolism, and MCT1 may be coordinately expressed with the heart forms of LDH and enzymes of oxidative metabolism.

Animals↗

PubMed1996

Reciprocal GLUT-1 and GLUT-4 expression and glucose transport in denervated muscles.

We investigated in 3-day-denervated muscles 1) the expression of GLUT-1 in perineurial sheaths (PNS) and muscle, 2) the muscle fiber-specific changes in GLUT-1 and GLUT-4, and 3) changes in basal and insulin-stimulated 3-O-methylglucose transport. GLUT-1 was increased in both the PNS (P < 0.05) and in the muscle membranes (P < 0.05). GLUT-1 and GLUT-4 concentrations were changed reciprocally, in a fiber-dependent fashion [GLUT-1: red gastrocnemius (RG), +31%; white gastrocnemius (WG), +10%; GLUT-4: RG, -53%; WG, -16%]. Basal glucose transport was increased (P < 0.05), and this increase was correlated with the oxidative nature of the muscles (r = 0.97). Insulin-stimulated glucose transport was decreased in denervated muscles (P < 0.05). This was also related to the oxidative nature of the muscles (r = -0.88). The increase in basal glucose transport was correlated with the loss of insulin-stimulated transport (r = 0.95). Thus the increase in GLUT-1 compensates for the loss of GLUT-4, resulting in a 56% regain of the reduced insulin-stimulated glucose transport.

3-O-Methylglucose↗

PubMed1996

Effect of overexpressing GLUT-1 and GLUT-4 on insulin- and contraction-stimulated glucose transport in muscle.

To examine the effects of GLUT-1 on GLUT-4-dependent, insulin-stimulated, and contraction-stimulated 2-deoxy-D-glucose (2-DG) transport, we overexpressed GLUT-1 in metabolically heterogeneous skeletal muscles [red and white tibialis anterior (TA) and extensor digitorum longus (EDL)] via 7 days of chronic electrical stimulation. GLUT-1 was increased 1.6- to 16.4-fold (P < 0.05). Basal 2-DG transport was increased 1.7- to 3.0-fold (P < 0.05) and was equal to (red TA and EDL; P > 0.05) or exceeded insulin-stimulated 2-DG transport by 50% (white TA; P < 0.05) in the control muscles. GLUT-4 was concomitantly overexpressed (2.1- to 4.4-fold; P < 0.05). Insulin-stimulated 2-DG transport was increased 1.6- to 2.5-fold (P < 0.05). During muscle contractions, 2-DG transport increased 9- to 12-fold (P < 0.05) in control muscles, but this was reduced by approximately 25% (P < 0.05) in muscles overexpressing GLUT-1 and GLUT-4 (red TA and EDL). In contrast, in the experiment, white TA contraction-stimulated 2-DG transport was increased 1.7-fold (P < 0.05). Therefore, overexpression of GLUT-1, when GLUT-4 is also overexpressed, does not impair insulin-stimulated 2-DG transport, although contraction-stimulated transport may be reduced in some muscles.

Animals↗

PubMed1996

Effect of cross-reinnervation on the expression of GLUT-4 and GLUT-1 in slow and fast rat muscles.

We determined whether the twitch-velocity phenotype or the metabolic phenotype of a muscle influences the content of GLUT-4 and GLUT-1 proteins. The soleus (Sol) and extensor digitorum longus (EDL) muscles were cross-reinnervated (X-Sol, X-EDL). After 3 mo the X-EDL had become enriched in slow-twitch oxidative (SO) fibers (70.5% SO) compared with its control (3.8% SO), whereas the X-Sol became enriched in fast-twitch oxidative-glycolytic (FOG) fibers (78.6% FOG) compared with its control (10% FOG). Thus the twitch phenotype of X-Sol shifted to fast-twitch muscle, whereas X-EDL shifted to a slow-twitch muscle. In the X-EDL, the oxidative nature of the X-EDL was increased to 97% oxidative fibers compared with 43% oxidative fibers in the normal EDL. In the Sol the oxidative nature of the X-Sol was retained at 100%. GLUT-4 content was increased 1.6-fold in the X-EDL (P < 0.05) but was not changed in the X-Sol (P > 0.05). GLUT-1 content was increased fourfold in X-EDL but was not altered in the X-Sol. We conclude that GLUT-4 and GLUT-1 content in muscle is related to the oxidative phenotype of the muscle rather than the twitch-velocity phenotype.

Animals↗

PubMed1996

Effect of electrical stimulation patterns on glucose transport in rat muscles.

Transport of 2-[3H]deoxy-D-glucose (2-DG) was investigated during supramaximal stimulations of different muscles. In addition, we varied the net stimulation time (NST). In different treatments, NST occupied either 5, 7.5, 10, 15, 20, 30, or 50% of a 20-min stimulation period. After a bolus injection of 3H-labeled 2-DG, the greatest transport occurred in the extensor digitorum longus. In red gastrocnemius (RG; type IIa fibers) and white gastrocnemius (WG; type IIb fibers), the 2-DG transport rate was highest at 10% NST (8- to 12-fold increase) and decreased thereafter. In soleus (type I fibers), the 2-DG transport increased from 5 to 50% NST. Below 30% NST, 2-DG transport was greater in RG and WG muscles than in soleus (P < 0.05). GLUT-4 and 2-DG transport were not correlated during the contractions. Therefore, the percent NST affects 2-DG transport differentially in muscles of varying fiber types, and the transport rate is not related to the GLUT-4 content of the muscles.

Animals↗

PubMed1995

Recreational exercise participation and aerobic fitness in men and women: analysis of data from a national survey.

We compared the relationship between predicted VO2 max (ml kg-1min-1) and recreational exercise patterns, using secondary data analysis of a comprehensive national survey (18,293 subjects aged 15-69 years). Exercise participation and predicted VO2 max data were available for about 50% of this sample (4933 females, 4738 males). As expected, VO2 max was significantly lower in the females than in the males at any age (P < 0.0001). Age was the most significant predictor of VO2 max (r = -0.71 for males, r = -0.73 for females). Adjusting the data for the body mass index (BMI) increased this relationship only slightly in the males (R = 0.75) and females (R = 0.79). The simultaneous inclusion of exercise participation data (intensity, duration, energy expenditure) did not increase the predictions meaningfully (R = 0.78 for the males, R = 0.81 for the females). These exercise participation parameters concomitantly accounted for only a very slight amount of the variance of VO2 max in both the females (3.0%) and males (4.5%). To minimize the effects of age, the data were analysed using 5-year intervals. Again, the exercise participation parameters accounted for only a small part of the variance in VO2 max (< 10%), except in the 15- to 19-year-old males (24%). These data suggest that VO2 max is not associated with participation in recreational exercise.

Adolescent↗

PubMed1995

L(+)-lactate binding to a protein in rat skeletal muscle plasma membranes.

Biochemical studies were conducted to determine the location of a putative lactate transport protein in rat skeletal muscle plasma membranes (PM). PM (50-100 micrograms protein) were incubated with [U-14C] L(+)-lactate, in the presence or absence of unlabeled monocarboxylates or potential inhibitors, after which proteins were separated by SDS-PAGE. Gel slices (2 mm) were cut and analyzed for 14C. [U-14C] L(+)-lactate was bound to plasma membranes in the 30 to 40 kDa molecular mass range. Binding of [U-14C] L(+)-lactate was inhibited by N-ethylmaleimide, unlabeled L-lactate and pyruvate, and in a dose dependent manner by alpha-cyano-4-hydroxycinnamate (r = 0.995), but not by cytochalasin-B. The inhibition of [U-14C] L(+)-lactate binding was similar to the inhibition of lactate transport. Therefore the transport of L(+)-lactate across skeletal muscle plasma membranes involves a polypeptide of 30 to 40 kDa.

Animals↗

PubMed1995

Vascular actions of insulin in health and disease.

Insulin has well known metabolic effects. However, depending on the magnitude and duration of the insulin stimulus, this hormone can also produce vasodilation and vascular smooth muscle growth. The association of hyperinsulinemia with the metabolic disorders of obesity and non-insulin-dependent diabetes, as well as with the cardiovascular pathologies of hypertension and atherosclerosis, has led to suggestions that perhaps elevated insulin levels are causally related to these diseases. Alternatively, insulin resistance may develop following an increase in skeletal muscle vascular resistance, with or without hypertension, such that a reduction in skeletal muscle blood flow leads to an attenuated glucose delivery and uptake. These hypotheses are explored in this review by examining the effects of insulin on vascular smooth muscle tissue during both acute and prolonged exposure. An interaction among hyperinsulinemia, hyperglycemia, and hyperlipidemia associated with the insulin resistant state is described whereby insulin resistance can be both a cause and a result of elevated vascular resistance. The association between blood flow and insulin stimulated glucose uptake suggests that therapeutic intervention against the development of skeletal muscle vascular resistance should occur early in individuals generally predisposed to cardiovascular pathology in order to attenuate, or avoid, insulin resistance and its sequelae.

Animals↗

PubMed1995

Benefits of exercise for type II diabetics: convergence of epidemiologic, physiologic, and molecular evidence.

In Canada diabetes affects approximately 5% of the population. The economic costs of diabetes and its attendant complications are significant, requiring approximately $1 billion a year from the health care system. Clearly the prevention and alleviation of diabetes is highly desirable. In the past few years there has been a remarkable convergence of physiologic, biochemical, molecular, and epidemiologic data, all of which indicate very strongly that exercise may be used as a therapeutic tool to prevent or alleviate non-insulin-dependent diabetes mellitus (NIDDM), or Type II diabetes. The evidence for this has been reviewed. Recently the significant therapeutic role of exercise for Type II diabetics has been endorsed by the medical community. However, there is virtually no education of exercise professionals in the area of diabetes and the benefits of lifestyle changes in treating Type II diabetics. This deficiency should be remedied. For the research community, the challenge now is to translate the physiologic, biochemical, and epidemiologic knowledge into optimally effective prescriptive exercise programs for Type II diabetic men and women.

Animals↗

PubMed1995

Substrate and hormonal responses during exercise classes at selected stages of pregnancy.

This study examined the substrate and hormonal responses during exercise classes for 36 pregnant women at selected stages of pregnancy. Six nonpregnant women served as controls. Blood samples were obtained before exercise, at the end of aerobic exercise, and 45 min after aerobic exercise. All samples were analyzed for substrates (glucose, lactate, FFA) and 7 hormones. Heart rates in the pregnant women were approximately 150 bpm, although some were as high as 180 bpm. Heart rates in the control group were approximately 130 bpm. Modest increments occurred in lactate and FFA during exercise, and decrements in glucose were observed in all groups. Insulin concentrations decreased with exercise and remained depressed after exercise whereas FFA remained elevated postexercise in the pregnant women. Modest transient changes were observed with exercise in the other hormones. A severely blunted norepinephrine response to exercise was observed in the third-trimester group. In conclusion, uncontrolled non-steady-state exercise does not disrupt the substrate and endocrine milieu markedly. Recovery in hormonal and substrate concentrations appears to occur rapidly after exercise. However, decrements in blood glucose do occur for a short time during prenatal exercise classes, most notably for women in the third trimester.

Adult↗

PubMed1995

Reduced lactate transport in denervated rat skeletal muscle.

The purpose of this study was to investigate the effect of the neural regulation of contractile activity on lactate transport in skeletal muscle. Contractile activity of the rat soleus muscle was abolished by denervating the hindlimb muscles in one leg (3 days) while the sham-operated contralateral hindlimb muscles served as a control. Three days after surgery, lactate transport into the soleus muscle was measured in vitro, using incubated soleus muscle strips. Lactate uptake by the denervated soleus muscle was reduced compared with control (P < 0.05). The diffusive component of lactate transport was unaltered by denervation (P > 0.05). These results translated into a reduction in lactate carrier-mediated transport capacity (-68%) in the denervated soleus (P < 0.05). These studies indicate that loss of contractile activity results in a decrement of lactate transport, which is probably due to a reduction in the number of lactate carriers in the sarcolemma. Our results suggest that the inherent activity of the muscle is important in maintaining the lactate transport system.

Animals↗

PubMed1995

Regulation of muscle glucose transport and GLUT-4 by nerve-derived factors and activity-related processes.

Glucose transport and GLUT-4 were examined in muscles in which activity and nerve-derived factors were eliminated (denervation) and in muscles in which only muscle activity was eliminated but in which nerve-derived factors were maintained [tetrodotoxin (TTX) treatment]. After 3 days of denervation, insulin-stimulated 3-O-methylglucose transport was markedly lowered in perfused rat hindlimb muscles (soleus, plantaris, and red and white gastrocnemius; < or = 35%). GLUT-4 was also decreased by 11-65% in denervated muscles. Blocking muscle activity with TTX superfusion of the sciatic nerve for 3 days reduced the insulin-stimulated glucose transport to the same extent as in the denervated muscles (P > 0.05). However, in soleus, plantaris, and red gastrocnemius muscles, GLUT-4 expression was reduced much less by TTX treatment than by denervation (P < 0.05). GLUT-4 mRNA abundance was decreased in denervated muscles but not in TTX-treated muscles. These results suggest that muscle activity largely regulates the insulin-signaling mechanisms of glucose transport and that nerve-derived trophic factors affect pretranslational events to regulate GLUT-4 expression.

3-O-Methylglucose↗

PubMed1994

Lactate transport in rat sarcolemmal vesicles and intact skeletal muscle, and after muscle contraction.

To determine whether it was possible to measure lactate transport rates into intact skeletal muscles, the transport of lactate (zero-trans) was determined in soleus muscle strips incubated in vitro and compared with lactate transport in sarcolemmal vesicles. In addition, the effects of muscle contractility on lactate transport were investigated in electrically-stimulated soleus muscle strips. In both the intact muscle and the sarcolemmal preparations the rates of transport were saturable, stereospecific, and inhibitable by monocarboxylates (pyruvate, alpha-cyano-4-hydroxycinnamate) and a protein modifier (N-ethylmaleimide; P < 0.05). The anion exchange inhibitor SITS had no effect on lactate uptake (P > 0.05). In both preparations lactate transport followed an inwardly directed proton gradient. Relative comparisons (%) between the preparations indicated a similar slope of increasing transport rates with increasing lactate concentrations and similar responses to a changing pH environment. These characterizations of L-lactate transport into isolated sarcolemmal vesicles and muscle strips revealed that both preparations yielded similar conclusions regarding the transmembrane movement of L-lactate. By using this more physiological muscle preparation, contractile activity, induced by electrical stimulation, did not increase lactate uptake in skeletal muscle in the post-exercise period whereas under similar conditions a marked increase in 2-deoxy-D-glucose uptake occurred (+ 47%; P < 0.05). These data suggest that the transport of glucose and lactate in contracting muscle is regulated differently. These studies also show that the incubated muscle strip preparation may be useful for studying lactate transport in an intact cell system during physiological experiments.

Animals↗

PubMed1994

Effects of exercise on lactate transport into mouse skeletal muscles.

Skeletal muscle lactate transport was investigated in vitro in isolated fast-twitch (EDL) and slow-twitch soleus (Sol) skeletal muscles from control and exercised mice. Exercise (23 m/min, 8% grade) reduced muscle glycogen by 37% in EDL (p < 0.05) and by 35% in Sol muscles (p < 0.05). Lactate transport measurements (45 sec) were performed after 60 min of exercise in intact EDL and Sol muscles in vitro, at differing pH (6.5 and 7.4) and differing lactate concentrations (4 and 30 mM). Lactate transport was observed to be greater in Sol than in EDL (p < 0.05). In the exercised muscles there was a small but significant increase in lactate transport (p < 0.05). Lactate transport was greater when exogenous lactate concentrations were greater (p < 0.05) and more rapid at the lower pH (p < 0.05). These studies demonstrated that lactate transport was increased with exercise.

Animals↗

PubMed1994

Experimental approaches in muscle metabolism: hindlimb perfusion and isolated muscle incubations.

The perfusion of rat hindlimb muscles and the isolated in vitro muscle preparation are usually the preferred methods for investigating muscle metabolism. In light of recent concerns about the incubated muscle preparation, we have examined the problems, the advantages, and the viability of these two experimental techniques, with focus on glucose metabolism. A major advantage of the hindlimb perfusion system is that it maintains its metabolic viability very well, and perfusions in resting muscles can be achieved successfully with cell-free media. However, variations in the perfused rat hindlimb procedures result in considerable differences in perfusate flow among muscles, making quantitative comparisons among perfusion procedures difficult. Metabolic viability has been identified as a problem in some isolated in vitro muscle preparations. We have provided criteria to avoid muscle hypoxia. Minimum levels of insulin seem to be a key requirement to maintaining the muscle's viability, and essential amino acids are required to retard an increase in the basal rate of glucose and amino acid uptake. Under such conditions metabolic viability can be maintained during prolonged incubations (9-30 h). Both the isolated in vitro muscle preparation and the hindlimb perfusion preparation are viable models for the study of muscle metabolism.

Animals↗