The elderly's physiological and perceptual responses to cooling during simulated activities of daily living in UK summer climatic conditions

OBJECTIVES

The elderly are the most at-risk population for heat-related illness and mortality during the periods of hot weather. However, evidence-based elderly-specific cooling strategies to prevent heat-illness are limited. The aim of this investigation was to quantify the elderly's physiological and perceptual responses to cooling through cold water ingestion (COLD) or an L-menthol mouth rinse (MENT) during simulated activities of daily living in UK summer climatic conditions.

STUDY DESIGN

Randomised, controlled repeated measures research design.

METHODS

A total of ten participants (men n = 7, women n = 3: age; 69 ± 3 yrs, height; 168 ± 10 cm, body mass; 68.88 ± 13.72 kg) completed one preliminary and three experimental trials; control (CON), COLD and MENT. Experimental trials consisted of 40 min rest followed by 30 min of cycling exercise at 6 metabolic equivalents and a 6-min walk test (6MWT), within a 35 °C, 50% relative humidity environment. Experimental interventions (every 10 min); cold water (4 °C) ingestion (total of 1.5L) or menthol (5 ml mouth swill for 5 s, menthol concentration of 0.01%).

RESULTS

Peak rectal temperature (T_re) was significantly (P < 0.05) lower in COLD compared with CON (-0.34 ± 0.16 °C) and MENT (-0.36 ± 0.20 °C). End exercise heart rate (HR) decreased in COLD compared with CON (-7 ± 9 b min^-1) and MENT (-6 ± 7 b min^-1). There was no difference in end exercise thermal sensation (TS) (CON; 6.1 ± 0.4, COLD; 6.0 ± 0.4, MENT; 6.4 ± 0.6) or thermal comfort (TC) (CON; 4 ± 1, COLD; 4 ± 1, MENT; 4 ± 1) between trials. The participants walked significantly further during the COLD 6MWT compared with CON (40 m ± 40 m) and MENT (40 m ± 30 m). There was reduced physiological strain in the COLD 6MWT compared with CON (T_re; -0.21 ± 0.24 °C, HR; -7 ± 8 b min^-1) and MENT (T_re; -0.23 ± 0.24 °C, HR; -4 ± 7 b min^-1).

CONCLUSION

The elderly have reduced physiological strain (T_re and HR) during activities of daily living and a 6MWT in hot UK climatic conditions, when they drink cold water. Furthermore, the elderly's perception (TS and TC) of the hot environment did not differ from CON at the end of exercise with COLD or MENT interventions. Menthol provided neither perceptual benefit to exercise in the heat nor functional gain. The TS data indicate that elderly may be at increased risk of heat illness, due to not feeling hot and uncomfortable enough to implement physiological strain reducing strategies such as cold-water ingestion.

Physiological and perceptual responses in the elderly to simulated daily living activities in UK summer climatic conditions

OBJECTIVES

The elderly population is at an increasingly significant health risk to heat-related illnesses and mortality when compared with younger people in the same conditions. This is due to an increased frequency and severity of heatwaves, attributed to climate change, and reduced ability of elderly individuals to dissipate excess heat. Consequently, most excess deaths and emergency visits during heatwaves occur in people aged more than 65 years. The aim of this investigation was to assess the physiological and perceptual responses of elderly people during exercise sessions equating to activities of daily living in UK summer climatic conditions.

STUDY DESIGN

Mixed-method, randomised research design.

METHODS

Twenty-eight participants (17 males, 10 females and 1 transgender female) were randomly assigned into three experimental groups; 15°C, 25°C or 35°C, with 50% relative humidity. Participants completed one preliminary and three experimental trials within their assigned environment. The data from the preliminary incremental recumbent cycling test was used to calculate participant's individual exercise intensities equating to 2, 4 and 6 metabolic equivalents (METs) for the subsequent trials. During experimental trials, participants completed 30-min seated rest and 30-min cycling.

RESULTS

No change was observed in thermal comfort ([TC] just uncomfortable in both trials), and only modest changes in ratings of perceived exertion (14 ± 2 vs 15 ± 2) at 6 METs in 25°C compared with those in 35°C were observed. In contrast, thermal strain markers did significantly increase (P < 0.05) across the same conditions, including change in rectal temperature (ΔT_re) during exercise (0.27 ± 0.17°C vs 0.64 ± 0.18°C) and peak skin temperature ([T_skin] 32.94 ± 1.15°C vs 36.11 ± 0.44°C).

CONCLUSION

When completing exercise that equates to activities of daily living, elderly people could have a decreased perceptual awareness of the environment even though physiological markers of thermal strain are elevated. Consequently, the elderly could be less likely to implement behavioural thermoregulation interventions (i.e. seek shade and/or remove excess layers) due to a decreased awareness of an increasingly thermally challenging environment.

Leukocyte Hsp72 mRNA transcription does not differ between males and females during heat acclimation

Purpose: Thermotolerance is an acquired state of increased cytoprotection achieved following single or repeated exposures to heat stress, in part characterized by changes in the intracellular 72 kda heat shock protein (HSP72; HSPA1A). Females have demonstrated reduced exercise induced HSP72 in comparison to males. This study examined sex differences in heat shock protein 72 messenger ribonucleic acid (Hsp72 mRNA) transcription during heat acclimation (HA) to identify whether sex differences were a result of differential gene transcription. Methods: Ten participants (5M, 5F) performed 10, 90 min controlled hyperthermia [rectal temperature (T_re) ≥ 38.5°C] HA sessions over 12 d. Leukocyte Hsp72 mRNA was measured pre and post D1, D5, and D10, via Reverse transcription polymerase chain reaction (RT-QPCR). Results: HA was evidenced by a reduction in resting T_re (-0.4 ± 0.5°C) and resting heart rate [(HR); -13 ± 7 beats.min^-1] following HA (p ≤ 0.05). During HA no difference (p > 0.05) was observed in ΔT_re between males (D1 = 1.5 ± 0.2°C; D5 = 1.6 ± 0.4°C; D10 = 1.8 ± 0.3°C) and females (D1 = 1.5 ± 0.5°C; D5 = 1.4 ± 0.2°C; D10 = 1.8 ± 0.3°C). This was also true of mean T_re demonstrating equality of thermal stimuli for mRNA transcription and HA. There were no differences (p > 0.05) in Hsp72 mRNA expression between HA sessions or between males (D1 = +1.8 ± 1.5-fold; D5 = +2.0 ± 1.0 fold; D10 = +1.1 ± 0.4-fold) and females (D1 = +2.6 ± 1.8-fold; D5 = +1.8 ± 1.4-fold; D10 = +0.9 ± 1.9-fold). Conclusions: This experiment demonstrates that there is no difference in Hsp72 mRNA increases during HA between sexes when controlled hyperthermia HA is utilised. Gender specific differences in exercise-induced HSP72 reported elsewhere likely result from post-transcriptional events.

Fat utilization during exercise: adaptation to a fat-rich diet increases utilization of plasma fatty acids and very low density lipoprotein-triacylglycerol in humans

1. This study was carried out to test the hypothesis that the greater fat oxidation observed during exercise after adaptation to a high-fat diet is due to an increased uptake of fat originating from the bloodstream. 2. Of 13 male untrained subjects, seven consumed a fat-rich diet (62 % fat, 21 % carbohydrate) and six consumed a carbohydrate-rich diet (20 % fat, 65 % carbohydrate). After 7 weeks of training and diet, 60 min of bicycle exercise was performed at 68 +/- 1 % of maximum oxygen uptake. During exercise [1-(13)C]palmitate was infused, arterial and venous femoral blood samples were collected, and blood flow was determined by the thermodilution technique. Muscle biopsy samples were taken from the vastus lateralis muscle before and after exercise. 3. During exercise, the respiratory exchange ratio was significantly lower in subjects consuming the fat-rich diet (0.86 +/- 0.01, mean +/- S.E.M.) than in those consuming the carbohydrate-rich diet (0.93 +/- 0.02). The leg fatty acid (FA) uptake (183 +/- 37 vs. 105 +/- 28 micromol min(-1)) and very low density lipoprotein-triacylglycerol (VLDL-TG) uptake (132 +/- 26 vs. 16 +/- 21 micromol min(-1)) were both higher (each P < 0.05) in the subjects consuming the fat-rich diet. Whole-body plasma FA oxidation (determined by comparison of (13)CO(2) production and blood palmitate labelling) was 55-65 % of total lipid oxidation, and was higher after the fat-rich diet than after the carbohydrate-rich diet (13.5 +/- 1.2 vs. 8.9 +/- 1.1 micromol min(-1) kg(-1); P < 0.05). Muscle glycogen breakdown was significantly lower in the subjects taking the fat-rich diet than those taking the carbohydrate-rich diet (2.6 +/- 0.5 vs. 4.8 +/- 0.5 mmol (kg dry weight)(-1) min(-1), respectively; P < 0.05), whereas leg glucose uptake was similar (1.07 +/- 0.13 vs. 1.15 +/- 0.13 mmol min(-1)). 4. In conclusion, plasma VLDL-TG appears to be an important substrate source during aerobic exercise, and in combination with the higher plasma FA uptake it accounts for the increased fat oxidation observed during exercise after fat diet adaptation. The decreased carbohydrate oxidation was apparently due to muscle glycogen sparing and not to diminished plasma glucose uptake.

Day length has a major effect on the response of protein synthesis rates to feeding in growing Japanese quail

We investigated the effect of day length on mixed protein fractional synthesis rates (K(S)) in 14- and 21-d-old Japanese quail (Coturnix c. japonica) habituated to either a long day length, 18 h light/6 h dark (LDL), or short day length, 6 h light/18 h dark (SDL), with free access to food during the light period. Rates of protein synthesis were measured by a flooding dose of L-[1-(13)C]leucine. In both groups, we measured K(S) of pectoral muscle, liver and heart after an overnight period of food deprivation and after 2-h food access at dawn. Rates of protein synthesis were also measured in LDL quail starved for 18 h and refed for 2 h. SDL chicks were smaller and had lower tissue weights at 2 wk of age than did LDL chicks (P<0.05). Starvation led to a lower rate of protein synthesis in those animals starved for 18 h. Food availability after starvation for 18 h induced a significant rise in tissue protein synthesis in both SDL and LDL quail (P<0.05). This increase was absent in LDL quail after a 6-h starvation period. There was an increase in K(S) to ad hoc changes in food supply. By determining the daily period in which feeding can occur, day length has a major effect on protein synthesis rates. This effect will determine the overall growth chicks are able to achieve that have been subjected to different day lengths.

Lactate transport in rat adipocytes: identification of monocarboxylate transporter 1 (MCT1) and its modulation during streptozotocin-induced diabetes

We have characterised L-lactate transport in rat adipocytes and determined whether these cells express a carrier belonging to the monocarboxylate transporter family. L-Lactate was taken up by adipocytes in a time-dependent, non-saturable manner and was inhibited (by approximately 90%) by alpha-cyano-4-hydroxycinnamate. Lactate transport was stimulated by 3.7-fold upon lowering extracellular pH from 7.5 to 6.5 suggesting the presence of a lactate/proton-cotransporter. Antibodies against mono carboxylate transporter 1 (MCT1) reacted positively with plasma membranes (PM), but not with intracellular membranes, prepared from adipocytes. MCTI expression was down-regulated in PM of adipocytes from diabetic rats, which also displayed a corresponding loss (approximately 64%) in their capacity to transport lactate. The data support a role for MCT1 in lactate transport and suggest that changes in MCT1 expression are likely to have important implications for adipocyte lactate metabolism.

Effect of oral glucose on leucine turnover in human subjects at rest and during exercise at two levels of dietary protein

The aim of this study was to determine the effect of glucose supplementation on leucine turnover during and after exercise and whether variation in the previous dietary protein content modulated this effect. Postabsorptive subjects received a primed constant [1-13C, 15N]leucine infusion for 6 h, after previous consumption of a high (1.8 g kg-1 day-1, HP, n = 16) or low (0.7 g kg-1 day-1, LP, n = 16) protein diet for 7 days. The subjects were studied at rest; during 2 h of exercise, during which half of the subjects from each dietary protocol received 0.75 g kg-1 h-1 glucose (HP + G, LP + G) and the other half received water (HP + W, LP + W); then again for 2 h of rest. Glucose supplementation suppressed leucine oxidation (P < 0.01) by 20% in subjects consuming the high protein diet (58.2 +/- 2.8 micromol kg-1 h-1, HP + G; 72.4 +/- 3.9 micromol kg-1 h-1, HP + W) but not the low protein diet (51.1 +/- 5.9 micromol kg-1 h-1, LP + G; 51.7 +/- 5.5 micromol kg-1 h-1, LP + W), with no difference in skeletal muscle branched-chain 2-oxo acid dehydrogenase (BCOADH) activity between groups. Glucose supplementation did not alter the rate of whole-body protein synthesis or breakdown. The sparing effect of glucose on leucine oxidation appears only to occur if previous protein intake was high. It was not mediated by a suppression of BCOADH fractional activity but may be due to reduced substrate availability.

Phosphorylation of the skeletal muscle glycogen-targetting subunit of protein phosphatase 1 in response to adrenaline in vivo

The protein G(M), which targets protein phosphatase 1 (PP1) to the glycogen particles and sarcoplasmic reticulum (SR) of striated muscles, is known to be phosphorylated at Ser48 and Ser67 in vitro by adenosine 3',5' cyclic monophosphate-dependent protein kinase (PKA) and at Ser48 by MAP kinase-activated protein kinase-1 (MAPKAP-K1, also called p90 RSK). The phosphorylation of Ser48 increases the rate at which the glycogen-associated PP1.G(M) complex dephosphorylates (activates) glycogen synthase, but the phosphorylation of Ser67 has the opposite effect, suppressing the activity of PP1 toward glycogen-bound substrates. The phosphorylation of Ser67 overrides the activating effect of Ser48 phosphorylation because it dissociates PP1 from G(M). Here, we use two phospho-specific antibodies to demonstrate that the SR-associated form of G(M), as well as the glycogen-associated form of G(M), becomes phosphorylated at Ser48 and Ser67 in response to adrenaline, supporting the view that the PKA-mediated regulation of the PP1.G(M) complex plays a role in the adrenergic control of glycogen metabolism and SR function. In contrast, Ser48 is not phosphorylated significantly in response to insulin, and neither is Ser67. Thus the phosphorylation of G(M) at Ser48 by MAPKAP-K1 or other insulin-stimulated protein kinases is not involved in the activation of glycogen synthase by insulin.

Interactions between the thiol-group reagent N-ethylmaleimide and neutral and basic amino acid transporter-related amino acid transport

The neutral and basic amino acid transport protein (NBAT) expressed in renal and jejunal brush-border membranes is involved in amino acid and cystine absorption. NBAT mutations result in Type 1 cystinuria. A C-terminal myc-tagged NBAT (NBATmyc) retains the amino acid transport and protein-protein interaction properties of NBAT when expressed in Xenopus oocytes. Neutral amino acid (Ala, Phe)-cationic amino acid (Arg) heteroexchanges related to NBATmyc expression in oocytes are inactivated by treatment with the thiol-group reagent N-ethylmaleimide (NEM), although significant Arg-Arg and Ala-Ala homoexchanges persist. Inactivation of heteroexchange activity by NEM is accompanied by loss of >85% of alanine and cystine uptake, with smaller (<50%) inhibition of arginine and phenylalanine uptake. NEM-sensitive cystine uptake and arginine-alanine heteroexchange (system b(0,+) activity) are not expressed by an NBAT truncation mutant (NBATmyc-Sph1) lacking the 13 C-terminal amino acid residues, but the mutant expresses NEM-resistant transport activity (system y(+)L-like) equivalent to that of full-length NBATmyc. The deleted region of NBATmyc-Sph1 contains two cysteine residues (671/683) which may be the targets of NEM action. The synthetic amino acid 2-trifluoromethylhistidine (TFMH) stimulated alanine efflux at pH 7.5 and arginine at pH 5.5, but not vice versa, establishing the existence of distinct pathways for cationic and neutral amino acid homoexchange (TFMH is zwitterionic at pH 7.5 and cationic at pH 5.5). We suggest that NBAT expresses a combination of system b(0,+) and y(+)L-like activities, possibly by interacting with different light-chain subunits endogenous to oocytes (as does the homologous 4F2hc protein). The C-terminus of NBAT may also have an additional, direct role in the mechanism of System b(0,+) transport (the major transport activity that is defective in Type 1 cystinuria).

Modulation of whole body protein metabolism, during and after exercise, by variation of dietary protein

The aim of this study was to investigate dietary protein-induced changes in whole body leucine turnover and oxidation and in skeletal muscle branched chain 2-oxo acid dehydrogenase (BCOADH) activity, at rest and during exercise. Postabsorptive subjects received a primed constant infusion of L-[1-13C,15N]leucine for 6 h, after previous consumption of a high- (HP; 1.8 g . kg-1 . day-1, n = 8) or a low-protein diet (LP; 0.7 g . kg-1 . day-1, n = 8) for 7 days. The subjects were studied at rest for 2 h, during 2-h exercise at 60% maximum oxygen consumption, then again for 2 h at rest. Exercise induced a doubling of both leucine oxidation from 20 micromol . kg-1 . h-1 and BCOADH percent activation from 7% in all subjects. Leucine oxidation was greater before (+46%) and during (+40%, P < 0.05) the first hour of exercise in subjects consuming the HP rather than the LP diet, but there was no additional change in muscle BCOADH activity. The results suggest that leucine oxidation was increased by previous ingestion of an HP diet, attributable to an increase in leucine availability rather than to a stimulation of the skeletal muscle BCOADH activity.

Insulin activates protein kinase B, inhibits glycogen synthase kinase-3 and activates glycogen synthase by rapamycin-insensitive pathways in skeletal muscle and adipose tissue

Insulin stimulated protein kinase B alpha (PKB alpha) more than 10-fold and decreased glycogen synthase kinase-3 (GSK3) activity by 50 +/- 10% in skeletal muscle and adipocytes. Rapamycin did not prevent the activation of PKB, inhibition of GSK3 or stimulation of glycogen synthase up to 5 min. Thus rapamycin-insensitive pathways mediate the acute effect of insulin on glycogen synthase in the major insulin-responsive tissues. The small and very transient effects of EGF on phosphatidylinositol (3,4,5)P3 PKB alpha and GSK3 in adipocytes, compared to the strong and sustained effects of insulin, explains why EGF does not stimulate glucose uptake or glycogen synthesis in adipocytes.

A novel, rapid, and highly sensitive mass assay for phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P3) and its application to measure insulin-stimulated PtdIns(3,4,5)P3 production in rat skeletal muscle in vivo

The pivotal role of phosphatidylinositol 3-kinase (PI 3-kinase) in signal transduction has been well established in recent years. Receptor-regulated forms of PI 3-kinase are thought to phosphorylate phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) at the 3-position of the inositol ring to give the putative lipid second messenger, phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4, 5)P3). Cellular levels of PtdIns(3,4,5)P3 are currently measured by time-consuming procedures involving radiolabeling with high levels of 32PO4, extraction, and multiple chromatography steps. To avoid these lengthy and hazardous procedures, many laboratories prefer to assay PI 3-kinase activity in cell extracts and/or appropriate immunoprecipitates. Such approaches are not readily applied to measurements of PtdIns(3,4,5)P3 in extracts of animal tissues. Moreover, they can be misleading since the association of PI 3-kinases in molecular complexes is not necessarily correlated with the enzyme's activity state. Direct measurements of PtdIns(3,4,5)P3 would also be desirable since its concentration may be subject to additional control mechanisms such as activation or inhibition of the phosphatases responsible for PtdIns(3,4,5)P3 metabolism. We now report a simple, reproducible isotope dilution assay which detects PtdIns(3,4,5)P3 at subpicomole sensitivity, suitable for measurements of both basal and stimulated levels of PtdIns(3,4,5)P3 obtained from samples containing approximately 1 mg of cellular protein. Total lipid extracts, containing PtdIns(3,4,5)P3, are first subjected to alkaline hydrolysis which results in the release of the polar head group Ins(1,3,4,5)P4. The latter is measured by its ability to displace [32P]Ins(1,3,4,5)P4 from a highly specific binding protein present in cerebellar membrane preparations. We show that this assay solely detects PtdIns(3,4,5)P3 and does not suffer from interference by other compounds generated after alkaline hydrolysis of total cellular lipids. Measurements on a wide range of cells, including rat-1 fibroblasts, 1321N1 astrocytoma cells, HEK 293 cells, and rat adipocytes, show wortmannin-sensitive increased levels of PtdIns(3,4,5)P3 upon stimulation with appropriate agonists. The enhanced utility of this procedure is further demonstrated by measurements of PtdIns(3,4,5)P3 levels in tissue derived from whole animals. Specifically, we show that stimulation with insulin increases PtdIns(3,4,5)P3 levels in rat skeletal muscle in vivo with a time course which parallels the activation of protein kinase B in the same samples.

Effects of limb immobilization on cytochrome c oxidase activity and GLUT4 and GLUT5 protein expression in human skeletal muscle

1. We investigated the effects of limb immobilization (for 1 or 6 weeks) in a long leg cast after a closed tibial fracture (n = 11). Biopsies of vastus lateralis were taken on admission and after either 1 week (n = 5) or 6 weeks (n = 6) and analysed for muscle fibre type characteristics, cytochrome c oxidase activity and the abundance of GLUT4 and GLUT5 hexose transporters. 2. After 1 week of immobilization there was a significant decrease (8%) in the cross-sectional area of type I, but not type II, muscle fibers and in the protein-DNA ratio (16%) compared with the initial biopsy. Six weeks of immobilization led to further muscle atrophy compared with the initial biopsy and a further reduction in the cross-sectional area of both type I and II fibres (29% and 36% decrease respectively) and in the protein-DNA ratio (25%). No changes were observed in the free leg after 1 week. However, at th end of the 6 week study period, the cross-sectional area of boty type I and II fibres of the free leg were increased (7% and 5%) and there was significant increase in the protein-DNA ratio (14%), indicating a net increase in muscle protein content. 3. Assay for cytochrome c oxidase activity showed significant reduction after 1 (30%) or 6 weeks (36%) of immobilization, reflecting a reduced capacity for oxidative metabolism. No significant changes in activity were observed in muscle from the free leg after 1 or 6 weeks of study. 4. The concentrations of GLUT4 and GLUT5 protein were determined by Western blot analysis. Limb immobilization induced a marked (50%) reduction in muscle GLUT4 protein concentration after 1 week that persisted for 6 weeks. A transient but significant increase (approximately twofold) in GLUT4 concentration was detected in muscle from the free leg after 1 week, but this returned to pre-imobilization values at 6 week. Unlike GLUT4, no significant changes in the abundance of the GLUT5 protein were detected in either the immobilized or free leg at the end of the 1 or 6 week periods. 5. The present findings indicate that disuse rapidly induces a selective loss of activity and abundance of some non-myofibrillar proteins in humans. The decrease in GLUT4 protein abundance and cytochrome c oxidase activity during muscle disuse is consistent with a decreased capacity for glucose uptake and with a lower oxidative potential of inactive muscle. The lack of any major changes in GLUT5 protein abundance during limb immobilization indicates that the expression of some non-myofibrillar proteins is differentially regulated in response to muscle disuse.

Pentagastrin increases pepsin secretion without increasing its fractional synthetic rate

We studied the effects of increasing doses of pentagastrin on gastric secretion of pepsin and on incorporation of L-[1-13C]leucine into gastric aspirate protein as an index of pepsin synthesis. Pentagastrin (0.25-4.0 micrograms.kg-1.h-1) significantly increased pepsin output from basal 76 mg/h to < or = 181 mg/h but did not significantly alter incorporation of L-[1-13C]leucine from the basal fractional synthetic rate of 3.63 +/- 0.05%/h. In four subjects in whom infusion of tracer leucine was continued for > 1 day, aspiration of pepsin between 24 and 27 h demonstrated that plateau 13C labeling of leucine in pepsin had been attained, but at a value that was only 48% of the 13C labeling of plasma alpha-ketoisocaproic acid (alpha-KIC) [0.730 +/- 0.02 (SE) vs. 1.520 +/- 0.14 atoms %excess]. This suggests that actual rates of pepsin synthesis were approximately double those calculated on the basis of alpha-KIC labeling. The results are consistent with an interpretation that increasing doses of pentagastrin cause increased secretion of pepsinogen by recruitment of gastric chief cells, each synthesizing pepsinogen at an unaltered rate. Plateau 13C enrichment of alpha-KIC may not be a valid surrogate for plateau 13C leucine enrichment when fractional synthetic rates of some secreted proteins are calculated.

Effects of prolonged treatment of lactating goats with bovine somatotropin on aspects of adipose tissue and liver metabolism

The effects of prolonged (22 weeks) treatment of lactating goats with bovine somatotropin on the metabolism of adipose tissue and liver has been investigated. Somatotropin treatment resulted in smaller adipocytes, decreased rate of fatty acid synthesis and decreased total acetyl-CoA carboxylase activity of adipocytes, but with no change in the proportion of this enzyme in the active state. The rate of acylglycerol glycerol synthesis from glucose of adipocytes tended to decrease as did total glucose utilization by the tissue. Glucose conversion to lactate was unchanged by somatotropin treatment but glucose conversion to other products was decreased. Maximum response of adipose tissue to insulin was unchanged but the sensitivity to insulin decreased on somatotropin treatment. Treatment with somatotropin had no effect on basal lipolysis and decreased maximum response to the beta-agonist isoproterenol, but this probably reflects the rate of isoproterenol-stimulated lipolysis varying with cell volume in adipocytes. No apparent change in response either to alpha 2-adrenergic agonists or to adenosine was apparent. The number of beta-adrenergic receptors was unchanged in adipocyte membranes but the number of alpha 2-adrenergic receptors increased. The rate of hepatic gluconeogenesis in vitro, the activity of key gluconeogenic enzymes and the modulation of the rate of gluconeogenesis by butyrate were unchanged except for the effect of this latter agent on gluconeogenesis from propionate. Hepatic ketogenic activity, as indicated by the activity of carnitine palmitoyl-CoA-transferase-1 and the concentrations of carnitine and acyl carnitines, was unchanged by treatment. Thus at the end of a prolonged period of treatment with somatotropin in lactating goats, lipid synthesis in adipose tissue is still decreased but no effects on liver lipid and carbohydrate metabolism were apparent.

Rapid tracer lactate influx into canine skeletal muscle

This study evaluated the effects of various lactate transport inhibitors and competitors on rapid tracer lactate influx into the canine gastrocnemius-plantaris muscle (GP). GPs of 25 anesthetized dogs were perfused with red blood cell-free media in situ. At 0.9 mM lactate concentration ([La]), GP oxygen uptake (2.6 +/- 0.1 ml.kg-1.min-1) and net lactate output (-0.039 +/- 0.007 mmol.kg-1.min-1) were similar to values during blood perfusion. Rapid tracer lactate influx was inferred by a paired-tracer dilution method at nominal perfusate [La] values of 1, 5, 10, 25, and 50 mM. The maximal tracer influx rate (Umax) decreased significantly with each increase in unlabeled [La]. A saturation effect was suggested by the fact that percent inhibition of Umax began to reach a plateau at the higher unlabeled [La] values. The inhibition of Umax was 20.5 +/- 2.9% at 5 mM, 34.1 +/- 3.3% at 10 mM, 47.3 +/- 2.7% at 25 mM, and 56.1 +/- 2.8% at 50 mM [La]. Umax was also inhibited by various inhibitors/competitors of lactate transport as follows (% inhibition): 50 mM alpha-cyano-4-hydroxy-cinnamate (69.2 +/- 4.9%), 1.5 mM phloretin (25.4 +/- 5.5%), 0.1 mM 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (0.3 +/- 1.9%), 0.5 mM p-chloromercuribenzenesulfonic acid (72.9%), 0.5 mM furosemide (+ 2.8%), 25 mM pyruvate (52.4 +/- 2.9%), and 50 mM DL-lactate (50.2 +/- 4.0%). These experiments support the notion that lactate influx into canine skeletal muscle is a function of both a linear (possible diffusive) component and a Michaelis-Menten (carrier-mediated) component.

Effects of flow and contraction on lactate transport in the perfused rat hindlimb

A perfused rat hindlimb preparation was used to assess the effects of perfusate flow and electrical stimulation to mimic exercise on the rates of lactate influx (measured by a dual tracer technique with [3H]mannitol as the extracellular marker) and net lactate production. The same perfused muscle system was also used for assessing the effects of alpha-cyano-4-hydroxycinnamate (CIN, 15 mM), phloretin (0.6 mM), and pyruvate on tracer lactate influx. Unidirectional lactate influx, oxygen uptake (VO2), and net lactate flux were all significantly dependent on perfusate flow rate (all P < 0.05). The hindlimb was in net lactate production at all flow rates studied. Electrical stimulation (60 Hz, 100 ms, 20 V trains at 0.6 min-1) at perfusate lactate concentration of 1 mM significantly increased the hindlimb VO2 from 8.0 +/- 1.1 to 16.0 +/- 2.2 ml.kg-1.min-1 and production of lactate from -69 +/- 31 to -823 +/- 77 nmol.min-1.g-1 (both P < 0.001) but did not affect tracer-measured unidirectional lactate influx (nonstimulated: 235.4 +/- 78.1; stimulated: 235.0 +/- 31.0 nmol.min-1.g-1). At a perfusate flow of 0.55 ml.g-1.min-1 the unidirectional influx of 1 mM lactate was markedly inhibited (90 +/- 5%) by 15 mM CIN. CIN also significantly reduced VO2 from 6.2 +/- 0.16 to 4.45 +/- 0.57 ml.kg-1.min-1 (P < 0.05, n = 5). Phloretin (0.6 mM, n = 3) had no significant effect on lactate influx.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of a flooding dose of leucine in stimulating incorporation of constantly infused valine into albumin

Recently, we demonstrated increased incorporation of [13C]valine tracer into muscle protein after administration of a flooding dose of L-leucine. We have now investigated the possibility of a similar effect on albumin synthesis in the same group of volunteers. We gave L-[1-13C]leucine (20 atom%, 0.05 g/kg) during the final 90 min of a 7.5-h primed constant infusion of L-[1-13C]valine (99 atom%, 1.5 mg/kg prime constant infusion of 1.5 mg.kg-1.h-1) in healthy male volunteers in the postabsorptive state. Blood samples, taken at 0.5- to 1-h intervals during the constant infusion and at 5- to 30-min intervals during the application of the flooding dose, were analyzed for the concentration and 13C enrichment of leucine, valine, and their ketoacids. Albumin was isolated and hydrolyzed, and the enrichments of incorporated valine and leucine were compared with the mean enrichment of various possible precursor pools to calculate the apparent rate of albumin protein synthesis according to the standard procedures. During constant infusion of [13C]valine tracer the rate of albumin synthesis (measured using alpha-ketoisovalerate labeling as a surrogate for the true precursor) was 0.250 +/- 0.041%h (SD), a value identical to that routinely obtained using constant leucine tracer infusion and alpha-ketoisocaproate labeling. During the application of the flooding dose of leucine, the rate of incorporation of tracer [13C]valine into albumin increased by 73% to 0.433 +/- 0.129%/h (P < 0.05); the apparent protein synthetic rate calculated from the incorporation of leucine applied during the flood was 0.402 +/- 0.057 (P < 0.001). These results raise further doubts about the validity of the flooding dose method for the measurement of rates of human protein synthesis.

Measurement of human tissue protein synthesis: an optimal approach

This paper reviews the evidence for and against the adoption of methods for the measurement of human tissue protein synthesis based upon the incorporation of stable isotopically labeled amino acids administered either as a continuous infusion or as a flooding dose. The practical advantages of the flooding dose method are the relative ease of application of the tracer and the ability to make a repeat measurement within approximately 2 h. For the method depending upon continuous infusion of labeled amino acid, the advantages include the use of labeled amino acids at true tracer doses (i.e., with no disturbance of metabolism) and the ability to make simultaneous measurements of whole body turnover and limb or organ turnover (given appropriate sampling techniques). The crucial question concerning the accuracy of the two methods (e.g., the 2-fold difference in the rate of skeletal muscle protein synthesis) remains unresolved, but in our opinion more evidence exists in favor of the values obtained from the continuous infusion method. Furthermore, as techniques for measurement of stable isotopically labelled amino acids improve, the length of time necessary for tracer infusion will fall, and the practical advantages of the flooding dose protocol will lessen in comparison.

Protein turnover in the human fetus studied at term using stable isotope tracer amino acids

Before elective cesarean delivery (4 h), we infused L-[1-13C]leucine and L-[15N]phenylalanine into the maternal circulation and measured enrichment and concentration of amino acids and carbon dioxide in cord blood of six normal human fetuses at delivery. There were net fetal uptakes of leucine (2.22 +/- 0.29 mumol.kg-1.min-1) and phenylalanine (0.80 +/- 0.11 mumol.kg-1.min-1) with net outputs of CO2 (6.11 +/- 1.12 ml.kg-1.min-1) and the transamination product of leucine, alpha-ketoisocaproate (1.04 +/- 0.32 mumol.kg-1.min-1). Fetal amino acid oxidation accounted for a substantial proportion of the flux from the mother (leucine, 0.36 +/- 0.09 mumol.kg-1.min-1 and phenylalanine, 0.18 +/- 0.04 mumol.kg-1.min-1). Fetal whole body accretion of leucine carbon (0.82 +/- 0.21 mumol.kg-1.min-1) was 69% of the umbilical uptake, and that of phenylalanine (0.62 +/- 0.08 mumol.kg-1.min-1) was 78%. Fetal whole body protein synthesis was approximately 13 g.kg-1.day-1, i.e., much faster than in adults but similar to that in the newborn. Net protein accretion was 2-4 g.kg-1.day-1. The placental supply of leucine and phenylalanine exceeds the fetal demand for protein synthesis by only a small amount, suggesting that the safety margin of placental transfer may be small for these amino acids. The results suggest that the method could be applied safely to studies of fetal growth retardation.

Mechanisms involved in the adaptations of the adipocyte adrenergic signal-transduction system and their modulation by growth hormone during the lactation cycle in the rat

The mechanisms responsible for the diminished lipolytic response of adipocytes to catecholamines after litter removal from lactating rats and their modulation by growth hormone have been investigated. Lactation, litter removal and growth-hormone treatment did not alter the ability of noradrenaline to activate protein kinase A (A-kinase), showing that the defect in signal transduction in rats after litter removal is after A-kinase. Litter removal had no effect on hormone-sensitive lipase activity itself, but the proportion of the lipase associated with the fat droplet was decreased; growth-hormone treatment increased hormone-sensitive lipase activity and the proportion associated with the fat droplet. In addition, a number of other adaptations in the beta-adrenergic signal-transduction system occur during the lactation cycle and in response to growth hormone treatment, including changes in receptor number, adenylate cyclase activity and cyclic AMP phosphodiesterase activity, but a defect in the ability of hormone-sensitive lipase to associate with the lipid droplet appears to be the major reason for the diminished response to catecholamines on litter removal.

Stimulation of protein synthesis in pig skeletal muscle by infusion of amino acids during constant insulin availability

Incorporation of L-[1-13C]leucine into muscle protein and leg exchange of L-[15N]phenylalanine were used to assess the effects over 240 min of amino acid supply on leg protein turnover in anesthetized, overnight-fasted (Landrace x Great White) female pigs. In all pigs, plasma insulin and glucagon stability was ensured by infusion of somatostatin (8 micrograms.kg-1.h-1), insulin (6 mU.kg-1.h-1), and glucagon (72 ng.kg-1.h-1). Mixed amino acid infusion (260 mg.kg-1.h-1) caused a 2- to 2.5-fold elevation of arterial plasma phenylalanine and leucine; in a control group (no amino acid infusion), an increase in phenylalanine and leucine concentration was observed as a result of the hormone clamp. Plasma insulin and glucagon concentrations were steady and not significantly different between control and amino acid-infused groups during the final 240 min, but plasma glucose fell (P less than 0.05) in both groups (4.57 +/- 0.17 to 3.15 +/- 0.73 mM). Muscle protein synthetic rate (estimated from the change in L-[1-13C]leucine incorporation compared with labeling of [13C]leucyl-tRNA) was greater in amino acid-infused (0.076%/h) than in control (0.053%/h) pigs. In the control group, leg amino acid balance was negative (Phe alone, -10.2 +/- 9.4 nmol Phe.100 g-1.min-1; total amino acids, -0.27 +/- 1.04 micrograms amino N.100 g-1.min-1), but during amino acid infusion, balance was positive (Phe alone, +33.6 +/- 8.8 nmol Phe.100 g-1.min-1; total amino acids, +58.2 +/- 4.9 micrograms amino N.100 g-1.min-1).(ABSTRACT TRUNCATED AT 250 WORDS)

Flooding with L-[1-13C]leucine stimulates human muscle protein incorporation of continuously infused L-[1-13C]valine

To investigate why flooding methods give higher rates than constant-infusion methods for muscle protein synthesis, we studied seven healthy postabsorptive male volunteers (20-42 yr; 67-74 kg) during a 7.5-h primed constant infusion of L-[1-13C]valine (99 atoms %, 1.5 mg/kg prime, 1.5 mg.kg-1.h-1); at 6.5 h they were given a flood of L-[1-13C]leucine (20 atoms %, 0.05 g/kg). Musculus tibialis anterior biopsies were taken at 0.5, 6, and 7.5 h, and blood was sampled as appropriate. The enrichment of valine and leucine in muscle protein (isotope ratio mass spectrometry of protein amino acid-derived 13CO2) was compared with the average enrichment of various amino acid pools (gas chromatography-mass spectrometry). During infusion of [13C] valine the rate of muscle protein synthesis measured using alpha-ketoisovalerate (alpha-KIV) as precursor surrogate was 0.043 +/- 0.002%/h (SE). After flooding with leucine, the incorporation rate of [13C]valine increased by 70% (P less than 0.05), i.e., apparent muscle protein synthetic rate (based on alpha-[13C]KIV) increased to 0.065 +/- 0.009%/h (P less than 0.05); the rate calculated from the [13C]leucine flood was 0.060 +/- 0.005%/h (P less than 0.01). The synthetic rates calculated using the constant-infusion method were higher after flooding, irrespective of the precursor chosen, raising serious concern about the validity of the flooding-dose method.

Isolation of aminoacyl-tRNA and its labeling with stable-isotope tracers: Use in studies of human tissue protein synthesis

We isolated aminoacyl-tRNA (60-70% yield) from human and rat tissues and measured, by GC/MS, its labeling in vivo by [15N]- and [13C]leucine. Tracer dilution artifacts seemed unlikely since, after infusion of L-[1-13C,15N]leucine into rats, (i) muscle leucyl-tRNA labeling exceeded tissue free leucine labeling, (ii) values were largely unaffected by storing over 5 min at 22 degrees C, and (iii) L-[2,4,5-methyl-13C]leucine was not incorporated into leucyl-tRNA during homogenization. Leucyl-tRNA labeling in liver and muscle suggested charging from extra- and intracellular pools: e.g., after infusing L-[1-13C,15N]leucine, rat muscle tissue free leucine 13C labeling (8.97 +/- 0.30 atom % excess) exceeded that by 15N (3.37 +/- 0.33 atom % excess), and both were significantly lower (P less than 0.02) than venous plasma (13C, 12.1 +/- 1.8; 15N, 5.54 +/- 0.6 atom % excess) indicating tracer dilution by transamination and by proteolysis; however, leucyl-tRNA labeling by either isotope (13C, 10.26 +/- 0.50; 15N, 4.72 +/- 0.72 atom % excess) was significantly above mixed tissue free leucine (P less than 0.05). Labeling of leucyl-tRNA in human erector spinae muscle (obtained after preoperative L-[1-13C]leucine infusion) was, at 4.98 +/- 0.43 atom % excess, lower (27%) than venous plasma leucine (P less than 0.05) and intermediate between muscle free leucine (9% lower; P less than 0.01) and venous alpha-ketoisocaproate (11% higher; P less than 0.02). Human placental leucyl-tRNA labeling (after predelivery tracer infusion) was 37% lower (P less than 0.05) than maternal uterine vein labeling but not significantly different from placental free leucine or umbilical arterial leucine.

Effects of corticosteroid on the transport and metabolism of glutamine in rat skeletal muscle

Intramuscular glutamine falls with injury and disease in circumstances associated with increases in blood corticosteroids. We have investigated the effects of corticosteroid administration (0.44 mg/kg dexamethasone daily for 8 days, 200 g female rats) on intramuscular glutamine and Na+, muscle glutamine metabolism and sarcolemmal glutamine transport in the perfused hindlimb. After dexamethasone treatment intramuscular glutamine fell by 45% and Na+ rose by 25% (the respective muscle/plasma distribution ratios changed from 8.6 to 4.5 and 0.12 to 0.15); glutamine synthetase and glutaminase activities were unchanged at 475 +/- 75 and 60 +/- 19 nmol/g muscle per min. Glutamine output by the hindlimb of anaesthetized rats was increased from 31 to 85 nmol/g per min. Sarcolemmal glutamine transport was studied by paired-tracer dilution in the perfused hindlimb: the maximal capacity (Vmax) for glutamine transport into muscle (by Na(+)-glutamine symport) fell from 1058 +/- 310 to 395 +/- 110 nmol/g muscle per min after dexamethasone treatment, accompanied by a decrease in the Km (from 8.1 +/- 1.9 to 2.1 +/- 0.4 mM glutamine). At physiological plasma glutamine concentration (0.75 mM) dexamethasone appeared to cause a proportional increase in sarcolemmal glutamine efflux over influx. Addition of dexamethasone (200 nM) to the perfusate of control rat hindlimbs caused acute changes in Vmax and Km of glutamine transport similar to those resulting from 8-day dexamethasone treatment. The reduction in muscle glutamine concentration after dexamethasone treatment may be primarily due to a reduction in the driving force for intramuscular glutamine accumulation, i.e., in the Na+ electrochemical gradient. The prolonged increase in muscle glutamine output after dexamethasone treatment (which occurs despite a reduction in the size of the intramuscular glutamine pool) appears to be due to a combination of (a) accelerated sarcolemmal glutamine efflux and (b) increased intramuscular synthesis of glutamine.

Adenosine and the control of adrenergic regulation of adipose tissue lipolysis during lactation

Adenosine is a locally active factor that is produced intracellularly and extracellularly in adipose tissue. Adenosine binds to receptors in the plasma membrane of adipocytes; this activates a guanine triphosphate binding protein that inhibits adenylate cyclase activity and, hence, lipolysis. Lactation results in an enhanced responsiveness of adipocytes to beta-agonists, which stimulate lipolysis, and, paradoxically, to adenosine, which inhibits lipolysis. These adaptations are partly due to increases in ligand binding and to changes in postreceptor components of the signal transduction systems. Somatotropin is implicated in the chronic adaptations of the beta-adrenergic system, whereas insulin, somatotropin, glucocorticoids, and at least one unidentified factor have a role in the chronic control of the adenosine system of adipocytes.

Chronic control of the beta- and alpha 2-adrenergic systems of sheep adipose tissue by growth hormone and insulin

1. Sheep adipose tissue retained responsiveness to catecholamines when maintained in tissue culture for 48 h; both the rate of basal lipolysis and sensitivity to beta-agonists were increased after tissue culture. 2. Tissue culture in the presence of growth hormone resulted in an increased maximum response and sensitivity to the beta-agonist isoprenaline, but had no effect on basal lipolysis. 3. Tissue culture in the presence of insulin increased the basal rate of lipolysis and increased the ratio of the rate of noradrenaline-stimulated/isoprenaline-stimulated lipolysis, indicating a decrease in the 2-adrenergic effect of noradrenaline. 4. Tissue culture in the presence of growth hormone increased ligand binding to beta-adrenergic receptors. 5. Tissue culture in the absence of exogenous hormones increased ligand binding to alpha 2-adrenergic receptors; this was prevented by actinomycin D and partly prevented by insulin. 6. These studies show that both growth hormone and insulin chronically modulate the adrenergic system of sheep adipose tissue; the effects of growth hormone are primarily on the beta-adrenergic system, whereas insulin modulates the alpha 2-adrenergic system.

Glutamine transport and metabolism in denervated rat skeletal muscle

Rat skeletal muscle glutamine fell by 40% from 4.18 to 2.5 mumols/g wet weight (P less than 0.01) after 4 days of denervation. Over the same period net glutamine efflux from denervated hindlimbs [i.e., arteriovenous (a-v) concentration differences x blood flow] increased 3.5-fold (from -6.72 +/- 1.73 to -26 +/- 4.81 nmol.min-1.g-1, P less than 0.001). Gastrocnemius glutamine synthetase activity fell 48% after denervation (from 475 +/- 81 to 248 +/- 39 nmol.min-1.g-1, P less than 0.001), but glutaminase activity was not significantly altered (17 nmol.min-1.g-1). The maximal activity (Vmax) of the unidirectional Na(+)-dependent glutamine transporter (system Nm) was depressed by 45% from 1,020 +/- 104 to 571 +/- 9 nmol.min-1.g-1 (P less than 0.01), but the concentration at which transport was half maximal (Km) was not significantly altered (control 8.1 +/- 0.6 mM; denervated 6.52 +/- 0.12). Hindlimb denervation resulted in an increase of intramuscular Na+ by 17% and a fall of K+ by 12%, and the resting membrane potential in isolated muscles decreased from -75 +/- 10 to -59.5 +/- 5.5 mV. Membrane potential of perfused denervated muscle, isolated after acute addition of the Na+ channel blocker tetrodotoxin (TTX, 3 microM), repolarized to -66.4 +/- 3.2 mV. In perfused denervated preparations TTX caused an acute recovery of Vmax of unidirectional glutamine transport to 848 +/- 75 nmol.min-1.g-1; Km was unaffected.(ABSTRACT TRUNCATED AT 250 WORDS)

The temperature dependence of the time course of growth and decay of miniature end-plate currents in carp extraocular muscle following thermal acclimation

1. The effect of temperature (5–35 degrees C) on the decay and growth phases of miniature end-plate currents (MEPCs) was investigated in extraocular muscle from freshwater carp acclimated to either high (28 degrees C) or low (8 degrees C) temperature. 2. The temperature dependence of the time constant of decay (TD) was found to follow an Arrhenius relationship; the relationship between logTD and reciprocal of absolute temperature (1/K) being linear in both groups. The TD of MEPCs recorded from cold-acclimated carp was not statistically significant from that of the warm group. 3. TD was moderately temperature-dependent. Regression gave a Q10 of 1.78 for the warm-acclimated carp, corresponding to an activation energy, Ea, of 41.15 +/− 2.17 kJ mol-1. For the cold-acclimated carp, the Q10 was 1.79, and Ea was 41.43 +/− 2.46 kJ mol-1. 4. Growth time (TG) was less susceptible than TD to temperature change. The relationship between growth time (taken as the time for MEPCs to rise from 20 to 80% of maximum) and temperature was linear for the cold-acclimated group, with a Q10 of 1.34 and Ea of 20.94 +/− 4.75 kJ mol-1. The data for the warm group, were, in contrast, best fitted by two linear regressions meeting at 15.1 degrees C. At temperatures below 15.1 degrees C Q10 was 3.16 and Ea was 82.20 +/− 15.47 kJ mol-1; above 15 degrees C, Q10 was 1.22 and Ea was 14.15 +/− 12.24 kJ mol-1. 5. The acetylcholinesterase inhibitor neostigmine increased TD by approximately twofold and raised TG to approximately 1.4 times control values. These effects were observed across the temperature range scanned for both groups. 6. The results are discussed with reference to the documented effects of temperature and temperature acclimation on membrane lipids and proteins.

Skeletal muscle glutamine transport, intramuscular glutamine concentration, and muscle-protein turnover

This article reviews work we have carried out to investigate (1) the transport mechanisms responsible for the high distribution ratio of free glutamine commonly observed in skeletal muscle; (2) the fall in the distribution ratio that accompanies starvation, injury and chronic disease, whether directly involving muscle or not; and (3) the effect of modulation of intracellular free-glutamine concentration on protein synthesis and breakdown in skeletal muscle. We suggest that the results are consistent with the controlling role of the muscle membrane glutamine-sodium cotransporter in the regulation of the intracellular glutamine pool, the existence of pathophysiological mechanisms for the modulation of intramuscular glutamine and anabolic effects of glutamine in promoting protein synthesis, with a smaller effect in reducing protein breakdown. The mechanisms by which glutamine affects skeletal muscle protein turnover, and thus muscle protein balance, and the extent of the net flow of amino acids between the periphery and the viscera are unknown as yet, but the results suggest that modulation of transporter activity may offer the possibility of therapeutic intervention to reduce muscle wasting associated with injury and disease.

Characteristics of acidic, basic and neutral amino acid transport in the perfused rat hindlimb

1. We have employed a paired-tracer isotope dilution technique in a perfused rat hindlimb preparation to obtain information on the kinetics of transport across the sarcolemmal membranes of acidic, neutral and basic amino acids. 2. We have defined the characteristics of the saturable transport of amino acids normally regarded as paradigm substrates for the A, ASC, L, y+(basic) and the dicarboxylic amino acid transport systems. Their maximal transport capacities (Vmax, nmol min-1 (g muscle)-1 and substrate concentrations for half-maximal transport (Km, mM) of representative amino acid substrates are as follows: 2-aminoisobutyrate (AIB), Vmax = 15 +/- 7, Km = 1.26 +/- 0.6; alanine, Vmax = 332 +/- 53, Km = 3.9 +/- 0.9; serine, Vmax = 410 +/- 61, Km 3.4 +/- 0.5; leucine, Vmax = 2800 +/- 420, Km = 20 +/- 2; lysine, Vmax = 136 +/- 46, Km = 2.1 +/- 1.3; glutamate, Vmax = 86 +/- 6, Km = 1.05 +/- 0.05; proline, Vmax = 196 +/- 48, Km = 4.1 +/- 0.6. 3. Glycine uptake was faster than expected on the basis of diffusion but was not saturable and showed uptake that could be best described by a first-order rate constant of 0.07 +/- 0.003 min-1. 4. We have attempted to discriminate kinetically between possible routes of entry for an amino acid on the basis of competitive and non-competitive interaction between substrates potentially sharing common routes. On this basis, the major routes of alanine entry appear to be via the ASC and L systems with the A system playing a quantitatively minor role. Glutamate and aspartate appear to be transported exclusively by a dicarboxylate amino acid carrier. The branched-chain amino acids (BCAA) and the aromatic amino acid, phenylalanine, are almost equivalent substrates for an L-like system. 5. Insulin had no detectable effect on the uptake of paradigm substrates for ASC, L, y+, the dicarboxylic amino acid or glycine transport systems. 6. Transport of serine and lysine was Na+ dependent. Lysine transport apparently occurred with a stoichiometry of 2 Na+: 1 lysine. With the exception of alanine, whose transport was partially Na+ dependent, all other amino acids examined in the present study were transported in a Na+-independent manner.

L(+)-lactate transport in perfused rat skeletal muscle: kinetic characteristics and sensitivity to pH and transport inhibitors

We have examined lactate uptake (as the rate of net muscle lactate accumulation) and unidirectional inward transport (measured by a paired-tracer dilution method) in muscle of the perfused skinned rat hindlimb. Inhibition of tracer influx (fractional uptake at 1 mM L(+)-lactate, 43.3 +/- 3.1% but only 32.9 +/- 1.8% at 50 mM lactate) suggested some competition between tracer and native forms of the carboxylate for transport. D(-)-lactate (50 mM) did not inhibit uptake of tracer L(+)-lactate. Pyruvate (25 mM), but none of five other monocarboxylates, inhibited uptake of tracer lactate, by 22% (P less than 0.01). Altering perfusate pH from 7.4 to 6.8 caused a 36% increase (P less than 0.001) in the unidirectional L(+)-lactate transport at 1 mM L(+)-lactate, whereas increasing pH to 7.7 reduced transport by 18% (P less than 0.01). Tracer lactate influx was inhibited by 500 microM 4-acetamido-4'-isothiocyanostilbene (SITS) (19%), 5 mM alpha-cyano-4-hydroxycinnamic acid (CIN) (20-30%), 1 mM amiloride (27%) and by a thiol group reagent p-chloromercuribenzenesulphonic acid (pCMBS) (26%). Overall the results indicate that at least two processes are involved in the transfer of lactate: one, saturable, with a Vmax of 0.84 mumol.min-1.g-1 and an apparent Km of 21 mM was sensitive to SITS, CIN, and a thiol group reagent; the other was non-saturable and insensitive to SITS and CIN with an apparent rate constant of 0.1 min-1.

Inhibition of protein breakdown by glutamine in perfused rat skeletal muscle

We have assessed the effects of glutamine (Gln) availability on protein breakdown in perfused rat hindlimb by measuring net phenylalanine (Phe) production (an index of protein balance), the dilution of [15N]Phe labelling (an index of mixed protein breakdown) and rate of production of 3-methylhistidine (3-MeH) (an index of myofibrillar breakdown). 15 mM Gln significantly inhibited net protein loss and protein breakdown compared to rates obtained in its absence (net protein loss, 200 +/- 230 vs 2080 +/- 200 nmol Phe/hindlimb per h; protein breakdown, 4566 +/- 480 vs 1614 +/- 180 nmol Phe/hindlimb per h; both p less than 0.01). Insulin (100 microU/ml) inhibited protein breakdown but less than Gln. The effects on protein breakdown of Gln and insulin together were not additive, suggesting a common mode of action. Production of 3-MeH (mean 20.3 +/- 2.8 nmol/hindlimb per h) was unaffected by Gln or insulin. Gln appears to inhibit protein breakdown of soluble rather than myofibrillar protein in muscle.

Effects of ageing and chronic dietary restriction on the morphology of fast and slow muscles of the rat

The soleus and extensor digitorum longus muscles of the hindlimb and the flexor digitorum profundus muscle of the forelimb were studied in ad libitum-fed control and age-matched diet-restricted male rats at various ages from weaning to senescence. Growth of individual muscles was accomplished by fibre hypertrophy and not hyperplasia. Between weaning and one year, fibre numbers remained constant in the soleus but fell by 50% in the extensor digitorum longus. Both muscles displayed increasingly oxidative fibre type profiles with advancing age, irrespective of dietary status. This was particularly noticeable in the soleus, which transformed its fibre population from one containing 35% fast fibres at weaning to one with no fast fibres at 91 weeks. In senility, however, the fibre type population again displayed 25% fast fibres. The capillary: fibre ratio and the capillary density were correlated with muscle fibre size in both hindlimb muscles. Although capillarity increased with age, expected differences between fast and slow muscles were probably minimised by the high proportion of FOG fibres in the extensor digitorum longus. Both hindlimb muscles displayed significant increases in the ratio of connective: muscle tissue with increasing age. The soleus invariably contained more connective tissue than the extensor digitorum longus. Dietary restriction reduced the rate of increase, so that the connective tissue content was approximately one half that found in control muscles at one year. Various pathological features associated with old age were delayed considerably in the muscles of the diet-restricted rats. It is concluded that chronic dietary restriction imposed directly after weaning has a dramatic effect on the normal growth and ageing of skeletal muscle.

Protein synthesis in tissues of fed and starved carp, acclimated to different temperatures

The temperature dependence of the rates of protein synthesis in the red and white skeletal muscle of the carp (Cyprinus carpio) was measured using a method which involved a single injection of tritiated phenylalanine. Plasma and muscle-free phenylalanine quickly reached a plateau level at all temperatures. During the plateau phase the incorporation of label into protein was liner. Muscle from fish previously acclimated to either a low temperature (8°C) or a high temperature (28°C), showed marked differences in the rates of protein synthesis. The results show that cold acclimation is associated with significantly higher rates of protein synthesis (p<0.001) in both red and white muscle. Arrhenius activation energies, derived from the rates of protein synthesis at the different experimental temperatures, were similar for both red and white muscle in fish acclimated to warm or cold temperatures. Measurements for both acclimated groups over the temperature range 8-34°C showed that the activation energy for the process of protein synthesis was 86.7 kJ/mol and 78.7 kJ/mol for the red and white muscle respectively.

Characteristics of L-glutamine transport in perfused rat skeletal muscle

1. We have investigated glutamine transport in the perfused rat hindlimb using the paired-tracer isotope dilution technique. 2. Uptake of L-glutamine was stereospecific, saturable, sodium dependent, insulin sensitive and pH insensitive in the physiological range. The maximum capacity of transport (Vmax) under normal perfusate conditions at 37 degrees C, 145 mM-Na+ and in the absence of insulin was 1156 +/- 193 nmol min-1 g-1 with transport being half-maximal at a perfusate glutamine concentration of 9.25 +/- 1.15 mM. 3. The kinetics of Na+ dependence strongly suggested co-transport of Na+ and glutamine with a stoichiometry of 1:1; furthermore, Na+ activated the carrier without any change in the concentration of glutamine at which transport was half-maximal, i.e. a 'Vmax effect' rather than a 'Km effect'. 4. The characteristics of glutamine transport, especially its substrate specificity and the pattern of competitive and non-competitive inhibition of glutamine transport by other amino acids, suggest that it is mediated by a carrier or carriers for which asparagine and histidine are also suitable substrates. 5. The characteristics of muscle glutamine transport are related but distinct from those of system N identified in hepatocytes; we suggest that they are sufficiently distinct to justify the identification of a new variant of mammalian amino acid transport systems which may be identified by the symbol Nm. 6. The kinetic characteristics of system Nm are such that glutamine is likely to be the most rapidly exchanging amino acid across the muscle membrane at physiological intra- and extracellular glutamine concentrations. Its hormone and ion sensitivities are likely to be important in the physiological modulation of whole-body glutamine metabolism and also during derangements observed in disease and after injury.

Decrease in human quadriceps muscle protein turnover consequent upon leg immobilization

Quadriceps muscle protein turnover was assessed in the post-absorptive state in six men immediately after the end of unilateral leg immobilization (37 +/- 4 days) in a plaster cast after tibial fracture. A primed-constant intravenous infusion of L-[1-13C]leucine was administered over 7 h. Quadriceps needle biopsies, taken bilaterally at the end of the infusion, were analysed for muscle protein leucine enrichment with 13C. Quadriceps muscle protein synthetic rate, calculated from the fractional incorporation of [13C]leucine into protein compared with the average enrichment of blood alpha-ketoisocaproate, was 0.046 +/- 0.012%/h in the uninjured leg, but was only 0.034 +/- 0.007%/h in the quadriceps of the previously fractured leg (P less than 0.05, means +/- SD). Muscle RNA activity (i.e. protein synthetic rate per RNA) fell from 0.27 +/- 0.08 microgram of protein synthesized h-1 microgram-1 of RNA in the control leg to 0.14 +/- 0.03 microgram of protein synthesized h-1 microgram-1 of RNA in the immobilized leg (P less than 0.02). Immobilization was associated with a significant atrophy of type I muscle fibres (mean diameter 69.5 +/- 21 microns immobilized, 81.1 +/- 18 microns control, P less than 0.05), but no significant change occurred in type II fibre diameter. Mean quadriceps fibre volume calculated from the values for fibre diameter and percentage of each fibre type, was smaller in the injured leg by 10.6%; this value was near to the calculated difference in muscle thigh volume (calculated from thigh circumference and skin-fold thickness) which was less by 8.3%.(ABSTRACT TRUNCATED AT 250 WORDS)

Characteristics of a glutamine carrier in skeletal muscle have important consequences for nitrogen loss in injury, infection, and chronic disease

A carrier for glutamine, identified in rat muscle, has properties in terms of kinetics, ion dependence and hormone sensitivity, and effects of endotoxin and branched-chain aminoacids that point to an important function in the control of whole-body aminoacid metabolism. The existence of a link between the size of the glutamine pool in muscle and the rate of muscle protein synthesis raises possibilities for therapeutic interventions to limit protein loss in injury, sepsis, and chronic disease.

Myofibrillar ATPase activity in the carp Cyprinus carpio: interactions between starvation and environmental temperature

The myofibrillar ATPase activity of the epaxial white muscle was measured in carp Cyprinus carpio L. acclimated to 10 degrees C or 28 degrees C. As previously reported, cold acclimation was associated with an increase in the ATPase specific activity and a decrease in the thermostability. The water content of the white muscle was significantly higher in cold-acclimated fish than in warm-acclimated fish (P less than 0.002). Starvation for 10 weeks resulted in a significant increase in the white muscle water content of both warm- and cold-acclimated fish (P less than 0.002). When carp were starved, the ability of the myofibrillar ATPase to show thermal compensation disappeared. Previously acclimated fish, when starved, showed steady alterations of the myofibrillar ATPase activity to a level mid-way between the acclimated extremes. Refeeding resulted in a gradual return to the normal acclimated level.

Changes in fiber type composition in growing muscle as a result of dynamic exercise and static overload

The effects of dynamic exercise (jumping) and static overload (synergist tenotomy) on rat soleus muscle were investigated during a growing phase in juvenile animals. The relative proportions of slow twitch (type 1), fast twitch (type 2a) and intermediate (type 2c) fibers were determined histochemically. The control animals, as part developmental process, lost type 2a fibers apparently by conversion to type 1 fibers via the type 2c form. Synergist tenotomy enhanced the conversion of type 2a fibers to type 2c fibers as these muscles displayed fewer type 2a fibers than control animals at the same age. In contrast, the dynamic exercise delayed the loss of type 2a fibers from the soleus. When the two exercises were combined the effects on fiber type proportions cancelled each other; this group also had a large number of type 2c fibers. This present study indicates that the activity imposed on a muscle influences the maturational development of the constituent fiber types.

Exercise-induced morphological and biochemical changes in skeletal muscles of the rat

Morphological and biochemical changes have been studied in two forelimb (i.e., brachialis and extensor carpi radialis) and two hindlimb (i.e., soleus and extensor digitorum longus) muscles of rats subjected to short bursts of high-intensity exercise over 2 wk. Regardless of muscle type, all four muscles grew significantly, accumulating protein, RNA, and DNA at faster rates than in the growing control tissues. Of the intrinsic fiber types within the individual muscles all increased their cross-sectional areas, but the fast-oxidative, glycolytic fibers (type IIa) showed marginally more hypertrophy than the slow-oxidative (I) or fast-glycolytic fibers (IIb). Induced changes in protein turnover were consistent with the additional growth of the exercised muscles. However, the precise alterations in the rates of protein synthesis and protein breakdown varied according to the fiber type composition of the muscle. The increased growth rate of the two principally fast-twitch muscles (i.e., brachialis and extensor digitorum longus) correlated solely with an enhancement of protein synthesis (measured in vivo). In contrast, the hypertrophy of the slow-twitch soleus appeared to relate only to a decrease in protein breakdown (a calculated value). In a more intermediate type of muscle (i.e., extensor carpi radialis) a complementary combination of an increase in synthesis and a decrease in breakdown was found.