The effects of muscle contraction and recombinant osteocalcin on insulin sensitivity ex vivo

We tested whether GPRC6A, the putative receptor of undercarboxylated osteocalcin (ucOC), is present in mouse muscle and whether ucOC increases insulin sensitivity following ex vivo muscle contraction. GPPRC6A is expressed in mouse muscle and in the mouse myotubes from a cell line. ucOC potentiated the effect of ex vivo contraction on insulin sensitivity.

INTRODUCTION

Acute exercise increases skeletal muscle insulin sensitivity. In humans, exercise increases circulating ucOC, a hormone that increases insulin sensitivity in rodents. We tested whether GPRC6A, the putative receptor of ucOC, is present in mouse muscle and whether recombinant ucOC increases insulin sensitivity in both C2C12 myotubes and whole mouse muscle following ex vivo muscle contraction.

METHODS

Glucose uptake was examined in C2C12 myotubes that express GPRC6A following treatment with insulin alone or with insulin and increasing ucOC concentrations (0.3, 3, 10 and 30 ng/ml). In addition, glucose uptake, phosphorylated (p-)AKT and p-AS160 were examined ex vivo in extensor digitorum longus (EDL) dissected from C57BL/6J wild-type mice, at rest, following insulin alone, after muscle contraction followed by insulin and after muscle contraction followed by recombinant ucOC then insulin exposure.

RESULTS

We observed protein expression of the likely receptor for ucOC, GPRC6A, in whole muscle sections and differentiated mouse myotubes. We observed reduced GPRC6A expression following siRNA transfection. ucOC significantly increased insulin-stimulated glucose uptake dose-dependently up to 10 ng/ml, in differentiated mouse C2C12 myotubes. Insulin increased EDL glucose uptake (∼30 %, p < 0.05) and p-AKT and p-AKT/AKT compared with rest (all p < 0.05). Contraction prior to insulin increased muscle glucose uptake (∼25 %, p < 0.05), p-AKT, p-AKT/AKT, p-AS160 and p-AS160/AS160 compared with contraction alone (all p < 0.05). ucOC after contraction increased insulin-stimulated muscle glucose uptake (∼12 % p < 0.05) and p-AS160 (<0.05) more than contraction plus insulin alone but without effect on p-AKT. In the absence of insulin and/or of contraction, ucOC had no significant effect on muscle glucose uptake.

CONCLUSIONS

GPRC6A, the likely receptor of osteocalcin (OC), is expressed in mouse muscle. ucOC treatment augments insulin-stimulated skeletal muscle glucose uptake in C2C12 myotubes and following ex vivo muscle contraction. ucOC may partly account for the insulin sensitizing effect of exercise.

Creatine transporter protein content, localization, and gene expression in rat skeletal muscle

The present study examined the gene expression and cellular localization of the creatine transporter (CreaT) protein in rat skeletal muscle. Soleus (SOL) and red (RG) and white gastrocnemius (WG) muscles were analyzed for CreaT mRNA, CreaT protein, and total creatine (TCr) content. Cellular location of the CreaT protein was visualized with immunohistochemical analysis of muscle cross sections. TCr was higher (P < or = 0.05) in WG than in both RG and SOL, and was higher in RG than in SOL. Total CreaT protein content was greater (P < or = 0.05) in SOL and RG than in WG. Two bands (55 and 70 kDa) of the CreaT protein were found in all muscle types. Both the 55-kDa (CreaT-55) and the 70-kDa (CreaT-70) bands were present in greater (P < or = 0.05) amounts in SOL and RG than in WG. SOL and RG had a greater amount (P < or = 0.05) of CreaT-55 than CreaT-70. Immunohistochemical analysis revealed that the CreaT was mainly associated with the sarcolemmal membrane in all muscle types. CreaT mRNA expression per microgram of total RNA was similar across the three muscle types. These data indicate that rat SOL and RG have an enhanced potential to transport Cr compared with WG, despite a higher TCr in the latter.

Muscle metabolism during prolonged exercise in humans: influence of carbohydrate availability

Eight endurance-trained men cycled to volitional exhaustion at 69 +/- 1% peak oxygen uptake on two occasions to examine the effect of carbohydrate supplementation during exercise on muscle energy metabolism. Subjects ingested an 8% carbohydrate solution (CHO trial) or a sweet placebo (Con trial) in a double-blind, randomized order, with vastus lateralis muscle biopsies (n = 7) obtained before and immediately after exercise. No differences in oxygen uptake, heart rate, or respiratory exchange ratio during exercise were observed between the trials. Exercise time to exhaustion was increased by approximately 30% when carbohydrate was ingested [199 +/- 21 vs. 152 +/- 9 (SE) min, P < 0.05]. Plasma glucose and insulin levels during exercise were higher and plasma free fatty acids lower in the CHO trial. No differences between trials were observed in the decreases in muscle glycogen and phosphocreatine or the increases in muscle lactate due to exercise. Muscle ATP levels were not altered by exercise in either trial. There was a small but significant increase in muscle inosine monophosphate levels at the point of exhaustion in both trials, and despite the subjects in CHO trial cycling 47 min longer, their muscle inosine monophosphate level was significantly lower than in the Con trial (CHO: 0.16 +/- 0.08, Con: 0.23 +/- 0.09 mmol/kg dry muscle). These data suggest that carbohydrate ingestion may increase endurance capacity, at least in part, by improving muscle energy balance.

Effect of timing of carbohydrate ingestion on endurance exercise performance

This study compared the effects of carbohydrate ingestion throughout exercise with ingestion of an equal amount of carbohydrate late in exercise. Eight well-trained men cycled 2 h at 70 +/- 1% VO2 peak, followed immediately by a 15-min performance ride, while ingesting either a 7% carbohydrate-electrolyte solution (CHO-7), an artificially sweetened placebo (CON), or the placebo for the first 90 min then a 21% glucose solution (CHO-0/21). At the start of the performance ride, plasma glucose averaged 4.2 +/- 0.2, 5.2 +/- 0.1, and 5.7 +/- 0.2 mmol.l-1 in CON, CHO-7, and CHO-0/21, respectively (all different, P < 0.05). Plasma insulin levels were similar just prior to the performance ride in CHO-7 and CHO-0/21, with both higher than CON. A similar pattern was observed with respiratory exchange ratio (RER). Work performed during the performance ride was significantly greater in CHO-7 (268 +/- 8 kJ) compared with CON (242 +/- 9 kJ). Performance in CHO-0/21 (253 +/- 10 kJ), however, was not improved compared with CON, despite higher plasma glucose levels and plasma insulin levels similar to CHO-7. Seven of the eight subjects performed best in CHO-7. In conclusion, performance was improved, relative to the control trial, only when carbohydrate was ingested throughout exercise. Carbohydrate ingestion late in exercise did not improve performance despite increases in plasma glucose and insulin.

Influence of muscle glycogen on glycogenolysis and glucose uptake during exercise in humans

To examine the effects of alterations in preexercise muscle glycogen availability on glycogenolysis and glucose uptake during exercise, 12 active but untrained men [22.8 +/- 1.6 (SE) yr, 71.7 +/- 2.0 kg, peak pulmonary oxygen uptake 3.85 +/- 0.16 l/min] were studied during 40 min of cycle ergometer exercise at 65-70% peak pulmonary oxygen uptake on two separate occasions, at least 1 wk apart. Preexercise muscle glycogen concentrations were manipulated by having the subjects perform glycogen-lowering exercise either 24 or 48 h before a trial, in combination with either high or low dietary carbohydrate intake. In series 1 (n = 7), increasing muscle glycogen from 90.3 +/- 6.0 to 124.7 +/- 10.8 mmol/kg wet wt increased muscle glycogenolysis during exercise (62.7 +/- 7.9 vs. 49.1 +/- 6.6 mmol/kg; P < 0.05). Similarly, in series 2 (n = 5) when muscle glycogen was reduced from 96.2 +/- 6.6 to 53.7 +/- 6.0 mmol/kg, glycogen utilization during exercise was reduced from 51.8 +/- 4.6 to 28.3 +/- 3.8 mmol/kg (P < 0.05). The altered muscle glycogen utilization was associated with alterations in carbohydrate oxidation during exercise, without effect on tracer ([3H]glucose)-determined glucose uptake. These results indicate that preexercise muscle glycogen availability influences muscle glycogenolysis, but not glucose uptake, during exercise.

Skeletal muscle GLUT-4 and glucose uptake during exercise in humans

The present study examined the relationship between total skeletal muscle GLUT-4 protein level and glucose uptake during exercise. Eight active non-endurance-trained men cycled at 72 +/- 1% peak pulmonary oxygen consumption for 40 min, with rates of glucose appearance and disappearance (Rd) determined by utilizing a primed continuous infusion of [3-3H]glucose commencing 2 h before exercise. Muscle glycogen content and utilization, citrate synthase activity, and total GLUT-4 protein were measured on muscle biopsy samples obtained from the vastus lateralis. A direct relationship existed between preexercise muscle glycogen content and glycogen utilization during exercise (r = 0.76, P < 0.05). Citrate synthase activity and glucose Rd at the end of exercise averaged 21.9 +/- 3.0 mumol.min-1.g-1 and 27.3 +/- 2.5 mumol.kg-1.min-1, respectively. There was a direct correlation between citrate synthase activity and GLUT-4 protein (r = 0.78, P < 0.05); however, at the end of exercise, glucose Rd was inversely related to both GLUT-4 (r = -0.89, P < 0.01) and citrate synthase activity (r = -0.72, P < 0.05). Plasma insulin, which decreased during exercise, was not related to glucose Rd. In conclusion, glucose uptake during 40 min of exercise at 72% peak pulmonary oxygen consumption was inversely related to the total muscle GLUT-4 protein level. This suggests that factors other than the total GLUT-4 protein level are important in the regulation of glucose uptake during exercise.

Effect of carbohydrate ingestion on glucose kinetics during exercise

Six well-trained men (peak pulmonary oxygen uptake = 5.03 +/- 0.11 l/min) were studied during 2 h of exercise at 69 +/- 1% peak pulmonary oxygen uptake to examine the effect of carbohydrate (CHO) ingestion on glucose kinetics. Subjects ingested 250 ml of either a 10% glucose solution containing 6-[3H]glucose (CHO) or a sweet placebo every 15 min during exercise. Glucose kinetics were assessed by 6,6-[2H]glucose infusion corrected for gut-derived glucose in CHO. Plasma glucose was higher (P < 0.05) in CHO from 20 min. Total glucose appearance was higher in CHO due to glucose delivery from the gut (68 +/- 7 g), since hepatic glucose production was reduced by 51% (29 +/- 5 vs. 59 +/- 5 g). Glucose uptake was higher in CHO (96 +/- 7 vs. 60 +/- 6 g) with the ingested glucose supplying 67 +/- 4 g and, with the assumption that it was fully oxidized, accounted for 14 +/- 1% of total energy expenditure. In conclusion, CHO ingestion during prolonged exercise results in suppression of hepatic glucose production and increased glucose uptake. These effects appear to be mediated mainly by increased plasma glucose and insulin levels.

Influence of sodium on glucose bioavailability during exercise

To examine the influence of beverage sodium content on glucose bioavailability during exercise, six trained men were studied during 30 min of cycle ergometer exercise at 65% VO2max. Immediately prior to exercise, subjects ingested 400 ml of a 10% glucose solution containing 100 microCi of D-(3-3H]-glucose, with a sodium concentration of either 0, 25, or 50 mmol.l-1. Trials were conducted in the morning after an overnight fast and in randomized order at least 1 wk apart. Blood samples were obtained from a forearm vein before and during exercise and subsequently analyzed for plasma glucose and 3H-glucose activity and blood lactate. No differences in oxygen uptake, heart rate, or blood lactate were observed between trials. Resting plasma glucose levels were not different between trials. The increases in plasma glucose and the plasma accumulation of 3H-glucose were similar in the three trials. These results indicate that alterations in beverage sodium content, from 0-50 mmol.l-1, have no effect on glucose bioavailability, as measured by increases in plasma glucose and 3H-glucose activity during moderate intensity exercise.

Carbohydrate ingestion during exercise: liquid vs solid feedings

Six trained men were studied during 2 h of exercise at 65% VO2max to examine the influence of the physical form of carbohydrate supplementation on blood glucose and insulin responses. Three exercise tests were performed in randomized order, at least 1 wk apart. Subjects ingested 25 g of carbohydrate or 500 ml of a sweet placebo (CON) at 0, 30, 60, and 90 min of exercise. Carbohydrate was ingested as 500 ml of a 5% rice-based liquid (L-CHO) or as a 31-g food bar (S-CHO). No differences in oxygen uptake, respiratory exchange ratio, or heart rate were observed between trials. Blood glucose levels were higher (P < 0.05) throughout exercise when carbohydrate was ingested, compared with CON. No differences in blood glucose during exercise were observed between L-CHO and S-CHO. Plasma insulin levels were higher (P < 0.05) after 120 min of exercise when carbohydrate was ingested. The results of this study indicate that carbohydrate supplements with differing physical form (liquid vs solid) but equal carbohydrate content produce similar blood glucose and insulin responses during exercise.