The effect of creatine supplementation on glucose uptake in rat skeletal muscle

Glucose transport in muscle is a function of the muscle metabolic state, as evidenced by the increase in glucose transport which occurs with conditions of altered aerobic metabolism such as hypoxia or contractile activity. The energy state of the muscle can be determined by the muscle phosphocreatine concentration. Dietary supplementation of creatine has been shown to increase both phosphocreatine (PCr) and creatine (TCr) levels in muscle, although not in the same proportion, so that the PCr/TCr ratio falls suggesting an altered energy state in the cell. The purpose of this study was to determine the effect of increased creatine content on glucose uptake in muscle. PCr and TCr were determined in plantaris muscles from rats following five weeks of dietary supplementation of creatine monohydrate (300 mg/kg/day). (3)H-2-deoxyglucose uptake was measured in epitrochlearis muscles incubated in the presence or absence of a maximally stimulating dose of insulin. Despite a significant increase in creatine content in muscle, neither basal nor insulin-stimulated glucose uptake was altered in creatine supplemented rats. Since PCr levels were not increased with creatine supplementation, these results suggest that the actual concentration of PCr is a more important determinant of glucose uptake than the PCr/TCr ratio.     

Creatine uptake and creatine transporter expression among rat skeletal muscle fiber types

Total creatine (Cr(total) = phosphocreatine + creatine) concentrations differ substantially among mammalian skeletal muscle. Because the primary means to add Cr(total) to muscle is uptake of creatine through the sodium-dependent creatine transporter (CrT), differences in creatine uptake and CrT expression could account for the variations in [Cr(total)] among muscle fiber types. To test this hypothesis, hindlimbs of adult rats were perfused with 0.05-1 mM [(14)C]creatine for up to 90 min. Creatine uptake rates at 1 mM creatine were greatest in the soleus (140 +/- 8.8 nmol x h(-1) x g(-1)), less in the red gastrocnemius (117 +/- 8.3), and least in the white gastrocnemius (97 +/- 10.7). These rates were unaltered by time, insulin concentration, or increased perfusate sodium concentration. Conversely, creatine uptake rates were correspondingly decreased among fiber types by lower creatine and sodium concentrations. The CrT protein content by Western blot analysis was similarly greatest in the soleus, less in the red gastrocnemius, and least in the white gastrocnemius, whereas CrT mRNA was not different. Creatine uptake rates differ among skeletal muscle fiber sections in a manner reasonably assigned to the 58-kDa band of the CrT. Furthermore, creatine uptake rates scale inversely with creatine content, with the lowest uptake rate in the fiber type with the highest Cr(total) and vice versa. This suggests that the creatine pool fractional turnover rate is not common across muscle phenotypes and, therefore, is differentially regulated.     

Growth hormone, IGF-I, and exercise effects on non-weightbearing fast muscles of hypophysectomized rats

Grossman, Elena J., Richard E. Grindeland, Roland R. Roy,Robert J. Talmadge, Juliann Evans, and V. Reggie Edgerton. Growth hormone, IGF-I, and exercise effects on non-weight-bearing fast musclesof hypophysectomized rats. J. Appl.Physiol. 83(5): 1522-1530, 1997.The effects ofgrowth hormone (GH) or insulin-like growth factor I (IGF-I) with orwithout exercise (ladder climbing) in countering the effects ofunweighting on fast muscles of hypophysectomized rats during 10 days ofhindlimb suspension were determined. Compared with untreated suspendedrats, muscle weights were 16-29% larger in GH-treated and5-15% larger in IGF-I-treated suspended rats. Exercise alone hadno effect on muscle weights. Compared with ambulatory control, themedial gastrocnemius weight in suspended, exercised rats was largerafter GH treatment and maintained with IGF-I treatment. The combinationof GH or IGF-I plus exercise in suspended rats resulted in an increasein the size of each predominant fiber type, i.e., types I, I+IIa andIIa+IIx, in the medial gastrocnemius compared with untreated suspendedrats. Normal ambulation or exercise during suspension increased theproportion of fibers expressing embryonic myosin heavy chain inhypophysectomized rats. The phenotype of the medial gastrocnemius wasminimally affected by GH, IGF-I, and/or exercise. These resultsshow that there is an IGF-I, as well as a GH, and exercise interactiveeffect in maintaining medial gastrocnemius fiber size in suspendedhypophysectomized rats.

     

Growth hormone control of glucose oxidation pathways in hypophysectomized rats.

An in vivo response of glucose oxidation to growth hormone has been demonstrated. Hypophysectomized rats were found to oxidize glucose at rates significantly higher than normal rats. Treatment with growth hormone 1 h before injection of 14C-U-glucose, 14C-6-glucose, or 14C-1-glucose caused a return to a normal oxidation pattern. This acute response was independent of insulin action but clearly time-dependent since no change from untreated hypophysectomized rats appeared when growth hormone was given at various times prior to administration of labeled glucose. The response observed for 14C-6-glucose was comparable to that observed for 14C-1-glucose with regard to dynamics but differed with respect to total 14C recovered as 14CO2. The cumulative percent 14CO2 recovered from oxidation of 14C-6-glucose 1 h after growth hormone injection exceeded that recovered from oxidation of 14C-1-glucose. These results suggest a change in glucose oxidation by a route that cannot be explained solely by changes in either the hexose monophosphate or Embden-Meyerhof pathways.     

Inhibitory effects of calcium channel blockers on thyroid hormone uptake in neonatal rat cardiomyocytes

The effects of the Ca2+ channel blockers verapamil, nifedipine, and diltiazem on triiodothyronine (T3) and thyroxine (T4) uptake were tested in cultured cardiomyocytes from 2-day-old rats. Experiments were performed at 37 degrees C in medium with 0.5% BSA for [125I]T3 (100 pM) or 0.1% BSA for [125I]T4 (350 pM). The 15-min uptake of [125I]T3 was 0.124 +/- 0.013 fmol/pM free T3 (n = 6); [125I]T4 uptake was 0.032 +/- 0.003 fmol/pM free T4 (n = 12). Neither T3 nor T4 uptake was affected by 1% DMSO (diluent for nifedipine and verapamil). Uptake of [125I]T3 but not of [125I]T4 was dose dependently reduced by incubation with 1-100 microM verapamil (49-87%, P < 0.05) or nifedipine (53-81%, P < 0.05). The relative decline in [125I]T3 uptake after 4 h of incubation with 10 microM verapamil or nifedipine was less than after 15 min or 1 h, indicating that the major inhibitory effect of the Ca2+ channel blockers occurred at the level of the plasma membrane. The reduction of nuclear [125I]T3 binding by 10 microM verapamil or nifedipine was proportional to the reduction of cellular [125I]T3 uptake. Diltiazem (1-100 microM) had no dose-dependent effect on [125I]T3 uptake but reduced [125I]T4 uptake by 45% (P < 0.05) at each concentration tested. Neither the presence of 20 mM K+ nor the presence of low Ca2+ in the medium affected [125I]T3 uptake. In conclusion, the inhibitory effects of Ca2+ channel blockers on T3 uptake in cardiomyocytes are not secondary to their effects on Ca2+ influx but, rather, reflect interference with the putative T3 carrier in the plasma membrane.     

Growth hormone and rat liver mitochondria effects on urea cycle enzymes

Effects of hypophysectomy and subsequent growth hormone administration on mitochondrial enzymes of the urea cycle were investigated in rat liver. Hypophysectomy increased the activities of the two mitochondrial enzymes, carbamyl phosphate synthetase and ornithine transcarbamylase but not of the cytosolic enzyme, argininosuccinate synthetase. The activity of mitochondrial phosphate dependent glutaminase was not affected. Administration of bovine growth hormone (100 μg/100 g body weight) for two weeks decreased the activities of carbamyl phosphate synthetase and ornithine transcarbamylase almost to the normal level. These results suggest a specific effect of growth hormone on mitochondrial enzymes of the urea cycle and serve to explain the increased urea formation in hypopituitarism.     

Palmitic acid uptake by the rat soleus muscle in vitro.

Abstract: The rate of fatty acid uptake, oxidation, and deposition in skeletal muscles in relation to total and unbound to albumin fatty acids concentration in the medium were investigated in the incubated rat soleus muscle. An immunohistochemical technique was applied to demonstrate whether the albumin-bound fatty acid complex from the medium penetrates well within all areas of the muscle strips. It was found that the percentage of incorporation of palmitic acid into intramuscular lipids was fairly constant, independently of the fatty acid concentration in the medium, and amounted to 63-72% for triacylglycerols, 7-12% for diacylglycerols-monoacylglycerols, and 19-26% for phospholipids. Both palmitic acid incorporation into the muscle triacylglycerol stores and its oxidation to CO2 closely correlated with an increase in both total and unbound to albumin fatty acid concentrations in the incubation medium. Under conditions of increased total but constant unbound to albumin palmitic acid concentrations, the incorporation of palmitic acid into triacylglycerols and its oxidation to CO2 were also increased, but to a lower extent. This supports the hypothesis that the cellular fatty acid metabolism depends not only on the availability of fatty acids unbound to albumin, but also on the availability of fatty acids complexed to albumin.     

Growth hormone enhances effects of endurance training on oxidative muscle metabolism in elderly women

The present study investigated whether recombinant human (rh) growth hormone (GH) combined with endurance training would have a larger effect on oxidative capacity, metabolism, and body fat than endurance training alone. Sixteen healthy, elderly women, aged 75 yr, performed closely monitored endurance training on a cycle ergometer over 12 wk. rhGH was given in a randomized, double-blinded, placebo-controlled design in addition to the training program. GH administration resulted in a doubling of serum insulin-like growth factor I levels. With endurance training, peak oxygen uptake increased by approximately 18% in both groups, whereas the marked increase in muscle citrate synthase activity was 50% larger in the GH group compared with the placebo group. In addition, only the GH group revealed an increase in muscle L-3-hydroxyacyl-CoA dehydrogenase activity. Body weight remained unchanged in both groups, but the GH group showed significant changes in body composition with a decrease in fat mass and an increase in lean body mass. Twenty-four-hour indirect calorimetry performed in four subjects showed a marked increase in energy expenditure with increased relative and absolute fat combustion in the two subjects receiving rhGH. In conclusion, rhGH adds to the effects of endurance training on muscle oxidative enzymes and causes a reduction in body fat in elderly women.     

Insulinomimetic effects of kaempferitrin on glycaemia and on 14C-glucose uptake in rat soleus muscle

Bauhinia forficata is one of the Bauhinia species mostly used as an antidiabetic herbal remedy in Brazil. Kaempferitrin (kaempferol-3,7-O-(alpha)-L-dirhamnoside) is the predominant flavonol glycoside found in the B. forficata leaves. The aim of the present work was to study the long-term effect of kaempferitrin on glycaemia in diabetic rats, as well as the in vitro effect of this compound on 14C-D-glucose uptake and 14C-leucine incorporation into protein in normal rat soleus muscle. Kaempferitrin was found to have an acute lowering effect on blood glucose in diabetic rats and to stimulate the glucose uptake percentile, as efficiently as insulin in muscle from normal rats. This compound did not have any effect on glucosuria or on protein synthesis in muscle from normal and diabetic animals. However, the protein synthesis in the kaempferitrin-treated groups was maintained at the same level as the respective controls. Thus, the hypoglycaemic effect and the prompt efficiency of the kaempferitrin in stimulating [U-14C]-2-deoxi-D-glucose uptake in muscle -constitute the first evidence to indicate that the acute effect of this compound on blood glucose lowering may occur as a consequence of the altered intrinsic activity of the glucose transporter (Vmax or glucose transporters translocation?) not involving directly the synthesis of new carriers.