Karin Söderlund

Creatine in Humans with Special Reference to Creatine Supplementation

SummarySince the discovery of creatine in 1832, it has fascinated scientists with its central role in skeletal muscle metabolism. In humans, over 95% of the total creatine (Crtot) content is located in skeletal muscle, of which approximately a third is in its free (Crf) form. The remainder is present in a phosphorylated (Crphos) form. Crf and Crphos levels in skeletal muscle are subject to individual variations and are influenced by factors such as muscle fibre type, age and disease, but not apparently by training or gender. Daily turnover of creatine to creatinine for a 70kg male has been estimated to be around 2g. Part of this turnover can be replaced through exogenous sources of creatine in foods, especially meat and fish. The remainder is derived via endogenous synthesis from the precursors arginine, glycine and methionine. A century ago, studies with creatine feeding concluded that some of the ingested creatine was retained in the body. Subsequent studies have shown that both Crf and Crphos levels in skeletal muscle can be increased, and performance of high intensity intermittent exercise enhanced, following a period of creatine supplementation. However, neither endurance exercise performance nor maximal oxygen uptake appears to be enhanced. No adverse effects have been identified with short term creatine feeding. Creatine supplementation has been used in the treatment of diseases where creatine synthesis is inhibited.

The role of phosphorylcreatine and creatine in the regulation of mitochondrial respiration in human skeletal muscle.

1 The role of phosphorylcreatine (PCr) and creatine (Cr) in the regulation of mitochondrial respiration was investigated in permeabilised fibre bundles prepared from human vastus lateralis muscle. 2 Fibre respiration was measured in the absence of ADP (V̇0) and after sequential additions of submaximal ADP (0.1 mm ADP, V̇submax), PCr (or Cr) and saturating [ADP] (V̇max). 3 V̇ submax increased by 55% after addition of saturating creatine (P < 0.01; n= 8) and half the maximal effect was obtained at 5 mm[Cr]. In contrast, V̇submax decreased by 54% after addition of saturating phosphorylcreatine (P < 0.01; n= 8) and half the maximal effect was obtained at 1 mm[PCr]. V̇max was not affected by Cr or PCr. 4 V̇ submax was similar when PCr and Cr were added simultaneously at concentrations similar to those in muscle at rest (PCr/Cr = 2) and at low‐intensity exercise (PCr/Cr = 0.5). At conditions mimicking high‐intensity exercise (PCr/Cr = 0.1), V̇submax increased to 60% of V̇max (P < 0.01) vs. rest and low‐intensity exercise). 5 Eight of the subjects participated in a 16 day Cr supplementation programme. Following Cr supplementation, V̇0 decreased by 17% (P < 0.01) vs. prior to Cr supplementation), whereas ADP‐stimulated respiration (with and without Cr or PCr) was unchanged. 6 For the first time evidence is given that PCr is an important regulator of mitochondrial ADP‐stimulated respiration. Phosphorylcreatine decreases the sensitivity of mitochondrial respiration to ADP whereas Cr has the opposite effect. During transition from rest to high‐intensity exercise, decreases in the PCr/Cr ratio will effectively increase the sensitivity of mitochondrial respiration to ADP. The decrease in V̇0 after Cr supplementation indicates that intrinsic changes in membrane proton conductance occur.

Slow-twitch fiber glycogen depletion elevates moderate-exercise fast-twitch fiber activity and O2 uptake.

PURPOSE We tested the hypotheses that previous glycogen depletion of slow-twitch (ST) fibers enhances recruitment of fast-twitch (FT) fibers, elevates energy requirement, and results in a slow component of VO2 during moderate-intensity dynamic exercise in humans. METHODS Twelve healthy, male subjects cycled for 20 min at approximately 50% VO2max with normal glycogen stores (CON) and with exercise-induced glycogen depleted ST fibers (CHO-DEP). Pulmonary VO2 was measured continuously and single fiber, muscle homogenate, and blood metabolites were determined repeatedly during each trial. RESULTS ST fiber glycogen content decreased (P < 0.05) during CON (293 +/- 24 to 204 +/- 17 mmol x kg d.w.), but not during CHO-DEP (92 +/- 22 and 84 +/- 13 mmol x kg d.w.). FT fiber CP and glycogen levels were unaltered during CON, whereas FT fiber CP levels decreased (29 +/- 7%, P < 0.05) during CHO-DEP and glycogen content tended to decrease (32 +/- 14%, P = 0.07). During CHO-DEP, VO2 was higher (P < 0.05) from 2 to 20 min than in CON (0-20 min:7 +/- 1%). Muscle lactate, pH and temperature, ventilation, and plasma epinephrine were not different between trials. From 3 to 20 min of CHO-DEP, VO2 increased (P <0.05) by 5 +/- 1% from 1.95 +/- 0.05 to 2.06 +/- 0.08 L x min but was unchanged during CON. In this exercise period, muscle pH and blood lactate were unaltered in both trials. Exponential modeling revealed a slow component of VO2 equivalent to 0.12 +/- 0.04 L x min during CHO-DEP. CONCLUSION This study demonstrates that previous glycogen depletion of ST fibers enhances FT fiber recruitment, elevates O2 cost, and causes a slow component of VO2 during dynamic exercise with no blood lactate accumulation or muscular acidosis. These findings suggest that FT fiber recruitment elevates energy requirement of dynamic exercise in humans and support an important role of active FT fibers in producing the slow component of VO2

Neuromuscular blockade of slow twitch muscle fibres elevates muscle oxygen uptake and energy turnover during submaximal exercise in humans.

We tested the hypothesis that a greater activation of fast‐twitch (FT) fibres during dynamic exercise leads to a higher muscle oxygen uptake () and energy turnover as well as a slower muscle on‐kinetics. Subjects performed one‐legged knee‐extensor exercise for 10 min at an intensity of 30 W without (CON) and with (CUR) arterial injections of the non‐depolarizing neuromuscular blocking agent cisatracurium. In CUR, creatine phosphate (CP) was unaltered in slow twitch (ST) fibres and decreased (P < 0.05) by 28% in FT fibres, whereas in CON, CP decreased (P < 0.05) by 33% and 23% in ST and FT fibres, respectively. From 127 s of exercise, muscle was higher (P < 0.05) in CUR compared to CON (425 ± 25 (±s.e.m.) versus 332 ± 30 ml min−1) and remained higher (P < 0.05) throughout exercise. Using monoexponential fitting, the time constant of the exercise‐induced muscle response was slower (P < 0.05) in CUR than in CON (55 ± 6 versus 33 ± 5 s). During CUR and CON, muscle homogenate CP was lowered (P < 0.05) by 32 and 35%, respectively, and also muscle lactate production was similar in CUR and CON (37.8 ± 4.1 versus 35.2 ± 6.2 mmol). Estimated total muscle ATP turnover was 19% higher (P < 0.05) in CUR than in CON (1196 ± 90 versus 1011 ± 59 mmol) and true mechanical efficiency was lower (P < 0.05) in CUR than in CON (26.2 ± 2.0 versus 30.9 ± 1.5%). In conclusion, the present findings provide evidence that FT fibres are less efficient than ST fibres in vivo at a contraction frequency of 1 Hz, and that the muscle kinetics is slowed by FT fibre activation.

Heterogeneous recruitment of quadriceps muscle portions and fibre types during moderate intensity knee-extensor exercise: effect of thigh occlusion

The involvement of quadriceps femoris muscle portions and fibre type recruitment was studied during submaximal knee‐extensor exercise without and with thigh occlusion (OCC) and compared with responses during intense exercise. Six healthy male subjects performed 90‐s of moderate exercise without (MOD; 29±4 W) and with thigh OCC, and moderate exercise followed by 90‐s of intense exercise (HI; 65±8 W). Temperatures were continuously measured in m. vastus lateralis (VL), vastus medialis (VM) and rectus femoris (RF) and successive muscle biopsies were obtained from VL. During MOD, muscle temperature increase (ΔTm) in RF was 0.52±0.09 °C, which was 57% and 73% higher (P<0.05) than in VL and VM, respectively. During OCC, ΔTm in RF was 0.39±0.05 °C, which was not different from VM but 54% higher (P<0.05) than in VL. After MOD, muscle CP in slow twitch (ST) and fast twitch (FT) fibres was 81% and 91% of resting levels, respectively, with lower (P<0.05) values after OCC (15% and 22%) and HI (24% and 13%). After MOD, OCC and HI, a total of 48%, 93% and 96% of the ST fibres had CP levels below mean‐1 SD, respectively, with corresponding values for FT fibres being 41%, 89% and 100%, respectively. In conclusion, a heterogeneous recruitment of the quadriceps muscle portions and muscle fibres was observed during submaximal knee‐extensor exercise, whereas recruitment pattern was homogenous during intense exercise. Thigh OCC caused an altered recruitment of fibres and muscle portions, suggesting a significant afferent response affecting the activation of fibres in the contracting muscles.

Plasma hypoxanthine and ammonia in humans during prolonged exercise.

Abstract In this study we examined the time course of changes in the plasma concentration of oxypurines [hypoxanthine (Hx), xanthine and urate] during prolonged cycling to fatigue. Ten subjects with an estimated maximum oxygen uptake (V˙O2max) of 54 (range 47–67) ml · kg−1 · min−1 cycled at [mean (SEM)] 74 (2)% of V˙O2max until fatigue [79 (8) min]. Plasma levels of oxypurines increased during exercise, but the magnitude and the time course varied considerably between subjects. The plasma concentration of Hx ([Hx]) was 1.3 (0.3) μmol/l at rest and increased eight fold at fatigue. After 60 min of exercise plasma [Hx] was >10 μmol/l in four subjects, whereas in the remaining five subjects it was <5 μmol/l. The muscle contents of total adenine nucleotides (TAN = ATP+ADP+AMP) and inosine monophosphate (IMP) were measured before and after exercise in five subjects. Subjects with a high plasma [Hx] at fatigue also demonstrated a pronounced decrease in muscle TAN and increase in IMP. Plasma [Hx] after 60 min of exercise correlated significantly with plasma concentration of ammonia ([NH3], r = 0.90) and blood lactate (r = 0.66). Endurance, measured as time to fatigue, was inversely correlated to plasma [Hx] at 60 min (r = −0.68, P < 0.05) but not to either plasma [NH3] or blood lactate. It is concluded that during moderate-intensity exercise, plasma [Hx] increases, but to a variable extent between subjects. The present data suggest that plasma [Hx] is a marker of adenine nucleotide degradation and energetic stress during exercise. The potential use of plasma [Hx] to assess training status and to identify overtraining deserves further attention.

ATP and phosphocreatine utilization in single human muscle fibres during the development of maximal power output at elevated muscle temperatures

Abstract In this study, we examined the effect of muscle temperature (T m) on adenosine triphosphate (ATP) and phosphocreatine utilization in single muscle fibres during the development of maximal power output in humans. Six male participants performed a 6-s maximal sprint on a friction-braked cycle ergometer under both normal (T m = 34.3°C, s = 0.6) and elevated (T m = 37.3°C, s = 0.2) muscle temperature conditions. During the elevated condition, muscle temperature of the legs was raised, passively, by hot water immersion followed by wrapping in electrically heated blankets. Muscle biopsies were taken from the vastus lateralis before and immediately after exercise. Freeze-dried single fibres were dissected, characterized according to myosin heavy chain composition, and analysed for ATP and phosphocreatine content. Single fibres were classified as: type I, IIA, IIAX25 (1 – 25% IIX isoform), IIAX50 (26 – 50% IIX), IIAX75 (51 – 75% IIX), or IIAX100 (76 – 100% IIX). Maximal power output and pedal rate were both greater (P < 0.05) during the elevated condition by 258 W (s = 110) and 22 rev · min−1 (s = 6), respectively. In both conditions, phosphocreatine content decreased significantly in all fibre types, with a greater decrease during the elevated condition in type IIA fibres (P < 0.01). Adenosine triphosphate content was also reduced to a greater (P < 0.01) extent in type IIA fibres during the elevated condition. The results of the present study indicate that after passive elevation of muscle temperature, there was a greater decrease in ATP and phosphocreatine content in type IIA fibres than in the normal trial, which contributed to the higher maximal power output.

Chronic Maxillary Sinusitis: Energy Metabolism in Sinus Mucosa and Secretion

The energy metabolism in maxillary sinus mucosa and secretion from 41 patients with, in most cases, recurrent or chronic sinusitis was studied and compared with that in mucosal samples from patients with no previous history of sinus abnormality or infection. In freeze-dried samples, glucose and lactate were measured by enzymatic assays and adenine nucleotides by HPLC. In chronic sinusitis the lactate concentration in the mucosa was increased, probably as a result of increased glycolysis or of impaired diffusion within the mucosa. The lactate concentrations in purulent and non-purulent secretions were 16.3 and 1.1 mmol x kg-1 w.w., respectively. The higher concentration in the purulent secretion was due mainly to leucocyte metabolism. The adenine nucleotide contents in the mucosa were similar in all groups studied. Anaerobic bacteria were isolated only in secretions with a high lactate concentration.

Inhaled Beta2-Agonist Increases Power Output and Glycolysis during Sprinting in Men.

PURPOSE The aim of the present study was to investigate the effects of the beta2-agonist terbutaline (TER) on power output and muscle metabolism during maximal sprint cycling. METHODS In a randomized double-blind cross-over design, nine moderately trained men (VO2max = 4.6 ± 0.2 L · min(-1)) conducted a 10-s cycle sprint after inhalation of either 15 mg of TER or placebo (PLA). A muscle biopsy sample was collected before and <10 s after the sprint and was analyzed for metabolites. RESULTS The mean power and peak power during the sprint were 8.3% ± 1.1% and 7.8% ± 2.5% higher (P < 0.05) with TER than with PLA, respectively. Moreover, the net rates of glycogenolysis (6.5 ± 0.8 vs 3.1 ± 0.7 mmol glucosyl units · kg dry weight(-1) · s(-1)) and glycolysis (2.4 ± 0.2 vs 1.6 ± 0.2 mmol glucosyl units · kg dry weight(-1) · s(-1)) were higher (P < 0.05) with TER than with PLA. After the sprint, adenosine triphosphate (ATP) was reduced with PLA (P < 0.05) but not with TER. During the sprint, there was no difference in the breakdown of phosphocreatine (PCr) between treatments. Estimated anaerobic ATP utilization was 9.2% ± 4.0% higher (P < 0.05) with TER than with PLA. After the sprint, ATP in Type II fibers was lowered (P < 0.05) by 25.7% ± 7.3% with PLA but was not reduced with TER. Before the sprint, PCr in Type II fibers was 24.5% ± 7.2% lower (P < 0.05) with TER than with PLA. With PLA, breakdown of PCr was 50.2% ± 24.8% higher (P < 0.05) in Type II fibers (vs Type I fibers), whereas no difference was observed between fiber types with TER. CONCLUSION The present study shows that a TER-induced increase in power output is associated with increased rates of glycogenolysis and glycolysis in skeletal muscles. Furthermore, as TER counteracts a reduction in ATP in Type II fibers, TER may postpone fatigue development in these fibers.

Lactic Acid Isomers and Fatty Acids in Sinus Secretion: A Longitudinal Study of Bacterial and Leukocyte Metabolism in Experimental Sinusitis

Concentrations of the two optic isomers of lactate (D- and L-form) as well as glucose, succinate, acetate, butyrate, isovalerate and valerate were examined in purulent sinus secretions. The samples were obtained from rabbit maxillary sinuses, experimentally infected with Streptococcus pneumoniae or Bacteroides fragilis. More soluble acids such as acetate displayed relatively low levels in the secretion, despite a high microbial production. A substantial increase in D-lactate concentration was found in secretions only the first day after induction of pneumococcal sinusitis, and not in bacteroides sinusitis. L-lactate levels were particularly high in secretions of a marked purulent character, and this isomer can be considered as indicator of anaerobic glycolysis in the leukocytes. Less diffusible fatty acids such as butyrate and isovalerate accumulated in the secretion, in spite of a relatively lower production rate, and are thus more reliable indicators of bacterial metabolism.