We tested the hypothesis that abdominal muscles are active during the expiratory phase of the respiratory cycle during exercise. Electromyographic (EMG) activities of external oblique and rectus abdominis muscles were recorded during incremental exercise to exhaustion and during 30 min of constant work rate exercise at an intensity of 85 % of the peak oxygen consumption rate (V̇O2). High amplitude intramuscular EMG activities of both abdominal muscles could be evoked with postural manoeuvres in all subjects. During cycling, respiratory‐related activity of the external obliques was evoked in four of seven subjects, whereas rectus abdominis activity was observed in six of the seven subjects. We measured only the activity that was confined exclusively to the expiratory phase of the respiratory cycle. Expiratory activity of both muscles increased with exercise intensity, although peak values averaged only 10‐20 or 20‐40 % of the peak activity (obtained during maximal, voluntary expiratory efforts) for the external oblique and rectus abdominis muscles, respectively. To estimate how much of the recorded abdominal muscle activity was supporting leg movements during exercise, we compared the activity at the very end of incremental exercise to that recorded during the first five respiratory cycles after the abrupt cessation of exercise, when ventilation was still very high. Although external oblique activity was reduced after exercise stopped, clear expiratory activity remained. Rectus abdominis activity remained high after exercise cessation, showing a gradual decline that approximated the decline in ventilation. During constant work rate exercise, EMG activities increased to 40‐50 and 5‐10 % of peak in rectus and external oblique muscles, respectively, and then plateaued for the remainder of the bout in spite of a continual upward drift in V̇O2 and pulmonary ventilation. Linear regression analysis showed that the rise in respiratory‐related expiratory muscle activity during progressive intensity exercise was significantly correlated with ventilation, although weakly. In constant work rate exercise, expiratory EMG activities increased, but the changes were highly variable and did not change as a function of exercise time, even though ventilation drifted significantly with time. These experiments suggest that abdominal muscles play a role in regulating the ventilatory response to progressive intensity bicycle exercise, although some of the observed activity may support postural adjustments or limb movements. The contribution of abdominal muscles to ventilation during constant work rate exercise is variable, and expiratory activity does not ‘drift’ significantly with time.

The total creatine pool size [Crtotal; creatine (Cr) + phosphocreatine (PCr)] is crucial for optimal energy utilization in skeletal muscle, especially at the onset of exercise and during intense contractions. The Crtotal likely is controlled by long-term modulation of Cr uptake via the sodium-dependent Cr transporter (CrT). To test this hypothesis, adult male Sprague-Dawley rats were fed 1% Cr, their muscle Crtotal was reduced by ∼85% [1% β-guanidinoproprionic acid (β-GPA)], or their muscle Crtotal was repleted (1% Cr after β-GPA depletion). Cr uptake was assessed by skeletal muscle 14C-Cr accumulation to Cr and PCr by using hindlimb perfusion, and CrT protein content was assessed by Western blot. Cr uptake rate decreased with dietary Cr supplementation in the white gastrocnemius (WG; 45%) only. Depletion of muscle Crtotal to ∼15% of normal increased Cr uptake in the soleus (21%) and red gastrocnemius (22%), corresponding to 70–150% increases in muscle CrT content. In contrast, the inherently lower Cr uptake rate in the WG was unchanged with depletion of muscle Crtotal even though CrT band density was increased by 230%. Thus there was no direct relationship between apparent muscle CrT abundance and Cr uptake rates. However, Cr uptake rates scaled inversely with decreases in muscle Crtotal in the high-oxidative muscle types but not in the WG. This implies that factors controlling Cr uptake are different among fiber types. These observations may help explain the influence of initial muscle Crtotal, time dependency, and variations in muscle Crtotal accumulation during Cr supplementation.

The study of cellular energetics is critically dependent on accurate measurement of high-energy phosphates. Muscle values of phosphocreatine (PCr) vary greatly between in vivo measurements (i.e., by nuclear magnetic resonance) and chemical measurements determined from muscles isolated and quick-frozen. The source of this difference has not been experimentally identified. A likely cause is activation of ATPases and phosphotransfer from PCr to ADP. Therefore, rat hindlimb skeletal muscle was perfused either with or without 2 mM iodoacetamide, a creatine kinase inhibitor, and muscle was freeze-clamped either at rest or after contraction. Creatine kinase inhibition resulted in approximately 6 micromol/g higher PCr and lower creatine in the freeze-clamped soleus, red gastrocnemius, and white gastrocnemius. This PCr content difference was reduced when the initial PCr content was decreased with prior contractions. Therefore, the amount of PCr artifact appears to scale with initial PCr content within a fiber-type section. This artifact directly affects the measurement and, thus, the calculations of muscle energetic parameters from studies using isolated and frozen muscle.

Skeletal muscle fiber types differ in their contents of total phosphate, which includes inorganic phosphate (P i) and high-energy organic pools of ATP and phosphocreatine (PCr). At steady state, uptake of Pi into the cell must equal the rate of efflux, which is expected to be a function of intracellular P i concentration. We measured 32 P-labeled Pi uptake rates in different muscle fiber types to determine whether they are proportional to cellular Pi content. Pi uptake rates in isolated, perfused rat hindlimb muscles were linear over time and highest in soleus (2.42 Ϯ 0.17, and lowest in white gastrocnemius (0.49 Ϯ 0.06Reasonably similar rates were obtained in vivo. Pi uptake rates at plasma Pi concentrations of 0.3-1.7 mM confirm that the P i uptake process is nearly saturated at normal plasma Pi levels. Pi uptake rate correlated with cellular Pi content (r ϭ 0.99) but varied inversely with total phosphate content. Sodium-phosphate cotransporter (PiT-1) protein expression in soleus and red gastrocnemius were similar to each other and seven-to eightfold greater than PiT-1 expression in white gastrocnemius. That the PiT-1 expression pattern did not match the pattern of P i uptake across fiber types implies that other factors are involved in regulating Pi uptake in skeletal muscle. Furthermore, fractional turnover of the cellular P i pool (0.67, 0.57, and 0.33 h Ϫ1 in soleus, red gastrocnemius, and white gastrocnemius, respectively) varies among fiber types, indicating differential management of intracellular Pi, likely due to differences in resistance to Pi efflux from the fiber. inorganic phosphate; sodium-inorganic phosphate transporters; PiT-2; inorganic phosphate efflux IN EVERY CELL, phosphate is necessary for structural and metabolic needs. In cellular energetics, phosphate forms the highenergy bonds of ATP and phosphocreatine (PCr), and inorganic phosphate (P i ) is a substrate for reactions in glycolysis, tricarboxylic acid cycle, and mitochondrial F 0 F 1 ATPase. Because cellular P i is a determinant of the free energy of ATP hydrolysis, its concentration is tightly controlled. However, alterations in total phosphate content in skeletal muscle, e.g., expansion of the PCr and/or reduction of the ATP pools, suggest that either P i uptake or P i loss from the cell is also modulated.In skeletal muscle, phosphate is distributed among the P i , ATP, and PCr pools primarily via ATPases, mitochondrial oxidative phosphorylation, and the creatine kinase reaction (21). Maintenance of the total phosphate contained in these pools depends on the balance of uptake and efflux of phosphate across the sarcolemma. Although most cellular phosphate is contained in organic phosphates, these molecules cannot easily cross the plasma membrane; rather, phosphate is transported as P i . Because P i is transported against a concentration and electrical gradient, it must be imported via active transport. This is presumably accomplished by the type III sodiumphosphate (Na-P i ) cotransporter (15) and is driven by the electrochemical ...

The influence of ribose supplementation on skeletal muscle adenine salvage rates during recovery from intense contractions and subsequent muscle performance was evaluated using an adult rat perfused hindquarter preparation. Three minutes of tetanic contractions (60 tetani/min) decreased ATP content in the calf muscles by approximately 50% and produced an equimolar increase in IMP. Effective recovery of muscle ATP 1 h after contractions was due to reamination of IMP via the purine nucleotide cycle and was complete in the red gastrocnemius but incomplete in the white gastrocnemius muscle section. Adenine salvage rates in recovering muscle averaged 45 +/- 4, 49 +/- 5, and 30 +/- 3 nmol. h(-1). g(-1) for plantaris, red gastrocnemius, and white gastrocnemius muscle, respectively, which were not different from values in corresponding nonstimulated muscle sections. Adenine salvage rates increased five- to sevenfold by perfusion with approximately 4 mM ribose (212 +/- 17, 192 +/- 9, and 215 +/- 14 nmol. h(-1). g(-1) in resting muscle sections, respectively). These high rates were sustained in recovering muscle, except for a small (approximately 20%) but significant (P < 0.001) decrease in the white gastrocnemius muscle. Ribose supplementation did not affect subsequent muscle force production after 60 min of recovery. These data indicate that adenine salvage rates were essentially unaltered during recovery from intense contractions.