GLYCOGEN DEPLETION IN SINGLE MUSCLE FIBER TYPES DURING LOW-FREQUENCY ELECTRICAL STIMULATION AND ENDURANCE EXERCISE IN TRAINED CYCLISTS577

Twelve healthy male volunteers cycled to exhaustion at a workload corresponding to 70% of maximal aerobic power after administration of 80 mg of the beta 1+2-adrenoceptor antagonist propranolol and after administration of placebo by mouth. Exercise times until exhaustion were 39 +/- 7 and 86 +/- 7 min in the propranolol and placebo groups, respectively. Muscle inosine 5′-monophosphate content was significantly increased above resting levels at exhaustion after placebo. At exhaustion after propranolol, inosine 5′-monophosphate was not increased significantly and was lower than at exhaustion after placebo. No changes in ATP and the total adenine nucleotide content during exercise were found in the two tests. Muscle glycogen content was significantly reduced at exhaustion after placebo as well as after propranolol, but the levels were still significantly higher at exhaustion after propranolol than after placebo. No evidence for a shift in glycogen utilization among types I, IIa, and IIb fibers after propranolol was found. The results show that neither an imbalance between ATP utilization and ATP regeneration nor premature glycogen depletion, either in the whole muscle or in specific muscle fiber types, provides a satisfactory explanation for the premature fatigue during endurance exercise after propranolol.

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Differential susceptibility of diaphragm muscle fibers to neuromuscular transmission failure

Journal of Applied Physiology ◽

10.1152/jappl.1993.75.1.341 ◽

1993 ◽

Vol 75 (1) ◽

pp. 341-348 ◽

Cited By ~ 49

Author(s):

B. D. Johnson ◽

G. C. Sieck

Keyword(s):

Muscle Fiber ◽

Nerve Stimulation ◽

Neuromuscular Transmission ◽

Rank Order ◽

Diaphragm Muscle ◽

Muscle Fiber Types ◽

Fiber Types ◽

Glycogen Depletion ◽

Stimulation Rate ◽

Transmission Failure

The pattern of glycogen utilization was used to determine whether various muscle fiber types in the rat diaphragm are differentially susceptible to neuromuscular transmission failure. Muscle segments from the midcostal region were repetitively stimulated directly or via the phrenic nerve at 10 or 75 Hz. Muscle fiber types were classified histochemically as type I, IIa, or IIb. The amount of muscle fiber glycogen depletion with direct stimulation depended on stimulation rate (75 Hz > 10 Hz) and fiber type (IIb > IIa > I). However, with nerve stimulation, muscle fiber glycogen depletion did not display the same dependency on stimulation rate (10 Hz > 75 Hz), although with stimulation at 10 Hz, the same rank order of fiber depletion was observed (IIb > IIa > I). This rank order of depletion was reversed (I > IIa > IIb) during repetitive stimulation of the nerve at 75 Hz. By intermittently stimulating the muscle directly during continuous nerve stimulation, we determined that neuromuscular transmission failure contributed significantly to the force decline after 2 min of stimulation at 75 Hz but relatively little to the force decline after 2 min of stimulation at 10 Hz. A significantly greater fraction of the force decline could be attributed to neuromuscular transmission failure with repetitive bouts of stimulation at 10 Hz. We conclude that neuromuscular transmission failure causes a significant portion of the force decline after 8 min of stimulation at 10 and 75 Hz, that all diaphragm fiber types are susceptible to neuromuscular transmission failure, but that type IIb fibers are particularly susceptible at higher frequencies of stimulation.

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Glycogen, lactate, and alanine changes in muscle fiber types during graded exercise

Journal of Applied Physiology ◽

10.1152/jappl.1977.43.2.288 ◽

1977 ◽

Vol 43 (2) ◽

pp. 288-291 ◽

Cited By ~ 29

Author(s):

K. M. Baldwin ◽

P. J. Campbell ◽

D. A. Cooke

Keyword(s):

Blood Lactate ◽

Muscle Fiber ◽

Fiber Type ◽

Lactate Production ◽

Female Rats ◽

Muscle Fiber Types ◽

Fiber Types ◽

Glycogen Depletion ◽

Experimental Conditions ◽

Graded Exercise

The effect of different speeds of incline running (16–48 m/min) on changes in glycogen, lactate, and alanine concentrations in different skeletal muscle fiber types and in blood were investigated in nontrained adult female rats during 5-min bouts of exercise. Glycogen depletion occurred extensively (25–86%) in fast-oxidative-glycogenolytic (FOG) and in slow-oxidative (SO) fibers over the range of speeds studied, whereas it occurred primarily (76%) at 48 m/min in the fast-glycogenolytic (FG) fiber type. No marked increases in lactate over resting control levels occurred in the FOG fibers. However, compared to resting levels, lactate increased over fourfold in FG muscle during running at 48 m/min and blood lactate concentrations paralleled these changes. No changes occurred in alanine concentration in FOG muscle and in blood under these experimental conditions, whereas there were approximately 70% increases in FG and SO fibers during moderate-to-heavy exercise. These findings suggest that elevations in blood lactate of rodents primarily result from lactate production in the FG fiber type.

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