title: Muscle contractile properties during intermittent nontetanic stimulation in rat skeletal muscle.
abstract: To examine changes in contractile properties and mechanisms of fatigue during submaximal nontetanic skeletal muscle activity, in situ perfused soleus (60-min protocol) and extensor digitorum longus (EDL; 10-min protocol) muscles of the rat were electrically stimulated intermittently at low frequency. The partly fused trains of contractions showed a two-phase change in appearance. During the first phase, relaxation slowed, one-half relaxation time increased, and maximal relaxation first derivative of force (dF/dt) decreased. Developed force during the trains was reduced and was closely related to the rate of relaxation in this first phase. During the second phase, relaxation became faster again, one-half relaxation time decreased, and force returned to resting levels between contractions in a train. In contrast, developed force remained reduced, so that peak force of the contractions was 51% (soleus) and 30% (EDL) of control. In the soleus muscle, the changes in contractile properties were not related to ATP, creatine phosphate, or lactate content. The changes in contractile properties fit best with a mechanism of fatigue involving changes in Ca(2+) handling by the sarcoplasmic reticulum.
authors.affiliation: Institute for Experimental Medical Research, University of Oslo, Ullevaal Hospital, N-0407 Oslo, Norway.
authors.forename: E
authors.initials: E
authors.lastname: Verburg
authors.forename: H M
authors.initials: HM
authors.lastname: Thorud
authors.forename: M
authors.initials: M
authors.lastname: Eriksen
authors.forename: N K
authors.initials: NK
authors.lastname: Vøllestad
authors.forename: O M
authors.initials: OM
authors.lastname: Sejersted
mesh_terms: D000818:Animals; D004558:Electric Stimulation; D008297:Male; D009119:Muscle Contraction / Q000502:physiology*; D018763:Muscle Fatigue / Q000502:physiology; D009126:Muscle Relaxation / Q000502:physiology; D018482:Muscle, Skeletal / Q000502:physiology*; D010477:Perfusion; D051381:Rats; D017208:Rats, Wistar; D013997:Time Factors