Crossbridge Properties during Fatigue and Recovery in Mouse Skeletal Muscle Fibres

Abstract

Fatigue occurring during exercise can be defined as the inability to maintain the initial force or power output. We showed recently that fatigue during repetitive tetanic stimulation at 24 °C of fibre bundles dissected from FDB mouse (C57BL/6) muscle, occurs in two phases: an initial one during which individual crossbridge force decreases, and a later phase during which also crossbridge number decrease (Nocella et al.,2011 J Physiol 589).The present experiments were made on the same preparation to compare the fatigue mechanism at 24°C and at physiological temperature of 35 °C and to investigate the mechanism of force recovery from fatigue at 24°C. Fatigue was induced with 105 consecutive isometric tetani evoked every 1.5 s. Force recovery was followed by tetani evoked every 90s until force recovered to 90-100% of pre-fatigue value. Stiffness was measured with small sinusoidal length oscillations at 6.5 kHz. At both temperatures fatigue occurred initially through the decrease of the individual crossbridge force followed by the reduction of crossbridge number. However the initial phase lasted for ∼40 tetani at 35°C and ∼20 tetani at 24°C. This suggests a greater resistance to fatigue of this mechanism at high temperature. In contrast, during the second phase the tension loss was faster at 35°C than at 24°C so that after 105 tetani tension was similar at both temperatures. Force recovery also occurred in two phases. The initial phase lasted from ∼1.5-4 min and recovered 40-90% of tension loss. The second phase lasted for ∼60 min and its amplitude was well correlated with tension decrease during the second phase of fatigue. Thus the mechanism of tension recovery after fatigue seems symmetrical to tension loss during fatigue.