Attenuation of myosin light chain phosphorylation and posttetanic potentiation in atrophied skeletal muscle

Abstract

Previously we have demonstrated that the absence of staircase potentiation in atrophied rat gastrocnemius muscle is accompanied by a virtual absence of phosphorylation of the regulatory light chains (R-LC) of myosin. It was our purpose in the present study to determine if posttetanic potentiation and corresponding R-LC phosphorylation were also attenuated in disuse-atrophied muscles. Two weeks after a spinal hemisection (T12), twitch and tetanic contractile characteristics were measured in situ in control, sham-treated and atrophied (hemisected) muscles. Posttetanic potentiation 20 s after a 2 s tetanic contraction (200 Hz) was depressed in atrophied muscles (128.7 +/- 2.6%; mean +/- SEM) when compared to sham-treated (149.9 +/- 2.4%) and control (142.9 +/- 2. 7%) muscles. Atrophied muscles demonstrated a significant increase in R-LC phosphorylation from rest (0.05 +/- 0.04 moles of phosphate/mole of R-LC) to posttetanic conditions (0.21 +/- 0.03 moles of phosphate/mole of R-LC), and less phosphorylation than control and sham-treated muscles (0.43 +/- 0.06 and 0.49 +/- 0.03 moles of phosphate/mole of R-LC, respectively) after tetanic stimulation. The preservation of the potentiation-phosphorylation relationship in atrophied muscles is consistent with the hypothesis that R-LC phosphorylation may be the principal mechanism for twitch potentiation.

Low frequency fatigue in human quadriceps is fatigue dependent and not task dependent

“…Moreover, our between session ratio reliability was excellent (ICC > 0.92). Therefore, we believe that the D/S ratio was highly sensitive to LFF as supported by others (Strojnik and Komi, 2000;Tubman et al, 1997).…”

“…However, LFF has also been shown not only after low intensity/long duration fatigue tasks, but also after high intensity/short-duration tasks. Skurvydas et al (2003) found a change in the ratio of the torque elicited by electrical stimulation at 20 Hz and 50 Hz (20/50) after a maximal voluntary isometric contraction of the quadriceps sustained for 60 s. Tubman et al (1997) found a similar change in the ratio of the torque elicited by a double pulse (166 Hz) and single pulse to support that the R-LC phosphorylation system is impaired with disuse.…”

“…It is named so because of a preferential decrease in the torque elicited with electrical stimulation at a low frequency, compared to a high frequency. The mechanism for LFF is not fully understood; however, a decrease of sarcoplasmic reticulum Ca 2+ release per action potential and/or impaired regulatory light chain phosphorylation (R-LC), which leads to excitationcontraction (E-C) coupling failure (Westerblad et al, 1993;Chin and Allen, 1996;Hill et al, 2001;Tubman et al, 1997), has been implicated.…”

Time-related changes of motor unit properties in the rat medial gastrocnemius muscle after the spinal cord injury. II. Effects of a spinal cord hemisection

“…However, so far still little is known about changes of MU contractile properties induced by partial spinal cord lesions. In rats 14 days following spinal cord hemisection changes in myosin regulatory light chain phosphorylation, depressed posttetanic potentation, and increased twitch-to-tetanus ratio, as well as lower tetanic force of the MG muscle (Tubman et al, 1997), and larger excitatory reflexes on the lesioned side in comparison to the contralateral side (Malmsten, 1983) were described. In comparison to the results of the total spinal cord transection, which evoked a complete lose of postural and locomotor hind limb functions, partial spinal cord injury induced less dramatic limitation in movement.…”

Postactivation potentiation of short tetanic contractions is differently influenced by stimulation frequency in young and elderly adults

“…In addition, a reduced myosin-RLC phosphorylation during the conditioning contractile activity has been observed in atrophied muscles and may have contributed to the diminished PAP in elderly individuals (Tubman et al 1996(Tubman et al , 1997. However, despite the diVerence in absolute PAP Fig.…”