Staircase in mammalian muscle without light chain phosphorylation

Abstract

In disuse atrophied skeletal muscle, the staircase response is virtually absent and light chain phosphorylation does not occur. The purpose of the present study was to determine if staircase could be restored in atrophied muscle with continued absence of myosin light chain phosphorylation, by reducing what appears to be an otherwise enhanced calcium release. Control (untreated) and sham-operated female Sprague-Dawley rats were compared with animals after 2 weeks of complete inactivity induced by tetrodotoxin (TTX) application to the left sciatic nerve. In situ isometric contractile responses of rat gastrocnemius muscle were analyzed before and after administration of dantrolene sodium (DS), a drug which is known to inhibit Ca 2+ release in skeletal muscle. Twitch active force (AF) was attenuated by DS from 2.2 ± 0.2 N, 2.7 ± 0.1 N and 2.4 ± 0.2 N to 0.77 ± 0.2 N, 1.05 ± 0.1 N and 1.01 ± 0.2 N in TTX (N = 5), sham (N = 11) and control (N = 7) muscles, respectively. Following dantrolene treatment, 10 s of 10-Hz stimulation increased AF to 1.32 ± 0.2 N, 1.52 ± 0.1 N and 1.45 ± 0.2 N for the TTX, sham and control groups, respectively, demonstrating a positive staircase response. Regulatory light chain (R-LC) phosphorylation was lower for TTX-treated (5.5 ± 5.5%) than for control (26.1 ± 5.3%) and sham (20.0 ± 5%) groups. There was no significant change from resting levels for any of the groups after DS treatment (P = 0.88). This study shows that treatment with dantrolene permits staircase in atrophied muscle as well as control muscle, by a mechanism which appears to be independent of R-LC phosphorylation.

Myosin light-chain phosphorylation and potentiation of dynamic function in mouse fast muscle

“…On the other hand, the magnitude of staircase potentiation in mouse EDL muscle (23-35°C) was reported to be inversely proportional to muscle length in the range 2.5 to 3.7 μm sarc −1 [34]. This latter result, moreover, is consistent with a number of results from rat gastrocnemius muscle examining the length-dependence of staircase potentiation [36]. The mechanism whereby twitch force potentiation is influenced by length is consistent with mechanistic models developed from permeabilized skeletal muscle fibers whereby the addition of a negatively charged phosphate moiety to the RLC propels the myosin motor domain toward actin binding sites, thus facilitating motor Fig.…”

“…For example, recent studies using mouse EDL devoid of skMLCK have shown that although PTP is reduced or eliminated, staircase potentiation is still observed, indicating the presence of redundant mechanism for enhancing isometric twitch force [11,49]. It is feasible that repetitive stimulation potentiates twitch force by augmenting the intracellular calcium transient [36] independent of any skMLCK-mediated increase in Ca 2+ sensitivity. The contribution of this mechanism to PTP is suspect, however [8].…”

Coexistence of potentiation and fatigue in skeletal muscle

“…This difference suggests that at least in fatigued muscle the mechanism for enhancement may be different for staircase and PTP. This conclusion is consistent with the observation that staircase can occur in the absence of RLC phosphorylation at low levels of Ca 2+ release (13). One explanation for this discrepancy is that there could be another mechanism contributing to staircase under these circumstances.…”

“…Our knowledge of the mechanisms of activitydependent potentiation cannot rule out the possibility of a contribution by other factors. In fact it has recently been demonstrated that staircase can occur without corresponding RLC phosphorylation (13). In spite of this, for the purposes of this review, increased Ca 2+ sensitivity in association with RLC phosphorylation will be considered to be the primary mechanism of activity-dependent potentiation.…”

Modeling of summation of individual twitches into unfused tetanus for various types of rat motor units

“…Nevertheless, the parameters of these six twitches are not constant. In both types of fast units, the staircase effect is visible [26]. The maximal force of the twitches decreases for FR MU up to the 5th impulse (from 26.05 to 23.73 mN).…”