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

Abstract

The intent of this study was to determine if the stimulation-induced increase or "potentiation" of dynamic function of mouse extensor digitorum longus muscle (in vitro 25°C) during work cycles is graded to myosin regulatory light-chain (RLC) phosphorylation. To do this, concentric force and muscle work output during sinusoidal length changes were determined before (unpotentiated) and after (potentiated) the application of conditioning stimuli (CS) producing incremental elevations in RLC phosphorylation from rest. Sine wave excursion was from 1.09 to 0.91 of L (o) with a period of 142 ms; stimulating muscles to twitch and generate force during these cycles produced plots of force × displacement termed work loops. Stimulation at 2.5-, 5.0-, and 100-Hz elevated RLC phosphorylation from 0.16±0.02 (rest) to 0.29±0.03, 0.45±0.02 and 0.56±0.02 mol phos per mole RLC, respectively (n= 6-7, P<0.05). These CS potentiated mean concentric force (at all lengths) to 1.14±0.02, 1.26±0.04 and 1.41±0.06 of pre-stimulus, control levels (all n= 5-7, P<0.05) while work was increased to 1.07±0.02, 1.17±0.02 and 1.34±0.03 of controls, respectively. In a No CS condition that did not elevate RLC phosphorylation, neither mean concentric force nor work was altered. Thus, strong correlations between RLC phosphorylation and mean concentric force and work support the hypothesis that this molecular mechanism modulates muscle power output. No length-dependence for concentric force potentiation was observed in any condition, an outcome suggesting that interactions between instantaneous variations in muscle length and shortening velocity during work cycles modulates the potentiation response.

The effect of work cycle frequency on the potentiation of dynamic force in mouse fast twitch skeletal muscle

“…Our results confirm previous results showing an association between RLC phosphorylation and the potentiation of dynamic force and work (Abbate et al, 2000;Grange et al, 1995;Grange et al, 1998;MacIntosh and Bryan, 2002;MacIntosh et al, 2008;Xeni et al, 2011). Moreover, the present results extend these observations by showing that the potentiation of concentric force and work is shortening speed dependent and that eccentric force and work are relatively insensitive to potentiation.…”

“…Isometric data were obtained during separate, fixed length contractions at L o . All data were obtained before and after a tetanic conditioning protocol (see Fig.1 with previous studies using mouse fast twitch muscle (Xeni et al, 2011). Interestingly, however, work at 1.5 and 3.3Hz was either not increased or increased only marginally relative to concentric force.…”

“…Work cycles were performed by cycling muscle length about L o using a software-generated sine-wave function (Grange et al, 1998;Xeni et al, 2011). Sine excursion (i.e.…”

“…All frozen muscles were stored at -80°C until being packed on dry ice and shipped to the laboratory of Dr James Stull (University of Texas, Southwestern Medical Center, Dallas, TX, USA) for quantification of RLC phosphorylation. Details regarding these methods and procedures have been presented elsewhere (Xeni et al, 2011).…”

“…More recent studies, however, demonstrate dynamic muscle function involving muscle shortening, work and power are also subject to modulation by activity-dependent potentiation. Relevant in this regard are studies showing that RLC phosphorylation is associated with increased dynamic force, work and power in both rat and mouse muscle (Abbate et al, 2000;Grange et al, 1995;Grange et al, 1998;MacIntosh and Bryan, 2002;MacIntosh et al, 2008;Xeni et al, 2011). In general, these studies show that the rate and extent of isotonic shortening and/or work/power are enhanced during or following contractile activity that phosphorylates the myosin RLC.…”