title: Potentiation in mouse lumbrical muscle without myosin light chain phosphorylation: is resting calcium responsible?
abstract: The increase in isometric twitch force observed in fast-twitch rodent muscles during or after activity, known universally as potentiation, is normally associated with myosin regulatory light chain (RLC) phosphorylation. Interestingly, fast muscles from mice devoid of detectable skeletal myosin light chain kinase (skMLCK) retain a reduced ability to potentiate twitch force, indicating the presence of a secondary origin for this characteristic feature of the fast muscle phenotype. The purpose of this study was to assess changes in intracellular cytosolic free Ca(2+) concentration ([Ca(2+)](i)) after a potentiating stimulus in mouse lumbrical muscle (37°C). Lumbricals were loaded with the Ca(2+)-sensitive fluorescent indicators fura-2 or furaptra to detect changes in resting and peak, respectively, intracellular Ca(2+) levels caused by 2.5 s of 20-Hz stimulation. Although this protocol produced an immediate increase in twitch force of 17 ± 3% (all data are n = 10) (P < 0.01), this potentiation dissipated quickly and was absent 30 s afterward. Fura-2 fluorescence signals at rest were increased by 11.1 ± 1.3% (P < 0.01) during potentiation, indicating a significant increase in resting [Ca(2+)](i). Interestingly, furaptra signals showed no change to either the amplitude or the duration of the intracellular Ca(2+) transients (ICTs) that triggered potentiated twitches during this time (P < 0.50). Immunofluorescence work showed that 77% of lumbrical fibers expressed myosin heavy chain isoform IIx and/or IIb, but with low expression of skMLCK and high expression of myosin phosphatase targeting subunit 2. As a result, lumbrical muscles displayed no detectable RLC phosphorylation either at rest or after stimulation. We conclude that stimulation-induced elevations in resting [Ca(2+)](i), in the absence of change in the ICT, are responsible for a small-magnitude, short-lived potentiation of isometric twitch force. If operative in other fast-twitch muscles, this mechanism may complement the potentiating influence of myosin RLC phosphorylation.
authors: Department of Kinesiology, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada.
authors: Ian C
authors: IC
authors: Smith
authors: William
authors: W
authors: Gittings
authors: Jian
authors: J
authors: Huang
authors: Elliott M
authors: EM
authors: McMillan
authors: Joe
authors: J
authors: Quadrilatero
authors: A Russell
authors: AR
authors: Tupling
authors: Rene
authors: R
authors: Vandenboom
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism / Q000502:physiology*; D003600:Cytosol / Q000378:metabolism / Q000502:physiology; D005453:Fluorescence; D016257:Fura-2 / Q000031:analogs & derivatives / Q000378:metabolism; D007537:Isometric Contraction / Q000502:physiology*; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D018656:Muscle Fibers, Fast-Twitch / Q000378:metabolism / Q000502:physiology*; D018994:Myosin Light Chains / Q000378:metabolism / Q000502:physiology*; D010766:Phosphorylation
chemical_list: D018994:Myosin Light Chains; C058776:2-(2-(5-carboxy)oxazole)-5-hydroxy-6-aminobenzofuran-N,N,O-triacetic acid; D002118:Calcium; D016257:Fura-2
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