University of Heidelberg (2006) Proc Physiol Soc 4, SA1
Research Symposium: Martin F. Schneider1, Lisa Brown2, Henrietta Cserne-Szappanos1, George G. Rodney1
1. Department of Biochemistry and Molecular Biology, University of Maryland School of Medicine, Baltimore, MD, USA. 2. Department of Biology, Morgan State University, Baltimore, MD, USA.
Activation of a skeletal muscle fibre is initiated by electrical depolarization of its transverse tubules (TTs), which penetrate the fibre at each sarcomere. Membrane voltage sensors within the TT dihydropyridine receptor (DHPR) molecules in the TT membrane respond to TT depolarization, and trigger sarcoplasmic reticulum (SR) Ca2+ release via the abutting ryanodine receptor (RyR) Ca2+ release channels in the adjacent SR membrane. In frog muscle, the 'macroscopic' Ca2+ release caused by fibre depolarization has been shown by us and others to be composed of huge numbers of discrete local Ca2+ release events, the Ca2+ sparks, which blend together to form the macroscopic Ca2+ transient. Ca2+ sparks also occur spontaneously in resting frog fibres, but at much lower frequencies. We have used ultra high speed confocal line scan imaging of fluo-4 fluorescence in frog fibres slightly depolarized by exposure to elevated K+ Ringer solution in an attempt to characterize the gating properties of the few channels that underlie each voltage-activated Ca2+ spark, and to determine whether the group of channels always gates in unison, or may sometimes gate independently during a spark, as indicated by a change in the rate of rise of fluorescence prior to the final exponential decay of spark fluorescence. In mouse muscle, Ca2+ sparks similar to those in frog muscle are extremely infrequent in resting adult muscle, but are present in embryonic muscle and may appear under abnormal or pathological conditions, possibly due to DHPR-RyR uncoupling. We have found that Ca2+ sparks reappear during dedifferentiation of adult mouse flexor digitorum brevis muscle fibres in culture, and that the sparks that appear are blocked by the plasma membrane (PM) and TT L-type Ca2+ channel blocker nifedipine, as well as by the non-specific PM and TT Ca2+ channel blocker Co2+. These agents were previously shown to block Ca2+ sparks in embryonic muscle. Thus the Ca2+ sparks that appear during fibre dedifferentiation may represent a reversal of the embryionic muscle development/maturation process. Adult mammalian fibres express predominantly only the RyR1 isoform, which is present in an alternating checkerboard pattern of RyRs that are coupled and not coupled to DHPRs. Frog fibres express similar amounts of both the homologs of RyR1 (in an alternating coupled pattern) and of RyR3 (all uncoupled), as does embryionic mammalian muscle. Arrays of uncoupled RyRs may be needed for production of frog-like Ca2+ sparks in either frog or mammalian muscle.
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Where applicable, experiments conform with Society ethical requirements.