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Biophys J, May 2002, p. 2428-2435, Vol. 82, No. 5

Ca2+ Activation of RyR1 Is Not Necessary for the Initiation of Skeletal-Type Excitation-Contraction Coupling

Jennifer J. O'Brien,* Wei Feng,dagger Paul D. Allen,Dagger S. R. Wayne Chen,§ Isaac N. Pessah,dagger and Kurt G. Beam*

 *Department of Anatomy and Neurobiology, Colorado State University, Fort Collins, Colorado 80523 USA;  dagger Department of Molecular Biosciences, University of California at Davis, Davis, California 95616 USA;  Dagger Department of Anesthesia, Brigham & Women's Hospital, Boston, Massachusetts 02115 USA; and  §Department of Physiology and Biophysics, University of Calgary, Calgary, Alberta T2N 4N1, Canada

Although an elevation in myoplasmic Ca2+ can activate the skeletal muscle ryanodine receptor (RyR1), the function of this Ca2+ activation is unclear because extracellular Ca2+ influx is unnecessary for skeletal-type EC coupling. To determine whether Ca2+ activation of RyR1 is necessary for the initiation of skeletal-type EC coupling, we examined the behavior of RyR1 with glutamate 4032 mutated to alanine (E4032A-RyR1) because this mutation had been shown to dramatically reduce activation by Ca2+. Proc. Natl. Acad. Sci. USA. 98:2865-2870). Analysis after reconstitution into planar lipid bilayers revealed that E4032A-RyR1 was negligibly activated by 100 µM Ca2+ (Po too low to be measured). Even in the presence of both 2 mM caffeine and 2 mM ATP, Po remained low for E4032A-RyR1 (ranging from <0.0001 in 100 µM free Ca2+ to 0.005 in 2 mM free Ca2+). Thus, the E4032A mutation caused a nearly complete suppression of activation of RyR1 by Ca2+. Depolarization of E4032A-RyR1-expressing myotubes elicited L-type Ca2+ currents of approximately normal size and myoplasmic Ca2+ transients that were skeletal-type, but about fivefold smaller than those for wild-type RyR1. The reduced amplitude of the Ca2+ transient is consistent either with the possibility that Ca2+ activation amplifies Ca2+ release during EC coupling, or that the E4032A mutation generally inhibits activation of RyR1. In either case, Ca2+ activation of RyR1 does not appear to be necessary for the initiation of Ca2+ release during EC coupling in skeletal muscle.

Biophys J, May 2002, p. 2428-2435, Vol. 82, No. 5
© 2002 by the Biophysical Society   0006-3495/02/05/2428/08  $2.00


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